Index of content:
Volume 89, Issue 6, 15 September 1988

Absolute intensities of H_{2}CO in the 9–11 μm region
View Description Hide DescriptionLine intensities of H_{2}CO are analyzed between 1148.4 and 1192.8 cm^{−} ^{1} by taking into account the Coriolis interaction involving the ν_{4} and ν_{6} fundamentals. Finally two intensity parameters can be significantly determined: ∂μ_{ y }/∂q _{4}=0.0592±0.0018 D and ∂μ_{ x }/∂q _{6}=0.0772±0.0019 D. From these results, the ‘‘deperturbed’’ band strenghts can be calculated: S ^{0} _{4}=20.9±1.3 cm‘u−^{2} atm^{−} ^{1} and S ^{0} _{6}=38.0±1.9 cm^{−} ^{2} atm^{−} ^{1} at T=297 K.

Absorption spectrum of Kr_{2}F(4 ^{2}Γ) in the near ultraviolet and visible (335≤λ≤600 nm): Comparison with Kr^{+} _{2}(1( 1/2 )_{ u }) measurements
View Description Hide DescriptionAbsolute photoabsorption cross sections for the lowest excited state (4 ^{2}Γ) of Kr_{2}F and the ground state (1( 1/2 )_{ u }) of Kr^{+} _{2} have been measured in the visible and ultraviolet (Kr_{2}F: 248, 308, 335–600 nm; Kr^{+} _{2} : 360–500 nm). As predicted by theory, the Kr_{2}F spectrum is dominated by the 9 ^{2}Γ←4 ^{2}Γ band which peaks below 340 nm and correlates with the Kr^{+} _{2} 2( 1/2 )_{ g } ←1( 1/2 )_{ u } transition. However, Kr_{2}F(4 ^{2}Γ) and Kr^{+} _{2} (1( 1/2 )_{ u }) both absorb more strongly for wavelengths beyond ∼450 nm than expected, apparently owing to a nonthermalized Kr^{+} _{2} vibrational distribution. Despite interference from Kr_{2}F stimulated emission between ∼350 and 460 nm, the experimental results show the Kr_{2}F(4 ^{2}Γ) and Kr^{+} _{2}absorption profiles to be similar for λ≥460 nm but diverging rapidly as λ increases from 335 to 360 nm. At 360 nm, the absorption cross sections differ by a factor of 3. Contrary to theoretical expectations, the Kr_{2}F absolute absorption cross section is consistently smaller (typically <40%) than that for Kr^{+} _{2} which suggests that the impact of F^{−} on the Kr^{+} _{2}oscillator strengths is significant.

Quasibound levels and shape resonances of ^{3} ^{9}K_{2}(B ^{1}Π_{ u }) crossed laser‐molecular beam studies and analytical interpretation
View Description Hide DescriptionQuasibound levels and shape resonances in the (B ^{1}Π_{ u } −X ^{1}Σ^{+} _{ g }) band system of ^{3} ^{9}K_{2} have been recorded by crossed laser‐molecular beam techniques. Using optical–optical double resonance, individual rovibrational levels (v‘=15–18, J‘=3–25) of the K_{2} state are prepared by Franck–Condon pumping (FCP) in a supersonic nozzle beam. Excitation into quasibound levels below and above the (B ^{1}Π_{ u }) state barrier is detected as molecular a n d atomic (K4 ^{2} P _{3} _{/} _{2}→4 ^{2} S _{1} _{/} _{2} only) fluorescence. The resonance transition frequencies and shapes are measured and the results are used (a) to determine the scattering resonance energies, widths, and lifetimes; (b) to compare them with values obtained by a ‘‘maximum internal amplitude’’ approach [R. J. LeRoy and R. B. Bernstein, J. Chem. Phys. 5 4, 5114 (1971)]; and (c) to check the agreement with exact calculations of the B state potential using the ‘‘inverted perturbation approach (IPA).’’ The bound and quasibound part of the B ^{1}Π_{ u } state including the locus (R=8.08±0.05 Å) of the barrier maximum (298±8 cm^{−} ^{1} above the adiabatic dissociation limit) is found in excellent agreement with previous results. The shape resonances are not highly sensitive to the long‐range interatomic forces, here the repulsive dipole–dipole resonance interaction.

A picosecond bleaching study of quantum‐confined cadmium sulfide microcrystallites in a polymer film
View Description Hide DescriptionWe report a picosecond pump–probe study of 55 Å cadmium sulfide microcrystallites embedded in polymer films. Large negative absorbance changes at wavelengths corresponding to energies near the band gap are observed. This absorption bleaching and the associated changes in refractive index are mainly responsible for the large nonlinearity observed in degenerate four‐wave mixing experiments. Based on photoluminescence data, the known electron‐trapping cross section of defects, and these pump–probe experiments, we show that the conventional carrier density‐dependent band‐filling mechanism cannot account for the data, and the absorption bleaching is due to the saturation of the excitonic transition. We further show that the phase‐space filling and exchange effects from exciton–exciton and exciton‐free carrier interactions fail to account for the observed data. Instead, we propose that the exciton‐trapped carrier interaction is mainly responsible for the observed bleaching of the excitonic absorption. This interaction is unique for small semiconductor clusters since the presence of a high density of defects (most likely on the surfaces) causes the extremely rapid trapping of free carriers. According to this model, the recovery time of the absorption bleaching is determined by the trapped‐carrier relaxation time, which is sensitive to the fabrication methods and can be controlled by surface chemistry. Our study also demonstrates that one needs to understand the effects of surfaces and control the surface chemistry before the important question of size effects on the nonlinear optical properties can be addressed.

The emission spectrum of helium hydride. I. Bands near 8000 Å
View Description Hide DescriptionSpectra of helium hydride were observed after neutralization of a mass‐selected HeH^{+}beam. The molecules were produced in a fast beam, and so a special setup had to be used to avoid Doppler broadening and a careful calibration procedure had to be applied to determine the line positions and linewidths. In an earlier paper, we reported the first observation of a discrete spectrum of helium hydride, which was discovered by means of an emission band near 8000 Å. In this paper, a detailed analysis of this band for all four stable isotopic mixtures is given. For the deuterides several vibrational bands were observed, which allowed equilibrium molecular constants to be determined. These constants agree with the results of recent a b i n i t i o calculations. The similarity of these constants to those of HeH^{+} in the ground state confirms that the observed states are Rydberg states. Comparison of the molecular constants for different isotopic mixtures shows deviations from the Born–Oppenheimer approximation. Born–Oppenheimer breakdown parameters were derived. Both the upper and lower states show predissociation to the repulsive ground state of helium hydride. The line intensities give evidence of a dependence of the electronic transition moment on the interatomic distance.

Vibrational circular dichroism and vibrational optical rotatory dispersion in molecular crystals. IV. Overtones of degenerate vibrations
View Description Hide DescriptionTwo‐quanta anharmonic spectra, namely overtones and combination bands with degenerate vibrations, in uniaxial gyrotropic crystals from point group of symmetry C _{3}, D _{3}, C _{4}, D _{4}, are investigated, with due account taken of the time inversion. The symmetry of two‐phonon states is studied. A general model of the anharmonicity and the influence of the crystal chiralstructure on intermolecular exchange of the circularly polarized degenerate vibrations is elaborated. The contribution of overtones and of the combination bands to dielectricpermittivitytensor, and to the gyration tensor is studied too. The paper also deals with the IR and vibrational circular dichroism (VCD) spectra near the frequencies of the unbound and of the bound two‐phonon states as well as the general features of two‐phonon circularly polarized states appearing in the linear and in nonlinear optical processes in gyrotropic crystals.

Emission spectroscopy of the predissociative Rydberg B state of CH_{3}I and CD_{3}I at 193.3 nm
View Description Hide DescriptionWe measured the emission spectra of CH_{3}I and CD_{3}I excited to a predissociative Rydberg state (E,1){2} near 193.3 nm and tabulated emission line positions with their assignments. The emission spectrum of CH_{3}I both provides previously unobserved energies of ground state combination bands in ν_{2} (CH_{3} umbrella) and ν_{3} (C–I stretch), and also suggests the excited level in the (E,1){2} Rydberg state is the previously unobserved 2^{1}3^{2} state. These results are discussed in relation to recent comparisons of UVabsorption spectra of monomeric and dimeric CH_{3}I. The CD_{3}I emission spectrum measured results from excitation of the combination mode of CD_{3} rocking (ν_{6}), umbrella motion (ν_{2}), and C–I stretching (ν_{3}) in the (E,1){2} rydberg state of CD_{3}I, followed by emission to a progression of 2_{ n }3_{1}6_{1} combination modes in the ground electronic state. A simulation of the line shape of the peaks in the CH_{3}I spectrum shows the width of the peaks results from the contribution of both inhomogeneous (K and J rotational structure) and homogeneous (lifetime) broadening. Emission from molecules after crossing to the repulsive surface was not observed.

One‐ and two‐color multiphoton ionization of argon
View Description Hide DescriptionA systematic explanation of the pressure‐dependent multiphoton ionization spectra of argon is given in terms of the phase matching requirements of third‐harmonic generation. Two‐color sum‐frequency generation in a highly dispersive medium and its effects on normal three‐photon excitation process are analyzed, using argon as an example. Finally an anomaly in the resonance line shapes of the two laser MPI spectra of argon is reported and attributed to the displacement of the foci of the two counterpropagating laser beams.

Molecular Rydberg states: Classical chaos and its correspondence in quantum mechanics
View Description Hide DescriptionThe Rydbergspectrum of Na_{2} has been shown previously to alternate when increasing energy between ‘‘stroboscopic fringes’’ which correspond to a well known separable Hund’s coupling case (a), and a complex, unidentifiable intermediate coupling. We use this system as a prototypic example to test some current ideas on the correspondence between classical chaos and properties of quantum spectra. We first determine the phase space structure and transition to chaos in classical mechanics. We then determine the change in line intensities and level spacing statistics in quantum mechanics. We show that this system has the expected behavior in the semiclassical limit in the presence of classical chaos, except for a peculiarity in level spacing statistics, but that this behavior is not a signature of chaos, since the same system shows similar behavior for some values of the parameters which correspond to a nonchaotic situation in classical mechanics. We discuss also some problems related to the nonvalidity of the semiclassical limit.

Infrared diode laser kinetic spectroscopy of the ν_{3} band of C_{3}
View Description Hide DescriptionInfrared diode laser kinetic spectroscopy was applied to reaction intermediates generated by the photolysis of diacetylene and allene at 193 nm. A strong band was observed around 2040 cm^{−} ^{1} and was assigned to the ν_{3} band of C_{3} . The observed spectrum was analyzed by using an energy level expression for a linear molecule; higher‐order centrifugal distortion terms were necessary to be included in the least‐squares fit. The band origin was determined to be 2040.0198 (8) cm^{−} ^{1}. The rotational and centrifugal distortion constants obtained suggest the presence of a potential barrier at the linear configuration in both the ground and ν_{3} states.

Quenching of C_{2}H emission produced by vacuum ultraviolet photolysis of acetylene
View Description Hide DescriptionExcited C_{2} H* is produced by vacuum ultraviolet photolysis of acetylene using a frequency tripled laser. Time‐dependent emission is measured from 400–940 nm. The use of a coherent photolysis source produces an excitation spectrum in which the rotational band contour is resolved. The absorptionspectrum of C_{2} H_{2} taken in the same apparatus closely resembles the excitation spectrum indicating a homogeneous predissociation. Time‐dependent quenching of the C_{2} H* emission by Xe, Kr, Ar, He, N_{2}, H_{2}, D_{2}, and C_{2} H_{2} is measured. The rapid quenching rates and lack of strong dependence on atomic weight suggest a spin‐allowed process is involved in this channel of C_{2} H_{2}photolysis. Quench rates are compared with several theoretical models.

Infrared spectra of ClF, Cl_{2}, and Cl complexes with HCl in solid argon
View Description Hide DescriptionArgon mixtures of ClF, Cl_{2}, and Cl atoms have been codeposited with Ar/HCl at 5 and 12 K. Infrared spectra in the HCl stretching region give evidence for the FCl‐‐HCl, Cl_{2}‐‐HCl, and Cl‐‐HCl hydrogen‐bonded complexes. The molecular complexes are more stable in the hydrogen‐bonded than antihydrogren‐bonded forms as no evidence for the latter were observed. The Cl‐‐HCl radical hydrogen‐bonded complex is considerably different from the centrosymmetric (Cl–H–Cl)^{−} anion.

The Ne–N_{2} potential energy surface from high‐resolution total differential scattering experiments and a close‐coupling and infinite‐order‐sudden analysis
View Description Hide DescriptionHigh‐resolution crossed molecular beammeasurements of the total differential cross section (DCS) for the scattering of Ne by N_{2} at a collision energy of E=75.8 meV are reported. Strongly quenched diffraction oscillations superimposed on the falloff of the main rainbow structure are clearly resolved. A reliable Ne–N_{2}potential energy surface (PES) is derived by simultaneously fitting second virial, diffusion, and viscosity coefficient data taken from the literature. Information on the anisotropy of the interaction is obtained from the quenching of the diffraction oscillations in the total DCS within the framework of the infinite‐order‐sudden (IOS) approximation. The reliability of the IOS approximation in deriving a fully anisotropicpotential energy surface from the measuredscatteringdynamics is examined and demonstrated by performing exact close‐coupling calculations for the present experimental conditions and then comparing both integral and differential total and rotationally inelastic cross sections. The derived PES is compared with recently proposed theoretical model potential surfaces. Although in satisfactory agreement with bulk properties, none of these surfaces predicts correctly the present scattering data, each having significantly different spherical and/or anisotropic components in comparison with the PES derived here.

The dynamics of polar solvation: Inhomogeneous dielectric continuum models
View Description Hide DescriptionThe influence of an inhomogeneous dielectric response on the dynamics of solvation of ions and dipoles is investigated. Solventmodels considered include discrete shell models as well as models in which the solventdielectric constant varies continuously as a function of distance from a spherical solute. The effect of such dielectric inhomogeneity is to introduce additional, slower relaxation times into the solvation response when compared to the homogeneous case. For all models studied, the deviation of the average relaxation time from that predicted for a homogeneous continuum solvent increases as the dielectric constant and the length parameter, which specifies the rapidity of approach to bulk behavior, increase. For a given solventmodel the solvation response to a change in a point dipole moment is slower than the response to a charge jump. The continuum results are compared to a recent molecular model based on the mean spherical approximation. The comparison suggests that deviations from homogeneous continuum behavior in the molecular model can be accounted for by inhomogeneity of the solventdielectric constant extending only over the first solvation shell. Predictions of inhomogeneous continuum models are also compared to experimental data. Both the observed dependence of average relaxation time on dielectric constant, and the detailed time dependence of the relaxation in high dielectric constantsolvents can be rationalized on the basis of such models.

State‐to‐state vibrational predissociation dynamics and spectroscopy of HeCl_{2}: Experiment and theory
View Description Hide DescriptionThe structure and vibrational predissociationdynamics of HeCl_{2} are studied by pump–probe spectroscopy and by three‐dimensional quantum mechanical calculations. Parity selected excitation spectroscopy is used to confirm the essential features of the previous analysis of the HeCl_{2} B←X laser excited fluorescence spectra. Product vibrational and rotational state distributions are measured for the v’=6, 8, 12, 20, and 24 levels of HeCl_{2} in the B state. For the v’=6 and 8 levels the dependence of the product state distribution on the initially excited rotational state is also measured. Although the dissociationdynamics are dominated by Δv=−1, V→T energy transfer, several interesting effects are revealed by monitoring the product rotational degrees of freedom. Due to the symmetry of the HeCl_{2} potential, the parity of the initially excited HeCl_{2} rotational state is conserved during the dissociationdynamics. Even when a single initial rotational state is excited, the observed product rotational state distribution is bimodal. The product rotational distribution is nearly independent of the amount of kinetic energy released to the product degrees of freedom. Three‐dimensional quantum mechanical calculations using a simple potential energy surface are remarkably successful at reproducing the details of the experimental measurements. Only five parameters of the potential were adjusted to calculate the excitation spectrum, the vibrational predissociation product state distributions, and the lifetimes of the excited states. Analysis of the dissociation mechanism in terms of simple models, however, is not straightforward. In particular, the impulsive, quasiclassical half‐collision model is not compatible with the observed independence of the product rotational state distribution from the amount of kinetic energy which is released. The close agreement between state‐to‐state experiment and quantum theory on the HeCl_{2}dynamics shows that the shortcomings of more approximate theories are fundamental and cannot be attributed to lack of knowledge of the true potential energy surface.

Experimental and theoretical studies of the doubly charged NO^{2+} _{2} ion
View Description Hide DescriptionSix features observed in the energy distribution of H^{−} ions arising from the single collision double charge transfer of 3–5 keV protons on nitrogen dioxide are interpreted as vertical double ionizationenergies of NO_{2} of 35.1, 38.1, 39.5, 41.0, 43.2, and 46.8 eV to form doublet states. Theoreticalenergies of the first 13 doublet and quartet electronic states of NO^{2+} _{2} have been calculated by the a b i n i t i o CIPSI method at a fixed N–O distance and for several O–N–O bending angles. Thresholds and kinetic energy releases observed in dissociative double photoionization of NO_{2} are interpreted by comparison with the calculations and charge transfer results. The N‐ and O‐Auger spectra of NO_{2} have been recorded and some of the 40 features observed have been assigned, confirming and improving the spectrum of NO^{2+} _{2}.

Time‐resolved resonance Raman study of the spectroscopy and kinetics of the Cl^{−} _{2} radical anion in aqueous solution
View Description Hide DescriptionThe resonance Raman spectrum of the chlorine molecular anion Cl^{−} _{2} has been obtained using pulsed laserphotolysis as a radical production method. The fundamental and nine overtone transitions are observed. Analysis of the vibrational progression gives the vibrational constants ω_{ e } =277.0±0.3 cm^{−} ^{1} and ω_{ e }χ_{ e } =1.83±0.05 cm^{−} ^{1}; these constants give an upper limit dissociation energy of 1.3±0.1 eV (all errors are 1σ). Time‐resolved resonance Raman spectroscopy has been employed to study the kinetics of Cl^{−} _{2} disproportionation. A disproportionation rate coefficient of (11±7)×10^{9} M^{−} ^{1} s^{−} ^{1} is obtained where the reported error includes 1σ precision and estimates of systematic errors. A resonance Raman detection limit for Cl^{−} _{2} of 5×10^{−} ^{6} M has been obtained.

Vibrational energy relaxation of highly compressed N_{2}–D_{2} mixtures
View Description Hide DescriptionVibrational energy relaxation timeT _{1} of nitrogen in highly compressed mixtures of D_{2}–N_{2} has been measured at room temperature as a function of gas density and mixture ratio. The first vibrational state of N_{2} has been populated by a rapid V–V transfer from D_{2} which has been vibrationally excited by collision‐induced absorption. The decay of the N_{2} vibrational excited population has been time resolved by monitoring the infrared collision‐induced fluorescence. The data are compared with attractive hard‐sphere models developed for energy relaxation in dense fluids.

Photon angular momentum approximations for molecular collisions in an intense nonresonant laser field
View Description Hide DescriptionWe consider the problem of photonangular momentum approximation for molecular collisions in an intense nonresonant laser field. A couple of orientation averaging schemes are proposed for dealing with the absence of rotational invariance in the full (inclusive of photonangular momentum) dynamical equations. Application of the schemes is made for reactive laser switching between a pair of one‐dimensional potential curves which are free to rotate in space. The preferred scheme of the two depends upon there being only a single initial or final state of interest but both are very effective over the range of intensities examined. A third approximation scheme which essentially blots out photonangular momentum effects is less satisfactory.

Quantum interference among competing unimolecular decay channels: Asymmetric S _{0} D_{2}CO decay profiles
View Description Hide DescriptionThe dynamics of a state undergoing decay to a manifold of states coupled to a dissociative continuum is investigated in the weak coupling limit. The decay rate is found to be composed of terms symmetric and asymmetric in the relative energy difference between the initial state and the states in the manifold. The well‐known symmetric terms arise from resonantly enhanced decay of the initial state through each state in the manifold to the continuum. The newly identified asymmetric terms arise from interfering couplings of the initial state to the continuum through different states in the manifold, i.e., from off‐diagonal elements of the width matrix Γ_{ k k’} of the effective Hamiltonian describing dissociation of the states in the manifold. The general line shape derived here is used to fit asymmetric features in Stark level‐crossing spectra of D_{2}CO. Statistical properties of the width matrix are summarized, and the relation between diagonal and off‐diagonal matrix element magnitudes is derived.