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We have studied the luminescence of molecular nitrogen nanoclusters containing stabilized nitrogen, oxygen, hydrogen, and deuterium atoms. Optical spectra were observed during the destruction of these ensembles of nanoclusters accompanied by a rapid release of chemical energy stored in the samples. Several interesting features were observed including a broad band near λ ~ 360 nm, which has been identified as emission corresponding to 2Ag →1Ag transition of N4(D2H) polymeric nitrogen. Also the sharp lines at λ = 336 nm and 473 nm were observed, and their assignments to ND radicals are discussed.
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Luminescence of Molecular Nitrogen Nanoclusters Containing
Stabilized Atoms
Published as part of The Journal of Physical Chemistry virtual special issue W. Lester S. Andrews Festschrift.
Patrick T. McColgan,
Adil Meraki,
Roman E. Boltnev,
,§
David M. Lee,
and Vladimir V. Khmelenko*
,
Department of Physics and Astronomy and Institute for Quantum Science & Engineering, Texas A&M University, College Station,
Texas 77843, United States
Department of Physics, Bilecik University, 11210 Gülümbe, Bilecik, Turkey
Talrose Institute for Energy Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Russia
§
Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow, Russia
ABSTRACT: We studied the luminescence of molecular
nitrogen nanoclusters containing stabilized nitrogen, oxygen,
hydrogen, and deuterium atoms. Optical spectra were
observed during the destruction of these ensembles of
nanoclusters accompanied by a rapid release of chemical
energy stored in the samples. Several interesting features were
observed including a broad band near λ360 nm, which was
identied as emission corresponding to 2Ag1Agtransition of
N4(D2h) polymeric nitrogen. Also the sharp lines at λ336
and 473 nm were observed, and their assignments to ND
radicals are discussed.
INTRODUCTION
Chemical and physical processes occurring in solid nitrogen
have stimulated a broad range of research including studies of
ices present in the interstellar medium,
1
as well as studies of
high-energy density materials (HEDM). A promising method
in the search for high-energy density systems has achieved local
concentrations as high as 2 ×1021 cm3of stabilized nitrogen
atoms in aerogel-like ensembles of nitrogen nanoclusters
submerged in superuid helium.
25
In this method the
products of a radio frequency (RF) discharge in nitrogen
helium gas mixtures were injected into bulk superuid helium,
resulting in the production of ensembles of molecular nitrogen
nanoclusters containing very high concentrations of stabilized
nitrogen atoms.
6,7
This method also holds promise for the
study of low-temperature chemical reactions in ensembles of
nanoclusters. For example, exchange tunneling reactions
between atoms and molecules of hydrogen isotopes were
studied in nanoclusters immersed in superuid helium.
811
Another possibility is the investigation of chemical reactions of
a variety of heavier atoms and molecules during the warming of
ensembles of nanoclusters containing stabilized atoms. Earlier
investigations of chemical reactions during warming were
performed in ensembles of molecular nitrogen and nitrogen
rare gas nanoclusters containing stabilized N and O atoms.
1215
Rapid release of stored chemical energy in the samples resulted
in intense thermoluminescence. In the optical spectra of
thermoluminescence the bands of N and O atoms as well as N2,
NO, and O2molecules were observed. During investigations of
the dynamics of spectra accompanying the destruction of these
samples, it was found that during the process of annealing by
raising the sample temperature, the emission of N and O atoms
and the VegardKaplan (V-K) bands of N2molecule were
present. At the end of the destruction process bright ashes
were observed, and in the spectra of these ashes, intense bands
of O atoms and NO and O2molecules were present.
16
It was
found that small changes in the oxygen content in the
nanoclusters drastically inuenced the optical spectra ob-
tained.
16
This earlier work provided examples of observations
of chemical reactions in collections of nanoclusters containing
stabilized nitrogen and oxygen atoms.
In the experiment presented in this work, we added
hydrogen or deuterium molecules into the gas mixtures used
for preparing the samples in bulk superuid helium. First, we
expected that the addition of a small quantity of hydrogen
isotopes would increase the eciency of dissociation of atoms
in the radio-frequency (RF) discharge zone, creating samples
with the highest energy content.
17
During destruction of these
samples, new nitrogen compounds such as polynitrogen
molecules might be formed. The synthesis of HEDMs is a
signicant problem, and a promising direction is to synthesize
polymeric nitrogen. A large energetic release from polynitrogen
molecule decomposition provides a strong motivation to study
Received: September 28, 2017
Revised: November 3, 2017
Published: November 7, 2017
Article
pubs.acs.org/JPCA
© 2017 American Chemical Society 9045 DOI: 10.1021/acs.jpca.7b09661
J. Phys. Chem. A 2017, 121, 90459057
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J. Phys. Chem. A
2017, 121, 9045-9057
This is an open access article published under an ACS AuthorChoice License, which permits
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polynitrogen as a clean HEDM. Such materials decompose into
environmentally clean N2and produce enormous amounts of
energy per unit mass without harmful waste.
18
Theoretical
calculations have evaluated the structure and stability of
numerous isomers of possible Nnmolecules with nranging
from 3 to 60, but only some of the isomers are good candidates
for HEDM. Neutral and ionic species N3,N
3
+,N
3
,N
4
+, were
detected and studied in solid nitrogen lms.
1924
A cation N5
+
has been synthesized as a part of a compound.
25,26
There are
some detailed reviews of experimental and theoretical work on
polynitrogen compounds in the literature.
27,28
Experimental
evidence was obtained for the existence of tetranitrogen
(tetrazete), N4, in the gas and solid phases.
2932
Matrix-isolated
tetranitrogen was obtained by condensing products of an
electrical discharge on a cold window,
31
or by bombarding solid
nitrogen by electrons.
32
In the later case an intense broad band
at λ= 360 nm was observed, which was assigned to the
emission of N4(D2h) tetranitrogen. There are several
approaches for the creating the N4species. The formation of
N4
+cations followed by a neutralization reaction with electrons
has been experimentally realized.
2932
However, the association
of two metastable N2(A3Σu
+) molecules was suggested as an
alternative mechanism for the formation of N4polynitrogen.
33
The conditions during our experiments are ideal for testing this
suggestion. In our experiments, molecular nitrogen nano-
clusters with high concentrations of stabilized N(4S) atoms
were formed. During destruction of the ensembles of
nanoclusters, N(4S) atoms recombine and create a large
quantity of metastable N2(A3Σu
+) molecules. Interactions of
pairs of these molecules during the explosive destruction of the
samples can lead to the formation of N4polymers, which could
be identied by the light emitted at λ= 360 nm.
32
Second, the addition of hydrogen or deuterium in the
nitrogenhelium gas mixture provides the possibility of
observing radicals containing H or D atoms such as the NH
(ND) radicals. Eight emission systems of the NH radical have
been found in the range from vacuum UV at 160 nm to near IR
at 1.2 μm.
34,35
The most intense transition is the triplet
system A3ΠX3Σwith a maximum around 336 nm. This
band is a chief characteristic of the NH radical.
17,3643
The
singlet systems c1Πa1Δat λ= 324 nm,
44,45
c1Πb1Σ+at λ=
450 nm,
46,47
and d1Σ+b1Σ+at λ= 162 nm
34,48
have been
observed in gas-phase spectra. The weak, forbidden transitions
a1ΔX3Σat λ= 795 nm
35,49
and b1Σ+X3Σat λ= 471
nm
35,50
were observed in the emission spectra of NH (ND) in
noble-gas matrices. Recently, the transition between the two
lowest metastable states b1Σ+a1Δwas detected at 1.17 μmin
the emission spectra of matrix-isolated NH.
35
In our experi-
ments, the spectra were thoroughly examined in these spectral
regions to observe the bands of NH (ND) radicals.
In this work we mainly studied the dynamics of optical
spectra during the destruction of ensembles of molecular
nitrogen nanoclusters containing stabilized nitrogen, oxygen,
and also hydrogen or deuterium atoms. We found that in the
spectra of the samples prepared from deuteriumnitrogen
helium gas mixtures the bands at λ= 336, 360, and 471 nm are
present in addition to bands observed during destruction of
nanoclusters containing only N and O atoms. The intensity of
all bands in the spectra were inuenced by the presence of
admixtures of hydrogen isotopes. We conclude that the band at
λ= 360 nm belongs to the emission of the N4compound,
supporting the results obtained in ref 32. Possible mechanisms
for the formation of the N4compounds are discussed. The
emission at λ= 336 nm was assigned to the A3Π,v=0
X3Σ,v= 0 transition of ND radicals. The assignment of 473
nm band is still controversial. Two species, namely, ND radicals
and Nanions,
51
may be responsible for this emission.
EXPERIMENTAL SETUP
Our experimental setup has been described elsewhere.
16
The
cryogenic portion of the experimental setup consists of two
silvered-glass double-walled Dewars. The outer Dewar is lled
with liquid nitrogen (LN2), and the inner Dewar is lled with
liquid helium (LHe). This inner Dewar is pumped by an
Edwards model E2M80 rotary pump. With this vacuum pump,
temperatures of 1.1 K of the liquid helium bath are
achievable.
Gas mixtures are prepared at room temperature using a gas
handling system. This system consists of a manifold that
connects storage tanks, mixing tanks, vacuum pumping lines,
and a ux controller for supplying and controlling gas mixtures
to the cryogenic system. In our experiments we used
hydrogennitrogenhelium gas mixtures. Research-grade
helium gas from Linde Electronics & Specialty Gases with
99.999% purity was used. The oxygen content in the gas
mixtures resulted from contamination in this gas (1 ppm).
The samples are created by injecting a gas mixture through
an RF discharge zone into superuid helium (HeII).
6,7
The
atomic source is made of a cylindrical outer quartz tube with a
concentric inner quartz capillary. At the bottom of the tube
there are two electrodes, which surround the capillary. The tube
is lled with LN2, which simultaneously cools the incoming gas
mixture and the discharge electrodes. The 75 W RF discharge
with frequency of 50 MHz is provided by an HP 8556B signal
generator amplied by an E&I 3100L amplier. The gas
mixture exits the discharge zone through an orice with
diameter 0.75 mm, which is 2 cm above the level of HeII
inside the beaker. A pressure gradient of 2 torr creates a well-
formed gas jet that penetrates the surface of the HeII, where the
gas mixture containing dissociated impurity atoms and excited
species forms ensembles of nanoclusters. The level of HeII in
the sample collection beaker was kept constant by lling with a
thermomechanical fountain pump, which sent HeII from the
bottom of the main bath. During sample preparation, the
temperature of HeII was maintained at 1.5 K. The temperature
inside of the beaker is measured by using a germanium
thermometer.
After the sample accumulates for 1030 min, the ow of gas
mixture is ceased, and the RF discharge is turned o. The
fountain pump is then turned o. Over the course of 2030
min, the HeII exits the beaker by processes of evaporation and
creeping lm, leaving a dry sample. When the liquid helium is
nearly removed from the beaker, the pumping line is closed
allowing the temperature of the sample to increase from 1.2 to
15 K in 50 s and initiate destruction of the sample.
Thermoluminescence from the sample increases with temper-
ature, until the sample is completely destroyed. This process is
accompanied by a series of bright ashes.
The optical spectra are obtained using a special bifurcated
optical ber assembly part of which was installed inside the
helium Dewar, where it was able to withstand liquid-helium
temperatures.
16
By employing this bifurcated ber assembly, we
could make simultaneous measurements in two dierent
spectrometers, the Andor Shamrock SR-500i spectrometer
with Newton EMCCD camera and the Ocean Optics HR2000+
spectrometer.
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The high-sensitivity Andor spectrometer can take high-
resolution spectra (0.53 nm, rst grating) and can capture
spectra with an exposure time of 3 ms in a limited (Δλ340
nm) spectral range. The Ocean Optics spectrometer can take
spectra over a large wavelength range (from 200 to 1100 nm)
with the resolution of 1.3 nm and generally uses an exposure
time varying from 100 to 300 ms.
EXPERIMENTAL RESULTS
Inuence of Hydrogen on Thermoluminescence
Spectra of Ensembles of Nitrogen Nanoclusters. First
we studied the destruction process of samples prepared from
the gas mixture H2/N2/He = 1:330:33 000. Figures 1 and 2
show the dynamics of luminescence during the destruction of
samples prepared from gas mixture H2/N2/He = 1:330:33 000
in the wavelength range from 240 to 580 nm and 540880 nm,
respectively. Figure 1a shows the dynamics of the emission of
α-group of N atoms, V-K bands of N2molecules, and β- and
β-groups of O atoms,
16
which were observed from the
beginning of sample thermoluminescence. The intensities of
these features increase with temperature. The spectrum of
emission observed during the early stage of destruction, with
identication of bands, is shown in Figure 1b. At the end of the
destruction during bright ashes, the intense β-group and the
band with maximum at λ= 360 nm were most intense. The
dynamics of thermoluminescence at the nal stage destruction
is shown in Figure 1c. During the time period of 60 ms the
correlation between the emission of the β-group and the band
with a maximum at λ= 360 nm were observed. The spectrum
of the most intense ash with identication of the bands is
presented in Figure 1d. The weak α-group of nitrogen atoms
and β-group of oxygen atoms are also present in this spectrum.
Figure 2a shows the dynamics of the emission of the β-group
of O atoms, α- and δ-groups of N atoms, and the γ-line of N
anions.
51
The intensity of all lines increased with temperature.
The spectrum of the emission at the beginning of the
destruction with the identication of all observed bands is
shown in Figure 2b. The dynamics of the thermoluminescence
during the last 900 ms of sample destruction is shown in Figure
2c with better time resolution. At the end of the destruction,
the intense β-group of O atoms and γ-line of Nanions, as well
as weaker δ-group of N atoms, and β-group of O atoms were
observed. The spectrum with identied bands corresponding to
largest ashes at the end of destruction is shown in Figure 2d.
The most striking change in the spectra compared to those
obtained for nitrogenhelium samples
13,14,16
was the appear-
ance of a broad band with maximum at λ= 360 nm. The
positions and origins of the atomic and molecular bands in the
spectra presented in Figures 1 and 2are listed in columns 2 of
Tables 1 and 35. The presence of α-, β-, β-, and δ-groups
of N and O atoms in the spectra indicates that N2molecules are
neighbors of these atoms.
16,52
As a next step, we observed the inuence of the presence of
molecular hydrogen in the initial gas mixtures on the
appearance of the band at λ= 360 nm. Thus, spectra were
studied during destruction of the samples prepared from
Figure 1. Spectra taken in the range of 240580 nm by the Andor spectrometer during destruction of the sample prepared from gas mixture [H2]/
[N2]/[He] 1:330:33 000. (a) Dynamics of luminescence spectra for the entire destruction process. Each spectrum was accumulated during a period
of 1.5 s. (b) Spectrum taken during destruction at t= 37.5 s with band identications. (inset) Temperature dependence on time during sample
destruction. (c) Dynamics of luminescence spectra during the nal period of destruction (150 ms) of the sample with exposure time of 3 ms. (d)
Spectrum taken at the end of sample destruction corresponds to t= 54.891 s with band identications. (inset) Quantum eciency of the Andor
spectrometer.
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dierent H2/N2/He gas mixtures. The ratio between H2/N2in
these mixtures was changed from 1/200 to 1/500.
Figure 3 shows the integrated spectra of the emission
observed with the Ocean Optics spectrometer during the
destruction of samples prepared from dierent H2/N2/He gas
mixtures. Lines and bands observed include the α-group and α-
group of N atoms, the γ-line of Nanions, the V-K bands of N2
molecules (v=0,v=212), and the β-group of O atoms.
This gure demonstrates the inuence of the concentration of
H2in the sample gas mixture on the intensity of the observed
lines in the spectra. The integrals of intensities of bands shown
in Figures 3 and 4are listed in Table 2. The maximum
intensities of the lines were observed for the gas mixture with
the ratio of H2/N2= 1/330 as shown in Table 2. The intensities
of bands from the sample prepared from gas mixture with H2/
N2ratio equal to 1/200 are 49 times smaller, and the
intensities of the bands of the sample prepared from gas
mixture with ratio H2/N2= 1/500 were twice smaller.
Figure 4 shows spectra of the emission of the largest ashes
observed with the Ocean Optics spectrometer accompanying
the destruction of samples prepared from gas mixtures with
dierent H2/N2/He ratios. Lines and bands observed include
the α-, δ-, and δ-groups of N atoms, the γ-line of Nanions,
51
the V-K bands of N2molecules, the β-group of O atoms, and
the M-bands of the NO molecule. It is also interesting to note
that for all samples made from gas mixtures with small
admixtures of molecular hydrogen the broad feature at λ= 360
nm was clearly observed only in the spectra of the largest
ashes. This gure demonstrates the same tendency between
the concentration of H2in the gas mixture used for sample
Figure 2. Spectra taken in the range of 540880 nm with the Andor spectrometer during destruction of the sample prepared from gas mixture [H2]/
[N2]/[He] = 1:330:33 000. (a) Dynamics of the luminescence spectra during the entire destruction process. Each spectrum has an exposure time of
1.5 s. (b) Spectrum taken during destruction at t= 37.5 s with identication of all bands observed with exposure time of 1.5 s. (c) Dynamics of the
luminescence spectra during the nal period of sample destruction with exposure time of 3 ms. (d) Spectrum taken at the end of sample destruction
at t= 37.491 s with band identications.
Table 1. Positions (nm) of the V-K Bands N2(A3Σu
+v=0X1Σgv) Observed during the Destruction of Samples Prepared
from Dierent Gas Mixtures
band in N2[H2]/[N2]/[He] [D2]/[N2]/[He] [D2]/[N2]/[Ar]/[He] [D2]/[N2]/[Ne]/[He]
v,vmatrix
53
1:330:33 000 1:2000:100 000 1:500:4500:225 000 1:500:5000:100 000
0,4 248.4 248.6 250.08 248.2 248.9
0,5 263.0 263.2 263.22 263 263.7
0,6 278.9 279.1 278.69 278.6 279.5
0,7 296.8 296.8 296.49 296.4 297.1
0,8 316.8 317.2 318.12 315.9 317.2
0,9 339.3 337.9 338.3 340.1
0,10 365.2 365.5 365.65 365.5
0,11 393.5 394.4 394.3 394.9 394.6
0,12 427.1 429.99
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preparation and the intensity of the lines in the emitted spectra
as was demonstrated in Figure 3.Themostintense
luminescence was observed for the sample prepared from the
gas mixture with ratio of H2/N21/330 as shown in Table 2
and Figure 4. In contrast to the integrated spectra, in the
spectra of the largest ashes, V-K bands are substantially
suppressed, but a new rather intense band with maximum at λ=
360 nm appears. The intensity of the band at λ= 360 nm for
gas mixture with H2/N2ratio of 1/330 was 3 times greater than
for the gas mixture with H2/N2ratio of 1/200 and 4 times
greater than for gas mixture with H2/N2ratio of 1/500.
Inuence of Deuterium on Thermoluminescence
Spectra of Ensembles of Nitrogen Nanoclusters. To
understand the origin of the band at λ= 360 nm we performed
experiments to study the inuence of the addition of D2
molecules to N2He gas mixtures used for sample preparation
on the spectra obtained during sample destruction. The isotope
shift eect can help in the identication of bands corresponding
to species containing hydrogen isotopes.
Figure 5a shows the dynamics of the emission of the α-group
of N atoms, the V-K bands of N2molecules, and the β-group of
O atoms, which were observed in the thermoluminescence of
the sample prepared from the gas mixture D2/N2/He =
1:2000:100 000. The intensities of the bands mentioned above
increase with temperature. The spectrum of the emission is
shown in Figure 5b near the end of destruction, where the
emission from the V-K band is most intense. This spectrum
also includes the strong α-group of N atoms, and the β-group
of O atoms as well as other weaker lines including the β-group
from oxygen, and the line at λ= 473 nm. The dynamics of
thermoluminescence at the end of destruction is shown in
Figure 5c. During the time period of 60 ms, the intensity of
emission of the β-group correlates with the intensity of the
band at λ= 360 nm. The spectrum of this emission is shown in
Figure 5d. The strong β-group of O atoms and the band at λ=
360 nm along with the weaker α-group of N atoms, and bands
at 337 and 473 nm, are also present in this spectrum. The
positions and origins of atomic and molecular bands in the
spectra presented in Figure 5 are listed in column 3 in Tables 1
and 35. It is interesting to note that the appearance of the
bands at λ= 336 nm and at λ= 473 nm in the samples
prepared with addition of D2molecules is in contrast with the
case of hydrogen-containing samples, where these bands were
not observed.
We also studied the inuence of the concentration of D2
molecules in the N2/He gas mixture used for sample
preparation on the intensity of the bands in the spectra
obtained during the destruction of the samples. Figure 6 shows
the integrated spectra of the emission observed with the Ocean
Optics spectrometer during the destruction of dierent D2/N2/
He samples. Lines and bands observed include the α- and α-
groups of N atoms, the γ-line of Nanions, the V-K bands of
Figure 3. Integrated spectra observed with the Ocean Optics
spectrometer during the entire destruction process for samples created
from dierent H2/N2/He gas mixtures: (a) [H2]/[N2]/[He] =
1:200:20 000. (b) [H2]/[N2]/[He] = 1:330:33 000. (inset) The
quantum eciency of the Ocean Optics spectrometer. (c)[H2]/
[N2]/[He] = 1:500:50 000.
Figure 4. Spectra of the largest ashes during the destruction of
samples formed by dierent hydrogennitrogenhelium gas mixtures:
(a) [H2]/[N2]/[He] = 1:200:20 000, (b) [H2]/[N2]/[He] =
1:330:33 000, and (c) [H2]/[N2]/[He] = 1:500:50 000. Spectra
were taken with the Ocean Optics spectrometer with exposure time
of 500 ms.
Table 2. Integrals of the Intensities of Bands (arb. units) Observed during the Entire Destruction Process (Int) and during the
Largest Flash (Flash) for Samples Prepared from Dierent HydrogenNitrogenHelium Gas Mixtures
[H2]/[N2]/[He] [H2]/[N2]/[He] [H2]/[N2]/[He]
1:200:20 000 1:330:33 000 1:500:50 000
band Int Flash Int Flash Int Flash
α1.92 ×1051.73 ×1031.74 ×1062.06 ×1049.02 ×1056.62 ×103
β1.41 ×1052.10 ×1049.03 ×1053.44 ×1054.73 ×1053.01 ×104
γ1.13 ×1054.01 ×1034.38 ×1057.53 ×1042.82 ×1055.42 ×103
λ= 360 nm 5.49 ×1041.75 ×1054.06 ×104
VK (v= 0,v= 7) 3.39 ×1059.76 ×1031.62 ×1066.00 ×1031.00 ×1061.22 ×104
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N2molecules (v=0,v=213), and the β-group of O atoms.
This graph demonstrates that there exists an optimal
concentration of D2in the gas mixture used for sample
preparation that produces a maximum intensity for all lines of
luminescence during sample destruction. This optimum
concentration of D2corresponds to the spectra shown in
Figure 6b. The integrals of intensities of the bands shown in
Figures 6 and 7are listed in Table 6.
Figure 7 shows spectra of the emission observed with the
Ocean Optics spectrometer for the largest ashes during
Figure 5. Spectra taken in the range of 240580 nm with the Andor spectrometer. Spectra were observed during the destruction of the sample
prepared from gas mixture [D2]/[N2]/[He] = 1:2000:100 000: (a) Luminescence dynamics for the entire destruction process. Each spectrum has an
exposure time of 1.5 s. (b) Spectrum taken at t= 34.5 s with 1.5 s exposure time with band identications. (c) Dynamics during the nal stage (60
ms) of the sample destruction. Each spectrum has exposure time of 3 ms. (d) Spectrum taken at the end of destruction at t= 37.509 s with 3 ms
exposure time with band identications.
Table 3. Positions (nm) of Lines of N and O Atoms, and Nitrogen Anion Observed during the Destruction of Samples Prepared
from Dierent Gas Mixtures
in N2[H2]/[N2]/[He] [D2]/[N2]/[He] [D2]/[N2]/[Ar]/[He] [D2]/[N2]/[Ne]/[He]
matrix
54
1:300:30 000 1:2000:100 000 1:500:4500:225 000 1:500:5000:100 000
α522.8 522 522.3 523.7 523.3
α594.4 594.5 592.1 594.5
δ857 857.2
β554.9 557.8 557.4 559.1 557.6
β636.7 633.2 632.7
β494
γ791 793 794.0 794.2 793.8
Table 4. Positions (nm) of NewLines Emitted during the Destruction of Samples Prepared from Dierent Gas Mixtures with
the Addition of Hydrogen and Deuterium Molecules
in N2[H2]/[N2]/[He] [D2]/[N2]/[He] [D2]/[N2]/[Ar]/[He] [D2]/[N2]/[Ne]/[He]
matrix 1:330:33 000 1:2000:100 000 1:500:4500:225 000 1:500:5000:100 000
336 (ND)
41
337.8 336.6 337.7
471 (ND)
41
472.9 473.9 472.4
360 (N4
+)
32
361.9 363.3 361.5 365.1
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destruction of samples prepared from dierent D2/N2/He gas
mixtures. Lines and bands observed include the broad band at λ
= 360 nm, the α-group of N atoms, the γ-line of Nanions, the
weak V-K bands of N2molecules (v=0,v=211), the
strong β-group of O atoms, and the M-bands of NO molecules
(v=0,v=610). This graph demonstrates the similar
relation between the concentration of D2in the gas mixture
used for sample preparation and the intensity of the
luminescence observed during sample destruction as was
shown in Figure 6. The optimum concentration of D2for
observation of the most intense bands corresponds to the
[D2]/[N2]/[He] = 1:2000:100 000 gas mixture as can be seen
from Table 6. The spectrum of the sample created from this gas
mixture is shown in Figure 7b. It is also interesting to note that
in all spectra of the largest ashes of the samples prepared from
D2/N2/He gas mixtures, the intense broad feature at λ= 360
nm was observed.
Destruction of Samples Prepared from Argon- and
Neon-Containing Mixtures. To observe possible matrix
eects on the positions of the atomic and molecular bands, we
also studied samples prepared from deuteriumnitrogen
helium gas mixtures with the addition of rare gases Ne and Ar.
Figure 8a shows the dynamics of the emission of the sample
prepared from gas mixture D2/N2/Ar/He =
1:500:4500:225 000. The α-group of N atoms, the V-K bands
of N2molecules, and the β-group of O atoms were observed
from the beginning of sample thermoluminescence. The
intensities of these features increase with temperature. The
spectrum of the sample emission with identication of bands is
shown in Figure 8b. The spectrum was observed near the end
of destruction, where the emission from the VK band is most
intense. This spectrum also includes the strong α-group of N
atoms, the β-group of O atoms, and in addition other weaker
lines at λ= 473 and the β-group of oxygen. The dynamics of
thermoluminescence during the brightest ashes is shown in
Figure 8c. During the time period of 60 ms the emission of α-
group, the β- and β-groups and bands at λ= 336 nm and λ=
473 nm, as well as the broad feature at λ= 360 nm, were
observed. The spectrum of this emission with band
identications is shown in Figure 8d.
Figure 9 shows a comparison of spectra taken during the
destruction of samples created from deuteriumnitrogen
helium gas mixtures as well as those formed with the addition
of rare gases Ne and Ar. Features observed in this spectra
include the strong α-group of N atoms and the β-group of O
atoms, the V-K bands of N2molecules (v=0,v=411), the
M bands of NO molecules (v=0,v=610), along with the
broad band with maximum at λ= 360 nm, and the bands at λ=
336 nm and at λ= 473 nm. The most intense ash was from
Table 5. Positions (nm) of NOM Bands (a4Π,v=0X2Π,v) and NO β-Bands (B2Π,v=0X2Π,v) Observed during the
Destruction of Samples Prepared from Dierent Gas Mixtures
N2[H2]/[14N2]/[He] [D2]/[N2]/[He] [D2]/[N2]/[Ar]/[He] [D2]/[N2]/[Ne]/[He]
v,vmatrix
55
1:300:30 000 1:2000:100 000 1:500:4500:225 000 1:500:5000:100 000
NOM bands
0,7 399.41 394.8 398.8 401.6 395.2
0,8 428.71 429.0 429.4 430.9 429.9
0,9 462.00 464.1 463.6
0,10 494.4
16
496.8 494.2 494.6 496.6
NO-βbands
0,8 322.40 323.5 324.4
0,9 340.40 340.7 339.4
0,10 359.30
0,11 383.75
16
384.0 384.0
Figure 6. Integrated spectra taken with the Ocean Optics
spectrometer during the destruction of samples created from dierent
deuteriumnitrogenhelium gas mixtures: (a) [D2]/[N2]/[He] =
1:750:50 000, (b) [D2]/[N2]/[He] = 1:2000:100 000, and (c) [D2]/
[N2]/[He] = 1:10 000:500 000.
Figure 7. Spectra taken with the Ocean Optics spectrometer of the
largest ashes during the destruction of samples created from dierent
deuteriumnitrogenhelium gas mixtures: (a) [D2]/[N2]/[He] =
1:750:50 000, (b) [D2]/[N2]/[He] = 1:2000:100 000, (c) [D2]/[N2]/
[He] = 1:10 000:500 000.
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the sample prepared from the gas mixture containing neon. The
positions and origins of atomic and molecular bands in the
spectra presented in Figures 5 and 9are listed in columns 35
of Tables 1 and 35.
DISCUSSION
We have studied the destruction of nanostructures with high
densities of stored chemical energy. The fast release of chemical
energy resulted in intense sample thermoluminescence. The
nanostructures were formed by nanoclusters of molecular
nitrogen in bulk superuid helium. The average size of these
nanoclusters was 5 nm with an overall density of impurity
material in liquid helium of order 1×1020 cm3as
determined form X-ray experiments.
5661
Nanoclusters form
porous aerogel-like structures with a wide distribution of pore
sizes from 8 to 800 nm.
62,63
The pores initially were lled with
liquid helium.
57
The stabilized atoms reside mostly on the
surface of the nanoclusters.
5,64,65
The average concentration of
stabilized nitrogen atoms was of order 1 ×1019 cm3,
3,4
and the
local concentrations of nitrogen atoms in the nanoclusters were
of order 1 ×1021 cm3as determined by the width of electron
spin resonance (ESR) spectra.
5
These samples contained O
atoms and H(D) atoms, whose concentrations were from 1 ×
103to 1 ×104times smaller than the N atom concentrations.
As a result, the porous structures formed by molecular nitrogen
nanoclusters containing stabilized atoms are characterized by
high specic energy content (up to 1 ×104J/g).
2,3
These
structures were stable while immersed in bulk superuid
helium. Removing liquid helium from the structures led to a
collapsing of the pores and the initiation of chemical reactions
of atoms residing on the surfaces of the nanoclusters. At the
beginning of the sample destruction we observed an increase of
the emission of the sample. In the earlier work these structures
were formed by nitrogen molecules and atoms with a small
Table 6. Integrals of the Intensities of Bands (arb. units) Observed during the Entire Destruction Process (Int) and during the
Largest Flash (Flash) for Samples Prepared from Dierent DeuteriumNitrogenHelium Gas Mixtures
[D2]/[N2]/[He] [D2]/[N2]/[He] [D2]/[N2]/[He]
1:750:50 000 1:2000:100 000 1:10 000:500 000
band Int Flash Int Flash Int Flash
α8.47 ×1041.41 ×1031.58 ×1061.76 ×1041.16 ×1061.19 ×104
β4.12 ×1042.58 ×1032.41 ×1055.81 ×1043.58 ×1055.05 ×104
γ2.24 ×1051.24 ×1042.86 ×1055.07 ×1032.01 ×1051.11 ×104
λ= 360 nm 2.58 ×1041.25 ×1056.77 ×104
V-K v= 0,v= 7 1.23 ×1055.13 ×1035.47 ×1059.48 ×1031.15 ×1054.86 ×103
Figure 8. Spectra taken in the range of 240580 nm with the Andor spectrometer. Spectra observed during the destruction of the sample prepared
from gas mixture [D2]/[N2]/[Ar]/[He] = 1:500:4500:225 000: (a) Dynamics of spectra during the entire destruction process with exposure time of
1.5 s. (b) Spectrum taken at t= 44.1 s with 1.5 s exposure time with identication of all observed bands. (c) Dynamics of spectra taken with exposure
time of 3 ms during the nal stages of the sample destruction. (d) Spectrum taken at t= 27.231 s with 3 ms exposure time with identication of
observed bands.
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9052
admixture of O atoms (N2/O21×103to 1 ×104). This
explains the spectra obtained during the destruction of the
samples, where in addition to the expected α- and α-bands of
nitrogen atoms and the V-K bands of N2molecules, the
β-,β-,β-bands of O atoms, M- and β-bands of NO molecules
and even the second Herzberg bands of O2molecules were
observed.
1214,16
The addition of hydrogen or deuterium
molecules into the gas mixtures used for sample preparation
can lead to the appearance of luminescence from some new
species formed in chemical reactions during the destruction of
these samples. It was found that a relatively small change of the
hydrogen and deuterium impurity content drastically inuenced
the spectra obtained (see Figures 3,4,6, and 7). Indeed, in the
spectra obtained during the destruction of samples prepared
from deuteriumnitrogenhelium gas mixtures we observed
new intense bands at λ= 336, 360, and 473 nm and for samples
prepared from hydrogennitrogenhelium gas mixtures we
observed only one new intense band at λ= 360 nm in addition
to the bands observed in the spectra obtained from samples
prepared from purenitrogenhelium gas mixtures.
14,16
Possible candidates for emission of these bands may be
species containing H and D atoms such as the radicals NH and
ND. The energy levels and transitions of the NH radical are
presented in Figure 10. The transitions A3ΠiX3Σand
b1Σ+X3Σof ND can be responsible for emission at λ= 336
and 473 nm, respectively. The position of the ND bands in
dierent matrices is shown in Table 7. The observed positions
of the new lines at λ= 336 and 473 nm shown in Table 4 are
close to the positions of ND radical lines in the rare-gas
matrices (see Table 7). Assignment of emission at λ= 473 nm
to the b1Σ+,v=0X3Σ,v= 0 transition of the ND radicals
are also supported by a decrease of the emission of δ- and δ-
groups of N atoms in the spectra when the band at λ= 473 nm
is present (see Figure 7). The b1Σ+-state of ND radical is
formed in the recombination of N(2P) and D(2S) atoms (see
Figure 10). As a result of this reaction the N(2P) atoms were
removed from the system, and thus the δ- and δ-emissions are
absent. In the emission from hydrogennitrogenhelium
samples the band at λ= 473 nm is absent, but the emission
of the δ- and δ-groups is present (see Figure 4), providing
evidence that N(2P) atoms have undergone radiative decay and
thus did not participate in the reactions with H(2S) atoms in
this case.
An alternative interpretation for the line at λ= 473 nm may
be the transition 1S03P1of Nanion.
51
Figure 11 shows the
energy diagram for the three lowest energy levels of the N
anion. If the line at λ= 473 nm belongs to the N(1S03P1)
transition we can calculate the line corresponding to the
N(1S01D2) transition, because we know the position of the
line corresponding to the N(1D23P1,2) transition (γ-line).
51
The position of the N(1S03D2) transition should be at λ=
1167 nm. The position of this line should be resolved even if
the line corresponding to the NH (ND) transition b1Σ+,v=
Figure 9. Spectra of the largest ash observed by the Andor
spectrometer with resolution of 0.5 nm during the destruction of
samples prepared from dierent gas mixtures: [D2]/[N2]/[Ne]/[He]
= 1:500:5000:100 000 (blue), [D2]/[N2]/[Ar]/[He] =
1:500:4500:225 000 (red), and [D2]/[N2]/[He] = 1:2000:100 000
(black).
Figure 10. Energy diagram for NH radicals.
66
The transitions observed
in this work are shown as thicker arrows.
Table 7. Positions (nm) of NH and ND Transition A3Πi,v=
0X3Σ,v= 0 and b1Σ+,v=0X3Σ,v= 0 in Dierent
Rare-Gas Matrices
35,41
radical matrix A3Πi,v=0X3Σ,v=0 b
1Σ+,v=0X3Σ,v=0
NH Ne 335.92
41
Ar 337.76
41
472.99
35
Kr 338.95
41
ND Ne 335.57
41
Ar 337.76
41
473.26
35
Kr 338.73
41
Figure 11. Energy diagram for Nanions.
51
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0a1Δ+,v= 0, which was observed in an Ar matrix at λ=
1173.58 nm (1170.47 nm), is present in the spectra.
35
Experiments with registration of lines from the samples in
the near-IR region should resolve this problem. At the present
time we do not have equipment to investigate emission in this
(11001200 nm) wavelength range. We plan to perform these
experiments in the future. We can conclude that, for the
identication of the emission at λ= 473 nm, there are two
possible candidates, the ND radical (transition b1Σ+,v=0
X3Σ,v= 0) or the Nanion (transition 1S03P1). Both of
these candidates have the same precursor N(2P) atom.
From the comparison of the observed bands at λ= 336 nm
with the results of emission from ND(A3Πi,v=0X3Σ,v=
0) obtained in rare-gas matrices (see Table 7) we can assign
this emission to the A3ΠiX3Σtransition of the ND radical.
The A3Πi-state of the ND radical is the result of the
recombination of N(2D) and D(2S) atoms. Indeed we observed
a correlation between intensities of emissions at λ= 336 nm
and that of the α-group of N atoms, which supports the
assignment for this emission.
Another candidate for emission of the band at 336 nm might
be the N2molecule transition C3Πg,v=0B3Πg,v= 0. In the
gas phase this band is located at λ= 337 nm,
32
but there is no
evidence for observation of this band in solid nitrogen.
The most interesting result obtained in this work is the
observation of the rather intense broad band with the maximum
at λ= 360 nm for hydrogennitrogen-helium and deuterium
nitrogenhelium samples. After analysis of our previous results
we found that we observed only very weak bands at 360 nm
during the destruction of nitrogen nanoclusters containing only
stabilized nitrogen atoms and small (1 ×103to 1 ×104)
admixtures of oxygen atoms.
13,16
The addition of H2or D2
molecules in the gas mixture used for sample preparation
resulted in a large enhancement of the intensity of the band
appearing at λ= 360 nm in the luminescence spectra in the nal
stage of sample destruction (see Figures 4,7, and 9). We found
that the maximum intensity of the 360 nm band corresponds to
some optimum quantity of H2or D2present in the makeup gas
mixture. An important observation is that the addition of rare
gases in the makeup gas mixture does NOT inuence
signicantly the position of this band (see Figure 9).
In the literature there are two explanations for the emission
bands at λ= 360 nm. The rst observation of this broad band
was obtained when solid nitrogen was irradiated by 400 eV
electrons, and the band was assigned to an unidentied
impurity.
67
Later this luminescence band was studied during
the excitation of solid nitrogen lms by 500 eV electrons.
32
The
band was assigned to the emission of polynitrogen N4
*, which
was formed as a result of a neutralization reaction of N4
+with
electrons in solid nitrogen. The scenario of hole self-trapping
for N2
+ions with the formation of N4
+in a nitrogen matrix was
quite recently proposed
23,68
because of the localized character
of positive charge carriers in solid N2.
69
This suggestion is in
good accordance with both the study of gas-phase equilibria of
solvation reactions of N2
+with N2molecules, which revealed
electrostatic bonding in the core clusters N4
+
70
and the laser-
induced dissociation experiments showing N4
+as the ionic core
for the even ion clusters N2
+(N2)n.
71
Another interpretation of the 360 nm band corresponding to
the transition ND (A3Π,v=0X3Σ,v= 1) was suggested
from the analysis of the luminescence spectra of the 0.1% N2-
doped solid deuterium irradiated by electrons.
72
Emission of
NH (ND) radicals was also studied in dierent solid rare gases.
It was found that the position of the (01) NH(A3ΠX3Σ)
transition is shifted by a few nanometers for dierent solid rare
gases and that the band had a resolved structure.
41
We rule out
this interpretation because of the unusual broad shape of the
observed band at λ= 360 nm and the absence of the more
intense emission of the (00) NH(A3ΠX3Σ) transition.
In our experiments the spectra of the 360 nm band were not
signicantly inuenced by the addition of rare gases or by the
replacement of hydrogen isotopes in the nitrogen nanoclusters.
This would also give a preference for the interpretation of the
observed band at 360 nm in our experiments as an emission of
N4polynitrogen. However, the question remains open as to
why this emission was enhanced when the impurities of H2or
D2were present in the molecular nitrogen nanoclusters. A
possible explanation of this behavior is the change of the
eciency of formation of N atoms in the discharge zone with
the addition of H2and D2molecules. It has been known for
many years that the presence of water vapor in nitrogen
discharges increases the production of N atoms via dissociation
of N2molecules.
17
For the conditions of our experiments, the formation
mechanism of N4polynitrogen predicted in earlier theoretical
work
33
can be realized. It was shown that the barrier for N4
compound formation from two nitrogen molecules in the
excited metastable A3Σu
+states is very small (0.25 eV).
33
Intense recombination of N(4S) atoms during the explosive
destruction of the samples results in high concentrations of
metastable N2(A3Σu
+) molecules
16
that can participate in
bimolecular reactions to form N4compounds. The potential
curves for dierent electronic congurations of N4(D2h)
polynitrogen is shown in Figure 12. The bound state of the
N4compound in the potential curve of two interacting triplet
molecules is located at a distance R= 1.83 Å between molecules
(see Figure 12). The lower potential curve of the N4compound
for the interaction of two N2molecules in the ground state at
this distance has a repulsive character (see red curve in Figure
12). Therefore, the emission from the bound triplettriplet 2Ag
state of N4to the singletsinglet 1Agstate involving the
dissociation of the compound into two molecules (transition 1
in Figure 12) should be broad and without any structure just as
Figure 12. Dependence of the system energy on the distance R
between N2molecules. Each curve corresponds to the indicated
symmetry of the wave function. The interatomic distance inside the N2
molecule was L= 1.274 Å. (inset) An N4cluster of D2hsymmetry.
33
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9054
for the case of excimer molecules. The predicted energy of the
emitted quantum is 3.1 eV. Both of these predictions are in
good agreement with the broad nonstructured emission with a
maximum at λ= 360 nm (E3.44 eV) observed in our
experiments. According to this analysis we can thus assign the
observed emission at λ360 nm to the 2Ag1Agtransition of
the N4(D2h) compound.
33
After emission the compound
dissociates into two N2molecules in the ground state.
In the case of formation of N4compounds as a result of the
interaction of N2(A3Σu
+) and N2(B3Δu), the potential curve has
a minimum at R= 1.55 Å. At this distance there is also a
minimum in the potential curve of N4resulting from the
interaction of two N2(X1Σg
+) molecules in the ground state
(see Figure 12). This means that after emission of the quantum
with energy of 3 eV (transition 2 in Figure 12) the N4
compound will remain in the metastable state providing
possibility for the formation of an HEDM. Further
experimental and theoretical work would be desirable to realize
HEDMs from N4polynitrogen.
CONCLUSIONS
1. The process of stabilizing high concentrations of ground-
state atoms in molecular nitrogen nanoclusters provides a
unique opportunity to study low-temperature reactions
and to produce a variety of unusual species in excited
states.
2. A weak broad band at 360 nm is observed during the
destruction of ensembles of molecular nitrogen nano-
clusters containing stabilized nitrogen and small
admixtures of oxygen atoms. This band was greatly
enhanced in the spectra of samples prepared from gas
mixtures that contained hydrogen or deuterium.
3. Since this band is not changed signicantly in rare-gas
matrices, we suggest that N4
*polynitrogen molecule is
responsible for the emission of the band at 360 nm. The
exact shape and location of the 360 nm band is not
altered by hydrogen isotope substitution. These N4
polynitrogen compounds are formed during the process
of sample destruction accompanied by fast chemical
reactions of nitrogen atoms and molecules.
4. The observed emission at λ336 nm was assigned to
the ND radical, A3Π,v=0X3Σ,v= 0 transition.
5. The observed emission at λ= 473 nm may be assigned to
the ND radical, b1Σ+,v=0X3Σ,v= 0 transition,
but the alternative assignment to the Nanion 1S03P1
transition, can also be considered.
AUTHOR INFORMATION
Corresponding Author
*E-mail: khmel@physics.tamu.edu.
ORCID
Vladimir V. Khmelenko: 0000-0002-3362-3891
Notes
The authors declare no competing nancial interest.
ACKNOWLEDGMENTS
This work was supported by NSF Grant No. DMR 1707565.
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... The most studied species is atomic nitrogen observable in solid films in each of the lowest states 4 S, 2 D, and 2 P [2][3][4]. Neutral and ionic molecular species + + -+ 2 3 3 3 4 N , N , N , N , N , were detected and studied in solid nitrogen films and nanoclusters [5][6][7][8][9][10][11][12][13][14][15]. A cation + 5 N has been synthesized as a part of compound [16,17]. ...
... Luminescence spectrum integrated during destruction of this sample is shown in Fig. 4 [15]. The bands at 291, 307, and 323 nm correspond to the well known transitions of the β-system of NO molecules. ...
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... 59 A broad band about λ % 360 nm was also detected in the luminescence of molecular nitrogen nanoclusters. 84 Optical spectra were observed during the destruction of nanoclusters accompanied by a rapid release of chemical energy stored in the samples. This emission was identified as corresponding to 2 A g ! 1 A g transition of N 4 (D 2h ) polynitrogen center formed in IHC during the process of sample destruction accompanied by fast chemical reactions of nitrogen atoms and molecules. ...
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