Conference PaperPDF Available

Rotationally cold ( > 99% J = 0) OH − molecular ions in a cryogenic storage ring


Abstract and Figures

We store a 10 keV OH⁻ ion-beam at 13.5 ± 0.5 K in one of the DESIREE storage rings. Using photodetachment thermometry we measure the effective relative photodetachment cross section at different storage times and determine the rotational temperature of the ions to be 13.4 ± 0.2 K in agreement with the macroscopic temperature. A model cross section in the threshold range taking into account the formation of excited neutral OH molecules is calculated as a function of rotational temperature in order to justify the use of the rotational thermometry method developed earlier by the group of Roland Wester at Innsbruck University in the present case. In addition, we apply a selective photodetachment technique to produce an ion beam with more than 99% of the ions in the rotational ground state. The intrinsic lifetime of the J = 1 rotational level is measured to be 145 ± 28 s.
Content may be subject to copyright.
Journal of Physics: Conference Series
Rotationally cold (
= 0) OH
molecular ions in a cryogenic
storage ring
To cite this article: Gustav Eklund et al 2017 J. Phys.: Conf. Ser. 875 012016
View the article online for updates and enhancements.
This content was downloaded from IP address on 01/12/2017 at 02:18
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Published under licence by IOP Publishing Ltd
ICPEAC2017 IOP Publishing
IOP Conf. Series: Journal of Physics: Conf. Series 875 (2017) 012016 doi :10.1088/1742-6596/875/2/012016
Rotationally cold (>99% J= 0) OHmolecular ions
in a cryogenic storage ring
Gustav Eklund1, Kiattichart Chartkunchand1, Emma K Anderson1,
Magdalena Kami´nska1,2, Nathalie de Ruette1, Richard D Thomas1,
Moa K Kristiansson1, Michael Gatchell1, Peter Reinhed1, Stefan
Ros´en1, Ansgar Simonsson1, Anders K¨allberg1, Patrik L¨ofgren1, Sven
Mannervik1, Henning Zettergren1, Henrik Cederquist1and Henning
T Schmidt1
1Stockholm University, Department of Physics, Stockholm 10691, Sweden
2Institute of Physics, Jan Kochanowski University, 25-369 Kielce, Poland
Abstract. We store a 10 keV OHion-beam at 13.5±0.5 K in one of the DESIREE storage
rings. Using photodetachment thermometry we measure the effective relative photodetachment
cross section at different storage times and determine the rotational temperature of the ions to
be 13.4±0.2 K in agreement with the macroscopic temperature. A model cross section in the
threshold range taking into account the formation of excited neutral OH molecules is calculated
as a function of rotational temperature in order to justify the use of the rotational thermometry
method developed earlier by the group of Roland Wester at Innsbruck University in the present
case. In addition, we apply a selective photodetachment technique to produce an ion beam with
more than 99% of the ions in the rotational ground state. The intrinsic lifetime of the J= 1
rotational level is measured to be 145 ±28 s.
1. Introduction
In experiments on molecular ions it would be of great advantage if all ions are populated in the
same quantum state. However, most available ion sources usually produce hot ions occupying
a large number of vibrational and rotational states. To relax these degrees of freedom many
experiments have been performed where ions are stored in traps and storage rings. Relaxation of
the vibrational degrees of freedom has been studied extensively in room temperature experiments
with storage times on the order of tens of seconds. For rotational relaxation the time scales are
much longer [1] and at room temperature, a large number of rotational states are occupied.
Recent development in cryogenic storage techniques [2–5] allows for ion-beam storage times of
hours [6] and has opened up new possibilities to study rotationally cold molecular ions.
In this work we present a study of rotational relaxation of the OHmolecular ion using the
photodetachment thermometry technique developed by Otto et al. [7] in one of the DESIREE
storage rings. The hydroxyl anion, OH, is a suitable molecular ion for photodetachment
studies with its well separated rotational levels and large rotational constant B= 18.7354(16)
cm1[8]. Several experiments set out to study cold OHhas been performed recently. Otto et
al. [7] performed photodetachment thermometry on OHstored in a 22-pole radiofrequency trap
using buffer gas cooling with He. They found that the rotational temperature agreed well with
ICPEAC2017 IOP Publishing
IOP Conf. Series: Journal of Physics: Conf. Series 875 (2017) 012016 doi :10.1088/1742-6596/875/2/012016
the macroscopic temperature of their environment except at the lowest temperatures where the
rotational temperature always remained higher for reasons that were not clear at the time [7].
Possible explanations for the lack of cooling to below about 20 K in the buffer gas trap was
further investigated by Endres et al. [9] but again no firm conclusion was reached. Recently,
Meyer et al. [10] studied the rotational relaxation of OHin the Cryogenic Storage Ring (CSR)
and found the effective radiative temperature of the blackbody field in the ring to be 15.1±0.1
K although the main part of their ring was substantially colder than this. They also measured
the intrinsic lifetime of the first three excited rotational levels of OH[10].
2. Experiment
Here, we use one of the DESIREE storage rings, shown schematically in Figure 1, to study the
cooling of OHions in a 10 keV beam. The storage rings are contained in a single cryogenically
cooled chamber at 13.5±0.5 K with a residual gas density of 104H2per cm3. The OHions
are produced in a Cesium sputter ion source, accelerated to 10 keV and mass selected by a
bending magnet. A bunch of 1-10 million ions is injected into the ring. During the present
experiment, a 1/e storage lifetime of approximately 10 minutes is achieved. Two laser and
two detection systems are used. The first is a tunable cw Ti:Sapphire laser overlapping the
ion beam collinearily in the straight section on the injection side of the ring. The neutral
OH produced from photodetachment hit a glass plate emitting secondary electrons which are
accelerated and detected by an MCP. The second system consists of a tunable pulsed OPO
interacting perpendicularly with the ion beam on the opposite straight section. The neutral
particles are detected by an Imaging Detector consisting of a triple stack MCP and phosphor
screen viewed by a CMOS camera and a photomultiplier tube.
MCP Collinear
cw Laser Beam
Perpendicular Pulsed
OPO Beam
Glass Plate
OH Injection
Figure 1. Schematic of one of
the DESIREE storage rings. Lasers
and detectors for rotational ther-
mometry and manipulation studies
are indicated.
3. Photodetachment cross section
The effective relative photodetachment cross section is modeled by [7]
eff (E, T )X
I(J00, J 0)P(J00, T )(EJ00 ,J 0)p(1)
where Eis the energy of the incoming photon, J00 is the initial rotational state of the ion, and J0is
the final rotational state in the neutral molecule. I(J00 , J 0) is an intensity factor for the transition
J00 J0[8, 11], P(J00 , T ) is the population in level J00,J00,J 0is the threshold energy for the
photodetachment transition, and p= 0.28 is an exponent for the cross section near threshold [12].
The allowed transitions are given by electric dipole selection rules ∆J=3/2,1/2,+1/2,+3/2
for the case of OH[8, 11]. The effective relative photodetachment cross section is dependent
on the populations P(J00 , T ) and hence the temperature Tof the ions. Assuming thermal
equilibrium the populations P(J00 , T ) are given by P(J, T )(2J+ 1) exp [J(J+ 1)B/kBT]
where kBis the Boltzmann constant. Using the experimental thresholds J00 ,J 0[13], σpd
eff (E, T )
can be modeled using Equation 1. Figure 2 shows calculated σpd
eff (E, T ) curves for photon
energies, E, near the electron affinity of OH for different rotational temperatures, T. At high T
ICPEAC2017 IOP Publishing
IOP Conf. Series: Journal of Physics: Conf. Series 875 (2017) 012016 doi :10.1088/1742-6596/875/2/012016
a large number of transitions to excited states in the neutral OH are contributing to σpd
eff (E, T ).
As can been seen in Figure 2, σpd
eff (E, T ) is completely dominated by three transitions (from
J00 = 0,1,2 to the ground state J0= 3/2 of OH) at T= 15 K.
To determine the rotational temperature of the ions we measure σpd
eff (E) as a function of
photon energy using the cw laser, correcting for the Doppler shift resulting from the collinear
geometry. The experiment is repeated for different storage times as shown in Figure 3. At photon
energies below 1.8137 eV the measured σpd
eff (E) is constant which is attributed to contamination
from 17Oin the ion beam. The differences in levels between the measurements in this energy
range are due to variations in ion source conditions. Three thresholds are clearly distinguishable
in the data which is consistent with the model in Figure 1. To extract the populations P(J00)
Equation 1 is fitted to the data using the three dominant transitions and a constant to account
for the 17Ocontamination. After 10 minutes of storage P(0) = 93.9±0.2 % and under the
assumption of thermal equilibrium this corresponds to a temperature of 14.1±0.2 K.
Figure 2. Modeled σpd
eff (E, T ) for different
rotational temperatures. As the temperature
decreases transitions from J00 = 0,1,2 to the
ground state J0= 3/2 dominates.
Figure 3. Measured σpd
eff (E) for 50, 300
and 600 s of storage. The thresholds for the
dominating transitions are marked by dashed
lines. The curves are fits to Equation 1.
4. Probing as function of time and selective photodetachment
To determine if thermal equilibrium is reached the populations need to be measured for longer
times. From the measurements described above it is clear that P(2) becomes negligible at
long storage times, thus for this measurement it is sufficient to consider J= 0 and J= 1. Using
the pulsed OPO two measurements are made, at λ= 677 nm (E= 1.8314 eV) and λ= 679 nm
(E= 1.8260 eV) for photodetachment of J0 and J1 respectively. Since the form of σpd
eff (E)
is known from Equation 1 the populations P(0) and P(1) can be extracted. The triangles in
Figure 4a show P(1) as a function of storage time.
In addition to probing the rotational relaxation of the ions we also apply the cw laser at
λ= 679 nm for selective photodetachment of ions with J1 (diamonds in Figure 4a). The
result is a faster decay and a lower equilibrium population than without the depletion laser.
The circles in Figure 4a show a measurement where the cw laser is turned on initially but
switched off after some time causing repopulation of the J= 1 level due to interaction with the
surrounding blackbody radiation. Figure 4b shows a similar measurement where the overlap
between the laser and ion beams is better optimized resulting in a more efficient depletion.
The lowest P(1) in this case is 0.9±0.1% and >99% of the ions are in the rotational ground
ICPEAC2017 IOP Publishing
IOP Conf. Series: Journal of Physics: Conf. Series 875 (2017) 012016 doi :10.1088/1742-6596/875/2/012016
state. There is a difference in the asymptotic value of P(1) between the spontaneous decay
and after repopulation. This is because the cw laser induce photodetachment of 17Oand
thus removes this beam contamination permanently. The repopulation measurement provides a
better estimate of the asymptotic P(1) and the final result is given by the weighted average of
the two repopulation measurements of 5.1±0.3%, which yields a temperature of 13.4±0.2 K of
the stored ensemble of OHions. Furthermore by considering the repopulation measurements in
a 13.4±0.2 K blackbody radiation field, we extract the intrinsic lifetime of the J= 1 rotational
level A1
10 = 145 ±28 s. [14]
Figure 4. Fractional population
in J= 1 as a function of storage
time. (a) Triangles: spontaneous
relaxation. Diamonds: cw laser
depleting J1. Circles: cw
laser depleting J1 until 440
(380) s, J= 1 repopulates due to
interaction with the blackbody
field. (b) A measurement with
improved overlap between the cw
laser and ion beam.
5. Conclusions
Using photodetachment thermometry we have measured the asymptotic rotational temperature
of the OHion beam to be 13.4±0.2 K, which demonstrates thermal equilibrium with the
13.5±0.5 K storage ring. Using selective photodetachment we have produced an ion beam with
>99% of the ions in the rotational ground state. This technique will be beneficial for studies of
ion interactions with photons, electrons, neutrals and other ions, as well as action spectroscopy
of ions at interstellar temperatures. We measure the intrinsic lifetime of the J= 1 rotational
level to be 145 ±28 s, which is about two standard deviations shorter than the corresponding
result of Meyer et al. [10].
[1] Amitay Z, Zajfman D and Forck P 1994 Phys. Rev. A 50 2304
[2] Thomas R D, Schmidt H T, Andler G, Bj¨orkhage M, Blom M, Br¨annholm L, B¨ackstr¨om E, Danared H, Das
Set al. 2011 Rev. Sci. Instrum. 82 065112
[3] Schmidt H T, Thomas R D, Gatchell M, Ros´en S, Reinhed P, L¨ofgren P, Br¨annholm L, Blom M, Bj¨orkhage
M, B¨ackstr¨om E et al. 2013 Rev. Sci. Instrum. 84 055115
[4] von Hahn R, Becker A, Berg F, Blaum K, Breitenfeldt C, Fadil H, Fellenberger F, Froese M, George S, G¨ock
Jet al. 2016 Rev. Sci. Instrum. 87 063115
[5] Nakano Y, Enomoto Y, Masunaga T, Menk S, Bertier P and Azuma T 2017 Rev. Sci. Instrum. 88 033110
[6] ackstr¨om E, Hanstorp D, Hole O M, Kaminska M, Nascimento R F, Blom M, Bj¨orkhage M, K¨allberg A,
ofgren P, Reinhed P et al. 2015 Phys. Rev. Lett. 114 143003
[7] Otto R, von Zastrow A, Best T and Wester R 2013 Phys. Chem. Chem. Phys. 15 612–18
[8] Goldfarb F, Drag C, Chaibi W, Kr¨oger S, Blondel C and Delsart C 2005 J. Chem. Phys. 122 014308
[9] Endres E, Egger G, Lee S, Lakhmanskaya O, Simpson M and Wester R 2017 J. Mol. Spectrosc. 332 134–38
[10] Meyer C, Becker A, Blaum K, Breitenfeldt C, George S, G¨ock J, Grieser M, Grussie F, Guerin E A, von
Hahn R et al. 2017 Phys. Rev. Lett. 119 023202
[11] Schulz P, Mead R D and Lineberger W 1983 Phys. Rev. A 27 2229
[12] Engelking P C 1982 Phys. Rev. A 26 740
[13] Schulz P, Mead R D, Jones P and Lineberger W 1982 J. Chem. Phys. 77 1153–65
[14] Schmidt H T, Eklund G, Chartkunchand K C, Anderson E K, Kami´nska M, de Ruette N, Thomas R D,
Kristiansson M K, Gatchell M, Reinhed P et al. 2017 Phys. Rev. Lett. 119 073001
... The s-wave threshold detachment of OH − has been extensively studied through photodetachment [27,28] and deviations from the Wigner threshold law have been observed due to the electron-dipole interaction [29,30]. With its large rotational constant, OH − has also been used to probe the rotational thermalization by buffer gas cooling [26,31] and in cryogenic storage rings [32,33]. In buffer gas at low temperatures, significant deviations of the rotational temperature from the thermal bath have been seen [34]. ...
Full-text available
The second vibrational overtone transition, ν′,J′=3,1←ν,J=0,0, in OH− confined in a 22-pole radio frequency ion trap has been measured using a laser induced reaction with molecular hydrogen. The resultant spectral line is found at 10 150.94(2) cm−1 and the excitation rate of this overtone excitation is obtained, which sensitively probes the molecular potential surface. The Doppler broadened line profile is used to determine the translational temperature of the ions in buffer gas temperatures ranging from 9(1) to 52(1) K. The translational temperatures closely follow those of the buffer gas over this range, including an offset caused by radio frequency heating. In comparison with previous measurements of the rotational temperatures of trapped ions, we show that translational and rotational temperatures do not equilibrate at the low buffer gas temperatures.
Full-text available
We apply near-threshold laser photodetachment to characterize the rotational quantum level distribution of OH− ions stored in the cryogenic ion-beam storage ring DESIREE at Stockholm University. We find that the stored ions relax to a rotational temperature of 13.4±0.2 K with 94.9±0.3% of the ions in the rotational ground state. This is consistent with the storage ring temperature of 13.5±0.5 K as measured with eight silicon diodes but in contrast to all earlier studies in cryogenic traps and rings where the rotational temperatures were always much higher than those of the storage devices at their lowest temperatures. Furthermore, we actively modify the rotational distribution through selective photodetachment to produce an OH− beam where 99.1±0.1% of approximately one million stored ions are in the J=0 rotational ground state. We measure the intrinsic lifetime of the J=1 rotational level to be 145±28 s.
Full-text available
Photodetachment thermometry on a beam of OH− in a cryogenic storage ring cooled to below 10 K is carried out using two-dimensional frequency- and time-dependent photodetachment spectroscopy over 20 min of ion storage. In equilibrium with the low-level blackbody field, we find an effective radiative temperature near 15 K with about 90% of all ions in the rotational ground state. We measure the J=1 natural lifetime (about 193 s) and determine the OH− rotational transition dipole moment with 1.5% uncertainty. We also measure rotationally dependent relative near-threshold photodetachment cross sections for photodetachment thermometry.
Full-text available
An electrostatic cryogenic storage ring, CSR, for beams of anions and cations with up to 300 keV kinetic energy per unit charge has been designed, constructed and put into operation. With a circumference of 35 m, the ion-beam vacuum chambers and all beam optics are in a cryostat and cooled by a closed-cycle liquid helium system. At temperatures as low as (5.5 $\pm$ 1) K inside the ring, storage time constants of several minutes up to almost an hour were observed for atomic and molecular, anion and cation beams at an energy of 60 keV. The ion-beam intensity, energy-dependent closed-orbit shifts (dispersion) and the focusing properties of the machine were studied by a system of capacitive pickups. The Schottky-noise spectrum of the stored ions revealed a broadening of the momentum distribution on a time scale of 1000 s. Photodetachment of stored anions was used in the beam lifetime measurements. The detachment rate by anion collisions with residual-gas molecules was found to be extremely low. A residual-gas density below 140 cm$^{-3}$ is derived, equivalent to a room-temperature pressure below 10$^{-14}$ mbar. Fast atomic, molecular and cluster ion beams stored for long periods of time in a cryogenic environment will allow experiments on collision- and radiation-induced fragmentation processes of ions in known internal quantum states with merged and crossed photon and particle beams.
Full-text available
We use a novel electrostatic ion storage ring to measure the radiative lifetime of the upper level in the 3p^{5} ^{2}P_{1/2}^{o}→3p^{5} ^{2}P_{3/2}^{o} spontaneous radiative decay in ^{32}S^{-} to be 503±54 sec. This is by orders of magnitude the longest lifetime ever measured in a negatively charged ion. Cryogenic cooling of the storage ring gives a residual-gas pressure of a few times 10^{-14} mbar at 13 K and storage of 10 keV sulfur anions for more than an hour. Our experimental results differ by 1.3σ from the only available theoretical prediction [P. Andersson et al., Phys. Rev. A 73, 032705 (2006)].
Full-text available
We report on the first storage of ion beams in the Double ElectroStatic Ion Ring ExpEriment, DESIREE, at Stockholm University. We have produced beams of atomic carbon anions and small carbon anion molecules (Cn (-), n = 1, 2, 3, 4) in a sputter ion source. The ion beams were accelerated to 10 keV kinetic energy and stored in an electrostatic ion storage ring enclosed in a vacuum chamber at 13 K. For 10 keV C2 (-) molecular anions we measure the residual-gas limited beam storage lifetime to be 448 s ± 18 s with two independent detector systems. Using the measured storage lifetimes we estimate that the residual gas pressure is in the 10(-14) mbar range. When high current ion beams are injected, the number of stored particles does not follow a single exponential decay law as would be expected for stored particles lost solely due to electron detachment in collision with the residual-gas. Instead, we observe a faster initial decay rate, which we ascribe to the effect of the space charge of the ion beam on the storage capacity.
A new electrostatic ion storage ring, the RIKEN cryogenic electrostatic ring, has been commissioned with a 15-keV ion beam under cryogenic conditions. The ring was designed with a closed ion beam orbit of about 2.9 m, where the ion beam is guided entirely by electrostatic components. The vacuum chamber of the ring is cooled using a liquid-He-free cooling system to 4.2 K with a temperature difference of 0.4 K at most within all the positions measured by calibrated silicon diode sensors. The first cryogenic operation with a 15-keV Ne⁺ beam was successfully performed in August 2014. During the measurement, the Ne⁺ beam was stored under a ring temperature of 4.2 K with a residual-gas lifetime of more than 10 min. This permits an estimation of the residual gas density at a few 10⁴ cm⁻³, which corresponds to a room-temperature-equivalent pressure of around 1×10−10 Pa. An effect of longitudinal pulse compression at the bunching cavity in the ring was clearly identified by monitoring the pick-up beam detector. The detailed design and mechanical structure of the storage ring, as well as the results from the commissioning run, are reported.
Cryogenic 22-pole ion traps have found many applications in ion-molecule reaction kinetics and in high resolution molecular spectroscopy. For most of these applications it is important to know the translational and internal temperatures of the trapped ions. Here, we present detailed rotational state thermometry measurements over an extended temperature range for hydroxyl anions in He, HD, and H2. The measured rotational temperatures show a termination of the thermalisation with the buffer gas around 25 K, independent of mass ratio and confinement potential of the trap. Different possible explanations for this incomplete thermalisation are discussed, among them the thermalisation of the buffer gas, room temperature blackbody radiation or warm gas entering the trap, and heating due to energy transfer from rotationally excited hydrogen molecules.
In this Brief Report we extend the work on the frame-transformation method to give an analytic form for the relative intensities of rotational transitions in photodetachment and photoionization. The specific case of photoabsorption by a 1Sigma species, resulting in a 2Pi species and an s electron, is treated analytically. The results are in good agreement with both threshold photodetachment cross sections and photoelectron spectra.
Strong coupling at large electron-molecule distances modifies the threshold laws for photodetachment of a molecular anion near a rotational threshold. If the final polar molecule possesses a symmetry axis containing a nonzero component of angular momentum, cross sections of the type σ∼kx with x<1 may result for certain ranges of molecular dipoles, depending on the specific rotational threshold considered.
A photodetachment experiment is performed on the v=0→v=0 OH− detachment threshold. The weak O and S branches provide a signal strong enough to make amplitude measurements on all five O, P, Q, R, and S branches possible, which are used to fix the formulas for their relative intensities. Photodetachment microscopy is applied to 15 different thresholds of the P, Q, and R branches. The quantitative analysis of the interference patterns obtained does not show any effect of the dipole moment of OH, but yields a new measurement of the rotational parameters of OH−(v=0) and of the electron affinity of the molecule. The new recommended value for the electron affinity of 16O1H is 14 740.982(7) cm−1 or 1.827 648 7(11) eV.© 2005 American Institute of Physics.