Volume 103, Issue 22, 08 December 1995
Index of content:

Resonance Raman spectrum and excitation profile of mass‐selected zirconium trimers
View Description Hide DescriptionWe present the resonance Raman and Raman excitation profile of mass‐selected zirconium trimers in argon matrices. In the Raman spectra, two fundamentals and one overtone are observed. Average Raman shifts, along with standard deviations (in parentheses) are 176.7 (13) cm^{−1} (ν_{2}), 258.0 (12) cm^{−1} (ν_{1}), and 516.1 (8) cm^{−1} (2ν_{1}). The ratio of the frequencies of the two lowest lines (ν_{1}/ν_{2}) is 1.46, which is very close to √2. This is indicative of a symmetrical equilateral geometry (D _{3h }). In such a case we expect two normal frequencies, one for a totally symmetric stretch of symmetry a′_{1} (ν_{1}) and a doubly degenerate bend of symmetry e′(ν_{2}). The Raman excitation profiles of the ν_{1}(a′_{1}) line shows two broad maxima: one near 491 nm and the other near 614 nm. The ν_{2} (e′) profile shows a broad region of intensity only near 614 nm. With the aid of theory we assign the 614 nm band to be ^{1} A′_{1}−^{1} E′ (x,y polarized) while the 491 nm band must be ^{1} A′_{1}−^{1} A″_{2} (z polarized).

A re‐examination of the S_{0}→S_{1} excitation spectrum of dimethylaniline
View Description Hide DescriptionA new assignment for the S_{0}→S_{1} transition of N,N‐dimethylaniline (DMA) and related derivatives is presented. The low frequency bands and long Franck–Condon envelope observed in DMA‐h _{6} and DMA‐d _{6} are assigned to the coupled methyl torsion mode of the amino group, not to torsion of the amino substituent about the C–N bond. This new assignment is consistent with the change in frequency of the excitation bands upon deuteration of the methyl groups and the strong origin transitions observed in the excitation spectra of other alkyl anilines. The assignment was confirmed by simulations of the excitation spectra of DMA‐h _{6} and DMA‐d _{6}, with parameters of the calculated potential energy surface determined to be V _{3}=148.0±0.5 cm^{−1}, V _{+}=−31.6±0.5 cm^{−1}, V _{−}=8.5±0.5 cm^{−1}, and V _{6}=−15±0.5 cm^{−1}. By Franck–Condon analysis, it was determined that the weak origin transition is due to the shifting of the S_{1} torsion minimum by 40° along the gearing coordinate relative to the corresponding minimum in the ground state.

The microwave spectrum and OH internal rotation dynamics of gauche‐2,2,2‐trifluoroethanol
View Description Hide DescriptionThe microwave spectra of CF_{3}CH_{2}OH and CF_{3}CH_{2}OD have been investigated from 5 to 26 GHz with a pulsed‐nozzle Fourier‐transform microwave spectrometer and from 26 to 42 GHz with an electric resonance optothermal spectrometer.Tunneling of the OH proton between the two isoenergetic gauche conformations splits the observed transitions into two tunneling components. An effective rotation‐tunneling Hamiltonian is used to fit the aan both isotopomers to better than 5 and 13 kHz for the OH and OD forms, respectively. The tunneling splittings determined from the fits for the OH and OD isotopomers are 5868.6952(16) and 208.5037(42) MHz, respectively. A structural analysis using the moments of inertia of the OH and OD isotopomers determines that the hydroxyl hydrogen is directed toward the fluorine with a F...H separation of 2.561(1) Å and a dihedral angle of φ(CCOH)=68.97(6)°. The observed tunneling splittings are fit to a double‐minimum potential, giving gauche–gauchetunneling barriers of 763 and 720 cm^{−1} and OH torsional fundamental frequencies of 364 and 271 cm^{−1} for CF_{3}CH_{2}OH and CF_{3}CH_{2}OD, respectively. The uncertainties shown in parentheses throughout the paper are one standard deviation.

Structure and spectroscopy of phosphorus cluster anions: Theory (simulated annealing) and experiment (photoelectron detachment)
View Description Hide DescriptionPhotoelectron detachment measurements have been performed on singly charged phosphorus cluster anions with up to nine atoms, generated by a pulsed arc cluster ion source (PACIS). Transitions between the anion ground states and states of the neutral clusters are observed for all clusters, and vibrational fine structure in both dimer and trimer. A comparison with the results of density functional calculations with simulated annealing—an extension to negative ions of earlier work on neutral and positively charged clusters—provides a consistent overall picture for all cluster sizes and the first experimental structural information on several.

Dipole broadened line shape of impurity chromophores isolated in a transparent matrix
View Description Hide DescriptionThe inhomogeneous broadening caused by dipole–dipole interactions between molecules having strongly allowed optical transitions is considered for the case in which the chromophores are randomly distributed at low concentration in a transparent matrix. Model calculations show that absorption in the wings of the spectrum is caused almost exclusively by strongly interacting pairs of chromophores. This result has important implications for the interpretation of recent experiments in which the spectra of single dye molecules have been observed in organic matrices. The generalized lineshape in these wings is shown to be proportional to the inverse square of the detuning from line center. The line shape is also somewhat asymmetric, with the red wing being more intense than the blue.

Boron nuclear quadrupole couplings and localized electronic wave functions in the boron clusters o‐carborane and o‐silaborane
View Description Hide DescriptionThe ^{11}B nuclear quadrupole couplings in ‘‘o‐silaborane’’ [1,2‐dimethyl‐1,2‐ disila‐closo‐ dodeca‐ borane(12), B_{10}Si_{2}(CH_{3})_{2}H_{10}] were measured at 77 K with a nuclear quadrupole double resonancespectrometer with rapidly switched magnet. Three ^{11}B transitions were found which were assigned by comparison with ab initio calculations of the electric field gradients. The surprising difference in the order of the quadrupole couplings of equivalent borons in o‐silaborane and the corresponding carbon compound o‐carborane (B_{10}C_{2}H_{12}) is explained by the different valence bond structures of the two boronclusters which were obtained by localization of the electronic wave functions.

Ultrasonic pulses in a glass forming salt melt near T _{ g }. A time and space resolved study using interferometry
View Description Hide DescriptionAn optoacoustic experiment was set up to study generation, propagation, and damping of broadband acoustic pulses in the MHz region. The sound pulse front is detected interferometrically. The analysis is based on linear hydrodynamictheory and comprises diffraction of the sound pulse. To introduce relaxational effects, it was extended by splitting the specific heat into a ‘‘fast’’ and a ‘‘slow’’ part. Thus, a good description of the observed pulse shapes and speeds could be obtained. Results for the glass forming 1‐methyl‐3‐ethyl‐imidazoliumchloride‐AlCl_{3} mixture at three concentrations and for temperatures between −80 °C and +20 °C are reported.

Molecular dynamics study of liquid carbon disulfide and benzene: Effect of pressure on the far‐infrared collision‐induced absorption
View Description Hide DescriptionWe have performed molecular dynamics calculations of liquid carbon disulfide and liquid benzene, intending to examine the effect of pressure on the far‐infrared collision‐induced absorption as well as on the local structure and on the molecular reorientational dynamics. The calculations reproduced well the experimental results of the reorientational correlation times of liquid carbon disulfide at pressures, of the reorientational anisotropy of liquid benzene, and of the density dependence of the collision‐induced dipole moments and their time derivatives of both liquids. Contributions of various interaction terms to the collision‐induced dipole moments were examined as a function of density. The experimental results that the collision‐induced dipole moments of liquid benzene decreases and those of liquid carbon disulfide changes little with increase in density are explained well by pressure‐dependent change in degree of the cancellation between the positive two‐molecule interaction term and the negative three‐molecule interaction terms.

Picosecond spectroscopy of the HgAr van der Waals complex
View Description Hide DescriptionWe have performed the time‐dependent spectroscopy of the B ^{3} P _{1} state of the HgAr van der Waals complex using a two‐color picosecond pump‐probe experiment. Recurrences can be observed for a short delay between pump and probe pulses and the oscillations disappear after 100 ps as a result of the large anisotropy of the weak B van der Waals well. The time‐resolved approach is well adapted to the observation of vibrational levels close to the dissociation limit. Moreover, the spectral bandwidth of the picosecond laser allows for a selective excitation of groups of levels in various parts of the potential well.

Rotationally resolved ultraviolet spectroscopy of indole, indazole, and benzimidazole: Inertial axis reorientation in the S _{1}(^{1} L _{ b })←S _{0} transitions
View Description Hide DescriptionRotationally resolved laser induced fluorescence excitation spectra of the S _{1}(^{1} L _{ b })←S _{0} origin bands of indole, indazole, and benzimidazole have been measured. From these spectra, the rotational constants in both electronic states have been determined. The spectra of all three molecules exhibit ‘‘anomalous’’ rotational line intensities. These intensity perturbations are a result of the reorientation, upon electronic excitation, of the inertial axes of the molecule. Intensity analysis of the rotational lines yielded information about the inertial axis reorientation, and the direction of the transition moment vector for each molecule.

Dynamics of aggregation of tungstic acid
View Description Hide DescriptionThe dynamics of aggregation of tungstic acid was investigated as a function of time, using static and dynamic light scattering. Two types of kinetic behaviors were observed depending on the magnitude of the system parameter: One is the power‐law growth and the other the exponential growth. At an intermediate value of the system parameter, the crossover from the power‐law behavior to the exponential one was also observed. However, none of the aggregation kinetics observed had the characteristic rates: Those were directly controlled by an amount of hydrogen chloride added. Thus the universality in aggregation may not hold for the present colloidal system.

The B(1/2 ^{2} P _{3/2})→X(1/2 ^{2}Σ^{+}) transition in XeBr
View Description Hide DescriptionThe B(1/2 ^{2} P _{3/2})→X(1/2 ^{2}Σ^{+}) transition in XeBr is recorded at high resolution, using a CCD array detector to record spectra from Tesla discharge sources containing isotopically pure ^{136}Xe with ^{81}Br_{2} or ^{79}Br_{2}. The high signal/noise capabilities of the detector permit the measurement of discrete vibrational structure in this system, which has normally been treated as a purely bound–free transition. The assignments comprise 119 υ′–υ″ bands for ^{136}Xe^{81}Br and 86 for ^{136}Xe^{79}Br, spanning υ′=0–33 and υ″=0–16. The van der Waals ground state is analyzed through fits to the customary polynomials in (υ+1/2) and to near‐dissociation expansions. Franck–Condon calculations are used to locate the X‐state potential on the internuclear axis relative to the B state, which is modeled as a Rittner potential. The following fundamental spectroscopic constants (units cm^{−1}, for ^{136}Xe^{81}Br) are obtained from the analysis:T _{ e } ^{′}=35 863.2, ω_{ e } ^{′}=135.72, ω_{ ex } _{ e } ^{′}=0.32, ω_{ e } ^{″}=25.7, ω_{ ex } _{ e } ^{″}=0.62. The ground state has a dissociation energyD_{ e } ^{″}=254±2 cm^{−1} and supports 24 bound vibrational levels.

Synchronization in the discrete chemical oscillation system
View Description Hide DescriptionThe properties of the coupling between chemical oscillators were studied in the discrete chemical oscillation system which was realized by immersing cation exchange beads loaded with ferroin in the Belousov–Zabotinskii reactionsolution. A phase diagram of coupling states was obtained as a function of natural frequencies of oscillators and the distance d between oscillators. The synchronization was found not to be attributed to a simple entrainment of the slower oscillator by the faster one. Various entrainments between oscillators occurred depending on ratios of natural frequencies in the uncoupled state. A chaotic behavior was found at the boundary between stably coupled regions with frequency ratios of n/1 where n is an integer. Furthermore, effects of external perturbation on the coupling were investigated. Irregular oscillations were induced by illumination with a He–Ne laser light, which strongly depended on the phase of oscillator at the beginning of illumination. Such irregular behaviors were localized within the illuminated bead. This indicates that two oscillators are decoupled by illumination.

Modern He–He potentials: Another look at binding energy, effective range theory, retardation, and Efimov states
View Description Hide DescriptionWe compare a number of helium–helium potentials with respect to their predictions of dimer binding energy, scattering length, effective range and Efimov states. We also study the effect of retardation on the ‘‘best’’ potential. All realistic potentials support a weakly bound dimer, while none supports an Efimov state. We agree with other authors that retardation decreases the binding energy by about 10%. Finally, we investigated the effect on the binding energy from the application of retardation over different ranges of separation. The precise effects of retardation at short range in realistic potentials require further study.

Reversible reactions of metastable reactants
View Description Hide DescriptionThe non‐Markovian encounter theory is reformulated and used to find the quantum yields and kinetics of reversible energy transfer. Assuming a short range interaction between the particles, the kinematic approximation is employed to determine the integral kinetic equations and following from them differential equations of encounter theory. The differential variant of the theory is shown to be less appropriate since it fails to describe delayed luminescence during encounter: The reaction constant diverges with time when the acceptor of energy decays slower than the energy donor. On the contrary, integral encounter theory is very suitable for calculating the quantum yields and dissipation kinetics at any ratio between decay times.

Thermal and vibrational‐state selected rates of the CH_{4}+Cl↔HCl+CH_{3} reaction
View Description Hide DescriptionWe present direct ab initio dynamics studies of thermal and vibrational‐state selected rates of the hydrogen abstraction CH_{4}+Cl↔CH_{3}+HCl reaction.Rate constants were calculated within the canonical variational transition state theory formalism augmented by multidimensional semiclassical tunneling corrections. A vibrational diabatic model was used for vibrational‐state selected rate calculations, particularly for exciting the CH_{4} symmetric stretching and umbrella bending modes. The potential energy information was calculated by a combined density functional and molecular orbital approach. Becke’s half‐and‐half (BH&H) nonlocal exchange and Lee–Yang–Parr (LYP) nonlocal correlation functionals (BH&HLYP) were used with the 6‐311G(d,p) basis set for determining structures and frequencies at the stationary points and along the minimum energy path (MEP). Energetics information was further improved by a series of single point spin‐projected fourth‐order Mo/ller–Plesset perturbation theory (PMP4(SDTQ)) calculations using the 6‐311+G(2df,2pd) basis set. We found that the calculated thermal rate constants have reasonable agreement with experimental results for both the forward and reverse reactions. Our results also predict that exciting the CH_{4}symmetric stretching mode will greatly enhance the hydrogen atom transfer rate. Surprisingly, exciting the CH_{4} umbrella bend mode is also predicted to have a noticeable enhancement factor at room temperature.

Product fine structure state populations and nonadiabatic dissociation dynamics: Cl*(^{2} P _{1/2})/Cl(^{2} P _{3/2}) branching ratio in the visible and near ultraviolet photodissociation of nitrosyl chloride
View Description Hide DescriptionChlorine atoms in their ^{2} P _{ j } states were observed using resonance‐enhanced multiphoton ionization and time‐of‐flight techniques, following the photodissociation of nitrosyl chloride at preselected wavelengths (i.e., 600, 562, 478, and 355 nm). The fine structure branching ratio and the spatial anisotropy parameter were determined. Combining these observations with results from previous studies, a complete picture about product fine structure state populations is obtained. The observed fine structure populations are explained with the aid of a new correlation diagram in the C _{ s } point group. The adiabatic, nonadiabatic, and diabatic dissociation dynamics involving several avoided crossings at large interfragment separations are discussed.

Quantum scattering calculations of energy transfer and dissociation of HCO in collisions with Ar
View Description Hide DescriptionWe report a quantum scattering calculation of energy transfer and collision‐induced dissociation of HCO in collisions with Ar. The HCO rotation is treated in the infinite order sudden approximation, and the HCO vibrations are treated by the coupled‐channel method. Sixty L ^{2} HCO vibrational wave functions are included in the coupled‐channel basis, of which 15 correspond to bound HCO states for zero HCO angular momentum, and the remainder represent a discretized continuum, which includes ten resonances. A simple ‘‘sum‐of‐pairs’’ potential is used to describe the Ar–HCO interaction, and the HCO intramolecular potential is the previous Legendre polynomial fit to ab initio calculations. Vibrational state‐specific and state‐to‐state cross sections, averaged over the orientation of Ar relative to the CO‐bond axis, are calculated over a range of translational energies. Collision‐induced dissociation cross sections to form H+CO from all HCO bound states are presented, and decomposed into components corresponding to dissociation via HCO resonances and dissociation via nonresonance states. The energy transfer from selected initial states is also calculated as a function of initial relative translational energy.

The utility of higher order derivatives in constructing molecular potential energy surfaces by interpolation
View Description Hide DescriptionIn this paper we evaluate the use of higher order derivatives in the construction of an interpolatedpotential energy surface for the OH+H_{2}→H_{2}O+H reaction. The surface construction involves interpolating between local Taylor expansions about a set of known data points. We examine the use of first, second, third, and fourth order Taylor expansions in the interpolation scheme. The convergence of the various interpolatedsurfaces is evaluated in terms of the probability of reaction. We conclude that first order Taylor expansions (and by implication zeroth order expansions) are not suitable for constructing potential energy surfaces for reactive systems. We also conclude that it is inefficient to use fourth order derivatives. The factors differentiating between second and third order Taylor expansions are less clear. Although third order surfaces require substantially fewer data points to converge than second order surfaces, this faster convergence does not offset the large cost incurred in calculating numerical third derivatives. We therefore conclude that, without an efficient means for calculating analytic third derivatives, second order derivatives provide the most cost‐effective means of constructing a global potential energy surface by interpolation.

Rate coefficients for state‐to‐state rovibronic relaxation in collisions between NO(X ^{2}Π, ν=2, Ω, J) and NO, He, and Ar at 295, 200, and 80 K
View Description Hide DescriptionThe state‐to‐state rates of collisional energy transfer within and between the rotational level manifolds associated with the Ω=1/2 and Ω=3/2 spin–orbit states of NO(X ^{2}Π, ν=2) have been measured using an infrared–ultraviolet double resonance (IRUVDR) technique. NO molecules were initially prepared in a specific rovibronic level, for example, ν=2, Ω=1/2, J=6.5, by tuning the output from an optical parametric oscillator(OPO) to a suitable line in the (2,0) overtone band. Laser‐induced fluorescence(LIF)spectra of the A ^{2}Σ^{+}–X ^{2}Π (2,2) band were then recorded at delay times corresponding to a small fraction of the average time between collisions in the gas sample. From such spectra, the relative concentrations of molecules in levels populated by single collisions from the initially prepared state could be estimated, as could the values of the rate coefficients for the state‐to‐state processes of collisional energy transfer.Measurements have been made with NO, He, and Ar as the collision partner, and at three temperatures: 295, 200, and 80 K. For all collision partners, the state‐to‐state rate coefficients decrease with increasing ΔJ (i.e., change in the rotational quantum number and rotational angular momentum) and increasing ΔE _{rot} (i.e., change in the rotational energy). In NO–NO collisions, there is little propensity for retention of the spin–orbit state of the excited molecule. On the other hand, with He or Ar as the collision partner, transfers within the same spin–orbit state are quite strongly preferred. For transfers between spin–orbit states induced by all collision partners, a propensity to retain the same rotational state was observed, despite the large change in internal energy due to the spin–orbit splitting of 121 cm^{−1}. The results are compared with previous experimental data on rotational energy transfer, for both NO and other molecules, and with the results of theoretical studies. Our results are also discussed in the light of the continuing debate about whether retention of angular momentum or of internal energy is the dominant influence in determining the rates of state‐to‐state rotational energy transfer.