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Volume 98, Issue 1, 01 January 1993

Spectroscopy of SH (A–X) transition in Ar and Kr matrices: The caging of predissociation
View Description Hide DescriptionThe A(^{2}Σ^{+})–X(^{2}Π) transition of SH isolated in Ar and Kr matrices is studied by laser induced fluorescencespectroscopy. The (0,0), (1,0), and (2,0) transitions are observed in excitation. These lines are structureless and homogeneously broadened (200–300 cm^{−1}). The (0,0), (0,1), and (0,2) emission bands, are strongly Stokes shifted and further broadened (≥1000 cm^{−1}). The spectra can be successfully interpreted in analogy with OH–Rg pair interactions. The SH(A)–Rg potential is deeply bound (∼1000 cm^{−1} in Ar and ∼2000 cm^{−1} in Kr) with nearly 1 Å compression along this coordinate. Franck–Condon analysis indicates that the equilibrium bond length of SH(A) is 0.055 Å longer than in the free radical. The A state, which is strongly predissociated in the free radical, relaxes radiatively in the matrix: τ=425 and 770 ns in Ar and Kr, respectively. Predissociation is treated as a two‐dimensional tunneling process between two electronic surfaces. Poor Franck–Condon factors along the solvent coordinate, due to differential solvation of electronic states, leads to complete arrest of electronic predissociation.

Studies of molecular conformations by ultraviolet laser spectroscopy and quantum chemical calculations: Inversion potentials in ground and excited singlet states of jet‐cooled 1,4‐dihydronaphthalene
View Description Hide DescriptionStudies of inversion potentials have been carried out for gas‐phase 1,4‐dihydronaphthalene (DHN) using ultraviolet laserspectroscopy and ab initio quantum chemical calculations. The analyses of the experimental fluorescence excitation and dispersed fluorescence spectra in a supersonic free jet indicate that the equilibrium conformation of DHN is nonplanar in both the ground and lowest excited singlet states. However, the barrier to inversion is smaller than the energy of the zero‐point vibrational level, so that DHN behaves as a quasiplanar molecule. The ground‐state structure of DHN computed at the correlated MP2/6‐31G(d) level of theory is also nonplanar, with a dihedral angle of 148° and an inversion barrier of 0.5 kcal/mol. Comparisons are made for the series 1,4‐cyclohexadiene (1,4‐dihydrobenzene), DHN, and 9,10‐dihydroanthracene.

Franck–Condon structure of the S _{0}→S _{1} and S _{0}→S _{2} transitions in norbornadiene
View Description Hide DescriptionWe report an ab initio study of the geometry and force field of the S _{0}, S _{1}, and S _{2} electronic states of norbornadiene at the Hartree–Fock/6–31G* and configuration interaction singles/6–31+G levels of theory. The calculated Franck–Condon structures of the S _{0}→S _{1} and S _{0}→S _{2} transitions agree well with the structures observed in the optical absorption and energy lossspectra. It is argued that the 47 000 cm^{−1} absorption band which undergoes changes in the vibronic structure upon going from the gas to the solid phase owes its different activity to the freezing of the angle between the two ‘‘wings’’ of the cyclohexadiene part of the molecule. The complete neglect of differential overlap/spectroscopic parametrization method is used to locate valence states of the title molecule and to calculate vibronic coupling parameters to predict vibronic activity of the nontotally symmetric modes in the spectra. It is demonstrated that the highest frequency C=C stretching mode of the b _{2} symmetry is the most effective in bringing intensity into the S _{0}→S _{1} transition. The positions of the five lowest valence states obtained by the complete neglect of differential overlap/spectroscopic parametrization method agree very well with the peaks of bands observed in the electron energy lossspectra.

Resonance Raman excitation profiles of methyl iodide in hexane
View Description Hide DescriptionResonance Raman spectra, including absolute scattering cross sections, have been measured for methyl iodide in hexane at nine excitation wavelengths from 204.2 to 274.0 nm. Spectra excited in the 246–274 nm region, on resonance with the directly dissociative A state, exhibit overtone progressions in the C–I stretch and its combination bands with the methyl umbrella mode. The fundamental of the C–I stretch is weak relative to the overtones when excited near the peak of the A band, an effect shown to arise from interferences between the resonant amplitude and preresonant contributions from higher electronic states. Both the absorptionspectrum and the Raman excitation profiles are quite similar in solution and vapor phases, suggesting that the dissociative A state is not strongly perturbed by solvation. This conclusion is consistent with a simple theoretical estimate of the solvation effect on the A state potential.

k dependence of Brillouin halfwidths
View Description Hide DescriptionWe find that at a given temperature the Brillouinlinewidths in triphenylphosphite are well described by the two‐parameter expression Bk ^{α}, where the scattering wave number k is varied over two and a half orders of magnitude. The data include measurements in all states of the system from low viscosity liquid to a glass; the parameters α(T) and B(T) exhibit extrema at similar temperatures.

Vibrational and orientational relaxation of monomeric water molecules in liquids
View Description Hide DescriptionA polarization resolved infrared double resonance experiment with picosecond pulses has been used to measure the vibrational and orientational relaxation times of monomeric water and heavy water molecules in different organic solvents after excitation of the antisymmetric ν_{3} vibration. In all investigated systems a fast (but definitely time resolved) equilibration among the OH (OD) stretch vibrations was found, followed by a rather slow decay of this ensemble. Furthermore hints to a medium lived intermediate state, most likely the bending vibration, are reported. The principal relaxation scheme is very similar to that of water vapor. Comparing H_{2}O and D_{2}O in different solvents significant differences were found, which can be understood at least semiquantitatively. The orientational relaxation times of H_{2}O and D_{2}O differ by a factor of 2, which can be explained by the influence of weak hydrogen bonds of different strength for H_{2}O and D_{2}O on the orientational relaxation. Additionally interesting aspects of the transient spectra are discussed.

On the quasiclassical calculation of fundamental and overtone intensities
View Description Hide DescriptionVarious approximations to the transition dipole moment matrix element <n’‖M‖n≳ are compared with each other and to exact (numerical) values of this overlap integral for different n→n’ transitions in a Morse potential with a linear dipole moment function. By partitioning the numerical integral into different contributions that involve the classically allowed and forbidden regions of each wave function, we have learned what conditions must be satisfied for validity of the different approximations. In particular, we consider the Landau approximation to the quasiclassical matrix element in which the exact wave function for the upper state is replaced by the Wentzel–Kramers–Brillouin (WKB) wave function in the classically allowed region of that state. We find that the Landau approximation is more accurate than might have been expected because of the compensation of the neglected tunneling contribution by the singular behavior of the WKB wave function in the classically allowed neighborhood of the turning point. Based on this study, we suggest an improved semiclassical approximation for transition dipole matrix elements that involve an arbitrary dipole moment function. This method is applied to the n’−0 transition of a Morse oscillator using a linear dipole moment function; it can reproduce the exact values of the transition dipole moment matrix element to better than 5% for n’=1 to n’=15. Under the condition that the dipole moment function is slowly varying or decreases monotonically with increasing internuclear separation, a simple expression is presented for estimating relative strengths of neighboring high overtone transitions.

Direct laser‐induced emission detection of the S _{1} and T _{1} states of germanium dichloride: Pyrolysis jet spectroscopy and ab initio studies
View Description Hide DescriptionSpectra of jet‐cooled germanium dichloride were obtained by pyrolysis of trichlorogermane in the throat of a supersonic jet. Laser‐induced emission excitation spectra were recorded for the weak 450–400 nm and strong 320–300 nm band systems, both of which were vibrationally analyzed.Ab initio predictions of the excited state geometries, vibrational frequencies, and excitation energies were made to aid in assigning the spectra. The strong ultraviolet band system is assigned as Ã ^{1} B _{1}–X̃ ^{1} A _{1} with upper state vibrational frequencies of ν_{1}=354 cm^{−1} and ν_{2}=104 cm^{−1}. It is the direct analog of the 580–440 nm band system of dichlorocarbene. The weaker band system in the visible is shown to be the ã ^{3} B _{1}–X̃ ^{1} A _{1} transition, with upper state vibrational frequencies of ν_{1}=393 cm^{−1} and ν_{2}=118 cm^{−1}. This is the first report of direct laser‐induced phosphorescence detection of the excited triplet state of any of the carbene or heavier carbene analogs.

Fourier transform infrared isotopic study of the ν_{4} and ν_{5} stretching modes of linear C_{6} in Ar at 10 K
View Description Hide DescriptionThe vibrational spectrum of the linear C_{6} cluster produced by trapping the products of the high temperature evaporation of graphite in Ar at ∼10 K has yielded a complete set of measurements of the ν_{4}(σ_{ u }) and ν_{5}(σ_{ u }) stretching modes for all possible ^{13}C‐substituted isotopomers. The observed isotopomer frequencies are in excellent agreement with the predictions of recent second‐order, Mo/ller–Plesset, ab initio calculations.

The methyl torsional levels of solid acetonitrile (CH_{3}CN)—A neutron scattering study
View Description Hide DescriptionNeutronpowderdiffraction has been used to obtain the thermal parameters for the deuterium atoms and to confirm the crystal structure of acetonitrile‐d _{3} at 4 K. Inelastic neutron scattering from both isotopic species is used to determine the energies of the first and second rotational levels of the methyl group. These four levels are reasonably reproduced by a threefold potential with V _{3}=125 meV. The activation energy derived from this potential is in agreement with that previously obtained from the temperature dependence of T _{1} in protonmagnetic resonance measurements.

Isolated effective Hamiltonians for two nearly degenerate modes coupled by Coriolis and centrifugal terms
View Description Hide DescriptionA quantum mechanical canonical transformation due to Bogoliubov and Tyablikov (BT), applied previously to eliminate a single Coriolis term coupling two nearly degenerate vibrational degrees of freedom of a polyatomic molecule [J. Chem. Phys. 94, 461 (1991); 95, 1884 (1991)], is generalized. First, we show how to use an angular momentum dependent BT transformation to effectively decouple two vibrational degrees of freedom Coriolis coupled by two components of the angular momentum. This is accomplished by choosing the molecule‐fixed axes so that, in the rotated frame, there is only one Coriolis term. Redefining the orientation of the molecule‐fixed axes at equilibrium and using the BT transformation enables us to move large off‐block matrix elements into vibrational blocks so that we can use second order perturbation theory to treat strong Coriolis coupling. Second, we develop a mixed BT transformation‐perturbation theory method to calculate energy levels for molecules for which both Coriolis and centrifugal coupling are present. The method is tested on a two‐mode model of formaldehyde using an ab initio Hamiltonian. Third, we combine our BT transformation method and conventional contact transformation theory to derive effective Hamiltonians for nearly degenerate vibrational states coupled by Coriolis and centrifugal terms without resorting to a reduced dimensional model. It is very difficult to use perturbation theory after having applied a BT transformation. We circumvent this problem by first using standard perturbation theory to decouple modes whose zeroth‐order energies are well‐separated and then applying a BT transformation for the strongly coupled modes. The theory is applied to an experimentally determined Hamiltonian for formic acid.

High resolution rotational and ν_{3} coherent Raman spectra of C_{2}H_{6}
View Description Hide DescriptionCoherent anti‐Stokes Raman spectroscopy has been used to study C_{2}H_{6} in the rotational and ν_{3} CC stretching regions. Pure rotational transitions for torsionally excited molecules are seen for the first time and direct analysis of the peak maxima gives B values of 0.663 06 and 0.660 37 cm^{−1} for the υ_{4}=0 and 1 torsional levels. The change in B is combined with ab initio calculations to show that the primary response of the molecule to the increased torsional amplitude is a CC extension of 0.0031 Å, plus a slight increase of 0.10 degrees in the CCH angle. For the ν_{3} vibrational Q branch, a complex mixture of four band systems is seen, a consequence of Fermi resonance interactions causing the predicted four torsional sublevels of this state to separate. Spectra of jet‐cooled samples aid in the assignment of the spectrum and the four torsional components are determined as 994.973 (E _{3s }), 994.878 (E _{3d }), 994.864 (A _{1s }), and 993.791 cm^{−1} (A _{3d }). The A and Brotational constants decrease by 0.003 27 and 0.006 21 cm^{−1}, respectively, when ethane is excited in the ν_{3} mode. These differences are used with the ab initio results to deduce that this transition results in a net increase of 0.010 Å in the average CC distance, accompanied by a decrease of 0.12 degrees in the CCH angle and a very small CH decrease of 0.0002 Å. The calculations also suggest that the V _{3} torsional barrier for the υ_{3}=1 state is about 3% smaller than for the ground state.

Matrix isolation spectroscopy of metal atoms generated by laser ablation. II. The Li/Ne, Li/D_{2}, and Li/H_{2} systems
View Description Hide DescriptionResults of experiments on lithium‐doped cryogenicsolids (Ne, D_{2}, and H_{2}) prepared by laser ablation of solidlithium are presented, including near UV‐visible absorption spectra and thermal annealing studies. Li atoms are found in more than one type of trapping site in each of the systems studied. In Li/Ne matrices deposited at T=5 K, the main optical absorption feature is a so‐called ‘‘triplet’’ absorption with peaks near 628, 641, and 650.5 nm. This observation extends this familiar result from heavier alkali/rare gas matrices to the lightest M/Rg matrix. In Li/D_{2} and Li/H_{2} matrices deposited at T=3 K, the main absorptions show an ‘‘asymmetrical doublet’’ pattern with peaks near 655 and 672 nm and 650 and 671 nm, respectively. The successful isolation of Li atoms in such light matrix host systems supports previous conjectures about the microscopic mechanism for trapping fast laser ablated atoms. Several highly symmetrical hypothetical Li atom trapping site structures are proposed and discussed in conjunction with the results of the classical Monte Carlo simulations reported in the following article of this journal. The observed absorption line shapes are best explained in the context of a Jahn–Teller effect caused by dynamic distortions of the trapping sites by lattice vibrations, with negligible matrix modification of the Li atom spin–orbit splitting.

Classical Monte Carlo simulations of relaxed trapping site structures in Li atom doped solid Ne
View Description Hide DescriptionResults of classical Monte Carlo simulations of Li atom doped fcc Ne solids are presented. Li atom trapping sites based on four‐atom and six‐atom vacancies required only minor relaxation of the matrix surroundings to reach equilibrium. Trapping sites based on one‐atom and two‐atom vacancies were unstable and rearranged to yield crowded versions of the relaxed four‐atom trapping site. The lattice strain associated with the crowded trapping sites could be relieved by the incorporation of nearby additional vacancies. These results are all consistent with the intuitive concept of a minimum volume trapping site in a van der Waals guest/host system; for the Li/Ne system apparently at least four Ne atoms must be removed to accommodate a Li atom. In all cases, the energies of the trapped Li atoms were at least ≊0.25 eV above the energy of a Li atom separated from close‐packed solid Ne, indicating that Li atoms are indeed unwelcome in solid Ne.

Luminescence from pure and doped solid deuterium irradiated by keV electrons
View Description Hide DescriptionSolid samples of pure and doped deuterium were irradiated by keV electrons and the luminescence of these samples was measured between 200 and 700 nm. In order to separate the intrinsic emissions from impurity‐induced luminescence, deuterium was doped with standard air impurities like N_{2}, H_{2}O, O_{2}, and CO. In purified solid deuterium, an emission at 275 nm was observed for the first time and attributed to a transition in D^{*} _{3} molecules produced via ionization of D_{2}, formation of D_{3} ^{+}, and subsequent recombination with electrons. Film‐thickness dependence as well as timing experiments corroborate this interpretation.

Sub‐Doppler optical double‐resonance spectroscopy and rotational analysis of Na_{3}
View Description Hide DescriptionBased on rotationally resolved sub‐Doppler spectroscopy and optical–optical double resonance (OODR) experiments on cold Na_{3} molecules in a collimated supersonic argon beam seeded with sodium, 51 rotational transitions selectively excited by OODR in the complex electronic A ^{2} A _{2}←X ^{2} B _{2} system of Na_{3} could be assigned unambigiously. Accurate values of the rotational constants and the molecular geometrical parameters have been derived from these measurements.

Determining repulsive potentials of InAr from oscillatory bound→continuum emission
View Description Hide DescriptionOscillatory bound→continuum emission from vibrational levels v’=0–6 of the B(^{2}Σ^{+}) state of InAr onto the repulsive walls of the X _{1}(^{2}Π_{1/2}), X _{2}(^{2}Π_{3/2}), and A(^{2}Σ^{+}) electronic states, has been measured. In the B(^{2}Σ^{+})→X _{1}(^{2}Π_{1/2}) spectrum, the intensity extrema have been associated with particular extrema and nodes of the radial wave functions of the emitting levels, and the resulting phase vs energy information directly inverted to yield a pointwise potential for the X _{1}(^{2}Π_{1/2}) state. Analysis of the observed peak heights then showed that on the range 2.9–3.8 Å the associated transition moment function is constant. The overlapping of the B(^{2}Σ^{+})→X _{2}(^{2}Π_{3/2}) and B(^{2}Σ^{+})→A(^{2}Σ^{+}) spectra prevents application of the above inversion procedure, but reliable estimates of these two final‐state potentials were obtained by matching quantum mechanical simulated spectra with experiment. The simulations also showed that the transition moment functions associated with all three transitions are of approximately equal strength.

Resonance hyper‐Raman scattering polarization. A measure of methyl iodide B‐state subpicosecond lifetimes
View Description Hide DescriptionThe resonance hyper‐Raman (RHR) scattering of CH_{3}I vapor is observed as blue incident radiation is tuned through two‐photon resonance with the ν_{2} vibronic absorption band of the predissociative B̃←X̃ Rydberg transition in the UV. In analogy to linear resonance Raman spectroscopy, the RHR band shapes and polarization are found to be a sensitive function of the two‐photon resonant vibronic state lifetime and detuning. The dependence of these scattering characteristics on the two‐photon excited‐state dephasing constant provides a technique for determining subpicosecond predissociation rates via nonlinear spontaneous Raman scattering. Theoretical fits to the observed resonant rovibrational hyper‐Raman depolarization ratio dispersion curves are obtained for a vibronic B‐state lifetime of 0.5±0.2 ps. This lifetime value is in agreement with previous results obtained from linear resonance Raman scattering studies. The polarization analysis of RHR scattering, in particular, provides a technique for measuring excited‐state lifetimes of molecular transitions in the VUV.

Rotational excitations in CH_{4}/krypton mixtures
View Description Hide DescriptionIn contrast to the CH_{4}/argon system, methane and krypton form homogeneous solutions.Inelastic neutron scattering (INS) has been used to observe the concentration dependence of the rotational spectra of CH_{4} in krypton. The methane concentrations (0.3%, 3.2%, and 6.5%) were chosen such that the INS spectra are dominated by scattering from isolated defects and methane dimers. A model based on a Gaussian distribution of local orientational potentials reproduces the measured intensities well and enables the separation of the contributions of the different configurations to the scattering. It is demonstrated that the substitution of one krypton atom on the nearest‐neighbor shell by a methane molecule reduces the energy of the J=1 level by about 5%.

The spectroscopy and relaxation dynamics of three‐phonon bound states in crystal CO_{2}
View Description Hide DescriptionThe high‐resolution Fourier‐transform infrared spectrum of a single CO_{2} crystal has been studied in the (ω_{1}+ω_{2});3ω_{2} Fermi resonance region. The spectrum shows three types of vibrational excitation, one where the three ω_{2} vibrons propagate freely (P+P+P) into the crystal, a second with two (out of three) coupled on the same molecule (BP+P), and the last with all three ω_{2} modes linked on one molecule and travelling as a single excitation (TP). Fermi resonance may split off the three‐phonon continuum (ω_{2}+ω_{2}+ω_{2}) both the biphonon+phonon (BP+P) band and the sharp triphonon (TP) peak. A theoretical analysis of triphonons following a Green‐function approach is proposed. The temperature dependence of triphonon bandwidths has been measured in the temperature range 20–200 K. This gives information on the relaxation mechanisms of bound crystal states. Our results emphasize the role of dephasing through bending mode in the vibrational relaxation of the upper triphonon. The lower triphonon has a behavior with temperature which may be explained as due only to dephasing by lattice phonons. Triphonon depopulation is treated theoretically in a highly simplified form to enlighten the main channels available for decay.