Volume 106, Issue 11, 15 March 1997
Index of content:

Pulsed discharge jet spectroscopy of DSiF and the equilibrium molecular structure of monofluorosilylene
View Description Hide DescriptionThe jetcooled laser induced fluorescence excitation spectrum of the Ã ^{1}A^{′′}−X̃ ^{1}A band system of DSiF has been observed using the pulsed discharge jet technique. Vibrational analysis of the spectrum yielded upper state harmonic vibrational frequencies of ω_{1}=1322, ω_{2}=444, and ω_{3}=867 cm^{−1}. Vibronic bands involving all of the upper state fundamentals of HSiF and DSiF have now been rotationally analyzed, allowing a determination of the excited state equilibrium structure as Å, Å, and The harmonic frequencies and centrifugal distortion constants were used to obtain harmonic force fields and average (r_{z}) structures for the ground and excited states. The ground state average structure was used to estimate the equilibrium structure of Å, Å, and

Laserinduced fluorescence spectrum of the FCO radical
View Description Hide DescriptionLaserinduced fluorescencespectra of FCO are reported between 29 000 cm^{−1} and 32 800 cm^{−1}. FCO was prepared by three separate procedures: photolysis of CF_{2}O and C_{2}F_{2}O_{2}, and photolysis of F_{2} in the presence of CO. The observation of the same spectral features from all three production schemes confirms the assignment of FCO as the spectral carrier. Although the LIFspectrum lies in the same wavelength region as the UVabsorptionspectrum, the two spectra do not have the same appearance and so represent different upper states. The LIFspectrum is assigned as a transition to the A ^{″} Renner–Teller component of a linear ^{2}Π state predicted by ab initio calculations. The spectrum shows a progression of bands at approximately 430 cm^{−1} intervals, in good agreement with the predicted spacing of bending levels in the ^{2}Π state. The lower frequency stretching mode ν_{1} occurs at approximately 960 cm^{−1}. The vibronic bands are strongly degraded to the red, consistent with the calculated geometry of the linear state but not the bent A state to which the absorptionspectrum in this region is assigned. Resolved emission from the longest wavelength prominent bandhead at 29 872 cm^{−1} shows progressions in the C–O stretch and bend modes of the ground state. The fluorescence lifetime of this band extrapolated to zero pressure is 95±4 ns, while that of the band at 31 136 cm^{−1} is 63±5 ns.

First observation and electronic spectroscopy of chromium mononitride: The A ^{4}Π_{r}←X ^{4}Σ^{−} transition near 745 nm
View Description Hide DescriptionWe report the first gas phase spectroscopic study of chromium mononitride. CrN molecules were generated in our laser vaporization molecular beamsource and studied using laser induced fluorescence techniques. Twelve vibronic subbands of Cr^{14}N have been rotationally analyzed in the 745–647 nm region. These bands are assigned to the A ^{4}Π_{r}←X ^{4}Σ^{−} transition. The assigned ground state electron configuration 1δ^{2}9σ^{1} and symmetry are identical to those of the isovalent VO, MoN, and WN molecules. The energy observed for the excited ^{4}Π _{r} state matches closely the first ^{4}Π state energy derived from a recent ab initio calculation [J. F. Harrison, J. Phys. Chem. 100, 3513 (1996)]. In addition to the AX transition, 11 other bands of Cr^{14}N have been identified and vibrationally assigned to transitions involving two other excited states, presumably the B ^{4}Σ^{−}, and a ^{2}Π state. Comparisons with the isovalent VO, MoN, and WN systems have provided more insights into the bonding and electronic structure of the CrN molecule.

Farinfrared absorption spectra of water, ammonia, and chloroform calculated from instantaneous normal mode theory
View Description Hide DescriptionInstantaneous normal mode (INM) theory was used to calculate absolute farinfrared absorption spectra of water, ammonia, and chloroform. Three procedures for weighting the INM density of states to yield absorption intensities were tested against spectra based on dipole time correlation functionsgenerated from molecular dynamics (MD) simulations. Weighting method I, which utilizes only the rotational character of a mode in determining its contribution to absorption, performed slightly better than method II, a more exact treatment which incorporates the extent to which a mode is IRactive. Method III, which includes the contributions of induced dipoles, was successful in describing the influence of induced dipoles on the farinfrared spectra of the model liquids. The contribution to absorption of unstable modes with imaginary frequencies was found to be significant at low frequencies, and was treated by a simple approximation. Agreement between INM theory and the MD analysis was quite good for chloroform and ammonia, but less so for water. Agreement with experimental data in the range of 4–100 cm was generally poor, but no worse than that for the MD calculations, primarily reflecting the simple intermolecular potentials used rather than the computational method itself.

Optical dephasing in pentacenedoped PMMA under high pressure
View Description Hide DescriptionPressure and temperaturedependent photon echo results are obtained for pentacene doped polymethyl methacrylate (PMMA). A unique pressure effect is observed in which the optical dephasing rate increases as the pressure is increased from ambient pressure to 4 kbar, above which the optical dephasing rate is pressure independent up to 43 kbar. The present results are also compared with pressure and temperaturedependent photon echo results for rhodamine 101 in PMMA, in which the optical dephasing rate was completely insensitive to pressure over the range 0 to 30 kbar. A negative correlation is also observed between the optical dephasing rate and the spectral hole burning efficiency. Line broadening due to pressure induced spectral diffusion may be responsible for both the increased dephasing rate and the reduced spectral holeburning at high pressure.

On the scattering phenomena for various kinds of polarized light in a nonpolar binary liquid mixture
View Description Hide DescriptionThe explicit result for the Mueller matrix for the sum of single and double scatterings is obtained in a nonpolar binary liquid mixture composed of a chiralsolute and an achiral solvent are obtained with the aid of the Ornstein–Zernike form for the correlation function of concentration fluctuations. The explicit result of the matrix shows that the law of reciprocity does not hold in the mixture. The scattering phenomena for six kinds of completely polarized light in a nonpolar binary liquid mixture are different from those in a pure chiral fluid because of the failure of the law and the large effect of concentration fluctuations. The scattering phenomena are in detail discussed in the region far from and near the critical point.

Multistate vibronic coupling effects in the Kshell excitation spectrum of ethylene: Symmetry breaking and corehole localization
View Description Hide DescriptionThe vibrational fine structure of the prominent absorption band of ethylene and some of its isotopomers has been investigated theoretically with the aid of a specific (linear) vibronic coupling model. The presence of two equivalent levels gives rise to two nearly degenerate electronic states of and symmetry, respectively, which can interact vibronically via the (planar) antisymmetric C–H stretching and bending modes ( and ). In addition to these states of immediate interest, the present model comprises three more pairs of “effective” states at higher energy allowing one to describe the excitation of additional asymmetric (nonplanar) modes as a result of linear vibronic coupling. To a good approximation the intrapair vibronic coupling can be eliminated by using a representation in terms of localized hole states. As a further result, the inplane and outofplane modes become separable. The required vibronic coupling constants have been determined with the aid of ab initio calculations at various nuclear conformations using a secondorder polarization propagator method. After slight readjustment of some of the parameters, the calculated spectral profiles are found to be in excellent agreement with the experimental findings. The theoreticalspectra reflect strong excitation of nontotally symmetric modes (and the concomitant symmetry lowering), involving mainly the planar C–H stretching mode and the outofplane C–H bending mode . While the planar distortion is a consequence of the equivalent core levels, the nonplanar symmetry lowering can be rationalized in chemical terms as a rehybridization effect in which the bonding scheme in the ethylene ground state is changed to in the excited state.

Absolute partial cross sections for electronimpact ionization of CH_{4} from threshold to 1000 eV
View Description Hide DescriptionAbsolute partial cross sections for the production of CH, CH, CH , CH^{+}, C^{+}, H, and H^{+} from electronimpact ionization of CH_{4} are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a timeofflightmass spectrometer and detected with a positionsensitive detector whose output demonstrates that all product ions are completely collected. The overall uncertainty in the absolute cross section values is ±3.5% for singly charged parent ions and is slightly greater for fragment ions. Although previous measurements are generally found to agree well with the present results for CH and CH, almost all previous work for the remaining fragment ions lies lower than the present results and in the case of H^{+} is lower by approximately a factor of 4.

Photodissociation dynamics in the UV laser photolysis of DNCO: Comparison with HNCO
View Description Hide DescriptionUsing the laser photolysis/laserinduced fluorescence (LP/LIF) pump–probe technique, the gas phase photodissociationdynamics of room temperature DNCO were studied at photolysis wavelengths of 193 and 248 nm. D atoms produced via DNCO were detected by using tunable narrowband Lymanα laser radiation ( nm) generated by resonant thirdorder sumdifference frequency conversion of pulsed dye laser radiation. By means of a photolytic calibration method absolute cross sections for the direct photochemical D atom formation were measured to be and . For the wavelength of 193 nm, the measurement of the optical absorption cross section allowed the direct determination of the D atom product quantum yield of . At the wavelength of 248 nm, where the optical absorption cross section was too small to be measured with reasonable accuracy, a kinetic calibration method was used to measure product quantum yields for D atom, , and for “spinforbidden” formation, . From the D atom Doppler profiles, measured under collisionfree conditions, the fraction of the available energy released as product translational energy was determined to be for 193 nm and for 248 nm photolysis wavelengths. In addition, using polarized photodissociation laser light, measurements were carried out in order to determine the anisotropy of the D atom product angular distribution. It was found that the value of the anisotropy parameter β decreases from −(0.45±0.09) for a photolysis wavelength of 193 nm to −(0.01±0.10) at 248 nm, suggesting a moderately anisotropic distribution at the shorter wavelength and an effectively isotropic one at the longer wavelength. Comparisons are made with results from recent photodissociationdynamics studies of the isotopomer HNCO.

Guided ion beam studies of the reactions of with : Iron cluster oxide and dioxide bond energies
View Description Hide DescriptionThe kinetic energy dependence of the reactions of with are studied in a guided ion beammass spectrometer. A variety of , and where , product ions are observed, with the dioxide cluster ions dominating the products for all larger reactant cluster ions. Reaction efficiencies are near unity for all but the smallest clusters. The energy dependence of the products is analyzed in several different ways to determine thermochemistry for both the first and second oxygen atom binding to iron cluster ions. The trends in this thermochemistry are discussed and compared to bulk phase values.

Application of symplectic integrator to stationary reactivescattering problems: Inhomogeneous Schrödinger equation approach
View Description Hide DescriptionThe FFTsymplectic integrator (SI) scheme devised for solving the wave packet propagation problem is applied to stationary reactivescattering problems. In order to relate the stationary problem to the timedependent problem, a class of Schrödinger equation with an inhomogeneous wave source term is introduced. By using the equivalence between the stationary scattering eigenstate and the equilibrium state of the inhomogeneous Schrödinger equation, the scattering eigenstates can be computed by integrating the inhomogeneous Schrödinger equation with the FFTSI scheme. A Gaussian wave source is proposed as an efficient wave source exhibiting rapid relaxation toward the eigenstate. Our method is tested by a onedimensional example which has an analytical solution, and great numerical accuracy is confirmed. It is further examined by an example of timedependent scattering and by a twodimensional example of chaotic tunnelscattering.

Electron energy loss rates in gaseous argon determined from transient microwave conductivity
View Description Hide DescriptionThermalization of highenergy electrons in gaseous Ar at room temperature has been investigated by analyzing the imaginary component of the transient microwaveconductivity produced by pulse radiolysis. The conductivity signal amplitude showing a peak due to the Ramsauer minimum has been correlated with the amplitude derived from calculations of the effective collision frequency using Margenau’s formula assuming Maxwellian velocity distribution of electrons. Two approaches, using the peak and the plateau of the signal, for absolute normalization of the conductivity amplitude give results consistent with each other. It has been found that the excess mean electron energy drops very rapidly to about 0.2 eV and then decreases exponentially with a constant relaxation time. The thermalization time for 1 eV electrons to relax to 10% thermal energy has been determined to be 5.8 ms at 1 Torr Ar. Electron energy loss rate coefficients have been derived as a function of the mean electron energy.

Photoemission cross sections for CH radicals produced by collisions of He atoms with CHX (X=H, Cl, Br, I)
View Description Hide DescriptionPhotoemission cross sections for the A–X, B–X, and C–X bands of CH resulting from the He+CHX (X=H, Cl, Br, I) reaction have been studied in the relative collision energy of 120–210 meV. Formation cross sections for CH(A, B, C) were evaluated from the ’s taking account of predissociation. A good correlation was found between the sum of the ’s for CH and the dipoleinduced dipole interaction of He with targets. The ’s from methane increase with while those from methyl halides decrease with increasing The positive energy dependence for methane implies that effective potentials leading to CH are repulsive, whereas attractive potentials play a dominant role in the reaction of He with methyl halides. Model potentials between CHCl and He^{*}(Li) calculated using ab initio molecular orbital methods indicate that He(2 approaches CHCl not from CH side but from Cl side.

Statistical modeling of collisioninduced dissociation thresholds
View Description Hide DescriptionAnalysis of the energy dependence of the cross sections for collisioninduced dissociationreactions has permitted the determination of quantitative thermodynamic information for a variety of ionic clusters. As such clusters become larger, the rate at which the decomposition occurs becomes comparable to the instrumental time available for observing the reaction. A method for incorporating statistical theories for energydependent unimolecular decomposition in this threshold analysis is reviewed and updated. The revision relies on the fact that for most ionic clusters, the transition state is a loose association of the products that can be located at the centrifugal barrier. This permits a straightforward estimation of the molecular parameters needed in statistical theories for the transition state. Further, we also discuss several treatments of the adiabatic rotations of the dissociating cluster. The various models developed here and previously are compared and used to analyze a series of data for Li^{+}(ROH) complexes, where ROH=methanol, ethanol, npropanol, ipropanol, nbutanol, ibutanol, sbutanol, and tbutanol. The trends in the bond energies derived by these various models are compared and their accuracy evaluated by comparison with relative values determined by equilibrium methods.

Quantum dynamical stereochemistry of atom–diatom reactions
View Description Hide DescriptionWe have used density matrix techniques and angular momentumalgebra to obtain quantum–mechanical equations describing the dynamical stereochemistry of the atom–diatom reactionA+BC⇌AB+C. The relative motions of reagents and products are specified by four vectors: rotational angular momenta of diatomic molecules and relative velocities of reagents and products. Our equations show how the correlations between the spatial distributions of these four vectors are related to the scattering matrix determined in quantum scattering calculations. We present three different expressions for the fourvectors correlation. One of them is appropriate to the helicity representation of the scattering matrix, while the others are appropriate to the orbital angular momentum representation with either spacefixed or bodyfixed reference frames. The formulation adopted allows for a rigorous comparison between theory and experiment. It takes mixed quantum–mechanical states and unobserved quantumnumbers into account, and all vector distributions are expressed in terms of measurable quantities (scattering angles and polarization moments of rotational angular momenta). Explicit expressions for most of the lowerorder vector correlations obtained by direct reduction of the fourvectors correlation formulas are also presented.

Control of longrange electron transfer in dynamically disordered molecular systems by an external periodic field
View Description Hide DescriptionThe influence of a strong periodic field on the longrange electron transfer is studied theoretically within the driven spinboson model extended by a dichotomically fluctuatingtunneling coupling. Applying the noninteracting blip approximation, a set of coupled kinetic equations is derived. It describes the timedevelopment of the electronic population difference between the donor and acceptor states averaged with respect to the stochastic process, the quantum fluctuations of the bath and over the fast oscillating field. The strength of the periodic field is taken into account in a nonperturbative manner. A detailed analysis is carried out for the case of a strong coupling of the transferred electron to a specific reaction coordinate. Analytical expressions for the effective transfer rate are obtained and their numerical analysis is presented. For the case of adiabatically controlled (gated) transfer the existence of a transfer regime is demonstrated where the strong external field does not influence the effective transfer rate, however, it can completely revert the direction of the transfer. Finally, it is shown that the periodic field can induce transitions between the quasiadiabatic (gated) and the nonadiabatic regime of electron transfer.

The intermolecular potential energy surface of the Ar⋅NO^{+} cationic complex
View Description Hide DescriptionThe intermolecular potential energy surface of the Ar⋅NO^{+} cationic complex has been calculated using ab initio methods: RHF, MP2, MP3, MP4, CCSD, and CCSD(T), using the ccpVDZ and ccpVTZ basis sets. An additional surface was calculated at the MP2/ccpVQZ level. All calculations were performed with a fixed NO^{+} bond length, but in one set of calculations the effect of variation of the NO^{+} bond length was studied. Finally, the MP2/ccpVQZ intermolecular potential energy surface was recalculated by performing a pointbypoint correction for basis set superposition error. All of these surfaces were used to extract anharmonic intermolecular vibrational frequencies, in order to compare to previous calculations, and experimental results. Rotational energy levels were also calculated.

An ab initio MO study on structures and energetics of , , and
View Description Hide DescriptionThe geometrical structures of cyclic and linear conformers in , , and are optimized using the fullvalence CASSCF and CCSD(T) levels of theory. The atomic natural orbital and correlation consistent basis (augccpVTZ) sets are used. The relative stability between the cyclic and linear conformers is investigated using the CCSD(T) and multireference CI levels of theory with the augccpVTZ basis set. The basis set dependency is checked with the basis set. The most stable conformer in and is cyclic with a ring, and that in linear. The energy difference between the cyclic and linear in radical is really small, being around 1.0 kcal/mol. The πelectron population on the ring in cyclic is 2.00, which is a typical value predicted on the basis of the aromaticrule. The πelectron population on the ring decreases in the order of , , and , consistent with the order of the stability of the cyclic conformers. The adiabatic electron affinity and ionization potential in are calculated to be 2.01 and 9.06 eV, respectively. The excitation energies from the most stable isomer are also calculated at the multireference CI level of theory with the augccpVTZ basis set.

A model study of quantum dot polarizability calculations using timedependent density functional methods
View Description Hide DescriptionWe compare two timedependent methods (timedependent Hartree and timedependent density functional methods) with a timeindependent density functional method for the calculation of the frequency dependent polarizability and resulting absorptionspectrum of two interacting quantum confined particles (quantum dots). The system is examined within the dipole approximation and the methods are evaluated in terms of the optical absorptionspectrum. The spectral noisegenerated by timedependent methods is a sensitive measure of the degree of broken correlation between the quantum degrees of freedom and the timedependent density functional method may help to quantify the efficacy of correlationexchange potentials that are used in density functional models. With respect to the quantum confinement issue, we find that increasing the interaction energy between nearest neighbor quantum dot sites represented in our model tends to shift absorption intensity to higher energy transitions.

Allelectron Dirac–Fock–Roothaan calculations on the electronic structure of the GdF molecule
View Description Hide DescriptionThe electronic structure of the GdF molecule is investigated using allelectron Dirac–Fock–Roothaan calculations. It is found that, in the ground state, the Gd atom transfers a electron to the spinors of the F atom, so that the molecule is ionic, having the configuration of Gd^{+}F^{−}. However, the molecule is not purely ionic, since the electrostatic field produced by Gd^{+} and F^{−} causes the spinor energies of and one of the Gd to be almost energetically degenerate so that these spinors strongly mix with each other and form covalent bonds. The electrostatic field also causes a large energy lowering for one of the spinors, giving further stability to GdF. The electrons of Gd should be regarded as valence electrons. The lower excited states and positively and negatively ionized states are found to be roughly described by Gd atomlike excitations, ionization, and electron attachments.