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Volume 103, Issue 9, 01 September 1995

Anisotropic anharmonicity of lattice and molecular vibrations of 1,2,4,5‐tetrabromobenzene determined by piezomodulated Raman spectroscopy
View Description Hide DescriptionStrain‐induced coupling constants for the anharmonicity of Raman‐active lattice and certain molecular modes of vibration in 1,2,4,5‐tetrabromobenzene (TBB) crystals have been determined using piezomodulated Raman spectroscopy. These constants, which are directly related to the first anharmonic term in the potential energy expansion for lattice dynamical calculations, are a quantitative measurement of the modal anharmonicities in the TBB molecular crystal. Application of uniaxial stress in the experiments permits the anisotropy of the anharmonicity to be determined as well as its magnitude. The TBB lattice modes are significantly coupled by the induced strains and the effects of coupling were observed to be dependent on the direction and symmetry of the strains. The molecular modes investigated were, by comparison, less coupled by the acoustic phonons and generally exhibited less anharmonic response with increasing frequency.

Absorption and Raman spectroscopy of mass‐selected tantalum tetramers in argon matrices
View Description Hide DescriptionWe have examined both the absorption and resonance Raman spectra of mass‐selected Ta tetramers. The tetramers are produced in a sputtering source and mass filtered with a Wien filter, then neutralized and deposited in an Ar matrix at low temperatures. The absorptionspectrum indicates two broad transitions, one in the red at 768 nm and another to the blue at 532 nm. Raman resonances could be excited in both regions giving three distinct fundamental frequencies at 270.2(1) cm^{−1}, 185.1(1) cm^{−1}, and 130.6(2) cm^{−1}. The lowest vibration shows a long progression (up to seven overtones) with alternating separations of 135 cm^{−1} and 126 cm^{−1}, indicative of a weak Jahn–Teller effect in the tetramer ground state. This, along with the observation that the fundamental frequency ratios are close to 2:√2:1 indicate that the molecule has a tetrahedral ground state geometry with an electronic E state symmetry. The appearance of three fundamentals in the resonance Raman spectrum indicates that both excited states corresponding to the observed absorption bands arise from severely distorted T electronic states.

Rotationally resolved photodissociation spectroscopy of Mg^{+}–Ar
View Description Hide DescriptionThe metal ion‐complex ^{24}Mg^{+}–Ar has been prepared in a pulsed nozzle/laser vaporization source, mass selected with a reflection time‐of‐flight mass spectrometer and studied with photodissociationspectroscopy at high resolution. The (5,0) band of the A ^{2}Π_{ r }←X ^{2}Σ^{+} transition has been rotationally analyzed and the rotational constants, B″=0.1409(7) cm^{−1} and B′=0.1836(8) cm^{−1}, and spin–orbit constant, A′=73.94(2) cm^{−1}, have been determined. The bond distances in the ground and excited states of the complex (r _{0} ^{″}=2.88 Å, r _{5} ^{′}=2.52 Å) compare well with the values predicted by theory, and they confirm the suspected nature of the electrostatic bonding in this system.

Periodic orbits, bifurcation diagrams and the spectroscopy of C_{2}H_{2} system
View Description Hide DescriptionThe principal families of periodic orbits that emerge from the stationary points of the six‐dimensional potential energy surface of the C_{2}H_{2} molecular system, as well as periodic orbits from saddle‐node bifurcations, have been located and propagated for an energy range up to 36 500 cm^{−1} above the absolute minimum of the potential. The bifurcation diagrams of these periodic orbits reveal the regions of phase space where the dynamics are regular or chaotic (with soft or hard chaos) for acetylene, vinylidene, and the region over these two isomers. An association of the structure of phase space with spectroscopic findings is made by calculating Gutzwiller’s semiclassical trace formula and classical survival probability functions.

Coherent ion dip spectroscopy of the ground state benzene–Ar complex: Vibration–rotation levels up to 130 cm^{−1} intermolecular energy
View Description Hide DescriptionCoherent ion dip spectroscopy(CIS) provides high sensitivity and high resolution for the investigation of vibrational overtones in molecular electronic ground states. For a special time sequence of two coherent narrow‐band Fourier transform limited nanosecond UV light pulses, with a modest delay of the pump pulse of 6.4 ns, a complete blocking of the population transfer to the upper state is achieved in the lambda‐type three‐level system ion dip experiment. This leads to ion dips with a depth as large as 95% and each dip represents an individual rovibronic transition. In this work, CIS is applied for the first time to a weakly bound van der Waals complex, benzene–Ar. We are able to observe six new van der Waals vibrational states up to an excess energy of 130 cm^{−1}. The assignments are made by comparison with recent S _{1} excited state data of benzene–Ar and p‐difluorobenzene–Ar and by analysis of the positions and intensities of the observed individual rotational lines. The frequency positions of the intermolecular vibrational states display a regular pattern up to 130 cm^{−1}.

The low frequency density of states and vibrational population dynamics of polyatomic molecules in liquids
View Description Hide DescriptionInstantaneous normal mode calculations of the low frequency solvent modes of carbon tetrachloride (CCl_{4}) and chloroform (CHCl_{3}), and experiments on the vibrational population dynamics of the T _{1u } CO stretching mode (∼1980 cm^{−1}) of tungsten hexacarbonyl in CCl_{4} and CHCl_{3} are used to understand factors affecting the temperature dependence of the vibrational lifetime. Picosecond infrared pump–probe experiments measuring the vibrational lifetime of the T _{1u } mode from the melting points to the boiling points of the two solvents show a dramatic solvent dependence. In CCl_{4}, the vibrational lifetime decreases as the temperature is increased; however, in CHCl_{3}, the vibrational lifetime actually becomes longer as the temperature is increased. The change in thermal occupation numbers of the modes in the solute/solvent systems cannot account for this difference. Changes in the density of states of the instantaneous normal modes and changes in the magnitude of the anharmonic coupling matrix elements are considered. The calculated differences in the temperature dependences of the densities of states appear too small to account for the observed difference in trends of the temperature dependent lifetimes. This suggests that the temperature dependence of the liquid density causes significant changes in the magnitude of the anharmonic coupling matrix elements responsible for vibrational relaxation.

The high resolution infrared spectroscopy of cyanogen di‐N‐oxide (ONCCNO)
View Description Hide DescriptionThe high‐resolution infrared absorptionspectrum of the oxalodinitrile di‐N‐oxide (ONCCNO) molecule has been recorded in the gas phase with a Fourier transformspectrometer at a resolution of 0.003 cm^{−1}. No previous high‐resolution spectra have been recorded for this semistable palindromic molecule. On the basis of the 2:1 intensity alternation in the rotational lines caused by nitrogen nuclear spin statistics, the ONCCNO molecule appears to be linear. A quasilinear structure, however, cannot be ruled out at this stage of the analysis. The ν_{4} and ν_{5} fundamental modes at 2246.040 55(23) cm^{−1} and 1258.475 30(11) cm^{−1} have been analyzed to give ground state rotational constants of B _{0}=0.042 202 10(96) cm^{−1} and D _{0}=8.77(70)×10^{−10} cm^{−1}. By fixing the CN and NO bond lengths to 1.1923 and 1.1730 Å, respectively, the C–C bond length was determined to be 1.3329 Å using the B _{0} value. This short C–C bond length is thus similar to that observed for a carbon–carbon double bond.

Solvent effects on nuclear shielding of neon
View Description Hide DescriptionComputer simulations of neon dissolved in a variety of organic liquids are used to explore the origins of solvent‐induced changes in the neon shielding parameter. Relying on recent theoretical calculations, it is demonstrated that short‐range (van der Waals) interactions between the rare gas atom and solvent molecules are the predominant source of the shielding parameter changes. The treatment used leads to calculated solvent‐induced changes for solvents as diverse as water and hexane that are in good agreement with experimental observations.

The long‐range potential of the K _{2} X ^{1}Σ^{+} _{ g } ground electronic state up to 15 Å
View Description Hide DescriptionTitaniumsapphirelaser induced fluorescencespectra of the A ^{1}Σ^{+} _{ u }→X ^{1}Σ^{+} _{ g } electronic system have been recorded at high resolution by Fourier transform spectroscopy.Ground state vibrational levels are observed up to v=81 corresponding to an internuclear distance of 15.4 Å and to 99.96% of the potential well depth. A long range study of the potential energy curves (RKR and IPA) allowed the determination of the coefficients of the dispersion energy (multipolar expansion representation) and of the exchange energy (exponential representation). The dissociation energy is found to be 4450.75±0.15 cm^{−1}.

Hyperfine polarization quantum beats in cyanogen
View Description Hide DescriptionHyperfine polarizationquantum beats caused by the reversible interchange of molecular polarization and nuclear spinpolarization are observed in the fluorescence of C_{2}N_{2}, following laser absorption on the 4^{1} _{0} Ã(^{1}Σ^{−} _{ u })←X̃(^{1}Σ^{+} _{ g }) band at 219 nm. Cross sections for collisional quenching and alignment depolarization of the fluorescence, determined by quantum beatspectroscopy, are 88 and 285 Å^{2}, respectively.

Group theoretical study of the radical cation of methane: The effect of tunneling motions on the hyperfine interaction
View Description Hide DescriptionThe electron spin resonance(ESR) spectrum of CH^{+} _{4} indicates that some large amplitude tunneling motions among Jahn–Teller distorted structures make the four protons equivalent. A group theoretical study using the permutation–inversion (PI) group is performed to analyze the hyperfine interaction of the nonrigid CH^{+} _{4}. It is shown that three patterns of the interaction are possible depending upon the type of tunneling motions. Only one of the three patterns is consistent with the experimental spectrum, which is presented in the accompanying paper [J. Chem. Phys. 103, 3377 (1995)].

Electron spin resonance studies of the methane radical cations (^{12,13}CH^{+} _{4}, ^{12,13}CDH^{+} _{3}, ^{12}CD_{2}H^{+} _{2}, ^{12}CD_{3}H^{+}, ^{12}CD^{+} _{4}) in solid neon matrices between 2.5 and 11 K: Analysis of tunneling
View Description Hide DescriptionThe radical cation of methane isolated in neon matrices exhibits highly unusual electron spin resonance(ESR) spectral features between 2.5 and 11 K. The anomaly has been clarified by invoking large amplitude tunneling motions of the hydrogens among several symmetrically equivalent Jahn–Teller distorted structures. The effect of the tunneling motions upon the ESR spectrum was investigated by an analysis scheme based upon permutation–inversion group theory. All the deuterium substituted cations, i.e., CDH^{+} _{3}, CD_{2}H^{+} _{2}, CD_{3}H^{+}, and CD^{+} _{4} were also studied. The hyperfine coupling constant of ^{13}C was obtained from the study of ^{13}CDH^{+} _{3} and ^{13}CH^{+} _{4}. Several independent generation methods were employed during the course of these methane cation studies, including photoionization, electron bombardment, x‐irradiation, and a pulsed lasersurface ionization technique.

Calculations of the spectra of rare gas dimers and trimers: Implications for additive and nonadditive intermolecular forces in Ne_{2}–Ar, Ne_{2}–Kr, Ne_{2}–Xe, Ar_{2}–Ne, Ar_{3}, Ar_{2}–Kr and Ar_{2}–Xe
View Description Hide DescriptionCalculations of ground‐state energies and rotational constants are carried out for a variety of van der Waals dimers and trimers formed from Ne, Ar, Kr and Xe. It is found that the existing pair potentials for Ne–Ar, Ne–Kr and Ne–Xe do not adequately reproduce the measuredrotational constants of the van der Waals dimers. Modified pair potentials, with equilibrium distances that differ from the originals by less than 1% but give much better rotational constants, are then proposed. Calculations of rotational constants for Ne_{2}–Ar, Ne_{2}–Kr and Ne_{2}–Xe are carried out using pairwise‐additive potentials constructed from both the original and the modified pair potentials. The modified pair potentials give much better agreement with experiment for the trimers as well as the dimers. The effect of an Axilrod–Teller triple‐dipole term on the rotational constants is considered, and found to be significant, especially for the Arotational constant. However, the best available Ne–Ne potential is not accurate enough to allow unambiguous information on three‐body forces to be extracted.

On the adequacy of pairwise additive potentials for rare gas–halogen systems: The effect of anisotropy of interactions between atoms
View Description Hide DescriptionA simple model is presented for the potential energy functions of rare gas dihalides RgX_{2}, which uses empirical potentials for diatomic fragments and takes properly into account anisotropic interactions between atoms, resulting in diabatic potentials which correlate with the ground state X_{2} molecule and Rg atom. Specific results are obtained for potential energy surfaces of ArX_{2} (X =F, Cl, Br, I) complexes and compared to those from several widely used models based on pairwise additive isotropic interactions. All these earlier models are found to underestimate the binding in the linear geometry, predicting a complete absence of a linear bound state; this feature is especially significant for ArF_{2} in which the anisotropic model predicts the linear configuration to be more stable. The new anisotropic model leads to Ar–X_{2}dissociation energies in good agreement with experiments.

Fully ab initio investigation of bound and predissociating states of the NeOH(X) complex
View Description Hide DescriptionNew ab initiopotential energy surfaces (PESs) are reported for the interaction of Ne(^{1} S _{0}) with the OH radical in its ground (X ^{2}Π) electronic state. These are then used in the variational calculation of the bound vibrational states of the NeOH(X) complex. The calculated dissociation energy (D _{0}) is 26.2 cm^{−1}, which lies within the experimental estimate (23–30 cm^{−1}). The ab initio PESs are also used to determine the positions and widths of the metastable levels of the complex which correlate with the first excited rotational state (j=5/2, ω=3/2) and the first excited spin–orbit state (j=1/2, ω=1/2) of OH(X ^{2}Π). The predissociation rates are strongly dependent on the bending motion, the intermolecular stretching vibrational quantum number and the parity. The predissociation lifetimes are in good agreement with estimates from stimulated emission pumping experiments of [Chuang, Andrews, and Lester, J. Chem. Phys. 103, 3418 (1995)]. A quantum flux method is used to study the redistribution of the predissociation flux as a function of the fragment separation.

Intermolecular vibrations and spin–orbit predissociation dynamics of NeOH (X ^{2}Π)
View Description Hide DescriptionStimulated emission pumping of NeOH is used to access the bound intermolecular vibrational levels supported by the OH X ^{2}Π_{3/2}+Ne potential energy surface as well as predissociative levels correlating with the spin–orbit excited state of OH X ^{2}Π_{1/2}+Ne which lie more than 100 cm^{−1} above the dissociation limit. Intermolecular stretching intervals and rotor constants yield the radial dependence of the average interaction between Ne and OH X ^{2}Π_{3/2,1/2}, while the spacings between angular levels provide information on the anisotropy of the potentials. The lifetime of spin–orbit predissociative levels is found to increase by a factor of 3 upon intermolecular stretching excitation, from 8.5 ps to more than 27 ps. This increase in lifetime is attributed to the fall‐off in the coupling between the two spin–orbit states with increasing intermolecular separation distance. The dominant coupling for spin–orbit predissociation is provided by the difference potential, the change in the intermolecular potential when the unpaired electron of OH lies in or out of the NeOH plane. The energies, rotor constants, and lifetimes of the bound and predissociative NeOH levels observed experimentally are compared with theoretical calculations of these quantities based on ab initio potentials for Ne+OH X ^{2}Π by Yang and Alexander (accompanying paper). Remarkably good agreement is found between experiment and theory, given the weakness of the Ne+OH X ^{2}Π interaction.

A variational centroid density procedure for the calculation of transmission coefficients for asymmetric barriers at low temperature
View Description Hide DescriptionThe low temperature behavior of the centroid density method of Voth, Chandler, and Miller (VCM) [J. Chem. Phys. 91, 7749 (1989)] is investigated for tunneling through a one‐dimensional barrier. We find that the bottleneck for a quantum activated process as defined by VCM does not correspond to the classical bottleneck for the case of an asymmetric barrier. If the centroid density is constrained to be at the classical bottleneck for an asymmetric barrier, the centroid density method can give transmission coefficients that are too large by as much as five orders of magnitude. We follow a variational procedure, as suggested by VCM, whereby the best transmission coefficient is found by varying the position of the centroid until the minimum value for this transmission coefficient is obtained. This is a procedure that is readily generalizable to multidimensional systems. We present calculations on several test systems which show that this variational procedure greatly enhances the accuracy of the centroid density method compared to when the centroid is constrained to be at the barrier top. Furthermore, the relation of this procedure to the low temperature periodic orbit or ‘‘instanton’’ approach is discussed.

Electron transfer in high n Rydberg states
View Description Hide DescriptionA model is developed to determine the rate of electron transfer between high n molecular Rydberg states and ions of the same species under zero kinetic energy pulsed field ionization (ZEKE‐PFI) experimental conditions. A simple hydrogenic model is used considering the two particles to be at rest with respect to each other. The results of accurate calculations for the u–g splitting in H^{+} _{2} at low principal quantum numbers (separated atom) as a function of radius are extrapolated to large values of n relevant to typical ZEKE‐PFI experimental conditions and used to determine the rate of electron transfer. Radius ≊5n ^{2} is arrived at as a simple expression to estimate the radius for a charge transfer rate of ≊10^{8} s^{−1} for lowest energy members of the Stark manifold (the fastest). Expressions are derived for other members of the manifold. Results are compared with the recent observation of 3% charge transfer in a ZEKE‐PFI experiment by Alt et al.

Ab initio molecular orbital study of potential energy surface for the reaction of C_{2}H_{3} with H_{2} and related reactions
View Description Hide DescriptionThe potential energy surface of the reaction C_{2}H_{3}+H_{2}→C_{2}H_{4}+H→C_{2}H_{5} has been investigated using various theoretical methods including QCISD(T), CCSD(T), RCCSD(T), Gaussian‐2 (G2), and the density‐functional B3LYP approach. The reaction of the vinyl radical with molecular hydrogen is shown to take place through the hydrogen atom abstraction channel leading to the formation of C_{2}H_{4}+H with the activation energy of 10.4 kcal/mol at all the G2, QCISD(T)/6‐311+G(3df,2p), and CCSD(T)/6‐311+G(3df,2p) levels. The rate constant, calculated using the variational transition state theory with tunneling correction, k=3.68⋅10^{−20}⋅T ^{2.48}⋅exp(−3587/T) cm^{3} molecule^{−1} s^{−1}, is in good agreement with the experimental estimates. C_{2}H_{5} cannot be formed directly by inserting C_{2}H_{3} to H_{2}, but can only be produced by addition of H to C_{2}H_{4}, with a barrier of 4.5–4.7 kcal/mol calculated at high levels of theory. In order to match the experimental rate constant, the activation energy needs to be adjusted to 2.8 kcal/mol. Generally, the B3LYP method is found to predict well the geometries and vibrational frequencies of various species. However, it is less reliable for energy calculations than the QCISD(T) and CCSD(T) methods.

Spin–orbit effects in the photodissociation of ionized rare gas trimers: Comparison of He^{+} _{3}, Ar^{+} _{3}, and Xe^{+} _{3}
View Description Hide DescriptionThe velocities of neutral and charged photofragments of the rare gas trimers He^{+} _{3}, Ar^{+} _{3}, and Xe^{+} _{3} have been examined in a comprehensive study for photonenergies ranging from 1.5 to 6 eV. For this purpose, a novel time‐of‐flight technique has been applied which allows the simultaneous examination of both neutral and charged fragments. The general fragmentation pattern of all three species was that of a linear trimer with a parallel transition moment and a totally repulsive excited state: In the course of the dissociation, two of the particles gain high velocities in opposite directions, while the third particle (the middle particle of the linear trimer) only obtains a small velocity. The positive charge generally localizes on one of the fast outer particles, as can be expected from the symmetry properties of the excited state. For Ar^{+} _{3} and Xe^{+} _{3}, however, also localization of the charge on the slow particle can be observed. This effect strongly depends on the energy of the absorbed photon, and can be quenched by decreasing the vibrational excitation of the trimer. Comparison of the results with new potential energy surface calculations indicate that mainly spin–orbit coupling induced conical intersections are responsible for this charge redistribution phenomenon.