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This paper confirms that the ultraviolet photolysis reactions of UF6 in the B band spectral region is simple bond cleavage to UF5 and F. The photolysis products may either recombine to UF6 or the UF5 may dimerize, and ultimately polymerize, to solid UF5 particles. We use four methods to set an upper limit for the rate constant for recombination of kr<2.0×10-12cm3 molecule-1 s-1. We measure the rate constant for UF5 dimerization to be kd=(1.0±0.2)×10-11 cm3 molecule-1 s-1. The principal method employed in these studies is the use of diode lasers to monitor, in real time, the changes in density of the species UF6 and UF5 after laser photolysis of the UF6 gas sample. [ABSTRACT FROM AUTHOR]

In a recent paper (Az I), well-structured T=300 K resonance Raman (RR) profiles for the 1400, 1260, 900, and 2×825 cm-1 lines of azulene in CS2 and for the 825 cm-1 line of azulene in methanol were reported. Previously developed transform techniques were used to (1) compute RR profile line shapes directly from measured optical absorption spectra, and (2) extract ratios of Stokes loss parameters from the line shape scale factors. The transform analysis indicated that (1) our model assumptions (adiabatic and Condon approximations, harmonic phonons, atomic equilibrium position shifts, and small vibrational frequency shifts upon excitation to a single electronic state) are basically correct allowing forminor modifications, and (2) any deviations from these assumptions are likely to be larger for the 900 cm-1 mode and smaller for the 1400 and 1260 cm-1 modes. In this paper (Az II), we report model calculations of the optical absorption spectra, RR profile line shapes, and relative RR intensities. In these calculations, we use a recently proposed nonzero temperature multimode time-correlator modeling procedure. Compared with the conventional sum-over-states method, our time-correlator modeling procedure is superior in that (1) our optical absorption spectra and RR profiles computed via fast Fourier transform techniques have a practically unlimited spectral range and (2) the computing times are short for nonzero temperature multimode calculations. In our basic model, we adopt the assumptions of Az I and use seven azulene modes to obtain simultaneous good fits of the well-structured RR profile line shapes and optical absorption spectra. However, we find that the basic model does not account for the intensity of the 900 cm-1 Raman line relative to that of the 1400 cm-1 line, even though the individual profile line shape fits for these modes are very good. The basic model is therefore modified to allow mixing of the normal coordinates of these two modes. By... [ABSTRACT FROM AUTHOR]

In a recent paper, Hardwick predicted that the optical spectrum of NO2 should display ‘‘quantum ergodicity’’, meaning that almost all eigenstates allowed by total symmetry should be seen in absorption. We have performed a test of this prediction by using the technique of microwave detected, microwave–optical double resonance. We have determined the spectrum over the range 16 810–17 100 cm-1 for the two ground vibrational states 91,9(F1) and 100,10(F1) of NO2. We observed 324 transitions from the 100,10 state and 364 transitions from the 91,9 state. The number of observed transitions is a factor of 8 greater than that expected if only the allowed rotational transitions to any B2 symmetry vibronic level were observed. The number of observed lines is a factor of 3 less than the number predicted if all selection rules were broken as predicted by Hardwick. Thus we see that Hardwick’s ‘‘ergodicity’’ conjecture is a useful starting point for understanding the spectrum of NO2, but the spectrum is intermediate between the regular and the ergodic limits. [ABSTRACT FROM AUTHOR]

The absorption and luminescence data reported in this paper definitely confirm that the precipitates formed during high-temperature aging of Pb-doped NaCl are PbCl2 microcrystals. A detailed comparison of the optical spectra of the precipitates and free PbCl2 is presented. The absorption and luminescence spectra are essentially similar in both cases, although the splitted structure of the prominent exciton band has now been resolved for the first time in the absorption spectrum of the precipitates. The splitting has been interpreted in terms of the noncubic crystal field effect on the 3P1 excited level of Pb+2 which is considered as a cationic exciton. The gap energy Eg and the binding energy εb of the exciton have also been determined (Eg=4.86 eV, εb=0.18 eV). The same UV and blue luminescence bands, as well as a thermally induced interconversion among them, have now been observed for the precipitates as well as for free PbCl2. This luminescence behavior, not previously understood, has been explained in terms of two coexisting AT and Ax minima in the adiabatic energy surface of the 3P1 level of a Pb+2 localized exciton (Fukuda’s model). [ABSTRACT FROM AUTHOR]

A theoretical classical stochastic trajectory study of collision induced desorption spectroscopy (CIDS) is presented in this paper. These theoretical results suggest that CIDS might provide useful experimental information on the binding energy and binding sites for adsorbates on surfaces. This information is obtained from analysis of the kinetic energy and angular distributions of both the collider atom and the desorbed adsorbate. Simulations of CIDS are reported for Xe colliding beams of 1 to 5 eV with monoatomic adsorbates on Si(111). [ABSTRACT FROM AUTHOR]

The near IR and visible vibrational absorption spectra of CHF3 were recorded up to wave numbers of 17 500 cm-1 providing complete frequency coverage, together with paper I, from the low frequency fundamentals to the N=6 CH stretching–bending overtone multiplet. All strong bands in the high overtone spectra could be predicted and assigned by means of the tridiagonal Fermi resonance Hamiltonian, including a few combinations with intense CF3 stretching vibrations already observed for the low overtones. Improved vibrational Fermi resonance constants are presented on the basis of a fit to 35 assigned bands. An analysis of the rotational fine structure of the 2ν4 (E) overtone component and several Fermi resonance component bands result in values for αb and αs, which allow us to determine Be. In the high overtone bands no rotational fine structure is observed. The bands can be understood by introducing additional homogeneous rovibrational structures of phenomenological widths Γ≊1 to 10 cm-1. The results are discussed in relation to the separation of time scales for mode selective vibrational redistribution and further evolution. The overtone band strengths are reported and analyzed approximately with the empirical local Mecke dipole function. [ABSTRACT FROM AUTHOR]

In this and the accompanying paper we present a theoretical treatment and experimental study, respectively, of the phenomenon termed purely rotational coherence. This phenomenon has been demonstrated to be useful as a time domain means by which to obtain high resolution spectroscopic information on excited state rotational levels of large molecules [Felker et al., J. Phys. Chem. 90, 724 (1986); Baskin et al., J. Chem. Phys. 84, 4708 (1986)]. Here, the manifestations in temporally resolved, polarization-analyzed fluorescence of coherently prepared rotational levels in samples of isolated symmetric and asymmetric top molecules are considered. These manifestations, for reasonably large molecules at rotational temperatures characteristic of jet-cooled samples, take the form of polarization-dependent transients and recurrences with temporal widths of the order of tens of picoseconds or less. The transients, which arise from the thermal averaging of many single molecule coherences, are examined with respect to their dependences on molecular parameters (rotational constants, transition dipole directions) and experimental parameters (polarization directions and temperature). A physical picture of rotational coherence as a reflection of the time-dependent orientation of molecules in the sample is developed. And, the influence of rotational coherence in experiments designed to probe intramolecular energy flow is discussed. In the accompanying paper, we present experimental results for jet-cooled t-stilbene and anthracene. For t-stilbene we determine rotational constants for vibrational levels in the S1 electronic state (from the recurrences) and we monitor the trends in rotational coherence vs vibrational coherence as the total energy in the molecule increases. [ABSTRACT FROM AUTHOR]

A model consisting in a quantum rigid rotor interacting with a quasiharmonic medium is presented to obtain the far-infrared spectra of diatomic polar molecules dissolved in rare-gas liquids. The absorption coefficient is calculated within the framework of two non-Markovian approaches called TTOC (total time ordered cumulants) and PTOC (partial time ordered cumulants). Both non-Markovian spectra contain the same correlation functions of the diatomic–medium interaction, which is expressed in terms of a Legendre polynomial series truncated up to second order. These correlation functions are calculated in paper II. The absorption cofficients appear as a sum of two spectral contributions. The first one, called secular contribution, is given as a juxtaposition of basic resonances, different in each of two non-Markovian approaches, associated to the absorption lines j→j+1. The second one, called interference contribution, takes into account the existence of a overlapping line effect. The contributions to the absorption coefficients from the P1 and P2 Legendre polynomials of the interaction are explicitly reported. [ABSTRACT FROM AUTHOR]

In paper I, the theoretical TTOC and PTOC far-infrared absorption coefficients of diatomic polar molecules in rare-gas liquid have been expressed in terms of correlation functions of the Hamiltonian which describes the diatomic–medium interaction. In this paper these correlation functions are derived using a harmonic model for the liquid structure which takes into account some peculiarities such as dynamical behavior and geometrical arrangement of atoms in the liquid. The correlation functions and the different spectral contributions which appear in the spectral theory of paper I are numerically analyzed for a HCl–Ar solution. Comparison between theoretical and experimental spectra shows that the best agreement is obtained with the PTOC approach. [ABSTRACT FROM AUTHOR]

Vibrational spectroscopy is increasingly used as a selective and sensitive probe of the interaction between a given molecule and its environment. For electrochemistry, in particular, vibrational spectroscopy has been useful in the process of sorting reactive intermediates and mechanisms out of the many possibilities which present themselves. In this paper we discuss a general approach to the analysis of environmental effects on molecular vibrations. The objective is to use vibrational spectroscopy to determine values of specific parameters in a set of intra- and intermolecular pair potentials. In the body of this paper, we discuss in detail how one can manipulate a variety of model potential energy functions in order to account for environmental effects. We then apply the method to the adsorption of tetrahedral molecules on a metal surface. We report infrared spectra for sulfate ion adsorbed on a platinum electrode, and we make use of our analysis of the spectra to suggest that the sulfate anion associates with the metal through only one oxygen atom. [ABSTRACT FROM AUTHOR]

This paper, the first in a series of three papers, gives a detailed account of our studies on picosecond photofragment spectroscopy. The unimolecular reaction NCNO→CN+NO is examined in detail here. Microcanonical state-to-state rates are measured in molecular beams at different energies in the reagent NCNO using pump–probe techniques: one picosecond pulse initiates the reaction from an initial (v,J) state and a second pulse, delayed in time, monitors the CN radical product in a specific rovibrational state, or the reagent NCNO (transient absorption). The threshold energy for reaction is determined to be 17 083 cm-1 (bond energy=48.8 kcal/mol). Measured rates are found to be sharply dependent on the total energy of the reagent, but independent of the rotational quantum state of product CN. Results of transient absorption measurements are used to argue that the ground state potential energy surface dominates the reaction in the range of excess energies studied. The energy dependence of the rates, kMC(E), is compared with that predicted by statistical theories. Both standard RRKM (tight transition state) and phase space theory (loose transition state) fail to reproduce the data over the full range of energies studied, even though nascent product state distributions are known to be in accord with PST at these energies. Furthermore, kMC(E) is not a strictly monotonically increasing function of energy but exhibits some structure which cannot be explained by simple statistical theories. We advance some explanations for this structure and deviations from statistical theories. [ABSTRACT FROM AUTHOR]

This paper, second in the series, reports on the picosecond time-resolved photofragmentation of the overtone (vOH=5) initiated reaction: HOOH+hν→2OH. The hydrogen peroxide is initially excited by way of a picosecond laser pulse to the fourth overtone level of the OH-stretch local mode. The subsequent unimolecular reaction behavior is obtained by monitoring the laser-induced fluorescence, caused by the picosecond probe–pulse electronic excitation of the OH radical photoproduct (in a given rotational state). The two pulses are scanned relative to one another in time thereby mapping out the product yield for the given delay-time interval. The resultant product formation behavior is found to be nonexponential, and may be modeled as a biexponential rise. Furthermore, the quasibiexponential behavior is sensitive to the exact excitation wavelength—slight variations of which result in large changes in the two time constants and the relative amplitudes of the fast and slow components. These experiments give direct evidence for the inhomogeneous nature of the overtone transition on the picosecond time scale, and provide the dissociation rate contribution to the homogeneous width (0.05–0.15 cm-1).The apparent width for the main band feature is about 200 cm-1. The rate of product formation (magnitude and form) is interpreted in terms of statistical and nonstatistical theories. The limitations of the applicability of each model is discussed. The fluctuations of the fitting parameters as a function of excitation wavelength may be simulated by a statistical model which considers all possible discrete optical transitions within the simulated laser bandwidth and the details of product formation from each state. For a nonstatistical interpretation, the biexponential form reflects a division of the vibrational phase space, and this is discussed in the spirit of a kinetic model. Finally, experimental results are reported for direct UV initiated... [ABSTRACT FROM AUTHOR]

This paper, last in this series, reports on the picosecond dynamics of vibrational predissociation in beam-cooled van der Waals’ clusters. Reaction rates have been measured for clusters (1:1) of phenol and cresol (p-methylphenol) with benzene by the picosecond pump–probe photoionization mass-spectrometry technique. Dissociation to form phenol (cresol) and benzene takes place from vibrational levels of the S1 state of phenol (cresol) prepared by the pump laser. The predissociation rates were measured for a number of different excess energies upto ∼2500 cm-1, and the reaction threshold was found to be 1400 cm-1 above the S1 origin for phenol–benzene and ∼1795 cm-1 for cresol–benzene, respectively. For phenol–benzene, the predissociation rates, following excitation of ring-type modes, vs excess energy vary more or less smoothly. Cresol–benzene exhibits biexponential decay, with the fast component becoming more dominant at higher energies. A non-RRKM model involving division of the vibrational phase space is discussed to explain this observation. [ABSTRACT FROM AUTHOR]

The application of nonlinear multiple irradiation electron spin resonance techniques to the case of inhomogeneously broadened spectra is studied. A detailed theoretical analysis within a unified method shows that the longitudinally detected electron spin resonance (LODESR) and the double modulation electron spin resonance (DOMESR) techniques represent two different aspects of the same physical effect and, under the same conditions, both give information on the longitudinal relaxation time T1 of the single spin packet. Previous papers, giving different interpretation for the double modulation spectrum, are critically reviewed. The usefulness of the two techniques in the case of inhomogeneously broadened lines is put into evidence by experiments with dextrose chars pyrolyzed at different temperatures. The results are in excellent agreement with theoretical results. The optimum application ranges of these nonlinear techniques are discussed and compared. [ABSTRACT FROM AUTHOR]

This is the second of three papers discussing our studies of overlayers of Ar, Kr, and Xe physisorbed on Ag(111). All of these rare gases form ordered structures, which are azimuthally aligned but translationally incommensurate with the Ag substrate. In this paper, we discuss experiments in which we utilized the angle resolved time of flight of inelastically scattered 18 MeV He atoms to examine the surface dynamics of multilayer rare gas films along the Γ–M direction. This was done on a layer-by-layer basis for 2, 3, and ≥20 layers for each of the rare gases. Unlike the monolayers, the vibrational modes observed for the multilayers show dispersion across the surface Brillouin zone, the amount of dispersion increasing with the number of adsorbed layers. These results reveal in detail how the surface dynamical properties of a thin film evolve towards those of a thick crystal as a function of increasing dimension. Lattice dynamics calculations, which utilize realistic gas phase pair potentials, reproduce the experimentally observed phonon dispersion relations quite well. We also examine the inelastic scattering probabilities and linewidths of the transitions. One of the more notable results is that the inelastic scattering probabilities vary by at most a factor of 2–3 across the entire surface Brillouin zone. Isothermal desorption measurements for the Xe overlayers are also discussed. Like the monolayer, the bilayer and trilayer exhibit nearly zeroth order desorption until ∼90% of the top layer has desorbed, where the desorption kinetics become first order. [ABSTRACT FROM AUTHOR]

Complicated spectral features appear in the CO photoionization spectra between 17 and 17.5 eV photon energy. In order to interpret these processes, we refine in the present paper our previous model [B. Leyh and G. Raşeev, Phys. Rev. A 34, 2920 (1986)] by introducing all the vibrational levels of the first three electronic states of CO+. This more accurate treatment leads to two major results. The resonances at 17.1 and 17.3 eV are complex resonances resulting from the interaction between the Rydberg series converging to the A 2Π and B 2Σ+ ionic states: they can no more be assigned simply to Rydberg states converging to CO+B 2Σ+. The intermediate experimental feature at 17.2 eV, missing in our previous calculation, is now present in the theoretical spectrum. Mainly, this feature, which we have called composite resonance, is a broadened (by the experimental resolution) superposition of Rydberg states converging to the A 2Π ν+=4 state. In this paper, we present and analyze the vibrationally resolved cross sections and asymmetry parameter for the X 2Σ+ν+=0–3 and A 2Π ν+=0–4 states. These theoretical results are compared to recent experimental data by Leyh et al. [Chem. Phys. 115, 243 (1987)] and by Hardis and co-workers (the preceeding paper). [ABSTRACT FROM AUTHOR]

This project was inspired by the widespread use of the properties of coated spheres in modeling the dielectric and spectroscopic behavior, including surface enhanced Raman spectroscopy, of a variety of dispersed systems. In this paper the self-polarizing contributions to the absorption and elastic scattering properties of a coated sphere are obtained in the large wavelength limit, or dipole approximation, in which the coating is treated as a polarizable, uniaxially anisotropic, shell with the local optic axis directed radially. Numerical calculations for CO on Ag are provided to illustrate some of the phenomena predicted by the equations. Of particular interest are the dependence of the absorption band for CO stretch upon particle size, surface coverage, and molecular orientation, and the dependence of the surface plasmon resonance absorption on particle size and coverage. Some limitations of the model are examined. [ABSTRACT FROM AUTHOR]

Studies the v[sub 2] vibrational spectrum of deuterated methyl radical CD[sub 3]. Vibrational potential function for the species; Positions of unobserved vibrational bands for all symmetric isotopic modifications; Population distribution of the v[sub 2] levels.

Studies the optical-optical double resonance spectroscopy of rotational autoionization of nitric oxide. Convergence of the rotational autoionization of Rydberg series to higher thresholds; Competing decay process between predissociation and vibrational autoionization; Electric field dependence and pressure dependence of the ionization.

This paper describes the experimental implementation and theory of a new technique for the time resolved study of intramolecular vibrational redistribution: time resolved fluorescence depletion (TRFD). Picosecond time resolution is obtained by using two pulses of identical wavelength and duration to create and then stimulate emission from a vibrational state in S1. Spontaneous emission intensity is measured as a function of delay between pulses. By monitoring the efficiency with which the second pulse stimulates emission, the evolution of the initial optically prepared state can be followed. A description of the apparatus built for these experiments is also presented. [ABSTRACT FROM AUTHOR]

We present in this paper an application of the Lanczos algorithm to the fitting of a potential energy surface from the experimental spectrum. This method presents two definite improvements on the conventional approach: (i) the Lanczos algorithm allows to treat ultra-large basis sets (120 000 states in this calculation) and (ii) it is possible to tune selectively the calculation to specific components of the spectrum (e.g., a given combination band nνi+mνj) thus reducing considerably the complexity of the fit. This method has been applied to the CD3H molecule, considering all the vibrational degrees of freedom. Converged line positions have been obtained for high overtones of the C–H stretching mode (up to 16 000 cm-1). The accuracy of the fitted surface is demonstrated by direct comparison of experimental and calculated spectroscopical parameters. [ABSTRACT FROM AUTHOR]

There has been a wealth of recent infrared experimental data on van der Waals and hydrogen bonded complexes obtained under cooled, supersonic jet conditions where only a small fraction of the total bound quantum states can be elucidated. This partial set of data can often be well fit to a traditional Watson Hamiltonian derived from a rigid rotor perspective with low order centrifugal distortion effects included. In this paper we show that even in extremely floppy molecular systems with wide amplitude vibrational motion, the quantum term values are very well fit by a rigid or semirigid rotor Hamiltonian over the limited range of energy states accessible in a cooled beam. We provide explicit examples of this behavior by full quantum solutions in two extremes of floppy motion: (1) a symmetric triatomic with a square well bending potential (‘‘hinge’’) and (2) a nearly free internal rotor (‘‘pinwheel’’). These results show that potentials with fundamentally different topologies can be consistent with same data, and indicate that even the limits of nearly rigid and floppy internal motion may be difficult to distinguish from a limited set of rovibrational eigenvalues. [ABSTRACT FROM AUTHOR]

The first paper in this series presented a detailed analysis of the diffraction efficiency of a single holographic grating obtained by spectral hole burning. This paper covers the influence of phase changes which occur during the burning of a single hologram, and the interaction of spectrally adjacent two laser induced gratings. A theoretical description of the holographic efficiencies which takes into account the influence of the relative phases between the two gratings. Experimental results are presented and discussed in terms of the theory. [ABSTRACT FROM AUTHOR]

We report here the first experimental observation of bound–bound transitions between the ground X 2Σ+g and excited 1 2Πu states of Na+2. The basis of our experiment is to study doubly excited Rydberg states of Na2 by preparing a well-defined nd 1Λg singly excited Rydberg state of Na2 and then, by exciting the Na+2 core with a tunable laser. In this paper, we show that the ionic transitions may be directly deduced from the doubly excited Rydberg states spectra [C. Bordas, J. L. Vialle, and M. Broyer (submitted)]. We demonstrate that this technique is one of the most powerful to study the excited states of diatomic ions which are not predissociated. A detailed spectroscopic analysis of the 1 2Πu state has been performed and the results are compared with the more recent ab initio and pseudo- (or model-) potential calculations. [ABSTRACT FROM AUTHOR]

Recently we proposed a new approach to deal jointly with the presence of conformational disorder and the requirements imposed by the electronic structure of conjugated polymers in solution. The description of the system is based on the one-dimensional tight binding Hamiltonian modified in such a way to account for dependence of the electronic hopping constants on the conformational degrees of freedom. We are able to treat numerically the full statistical problem resulting from such a microscopic description. Effects arising from steric repulsion and other possible sidegroup interactions can be dealt with, using an appropriate effective potential. In this paper we present a number of new results, that we have obtained using this approach. In particular, we analyze in detail the dependence of both conjugation and persistence length on temperature and on the strength of the steric potential. These and the corresponding results for the structure of the energy spectrum suggest the existence of joint electronic and conformational changes in qualitative agreement with the thermochromic effects experimentally observed. [ABSTRACT FROM AUTHOR]

In this paper, three distinct phases of biphenyl on the Pd (110) surface are characterized and studied by angle-resolved ultraviolet photoemission (ARUPS); a disordered condensed multilayer which desorbs at 218 K, a bilayer that desorbs at 240 K, and a strongly bound monolayer which breaks up at the surface for temperatures greater than 400 K. The multilayer ultraviolet photoemission (UP) spectrum is almost identical to that of the gas phase spectrum and it is inferred that the torsional angle between the phenyl rings in the condensed phase is unaltered from that in the gas phase. Changes observed in the π orbital emissions on the formation of the bilayer indicate a significant reduction in the torsional angle. Application of the selection rules for ARUPS to the strong angular effects observed in the emissions from the highest-lying π orbitals suggests that the molecules in this second layer have their molecular axes parallel to and molecular planes perpendicular to the surface, that is, edge-on to the molecules in the monolayer which are shown to be flat lying to the metal surface and bonded via the π orbitals. [ABSTRACT FROM AUTHOR]

Comments on a paper on spectrum attributed to bound-free excimer emission of the van der Waals complex. Potential energy curves for the ground and excited states resulting from an analysis of the spectra; Importance of argon metastables in the fluorescence excitation in explaining the absence of excimer bands in the experiment; Time behavior of the nitrogen emission.

This paper presents a method to analyze thermal desorption spectra based on area integration of the desorption peaks. An expression is derived for the rate of desorption in a thermal-activated process which permits evaluation of the rate from the intensity and area values for each point of the curve. Arrhenius plots of the rate data yield the activation energy and attempt frequency for desorption. The order of the desorption process is determined from the best linear fit in the Arrhenius plots. Included are applications for analyzing thermal desorption spectra obtained with a thermal conductivity detector or a mass spectrometer. Analyses of NO desorption from KNbO3 and H2 desorption from Ni surfaces illustrate the method. [ABSTRACT FROM AUTHOR]

The formulation developed in the preceding paper [H. J. Kim and J. T. Hynes, J. Chem. Phys. 93, 5194 (1990)] is applied to describe the electronic structure and spectroscopic features of a model symmetric electron–donor–acceptor solute system D-A⇋DA- in solution in the strong coupling limit. In this limit, the electronic coupling is sufficiently strong to overcome the localizing influence of the solvent polarization, and two stable delocalized solute electronic states are found in the presence of either nonequilibrium or equilibrium solvation. The nonlinear influence of the equilibrated solvent electronic polarization and of exchange contributions to the solute electronic distribution incorporated in the theory lead to several consequences absent in standard descriptions. Among these are the necessity of two solvent coordinates to describe the system, and the prediction of solvent-dependent spectral shifts and the appearance of solvent relaxation dynamics after a Franck–Condon transition between the delocalized electronic states. Estimates of the magnitude of these new effects are provided, and the possibility for their experimental observation is briefly discussed. [ABSTRACT FROM AUTHOR]

A general method is developed for determination of cylindrically symmetric velocity distributions from Doppler profile measurements. This method applies Kinsey’s Fourier transform Doppler spectroscopy [J. L. Kinsey, J. Chem. Phys. 66, 2560 (1977)] to distributions arising from photodissociation and uses an orthogonal polynomial expansion to perform the integral transforms analytically. This method is shown to offer an improvement in stability over direct numerical solution of the integral equation and to have applicability to distributions which are not ‘‘separable,’’ that is, which cannot be separated into a product of speed- and angle-dependent factors. The method is applied to experimental measurements of the collisional relaxation of a fast anisotropic distribution of I[2P1/2] atoms in a thermal bath (preceding paper). It is shown that the nascent distribution is separable, but the distribution does not remain separable throughout the relaxation process. [ABSTRACT FROM AUTHOR]

The rotationally resolved, gas phase vibronic spectrum of dichlorocarbene has been recorded using the technique of pyrolysis jet spectroscopy. The rovibronic analysis has established the band origin of the A 1B1–X 1A1 electronic transition to be at 17 255.67 cm-1. The observed vibrational frequencies in the ground and excited states are very close to published values from matrix isolation data. The excited state structure has a 131.4° ClCCl angle and a 1.652 Å CCl bond length. [ABSTRACT FROM AUTHOR]

A theory of optical echo spectroscopies of large polyatomic molecules in condensed phases is developed. Using phase space correlation functions, we examine the interrelationships among the following optical measurements: ordinary photon echo, stimulated photon echo, accumulated photon echo, incoherent accumulated photon echo, and pump–probe absorption. Conditions for the elimination of inhomogeneous broadening in these experiments are specified. A multimode Brownian oscillator model is used to account for high frequency molecular vibrations, as well as solvent modes, and electronic dephasing processes. The effects of quantum beats, spectral diffusion, and homogeneous dephasing on the echo signals are studied and compared in detail with pump–probe and hole burning spectroscopy. [ABSTRACT FROM AUTHOR]

Three-dimensional potential energy and dipole moment surfaces have been calculated for the 24 electron triatomics O3, CF2, NO-2, and NF+2 using complete active space self-consistent field wave functions (CASSCF) and a basis set of 87 (99 for NO-2) contracted Gaussian-type orbitals (cGTOs). The analytical potential energy functions (PEFs) have been used in perturbation and variational calculations of anharmonic spectroscopic constants and vibrational energy levels. The results for O3 and CF2 are compared to the available experimental data, and predictions of the rotational and vibrational spectra of NO-2 and NF+2 have been made by comparison to these species. The equilibrium geometries of NO-2 and NF+2 are predicted to be re=1.260 Å, θe=116.5° and re=1.242 Å, θe=108.0°, respectively. The fundamental vibrational frequencies (ν1,ν2,ν3) and absolute band intensities (at 300 K) of these two ions are predicted to be 1286 cm-1/31 cm-2 atm-1, 782 cm-1/13 cm-2 atm-1, 1232 cm-1/2900 cm-2 atm-1 (NO-2) and 1272 cm-1/181 cm-2 atm-1, 711 cm-1/25 cm-2 atm-1, 1210 cm-1/1230 cm-2 atm-1 (NF+2). Band intensities for several low-lying vibrational transitions have been calculated for all four species from the CASSCF dipole moment functions, and comparisons to experiment have been made for O3 and CF2. [ABSTRACT FROM AUTHOR]

The virial theorem has been used to derive sum rules for dipole- and mixed-dipole–quadrupole nuclear electric shieldings and corresponding geometrical derivatives of dipole and quadrupole moments in a molecule. Test calculations have been carried out on a series of first- and second-row hydrides. The virial sum rules can be effective tools to prove the accuracy of theoretical nuclear shieldings and analytic geometrical derivatives. As the latter are related to ir intensities, the virial sum rules can give important indications on the reliability of theoretical predictions for this spectroscopical parameter. [ABSTRACT FROM AUTHOR]

Starting from an ab initio anisotropic potential surface, we have calculated the bound rovibrational states of the Ar–NH3 dimer for J=0, 1, 2, and 3. There is good agreement with data available from molecular beam spectroscopy in the microwave and far-infrared regions. The nature of the states is discussed and they are correlated with free internal rotor states. In spite of the substantial barriers to internal rotation, Ar–NH3 resembles much more a free internal rotor complex than a nearly rigid dimer. Still, the van der Waals vibrations show characteristic differences from free internal rotations. In particular, there is strong mixing between the fundamental stretch vibration of the dimer and the lowest bending mode. We assign the band observed in the far-infrared region to the lowest, primarily bending, of these modes. Further transitions in the yet unexplored far-infrared regions are predicted. [ABSTRACT FROM AUTHOR]

Coadsorption of H2O and Cl was studied on a Ag(110) surface under conditions of ultrahigh vacuum with thermal desorption spectroscopy, low energy electron diffraction, and electron stimulated desorption ion angular distribution. The experiments were conducted over the temperature range of 100–650 K for water coverages ranging from zero to several multilayers and chlorine coverages θCl of 0–0.75 monolayers (ML). Water adsorption is stabilized by chlorine; the thermal desorption peak for water interacting with chlorine, called the α2 state, shifts to higher temperature by 25–40 K from the α1 state for desorption from the clean surface. A c(2×2) bilayer for H2O forms for coadsorption with less than 0.25 ML of Cl. The surface solvation number (SSN), defined as the number of stabilized water molecules per chlorine atom, varies from 13 to about 4 as θCl increases from 0 to 0.25. The unusually large SSN and the c(2×2) structure is evidence that Cl(a) promotes water adsorption to the metal surface itself in an effect called adsorbate-induced hydrophilicity. Coadsorption with higher chlorine coverages produces a p(4×3) structure for 0.25<θCl<0.4 and a c(4×4) structure for 0.4<θCl<0.5. Chlorine interacts directly with water in these structures in the form of surface solvation seen in previous studies of water coadsorption. The p(4×3) and c(4×4) patterns are evidence that coadsorbed water alters the distribution of chlorine on the surface. These results are interpreted in terms of the balance of forces among the two adsorbed species and the surface. [ABSTRACT FROM AUTHOR]

The intensity of infrared absorption bands, observed for large gas-phase ice I clusters and attributed to OH (or OD) groups dangling at the cluster surfaces, has been used to estimate the average size of ice clusters formed under fairly typical static cell conditions. The assignment of bands at 3692 cm-1 (2425 cm-1) to dangling surface groups has been established as credible by comparison with absorption bands presumably produced by dangling groups at the surfaces of pores in microporous amorphous ice. Using fairly standard assumptions about absorptivities and molecular sizes, an estimated cluster size of 25 nm has been obtained for clusters formed at 280 Torr and 80 K in a gaseous mixture containing a He to H2O ratio of 200. [ABSTRACT FROM AUTHOR]

A structureless band observed at 180 nm in the absorption and excitation spectra of Cl2/Ar matrices is tentatively assigned as the spin forbidden 3Σ+u ← X 1Σ+g transition of molecular chlorine. Having an absorption cross section of approximately 10-18 cm2 in the solid, a twofold order of magnitude increase in the transition probability is observed relative to the gas phase. Wavelength specific measurements of the photodissociation of molecular chlorine in crystalline argon samples showed that a dominant threshold exists in the 130 nm band at 9.2 eV corresponding to absorption into the bound 1 1Σ+u state. The maximum quantum yield for permanent dissociation in the 130 nm band was found to be 0.3. Luminescence evidence indicates that this dissociation does not involve a charge-transfer mechanism but a crossing from the bound 1 1Σ+u state to a repulsive potential on which an impulsive cage escape occurs. Photoexcitation in the 180 nm band also results in the permanent dissociation of chlorine as well as the molecular A→X emission. The dissociation efficiency of this band was found however to be significantly less than in the 130 nm band and very sample preparation dependent, viz., sample crystallinity. In crystalline samples dissociation efficiencies were typically two orders of magnitude less than in noncrystalline samples. No dissociation occurs following excitation into the lower energy 1Π(1u) band at 300 nm irrespective of sample preparation conditions. [ABSTRACT FROM AUTHOR]

The spectroscopic spin-boson model [J. Chem. Phys. 86, 995 (1987)] for quantum dynamics of nuclei on potential surfaces coupled by nonradiative effects is studied via path integral techniques. In particular, long time recurrence dynamics of the electronic absorption correlation function of a multimode system characterized by strong nonradiative coupling is computed by both direct enumeration of spin configurations and by a novel Monte Carlo method based on the extended adiabatic formulation of the spin-boson problem [J. Chem. Phys. 92, 4993 (1990)]. Besides providing a demonstration of the utility of path integration for studying detailed spectroscopic signatures of nuclear motion on excited electronic potential surfaces, it is argued that the basic technical procedures can be applied to a number of problems in which a quantum system is coupled to an environmental oscillator bath. [ABSTRACT FROM AUTHOR]

An investigation concerning nine electronically low-lying doublet states of the SNS molecule appeared recently. The present paper represents the conclusion of studies on the NS2 molecule, focusing on the low-lying SNS quartet states. Nine quartet states, including six bent structures (4A2, 4B2, two 4A1, and two 4B1 ) and three linear structures (4Πg and two 4Πu ), have been investigated at the self-consistent field (SCF), single and double excitation configuration interaction (CISD), and complete active space (CAS) SCF levels of theory with five basis sets, double zeta (DZ) through triple zeta plus double polarization (TZ+2P). Four of these quartet states lie within 1.8 eV (42.0 kcal mol-1 ) of the 2A1 ground state of NS2. Thus one or more of these quartet states (which lie much lower than for the valence isoelectronic NO2 molecule) should be observable. At the SCF level of theory the stability of the wave functions and the relative energies of the nine quartet states are discussed in conjunction with the molecular orbital (MO) Hessian. [ABSTRACT FROM AUTHOR]

Metal-alkyl molecules adsorbed on chemically prepared silicon surfaces have been studied using infrared total-internal reflection and ultraviolet transmission spectroscopies, temperature-programmed desorption spectroscopy (TPD), and mass spectroscopy of laser induced desorption (LID) products. For chemisorbed species, the surface hydroxyl groups have been shown to be the prime adsorption sites. In addition, we determined the identity of the chemisorbed species and their fragments under ultraviolet photodissociation. Surface photodissociation was observed to be strongly wavelength dependent at 193 and 248 nm. [ABSTRACT FROM AUTHOR]

The structure and spectroscopy of van der Waals clusters between argon and 3-amino-s-tetrazine (AT) has been investigated in a supersonic jet. Low resolution vibrationally resolved fluorescence excitation spectra of AT at a variety of argon gas concentrations enable us to assign the origin transitions for the clusters AT–Arn, where n=1 through 6. The frequencies of the van der Waals vibrational modes of these clusters are determined, both in the excited states from the excitation spectra, and in the ground state from single vibronic level emission spectra. High resolution spectroscopy is used to resolve rotational structure in the origin transitions of some of the clusters, and this provides structural information about the clusters. The argon atom in AT-Ar is found to be above the tetrazine ring, and the argon atoms in AT–Ar2 are found to be above and below the tetrazine ring, in all cases having a van der Waals bond length of about 3.3 Å. This work is useful in interpreting the results of our previous study on the photodissociation dynamics of van der Waals clusters between argon and AT. [ABSTRACT FROM AUTHOR]

A laser vaporization/photolysis technique for obtaining spectra of jet-cooled organometallic molecules is reported. It makes use of an excimer laser which vaporizes a metal sample and simultaneously photolyzes an organic precursor passing over the surface of the metal. The metal atoms and organic fragments then react to yield the organometallic species. In principle, this approach could be used to prepare a wide variety of gas-phase compounds of any metal for spectroscopic study. To illustrate its capabilities, laser excitation spectra of the monocyclopentadienide and monomethyl derivatives of Ca and Cd are presented. In the case of cadmium cyclopentadienide, this report constitutes the first spectroscopic observation of this molecule. [ABSTRACT FROM AUTHOR]

Rotational tunneling of methyl groups in partially deuterated tetramethyltin compounds, (CH3)xSn(CD3)4-x with x=1 and 3, and in various isotopic mixtures, [(CH3)4Sn]x[Sn(CD3)4]1-x, x=0.027, 0.20, and 0.50, and [CH3Sn(CD3)3]x[Sn(CD3)4]1-x, x=0.10, respectively, has been studied using the inelastic neutron-scattering (INS) technique. Compared with the INS spectrum of fully protonated tetramethyltin, (CH3)4Sn, the spectra obtained in the present study show remarkable shifts and broadenings or splittings of tunnel lines. The interpretation of the INS results is based on the assumption that all partially deuterated compounds and isotopically mixed systems have the same crystal structure as pure (CH3)4Sn. Combining the INS results with a detailed discussion of the crystal structure, all features can be explained in terms of both the intra- and intermolecular interactions of methyl groups. The overall decrease of tunnel splittings with deuteration is explained by the reduction of the lattice parameter and the increased octopole moment of the CD3 groups. The interaction between different molecules is mediated by a subset of methyl group pairs only. [ABSTRACT FROM AUTHOR]

Photofragment excitation (PHOFEX) spectra of the jet-cooled formyl (HCO and DCO) radical have been measured by monitoring laser-induced fluorescence of the CO fragment. The following A(A‘)←X(A’) vibronic transitions were measured: (v1,v2,v3)←(0,0,0) where for HCO v2=6–16 with v1,v3=0; v2=7–12 with v1=1, v3=0; v2=9–12 with v1=0, v3=1 and for DCO v2=14–18 with v1,v3=0. The PHOFEX technique provides a powerful method for discriminating against strong transitions so as to allow assignment and measurement of many weaker Franck–Condon bands. The linewidths of the diffuse transitions lie in the 16–150 cm-1 (FWHM) range; they increase strongly with K’ (the projection of the total angular momentum onto the a axis) and decrease slightly with v’2 (the number of HCO/DCO bending quanta). The linewidths are interpreted as radiationless transition rates and yield upper A-state lifetimes ranging from 70 to 700 fs. These data are in excellent agreement with a dynamical model of HCO/DCO curve crossing developed by R. N. Dixon [Mol. Phys. 54, 333 (1985)]. [ABSTRACT FROM AUTHOR]

A tunable far-infrared difference frequency spectrometer has been used to examine the fully protonated form of malonaldehyde in the region near the ground-state tunneling frequency (21 cm-1). An extremely dense and complex spectrum is observed in which the strongest features have been assigned as pure rotational lines involving high values of J and K-1. These transitions, which occur within the individual rotational manifolds of the two halves of the ground-state tunneling doublet, have been analyzed simultaneously with existing microwave data for this species. The value of the ground-state tunneling splitting, determinable indirectly from analysis of vibration–rotation interactions, is 21.584 76(17) cm-1, and is in close agreement with that similarly obtained in previous microwave work. A thorough treatment of the centrifugal distortion in this system significantly extends the range of rotational states whose energies may be reliably calculated, and should therefore be valuable in the future direct measurement of the tunneling frequency. Aspects of the far-infrared spectrum of this species, and of the indirect method of determining the tunneling splitting, are discussed. [ABSTRACT FROM AUTHOR]

A quantitative analysis of the bending mode of H2O+ is made using the Coulomb Explosion technique. The full extent of the angular distribution is measured and is in good agreement with the previously measured bending frequency. The equilibrium value for the angle is extracted from the data with high precision and is in excellent agreement with theoretical predictions as well as spectroscopic measurements. [ABSTRACT FROM AUTHOR]

The vibration–rotation spectrum of the GeH radical for the fundamental band in the 2Π1/2 spin state was observed using an infrared diode laser spectrometer. The GeH radical was produced by ac glow discharge in the mixture of GeH4, He, and H2. The signals were detected by the discharge modulation method. In total 94 GeH lines were identified for five isotopic species, and were simultaneously analyzed to determine effective molecular constants in the 2Π1/2 spin state. [ABSTRACT FROM AUTHOR]

The photoionization efficiency spectra of nickel clusters containing adsorbed ammonia have been recorded near threshold. Ionization potentials (IPs) have been assigned for 97 selected Nin(NH3)m species, with n up to 56 atoms. We observe that nickel cluster IPs decrease upon ammonia adsorption, with the magnitude of the IP shifts varying linearly with the number of adsorbed molecules, in good agreement with a simple electrostatic model. Deviations from this behavior are observed for Ni5(NH3)m and Ni6(NH3)m clusters, in which adsorption of the first ammonia molecule produces a significantly larger IP drop than do subsequently adsorbed molecules. Such behavior may indicate the presence of unique adsorption sites on Ni5 and Ni6 preferentially occupied by the first ammonia molecule adsorbed onto these clusters, or alternatively, may indicate cluster rearrangement occurring as the result of chemisorption. Our results are discussed in light of studies of ammonia adsorption onto single crystal nickel surfaces. [ABSTRACT FROM AUTHOR]

Presents a study which examined rotational coherence spectroscopy of 1-naphthol-(water)[sub2] clusters as a structural evidence for a cyclic hydrogen-bonded trimer. Method of the study; Results and discussion; Conclusion.