Volume 92, Issue 9, 01 May 1990
Index of content:

A chemically induced dynamic electron polarization study on the acetone ketyl radical and radical pair in an alcohol solution
View Description Hide DescriptionA quantitative study of the chemically induced dynamic electron polarization (CIDEP) spectra of acetone and deuterated acetone ketyl radicals in i‐propanol over a wide temperature range has been done to understand the properties of the radical pairs in solution, e.g., the interactions between the radicals, the diffusional and rotational motions of the radicals of the geminate radical pairs, and the microscopic solvent structure. The spin polarization of the separated radical is analyzed in terms of the stochastic‐Liouville equation. It is shown that the observed polarization cannot be explained on the basis of the normal diffusion of the radicals in the pair. The modified diffusion models which presuppose slow diffusion of the radicals in a microscopic solvent structure can account for the observed polarization. The radical pair electron paramagnetic resonance(EPR) spectra observed at very low temperatures (<∼−70 °C) are explained by assuming that a considerable fraction of the radicals are held together in solvent structures whose lifetimes are a few microseconds. From the comparison with other alcohol and amine systems, we conclude that the observation of the radical pair EPR spectra is characteristic of alcohol solutions at very low temperatures.

Analysis of vibrational correlations and couplings in the lowest two singlet states of pentacene by high resolution, fully resonant, coherent four‐wave mixing spectroscopy
View Description Hide DescriptionCoherent, fully resonant four‐wave mixing (FRFWM) spectroscopy is used to probe the vibrational and vibronic state of the pentacene S _{0} and S _{1} electronic states. High resolution is obtained using mixed molecular crystals at cryogenic temperatures. The FRFWM spectra combined with conventional absorption and fluorescence spectra allow correlation of the S _{1} fundamentals to the corresponding ground state normal coordinates. Couplings between modes are indicated and may be related to Fermi resonance of the pentacene vibrations by means of host librational phonons.

Production of excited neutral and ionic photofragments following core‐level excitation in molecules
View Description Hide DescriptionWe have performed the first experiments to examine the n e u t r a l fragmentation paths following direct core‐level excitation in a molecule. Using monochromatized synchrotron radiation in the range 100–140 eV, we have monitored the dispersed UV/optical fluorescence resulting from excitation of a Si 2p electron in SiF_{4}. The main features in the fluorescencespectrum have been identified as emission from the SiF^{+} _{4} D state and from excited SiF, Si, F, and Si^{+}. Features in the fluorescence excitation spectra are assigned to excitation of a Si 2p electron to unoccupied valence orbitals, Rydberg orbitals, and shape resonances. There is a large enhancement in the yield of excited‐state fragments following core‐to‐Rydberg excitation, which is due to the greater probability of the core‐excited Rydberg state decaying, via a resonant Auger process, to highly excited, unbound states of SiF^{+} _{4}.

Reduced dimensionality quantum reactive scattering: H_{2}+CN→H+HCN
View Description Hide DescriptionWe apply a recently developed, reduced dimensionality quantum theory of diatom–diatom reactive scattering [Q. Sun and J. M. Bowman, Int. J. Quantum Chem., Symp. 2 3, 115 (1989] to the exoergic H_{2}+CN→H+HCN reaction, for zero total angular momentum. A new semiempirical, three‐dimensional potential surface, which is based in part on a b i n i t i o calculations of the saddle point properties is also reported. Reaction probabilities for the ground and first excited bending states of HCN are calculated for total energies up to 1.0 and 1.06 eV, respectively. The results show a strong preference for formation of HCN (0v _{ b }1) and HCN (0v _{ b }2), v _{ b }=0 and 1, starting with ground vibrational state reactants. Reaction probabilities for vibrational excitation of H_{2} or CN are also reported for both bending states of HCN. Vibrational excitation of H_{2} is found to be far more effective in promoting reaction than vibrational excitation of CN.

Temperature dependent quenching of the A ^{2}Σ^{+} and B ^{2}Π states of NO
View Description Hide DescriptionCollisional quenching of the v’=0 vibrational levels of the A ^{2}Σ^{+} and B ^{2}Π states of nitric oxide has been studied over the temperature range 300 to 750 K. The pressure dependence of the time decay of laser‐induced fluorescence, in a slowly flowing heated cell, furnished the quenching cross sections σ_{ Q }. NO and O_{2} quench the A state rapidly but with no temperature dependence; σ_{ Q }=37 and 21 Å^{2}, respectively. σ_{ Q A } for H_{2}O drops from 105 Å^{2} at 300 K to 34 Å^{2} at 750 K. σ_{ Q B } for O_{2} is independent of temperature but σ_{ Q B } for NO drops twofold and for H_{2}O decreases by a factor of 3 over the temperature range studied. This variation among these colliders cannot be explained by a uniform, simple picture of the collision dynamics. Evidence is seen for B→A transfer proceeding through an intermediate state, perhaps a ^{4}Π.

Quenching of A ^{2}Σ^{+} OH at 300 K by several colliders
View Description Hide DescriptionCollisional quenching of the v’=0 level of the A ^{2}Σ^{+} state of the OH molecule has been studied for a variety of collision partners. The pressure dependence of time‐resolved, laser‐induced fluorescence furnishes the quenching cross sections σ_{ Q }. OH radicals are produced in a microwavedischarge or by photolysis of HNO_{3} at 193 nm, always in sufficient Ar bath to produce a thermal rotational population at 300K in the laser‐excited A ^{2}Σ^{+} state. For Kr and Xe, the σ_{ Q } are 8 and 27 Å ^{2}, respectively; comparison with a prior study suggests a decrease in σ_{ Q } with increasing rotational level and/or increasing temperature. σ_{ Q } (O_{2})=18 Å ^{2},and σ_{ Q } (H_{2}O)=80 Å ^{2}; cross sections were measured for selected freons and butanes also important in tropospheric laser measurements of OH.

Intramolecular dynamics of collisionally excited metal⋅⋅⋅ligand complexes in the energy localization range
View Description Hide DescriptionWe report trajectory calculations on intramolecular energy flow and bonddissociation of a model system OC⋅⋅⋅Pt⋅⋅⋅CO in a collision energy range, where the energy transfer to the complex localizes in one of the two Pt⋅⋅⋅CO bonds. The collision model is an Ar atom incident on the left‐hand‐side terminal atom of the complex containing a vibrationally excited ligand (OC*⋅⋅⋅Pt⋅⋅⋅CO or OC⋅⋅⋅Pt⋅⋅⋅CO*) in a collinear configuration. In this energy range, essentially all of the energy transferred to the complex localizes in the right‐hand‐side Pt⋅⋅⋅CO bond for a time corresponding to many vibrational periods before dissociation or intramolecular energy flow occurs. The existence of this energy localization time zone and its effect on intramolecular dynamics are discussed in detail at various collision energies.Energy localization leads to long delay for bonddissociation and strong dependence of bonddissociation on the initial excitation site. Time delay for the dissociation of the left‐hand‐side Pt⋅⋅⋅CO bond is particularly long, and this inefficient dissociation process is discussed in terms of intramolecular recrossing of the vibrational energy across the heavy mass barrier. At the threshold, dissociation always occurs at the side where the excited ligand is present: Ar+OC*⋅⋅⋅Pt⋅⋅⋅CO→Ar+OC*+Pt⋅⋅⋅CO or Ar+OC⋅⋅⋅Pt⋅⋅⋅CO* →Ar+OC⋅⋅⋅Pt+CO*.

A generalized radiation boundary condition
View Description Hide DescriptionThe radiation boundary condition was originally proposed by Collins and Kimball as a means of avoiding the unphysical prediction of the Smoluchowski model for reaction rates that the calculated rate in three dimensions k(t) has the property k(0)=∞. A microscopic model that can be used to derive the boundary conditions uses the tacit assumption that an encounter between two molecules A and B gives rise to a reaction with a probability α<1. We consider a non‐Markovian model in which the probability that exactly n encounters between A and B are required to produce a reaction is equal to θ_{ n }. We show that when the expected number of such encounters is finite, one gets the usual radiation boundary condition. When the expected number is infinite, one finds a boundary condition that is nonlocal in time. The extension of our analysis to higher dimensions as well as to the Smoluchowski equation is readily generated.

Real‐time clocking of bimolecular reactions: Application to H+CO_{2}
View Description Hide DescriptionAn experimental methodology is described for the real‐time clocking of elementary bimolecular reactions, i.e., timing the process of formation and decay of the collision complex. The method takes advantage of the propinquity of the potential reagents in a binary van der Waals (vdW) ‘‘precursor’’ molecule. An ultrashort pump laser pulse initiates the reaction, establishing the zero‐of‐time (e.g., by photodissociating one of the component molecules in the vdW precursor, liberating a ‘‘hot’’ atom that attacks the nearby coreagent). A second ultrashort, suitably tuned, variably delayed probe laser pulse detects either the intermediate complex or the newly born product. From an analysis of this temporal data as a function of pump and probe wavelengths, the real‐time dynamics of such a ‘‘van der Waals‐impacted bimolecular (VIB)’’ reaction can be determined. Chosen as a demonstration example is the VIB reaction H+CO_{2}→HOCO^{‡}→HO+CO, using the HI⋅CO_{2} vdW precursor. The pump laser wavelength was varied over the range 231–263 nm; the probe laser detected OH in two different quantum states. The measured rates of formation and decay of the HOCO^{‡} complex are characterized by time constants τ_{1} and τ_{2}; τ_{2} spanned the range 0.4–4.7 ps, varying with the available energy. The dynamics of the HOCO^{‡} decay are discussed.

Individual cross sections for ^{1} D _{2} sublevels (M _{ L }=0, ±1, ±2) in the alignment‐dependent process: Ca(4p ^{2} ^{1} D _{2})+Rg→Ca(3d4p ^{1} F _{3})+Rg as a function of rare gas
View Description Hide DescriptionThe Ca(4p ^{2} ^{1} D _{2}) state is prepared in a two‐step excitation with linearly polarized lasers. Two different angular wave functions are selected, Y_{2,0} or (Y_{2,−1}−Y_{2,1})/, by using parallel or perpendicular laser polarizations, respectively. Subsequent collision with a rare gas atom (He, Ne, Ar, Kr, or Xe) populates the near‐resonant Ca(3d4p ^{1} F _{3}) state. The dependence of the collisional energy transfer process is measured as a function of the alignment of the initial ^{1} D _{2} state wave function with respect to the average relative velocity vector. The laser‐selected Y_{2,0} and (Y_{2,−1}−Y_{2,1})/ angular wave functions display dramatically different alignment dependences, which are understood by an analysis of the rotation properties of these wave functions. The relative contributions to the cross section of the individual ^{1} D _{2} sublevels, M _{ L }=0, ±1, and ±2, are extracted, and these vary considerably depending on the rare gas. For He, the M _{ L }=±2 sublevel (asymptotic Δ molecular state) contributes the most to the total cross section, while for all the other rare gases, the M _{ L }=0, ±1 sublevels (asymptotic Σ and Π molecular states, respectively) are more important. The contribution of the M _{ L }=0 sublevel increases smoothly with increasing mass of the rare gas collision partner, becoming the largest contributor for Xe.

Theory of reversible diffusion‐influenced reactions
View Description Hide DescriptionA unified theory of reversible diffusion‐influenced geminate and pseudo‐ first‐order reactions is developed. Explicit results are presented for the time dependence of the fraction of molecules that are dissociated at time t for a variety of initial conditions. To introduce the basic ideas of our approach, an elementary and rather complete treatment of the irreversible reaction between a pair of interacting, spherically symmetric particles is presented. The focus is on deriving relations among survival probabilities and bimolecular time‐dependent rate coefficients for the radiation and absorbing boundary conditions and the asymptotic behavior of these quantities. These relations are then generalized to reversible geminate reactions. For example, it is shown that the separation probability for an initially bound pair satisfies a simple convolution relation involving the survival probability of an irreversibly reacting geminate pair initially at contact. An analytic expression is obtained for this separation probability that is exact for free diffusion and is an accurate approximation for interacting particles. Finally, the Smoluchowski approach to irreversible pseudo‐first‐order reactions is extended to reversible reactions. The analysis is based on the generalization of the convolution relations that are rigorously valid for isolated pairs.

Multiphoton ionization and dissociation of mixed van der Waals clusters in a linear reflectron time‐of‐flight mass spectrometer
View Description Hide DescriptionSlow metastable fragmentation of benzene/toluene and benzene/para‐difluorobenzene clusters is observed in a newly developed linear reflectron time‐of‐flight mass spectrometer after two‐photon ionization. The breakdown of the metastable intensity with decreasing two‐photon energy is measured and yields the appearance potential for the main dissociation channels of the homo‐ and the heterodimers. Based on these values, the dissociation energies of the neutral dimers are obtained and shown to be consistent with the changes of the polarizability and dipole moment of the components. In addition, from the appearance potentials and the measured ionization potentials, the dissociation energies of the charged dimer clusters are found. The binding energies of the heterocluster ions and the para‐difluorobenzene homodimer ion are smaller than the respective binding energies in the benzene and toluene homodimer ions. This is explained by a larger contribution of charge transfer resonance interaction to the binding energy of the latter homodimer ions. On the basis of these results we present an energetic scheme for prediction of the dissociation pathways of the trimer ions in agreement with the measured results.

Time‐of‐flight spectra of a particle scattering from a collinear harmonic lattice at finite temperature
View Description Hide DescriptionA new formalism is developed for computing the time‐of‐flight spectrum of a particle scattering from a collinear harmonic lattice prepared at finite temperature. We use a time‐domain transcription to construct an S‐matrix formalism that can be easily implemented via Gaussian wave packet dynamics. Numerical results are presented for a particle scattered from a lattice containing 100 oscillators at several temperature values.

Generalized reactive flux method for numerical evaluation of rate constants
View Description Hide DescriptionReactive flux method for numerical evaluation of rate constants is generalized to arbitrary underlying dynamics. The feasibility of the method is illustrated by numerically evaluating rate constants for a one‐dimensional jump process and a diffusion process. In both cases, we find excellent agreement with exact results known in these cases.

Silicon dimer formation by three‐body recombination
View Description Hide DescriptionThe rates and dynamics of three‐body thermal recombination of silicon atoms to form dimers is investigated at temperatures of 800, 1000, and 1200 K with Ar and Si atoms acting as the third body. A previously reported global potential‐energy surface fitted to the results of a b i n i t i o calculations at the MP4/6‐31G* level and experimental data are employed for the [Si,Si,Si] system. A simple, pairwise potential is used for the [Ar,Si,Si] system. The calculated rate coefficients for the [Ar,Si,Si] system all lie in the range of 1.34–1.46×10^{1} ^{6} cm^{6}/mol^{2} s. If rotationally trapped dimers are included, the results are in the range of 2.51–2.68×10^{1} ^{6} cm^{6}/mol^{2} s. The weak temperature dependence is characterized by an activation energy of 1.2 kcal/mol. When silicon is the third body, the rates are more than an order of magnitude larger due to the increased interaction and the opening of a complex formation channel for recombination. Four mechanistic pathways leading to recombination are identified. These are direct energy exchange, direct atom exchange, complex formation, and metastable formation due to a rotational barrier. For the [Si,Si,Si] system at 800 K, the contributions of these pathways to the total recombination rate are: direct energy and atom exchange (65.5%), complex formation (6.5%), and metastable formation (28%). Internal energy distributions for product Si_{2} dimers are reported. In every case, these distributions exhibit a prominent maximum at the Si_{2}dissociation threshold. The falloff at energies below the maximum reflects the expected exponential distribution of translational energies in unimolecular dissociation processes. The distributions for the [Si,Si,Si] system are broader than those obtained when Ar is the third body. This increased breadth is interpreted to be due to the increased interaction and complex formation that is not present for the [Ar,Si,Si] system.

Multiplet‐specific shape resonance and autoionization effects in (2+1) resonance enhanced multiphoton ionization of O_{2} via the d ^{1}Π_{ g } state
View Description Hide DescriptionIn this paper we discuss the single‐photon ionization dynamics of the d ^{1}Π_{ g }Rydberg state of O_{2}. Comparision is made with vibrationally resolved measurements of photoelectron spectra which employ (2+1) resonance enhanced multiphoton ionization (REMPI) through the d ^{1}Π_{ g } state. A σ_{ u } shape resonance near the ionization threshold leads to non‐Franck–Condon vibrational branching ratios and a substantial dependence of photoelectron angular distributions on the vibrational state of the X ^{2}Π_{ g } ion. Significant differences exist between our one‐electron predictions and experiment. These are mainly attributed to electronic autoionization of repulsive ^{1}∑^{−} _{ u }, ^{1}∑^{+} _{ u }, and ^{1}Δ_{ u } states associated with the 1π^{3} _{ u }1π^{3} _{ g } configuration. A proposed singlet ‘‘K’’ ^{1}Π_{ u }Rydberg state converging to the A ^{2}Π_{ u } ion probably also contributes to autoionization in the d ^{1}Π_{ g } state spectrum. We also show that autoionizing H and J ^{3}Π_{ u }Rydberg states of O_{2} converging to the a ^{4}Π_{ u } and A ^{2}Π_{ u } ionic thresholds, respectively, may play a previously unsuspected role in the C ^{3}Π_{ g } state one‐color REMPI spectra. We discuss multiplet‐specific (spin‐dependent) effects via comparision of these results with recent experimental and theoretical studies of O_{2} C ^{3}Π_{ g }photoionization.

Vibrational and rotational energy distributions of CH_{3} and IF formed in the reactions of F atoms with CH_{4} and CH_{3}I
View Description Hide DescriptionThe two reactions F+CH_{4}→CH_{3}+HF (1) and F+CH_{3}I→CH_{3}+IF (2) have been investigated by using time‐resolved diode laserabsorption spectroscopy to probe the reaction products CH_{3} and IF. The fluorine atoms have been generated by the pulsed CO_{2} laser photolysis of SF_{6} in the presence of CH_{4} or CH_{3}I at a total pressure of 5 Pa. Rotational lines of CH_{3} in the v _{2} =1–0, 2–1, and 3–2 bands (out‐of‐plane bending vibration) and those of IF in the v=1–0, 2–1, 3–2, 4–3, 5–4, and 6–5 bands have been observed and analyzed to determine time‐dependent product energy distributions. No clear evidence for population inversion has been found in the vibrational levels of both CH_{3} and IF. Nascent vibrational distributions of v _{2}=0:1:2 of CH_{3} are 1:0.36:0.15 for reaction (1) and 1:0.32:0.14 for reaction (2), and that of IF v=0:1:2:3:4:5 is 1:0.70:0.40:0.22:0.10:0.04. The nascent CH_{3} produced in reaction (1) is rotationally cold (around room temperature) and only 3% and 2% of total available energy are partitioned to the ν_{2} vibrational mode and rotational motion of CH_{3}, respectively. The vibrational and rotational energy distributions of CH_{3} and IF produced in reaction (2) have been well reproduced by a statistical model calculation.

A molecular beam study of the one, two, and three photon photodissociation mechanism of the group VIB (Cr,Mo,W) hexacarbonyls at 248 nm
View Description Hide DescriptionPhotodissociation of the group VIB (Cr,Mo,W) hexacarbonyls has been studied at 248 nm using molecular beam photofragment spectroscopy. One, two and three photon processes have been observed. Analysis of the product velocity distributions shows that the photodissociation mechanism consists of sequential CO eliminations with the nth photon channel best described as the single photonphotodissociation of the stable products of the n‐1st photon channel. The product translational energy distribution for the first CO elimination step is quantitatively similar for all three hexacarbonyls and characteristic of a repulsive translational energy release. The product translational energy distributions of all subsequent CO elimination steps are accurately described by a simple, microcanonical model. Qualitative molecular orbital considerations suggest that the large product translational energy observed in the first CO elimination step results from a repulsive σ interaction between the closed shell CO ligand and an excited molecular orbital which has a significant admixture of metal (n+1)p_{z}, (n+1)s and n d _{ z } ^{2} orbitals. This repulsive interaction is absent in the remaining CO elimination steps because there are vacancies in the coordination shell along the z axis.

Population transfer between molecular vibrational levels by stimulated Raman scattering with partially overlapping laser fields. A new concept and experimental results
View Description Hide DescriptionThe feasibility of a novel technique for efficient and selective population transfer from a thermally populated level 1 via an intermediate state 2 to level 3 is experimentally demonstrated. It is shown for sodium dimers that the process of on‐ or near‐resonance stimulated Raman scattering with only partially overlapping laser beams is, in particular, useful for the selective population of high vibrational levels of particles in a molecular beam. This is achieved when the interaction with the Stokes laser, coupling levels 2 and 3, begins e a r l i e r than the interaction with the pump laser. The phenomenon, which is closely related to the formation of ‘‘trapped states,’’ is quantitatively explained using the basis of eigenstates of molecules strongly coupled to the radiation fields. The similarity and difference to related techniques such as rapid adiabatic passage phenomena in two‐level systems, off‐resonant stimulated Raman scattering, or stimulated emission pumping is briefly discussed.

On the dissociation energy of Mg_{2}
View Description Hide DescriptionThe bonding in the X ^{1}Σ^{+} _{ g } state of Mg_{2} is investigated using near‐complete valence one‐particle Slater and Gaussian basis sets containing up to h functions. Full configuration interaction (FCI) calculations are used to calibrate four‐electron correlation treatments. We show that the four‐electron complete CI limit can be approached using a sequence of either second‐order CI (SOCI) or interacting correlated fragment (ICF) calculations. At the valence level, our best estimate of the dissociation energy (D _{ e }) is 464 cm^{−1}. We show that this is a lower limit and probably within 5 cm^{−1} of the complete basis value. The inclusion of core–valence correlation using a model operator approach decreases D _{ e } by about 35 cm^{−1} and increases the bond length by 0.03 a _{0}, thereby yielding spectroscopic constants in good agreement with experiment. Attempts to compute the core–valence effect accurately by expanding the CI treatment were unsuccessful.