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ABSTRACT: Electronic and vibrational structures in the S(0) (1)A(1) and S(1) (1)A(1) states of jet-cooled phenanthrene-h(10) and phenanthrene-d(10) were analyzed by high-resolution spectroscopy using a tunable nanosecond pulsed laser. The normal vibrational energies and molecular structures were estimated by ab initio calculations with geometry optimization in order to carry out a normal-mode analysis of observed vibronic bands. The rotational structure was analyzed by ultrahigh-resolution spectroscopy using a continuous-wave single-mode laser. It has been demonstrated that the stable geometrical structure is markedly changed upon the S(1) ← S(0) electronic excitation. Nonradiative internal conversion in the S(1) state is expected to be enhanced by this structural change. The observed fluorescence lifetime has been found to be much shorter than the calculated radiative lifetime, indicating that the fluorescence quantum yield is low. The lifetime of phenanthrene-d(10) is longer than that of phenanthrene-h(10) (normal deuterium effect). This fact is in contrast with anthracene, which is a structural isomer of phenanthrene. The lifetime at the S(1) zero-vibrational level of anthracene-d(10) is much shorter than that of anthracene-h(10) (inverse deuterium effect). In phenanthrene, the lifetime becomes monotonically shorter as the vibrational energy increases for both isotopical molecules without marked vibrational dependence. The vibrational structure of the S(0) state is considered to be homogeneous and quasi-continuous (statistical limit) in the S(1) energy region.
The Journal of chemical physics 04/2012; 136(15):154301. · 3.09 Impact Factor
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ABSTRACT: Geometrical structures of the isolated benzene and naphthalene molecules have been accurately determined by using ultrahigh-resolution laser spectroscopy and ab initio calculation in a complementary manner. The benzene molecule has been identified to be planar and hexagonal (D(6h)) and the structure has been determined with accuracies of 2 × 10(-14) m (0.2 mÅ; 1 Å = 1 × 10(-10) m) for the C-C bond length and 1.0 × 10(-13) m (1.0 mÅ) for the C-H bond length. The naphthalene molecule has been identified to be symmetric with respect to three coordinate axes (D(2h)) and the structure has been determined with comparable accuracies. We discuss the effect of vibrational averaging that is a consequence of zero-point motions on the uncertainty in determining the bond lengths.
The Journal of chemical physics 08/2011; 135(5):054305. · 3.09 Impact Factor
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Masaaki Baba
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ABSTRACT: Fast intersystem crossing is observed in the S(1)(1)nπ* state of N-heterocyclic aromatic hydrocarbons and carbonyl compounds. It is attributed to spin-orbit coupling with the (3)ππ* state in the same energy region. The strong singlet-triplet mixing was confirmed by large Zeeman splitting of rotational lines in a high-resolution spectrum. For the S(1)(1)ππ* state of aromatic hydrocarbons, the observed Zeeman splitting was found to be considerably small, and intersystem crossing was considered to be minor. These facts are in accordance with El-Sayed's rule, which states spin-orbit coupling is forbidden between the (1)ππ* and (3)ππ* states. The Zeeman splitting of several derivatives was also observed and the substitution effect on the intersystem crossing rate is discussed.
The Journal of Physical Chemistry A 03/2011; 115(34):9514-9. · 2.95 Impact Factor
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ABSTRACT: The rotationally resolved high-resolution fluorescence excitation spectrum of the 0-0 band in the S(1)← S(0) electronic excitation of thioanisole was observed using the techniques of a collimated supersonic jet and a single-mode ultraviolet laser for the first time. High accurate rotational constants for the S(0) and the S(1) states have been determined by precisely calibrated transition energies of about 1000 assigned rotational lines. The molecular structure of thioanisole has been estimated by high-level MO calculations. The planarity of thioanisole in the S(0) and the S(1) states was also demonstrated clearly. The lifetime of the S(1) state was estimated to be 2.0 ns from the observed line width. This line shape did not change with the magnetic field of 1 Tesla, suggesting that the main radiationless process should be internal conversion to the S(0) state.
Physical Chemistry Chemical Physics 10/2010; 12(40):13243-7. · 3.57 Impact Factor
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ABSTRACT: Pyrene is one of the prototypical compact polycyclic aromatic
hydrocarbons (PAHs), and It is important to investigate its molecular
structure precisely, because it does not conform to Hückel's 4n+2
rule. We analyzed high-resolution and ultrahigh-resolution spectra of
jet-cooled pyrene and elucidated the vibrational and rotational
structures in the S_0 ^1A_g and S_1 ^1B3u states. We conclude
that the molecule is planar with D2h symmetry. The rotational
constants and vibrational normal energies are very similar for the S_0
and S_1 states, indicating that its geometrical structure and potential
energy curves are not changed much upon electronic excitation. This
small change is common to large PAH molecules because the changes of
bond orders by one electron excitation is diluted with a large number of
π electrons. The rates of Radiationless transitions in the S_1 state
are closely related with the molecular structure and the potential
energy curves. Intersystem crossing (ISC) to the triplet state is
expected to be very slow in planar PAHs. Internal conversion (IC) to the
S_0 state does not occur, if the molecular structure and potential
energy curves are identical for the S_0 and S_1 states. In perylene, the
fluorescence lifetime is 1400 ns, and the fluorescence quantum yield is
considerably high. These properties are attributed to its small changes
in molecular structure and potential energy curves upon S_1 ← S_0
excitation.
M. Baba, Y. Kowaka et al., J. Chem. Phys., 131, 224318 (2009)
05/2010; -1.
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Masaaki Baba,
Motohisa Saitoh,
Yasuyuki Kowaka,
Kunio Taguma,
Kazuto Yoshida,
Yosuke Semba,
Shunji Kasahara,
Takaya Yamanaka,
Yasuhiro Ohshima,
Yen-Chu Hsu,
Sheng Hsien Lin
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ABSTRACT: Vibrational level structure in the S(0) (1)A(g) and S(1) (1)B(3u) states of pyrene was investigated through analysis of fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation in a supersonic jet and through referring to the results of ab initio theoretical calculation. The vibrational energies are very similar in the both states. We found broad spectral feature in the dispersed fluorescence spectrum for single vibronic level excitation with an excess energy of 730 cm(-1). This indicates that intramolecular vibrational redistribution efficiently occurs at small amounts of excess energy in the S(1) (1)B(3u) state of pyrene. We have also observed a rotationally resolved ultrahigh-resolution spectrum of the 0(0) (0) band. Rotational constants have been determined and it has been shown that the pyrene molecule is planar in both the S(0) and S(1) states, and that its geometrical structure does not change significantly upon electronic excitation. Broadening of rotational lines with the magnetic field by the Zeeman splitting of M(J) levels was very small, indicating that intersystem crossing to the triplet state is minimal. The long fluorescence lifetime indicates that internal conversion to the S(0) state is also slow. We conclude that the similarity of pyrene's molecular structure and potential energy curve in its S(0) and S(1) states is the main cause of the slow radiationless transitions.
The Journal of chemical physics 12/2009; 131(22):224318. · 3.09 Impact Factor
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ABSTRACT: We have observed rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the 0(0)(0) (a-type) and 0(0)(0)+467 cm(-1) (b-type) bands of the S(2) (1)A(1)<--S(0) (1)A(1) transition of jet-cooled azulene. The observed linewidth is 0.0017 cm(-1), which corresponds to the lifetime of 3.1 ns in the S(2) state. Zeeman splitting of rotational lines is very small so that intersystem crossing to the triplet state is considered to be very slow. Inertial defect is very small and the molecule is considered to be planar in the S(0) and S(2) states (C(2v) symmetry). Rotational constants of the S(2) state are almost identical to those of the S(0) state, indicating that geometrical structure is similar in both electronic states. In this case, internal conversion (IC) by vibronic coupling is thought to be inactive. Therefore, the main radiationless transition process in the S(2) (1)A(1) state of azulene was identified to be IC to the S(1) (1)B(2) state. However, this S(2)-->S(1) IC is still slower than that of conventional polycyclic aromatic hydrocarbons. We consider it to be due to the shallower potential energy curve in the S(1) (1)B(2) state, which is also responsible for the extraordinarily fast S(1)-->S(0) IC in the isolated azulene molecule.
The Journal of chemical physics 07/2009; 131(2):024303. · 3.09 Impact Factor
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ABSTRACT: Rotationally resolved high-resolution spectra and fluorescence decay curves have been observed for weak and short-lived vibronic bands of the S(1) (1)B(3u) <-- S(0) (1)A(g) transition of naphthalene. Fluorescence lifetime of the vibronic band with an excess energy of 1390 cm(-1) (0(0)(0) + 1390 cm(-1) band) is remarkably shorter than that of other bands. Zeeman splitting of rotational lines is very small, so that the main radiationless process is not intersystem crossing to the triplet state but internal conversion to the ground state. The lifetime is thought to be governed by the strength of vibronic coupling between vibrational levels of the S(0) and S(1) states. As for the 0(0)(0) + 2570 cm(-1) band, energy shifts were found in only a few rotational levels although the excess energy was higher than the threshold of intramolecular vibrational redistribution. We conclude that all of the rotational levels are mixed with other vibrational levels. The 0(0)(0) + 3068 cm(-1) band spectrum is fairly complicated with numerous rotational lines, which is attributed to strong vibronic coupling with the S(2) (1)B(2u) state.
The Journal of chemical physics 05/2009; 130(19):194304. · 3.09 Impact Factor
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Masaaki Baba,
Motohisa Saitoh,
Kunio Taguma,
Keisuke Shinohara,
Kazuto Yoshida,
Yosuke Semba,
Shunji Kasahara,
Naofumi Nakayama,
Hitoshi Goto,
Takayoshi Ishimoto,
Umpei Nagashima
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ABSTRACT: Rotationally resolved ultrahigh-resolution spectra of the S1 1B2u←S0 1Ag transition of anthracene-h10 and anthracene-d10 have been observed using a single-mode UV laser and a collimated supersonic jet. We have determined rotational constants of the zero-vibrational levels of the S0 and S1 states by analyzing the precisely calibrated transition wavenumbers of rotational lines. We measured Zeeman splitting of each rotational line in the external magnetic field, of which the magnitude was small and strongly dependent on the rotational quantum numbers. We have shown that the magnetic moment in the S1 1B2u state arises from J-L coupling with the S2 1B3u state and that mixing with the triplet state is negligibly small. We concluded that the main radiationless transition in the S1 state of anthracene is not intersystem crossing to the triplet state but internal conversion to the ground state. We also examined methods of ab initio theoretical calculation to determine which method most closely yielded the same values of rotational constants as the experimentally obtained ones. Moller–Plesset second-order perturbation method with a 6-31G(d,p) basis set yielded approximately the same values for the S0 1Ag state with an error of less than 0.04%. Geometrical structure in the S0 1Ag state of the isolated anthracene molecule has been accurately determined by this calculation. However, configurational-interaction with single excitations, time-dependent Hartree–Fock, and time-dependent density-function-theory methods did not yield satisfactory results for the excitation energy of the S1 1B2u state. Symmetry-adapted-cluster configuration-interaction calculation was sufficiently good for the excitation energy and rotational constants.
The Journal of Chemical Physics 04/2009; 130(13):134315-134315-9. · 3.33 Impact Factor
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ABSTRACT: Fluorescence excitation spectra and dispersed fluorescence spectra of jet-cooled 9-methylanthracene-h12 and -d12 (9MA-h12 and 9MA-d12) have been observed, and the energy levels of methyl internal rotation (CH3 torsion) in the S0 and S1 states have been analyzed. The molecular symmetry of 9MA is the same as that of toluene (G12). Because of two-fold symmetry in the pi system, the potential curve has six-fold barriers to CH3 rotation. In toluene, the barrier height to CH3 rotation V6 is very small, nearly free rotation. As for 9MA-h12, we could fit the level energies by potential curves with the barrier heights of V6(S0) = 118 cm(-1) and V6(S1) = 33 cm(-1). These barrier heights are remarkably larger than those of toluene and are attributed to hyperconjugation between the pi orbitals and methyl group. The dispersed fluorescence spectrum showed broad emission for the excitation of 0(0)(0) + 386 cm(-1) band, indicating that intramolecular vibrational redistribution efficiently occurs, even in the vibronic level of low excess energy of the isolated 9MA molecule.
The Journal of Physical Chemistry A 03/2009; 113(11):2366-71. · 2.95 Impact Factor
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ABSTRACT: We analyzed the H/D isotope effect in the methyl torsional interactions accompanying two methyl internal rotations for acetone (CH(3)COCH(3)) and deuterated acetone (CD(3)COCD(3) and CH(3)COCD(3)) in the ground state by means of the multicomponent molecular orbital (MC_MO) method, which directly accounts for the quantum effects of protons and deuterons. Our estimated rotational constants and moments of inertia for CH(3)COCH(3) and CD(3)COCD(3) agreed well with the experimental results because of the adequate treatment of protonic and deuteronic quantum effects afforded by the MC_MO method. Because the C-D bond distance in the CD(3) group was shorter than the C-H distance in CH(3) owing to the anharmonicity of the potential, the difference in potential energy surfaces of CH(3)COCH(3), CD(3)COCD(3), and CH(3)COCD(3) was strongly related to the differences induced in geometrical parameters by the H/D isotope effect. The potential energy obtained by the MC_MO method was estimated as 290.88 cm(-1) for CH(3)COCH(3), which is in excellent agreement with the experimental results. For CH(3)COCD(3), two potential energies were obtained for CH(3) and CD(3) internal rotations. The MC_MO method successfully elucidated the H/D isotope effect for methyl-methyl repulsive interactions by allowing the adequate treatment of protonic and deuteronic wave functions. The potential energies and bond distances associated with methyl internal rotation induced by the H/D isotope effect were also controlled by the distribution of wave functions of protons and deuterons.
The Journal of chemical physics 01/2009; 129(21):214116. · 3.09 Impact Factor
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ABSTRACT: Electron spin resonance (ESR) features in heavily doped conjugated polymers are investigated through the comparison of temperature dependences of ESR spectra between head-to-tail coupled regioregular (RR) and regiorandom (RRa) poly(3-octylthiophenes) (P3OTs). RR-P3OT, used as a model of having crystalline grains in the solid film, is found to exhibit anisotropic ESR spectra, whereas RRa-P3OT gives almost isotropic ESR spectra similar to those of usual heavily doped conjugated polymers. This difference in the degree of spectral anisotropy primarily arises from a difference in their film morphology. Spectral simulations show the anisotropy observed in RR-P3OT to be caused by g-anisotropy. The presence of the g-anisotropy in RR-P3OT indicates that its polarons spend most of the time within a single crystalline grain that has some domains with a common direction of the g-tensor. The g-anisotropy turns out to decrease with increasing temperature. This result is explained by thermally activated hopping motions between crystalline grains. We emphasize that the decrease in the g-anisotropy with temperature should be associated with its activated type of temperature dependence of conductivity. In RRa-P3OT, its isotropic ESR spectra are suggested to be caused by the interchain motion as well as the intrachain one.
The Journal of Physical Chemistry B 10/2008; 112(35):10922-6. · 3.70 Impact Factor
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ABSTRACT: We have analyzed the differences in the methyl internal rotation induced by the HD isotope effect for acetaldehyde (CH(3)CHO) and deuterated acetaldehyde (CD(3)CDO) in ground state by means of the multicomponent molecular orbital (MC_MO) method, which directly accounts for the quantum effects of protons and deuterons. The rotational constant of CH(3)CHO was in reasonable agreement with experimental one due to the adequate treatment of the protonic quantum effect by the MC_MO method. The C-D bond distances were about 0.007 A shorter than the C-H distances because of the effect of anharmonicity of the potential. The Mulliken population for CD(3) in CD(3)CDO is larger than that for CH(3) in CH(3)CHO because the distribution of wavefunctions for the deuterons was more localized than that for the protons. The barrier height obtained by the MC_MO method for CH(3)CHO was estimated as 401.4 cm(-1), which was in excellent agreement with the experimentally determined barrier height. We predicted the barrier height of CD(3)CDO as 392.5 cm(-1). We suggest that the internal rotation of the CD(3) group was more facile than that of the CH(3) group because the C-D bond distance was observed to be shorter than the C-H distance. Additionally the localized electrons surrounding the CD(3) group in CD(3)CDO caused the extent of hyperconjugation between the CD(3) and CDO groups to be smaller than that in the case of CH(3)CHO, which may have also contributed to the observed differences in methyl internal rotation. The differences in bond distances and electronic populations induced by the H/D isotope effect were controlled by the difference in the distribution of wavefunctions between the protons and deuterons.
The Journal of Chemical Physics 06/2008; 128(18):184309. · 3.33 Impact Factor
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ABSTRACT: The substituent effect on the g-tensor of polycrystalline 2,6-di-tert-butyl phenoxyl radical derivatives diluted in diamagnetic crystals was investigated using multifrequency ESR spectroscopy and DFT calculations. It was revealed that the g-tensors of the series of phenoxyl radical derivatives essentially have an orthorhombic symmetry. For some radicals, the hyperfine-splitting tensors from the para groups were resolved. The interpretations and the assignments of the spin-Hamiltonian parameters were confirmed with computer simulations in all bands. The DFT-calculated g-tensors were consistent with the experimental g-tensors. Furthermore, the shifts Delta(g) from the free electron ge were analyzed in details as the sum of three contributions. The spin-orbit interactions were found to be the dominant factor with regard to the Delta(g). With a focus on the s-o term, thus, the relationship of the g-values and the electronic excited states was explained by visualizing the molecular orbitals of the phenoxyl radical derivatives. This study thus showed the very significant potential of the combination of a multi-frequency ESR approach and a DFT calculation to advanced ESR analysis, particularly, g-tensor analysis, even for a powder-sample radical.
The Journal of Physical Chemistry A 06/2007; 111(21):4612-9. · 2.95 Impact Factor
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ABSTRACT: By EPR spectroscopy, we have developed a new method for determining the molecular orientation in a surface-stabilized liquid crystal (LC) cell, which includes a paramagnetic LC, (2S,5S)-2,5-dimethyl-2-heptyloxyphenyl-5-[4-(4-octyloxybenzenecarbonyloxy)phenyl]pyrrolidine-1-oxy (1), whose spin source is fixed in the rigid core. For each phase of racemic [(+/-)] and enantiomerically enriched [(S,S)] 1 in a surface-stabilized LC cell (4 microm thickness), the observed g-value profiles depending on the angle between the applied magnetic field and the cell plane were successfully simulated by the orientation models: (i) the LC molecule in the nematic (N) phase of (+/-)-1 freely rotates around the long axis, which is always parallel to the rubbing direction; (ii) the long axis of the freely rotating LC molecule in the chiral nematic (N*) phase of (S,S)-1 is always parallel to the cell plane but rotates in the plane to form a helical superstructure; and (iii) in the crystalline phase of (S,S)-1, the molecular long axis forms a helical superstructure similar to that of the N* phase, but the molecule is fixed around the long axis so that the NO bond lies in the cell plane. Fitting the temperature profile of the g-value in the N phase of (+/-)-1 by use of the Haller equation, we determined the molecular g-values along the molecular long axis (g(parallelM)) and short axis (g(perpendicularM)), which were successfully reproduced by the use of the set of principal g-values of a similar nitroxide with consideration of the structure of the LC molecule optimized by Molecular Mechanics 3 (MM3).
The Journal of Physical Chemistry B 12/2006; 110(47):23683-7. · 3.70 Impact Factor
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ABSTRACT: Fluorescence excitation spectra of dibenzofuran in a supersonic jet are observed and the vibronic structure is analyzed for the S(1) (1)A(1) (pipi) and S(0) states. An observation of the rotational envelopes reveals that the band is a B-type band. However, it is shown that most of the strong vibronic bands are A-type bands. The intensity arises from vibronic coupling with the S(2) (1)B(2) state. We find a broad emission in the dispersed fluorescence spectrum for the excitation of the high vibrational levels in the S(1) state. This indicates that intramolecular vibrational redistribution (IVR) occurs efficiently in the isolated dibenzofuran molecule.
The Journal of Physical Chemistry A 09/2006; 110(33):10000-5. · 2.95 Impact Factor
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ABSTRACT: Fluorescence excitation spectra of dibenzofuran in a supersonic jet are observed and the vibronic structure is analyzed for the S1 1A1 (ππ*) and S0 states. An observation of the rotational envelopes reveals that the band is a B-type band. However, it is shown that most of the strong vibronic bands are A-type bands. The intensity arises from vibronic coupling with the S2 1B2 state. We find a broad emission in the dispersed fluorescence spectrum for the excitation of the high vibrational levels in the S1 state. This indicates that intramolecular vibrational redistribution (IVR) occurs efficiently in the isolated dibenzofuran molecule.
07/2006;
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ABSTRACT: Doppler-free two-photon excitation spectra and the Zeeman effects for the 1 band of the S1 1B2u <-- S0 1A1g transition in gaseous benzene-d6 were measured. Although the spectral lines were strongly perturbed, almost all of the lines near the band origin could be assigned. From a deperturbation analysis, the perturbation near the band origin was identified as originating from an anharmonic resonance interaction. Perturbation centered at K = 28-29 in the 14(0)1 band was analyzed, and it was identified as originating from a perpendicular Coriolis interaction. The symmetry and the assignment of the perturbing state proposed by Schubert et al. (Schubert, U.; Riedle, E.; Neusser, H. J. J. Chem. Phys. 1989, 90, 5994.) were confirmed. No perturbation originating from an interaction with a triplet state was observed in both bands. From the Zeeman spectra and the analysis, it is demonstrated that rotationally resolved levels are not mixed with a triplet state. The intersystem mixing is not likely to occur at levels of low excess energy in the S1 state of an isolated benzene. Nonradiative decay of an isolated benzene in the low vibronic levels of the S1 state will occur through the internal mixing followed by the rotational and vibrational relaxation in the S0 state.
The Journal of Physical Chemistry A 08/2005; 109(32):7127-33. · 2.95 Impact Factor
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ABSTRACT: Doppler-free two-photon excitation spectrum and the Zeeman effect of the S1 1B1u(v21=1) <-- S0 1Ag(v=0) transition of naphthalene-d8 have been measured. 908 lines of Q(Ka)Q(J)KaKc transition of J=0-41, Ka=0-20 were assigned, and the molecular constants of the S1 1B1u(v21=1) state were determined. Perturbations were observed, and those were identified as originating from Coriolis interaction. No perturbation originating from an interaction with triplet state was observed. The Zeeman splittings from lines of a given J were observed to increase with Kc, and those of the Kc=J levels increased linearly with J. The Zeeman effects are shown to be originating from the magnetic moment of the S1 1B1u state, which is along the c axis and is induced by mixing of the S2 1B3u state to the S1 1B1u state by J-L coupling. Rotationally resolved levels were found not to be mixed with a triplet state from the Zeeman spectra. Accordingly, it is concluded that nonradiative decay of an isolated naphthalene excited to low rovibronic levels in the S1 1B1u state does not occur through the intersystem mixing. This is at variance with generally accepted understanding of the pathways of the nonradiative decay.
The Journal of Chemical Physics 04/2005; 122(14):144303. · 3.33 Impact Factor
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The Journal of Chemical Physics 12/2004; 121(18):9188-90. · 3.33 Impact Factor
