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Volume 104, Issue 10, 08 March 1996

Photoelectron spectroscopy of iron–sulfur cluster anions
View Description Hide DescriptionIron–sulfur cluster anions (Fe_{ n }S^{−} _{ m }, n=1–6, m=1–6) have been studied using photoelectron spectroscopy (PES) with a magnetic‐bottle‐type time‐of‐flight electron spectrometer. The Fe_{ n }S^{−} _{ m } cluster anions were formed in a laser vaporization cluster source. It was found that the stable cluster ions are the ones with compositions of n=m and n=m±1. The electron affinities were measured from the onsets of the PES spectra. Three low‐lying electronic states were observed for FeS. The PES spectra of Fe_{ n }S^{−} (n=1–4) series show a unique similarity, indicating that the Fe atom addition to FeS^{−} has little effect on the electronic property of FeS. The PES spectra of Fe_{ n }S^{−} _{2} series show a similarity among the cluster anions with n=2–4, showing that Fe_{2}S^{−} _{2} is the structural framework of these clusters. For Fe_{ n }S^{−} _{3} series, Fe_{3}S^{−} _{3} is proposed to be the structural framework. The electronic properties and geometricalstructures of the clusters are discussed.

Raman spectra of mass‐selected nickel dimers in argon matrices
View Description Hide DescriptionThe absorption and Raman spectra for nickel dimers in an argon matrix prepared by the mass‐selected ion deposition technique have been measured. A weak dimer absorption band is centered around 480 nm. ResonanceRaman spectra obtained from exciting into the dimer 480 nm band show a vibrational progression for which ω_{ e }=259.2±3.0 cm^{−1} with ω_{ ex } _{ e }=1.9±0.7 cm^{−1}. The dinickel Raman spectra are discussed in terms of isotopic shifts, as well as the changeover from d–d bonding to s–s bonding along the series Fe_{2}→Cu_{2}.

The delayed ionization and atomic ion emission of binary metal metallocarbohedrenes Ti_{ x }M_{ y }C_{12} (M=Zr, Nb; 0⩽y⩽4; x+y=8)
View Description Hide DescriptionInvestigations of the photophysical character of binary metal containing metallocarbohedrene clusters (Met‐Cars), reveal the phenomena of delayed ionization. Additionally, the onset of a channel corresponding to delayed atomic ion emission is observed at high laser fluence, becoming progressively more significant as the fluence is raised. The fluence dependence of both the delayed electron and delayed atomic ion emission channels at 355 and 532 nm are reported, and the Met‐Car delayed ion yield estimated. At moderate fluences, the yield of the delayed electron emission channel leading to the Met‐Car ion is found to be on the order of 70% of the total Met‐Car signal. All of the experimental findings point to thermionic emission being the operative mechanism for the delayed ionization, while the delayed atomic ion emission is evidently a new mechanism [B. D. May, S. F. Cartier, and A. W. Castleman, Jr., Chem. Phys. Lett. 242, 265 (1995)] which may be due to the collective electronic excitation of these cluster materials.

Threshold zero‐kinetic‐energy photoelectron spectroscopy of the a ^{3}Σ^{+} state of NO^{+}
View Description Hide DescriptionResults of calculations of the photoelectron spectra for single‐photon pulsed‐field ionization of the 1π orbital of the X ^{2}Π (v″=0) ground state NO leading to the a ^{3}Σ^{+} (v ^{+}=0–2) excited ionic state are reported. Agreement between these calculated and recently measured spectra is very encouraging. Comparison of these spectra for ionization of the 1π level of NO (X ^{2}Π) with those for the 2π orbital reveal significantly different spectral profiles and underlying dynamics for these two cases.

Up–down–up–... multiphoton excitation of two‐level systems
View Description Hide DescriptionMultiphoton up–down–up–...(UDU) excitation in a two‐level system was realized under adiabatic conditions. Spectra of C_{2}H_{4} excitation were investigated and UDU peaks were observed. The dependence of peak amplitudes on the powers and the frequencies of exciting fields were investigated experimentally as well as by computer simulation based on the rotating wave approximation (RWA). Agreement between experimental and theoretical results was observed.

Theory of polarization measurements of second‐order nonlinear light scattering
View Description Hide DescriptionWe present a complete theory of polarizationmeasurements of second‐order nonlinear light scattering in isotropic solutions of nonlinear molecules. The nonlinear interaction between the molecules and input beams at frequencies ω_{1} and ω_{2} gives rise to incoherently scattered radiation at frequency ω_{1}+ω_{2}. The fundamental quantity measured by such experiments in the laboratory frame of reference is the quadratic orientational average 〈β_{ ikl }β_{ jmn } ^{*}〉 of the hyperpolarizability tensor. The number of independent orientational averages that can be measured is shown to be equal to the number of rotational invariants of sixth rank quantities. The absolute maximum number of independent measurements is 15 and occurs for the most general case in which the hyperpolarizability tensor is complex and the dispersion between all three frequencies is important. This number is reduced to eleven for the case of a real hyperpolarizability tensor and to six and five for the case of hyper‐Rayleigh scattering and complex and real tensors, respectively. For the case of planar molecules, these numbers are further reduced to ten, seven, five, and four, respectively. We present explicit expressions that relate the rotational invariants to the components of the hyperpolarizability tensor in the molecular frame of reference. We also present practical measurement schemes that can be used to determine all rotational invariants experimentally and discuss the possibilities and limitations of nonlinear light scattering in determining the values of individual components of the molecular hyperpolarizability tensor.

Adiabatic nuclear magnetic resonance linewidth contributions for central transitions of I>1/2 nuclei
View Description Hide DescriptionThe loss of coherence between any pair of spin‐inverted states is independent of adiabatic (secular), second‐order perturbations associated with the electric quadrupole interaction. Thus, consideration of various dephasing mechanisms, commonly ignored for NMR studies of quadrupolar nuclei (I≳1/2), may be important whenever extreme narrowing approximations fail. In this work, the consequences of various magnetic and higher‐order electric effects, as well as mutual interferences, are considered in greater detail.

Multiple quantum filtered nuclear magnetic resonance spectroscopy of spin 7/2 nuclei in solution
View Description Hide DescriptionMultiple quantum dynamics of spin I=7/2 are presented considering modulation of quadrupolar interaction as the relaxation mechanism. The equation of motion of the spin system is calculated in Liouville space using irreducible spherical tensor operators as the orthonormal basis. Relaxation matrices are given explicitly for coherences one to seven. Experiments for measuring the creation of multiple rank tensors and multiple quantum relaxation for spin 7/2 are described and analyzed. Results of double quantum and triple quantum filtered spectra of Cs^{+} bound to the crown ether 18‐crown‐6, dissolved in glycerol, were analyzed in terms of the microscopic parameters of the system, the reorientation correlation time and the quadrupolar coupling constant. Numerical calculations indicated that the expected MQF signal intensities decrease in the order TQF≳DQF≫5QF≳4QF≫7QF≳6QF.

Experimental and theoretical vibrational overtone spectra of v _{CH}=3, 4, 5, and 6 in formaldehyde (H_{2}CO)
View Description Hide DescriptionWe present the first photoacoustic absorption spectra of higher C–H stretching overtones in formaldehyde up to the visible spectral region. The analysis of the coarse rotational structure on the basis of asymmetric rotor simulations provides band centers up to the fifth overtone (v _{CH}=6). A two dimensional normal coordinate model for the C–H stretching degrees of freedom explains the observed polyad structure of the spectrum using ab initio (MP2/6‐311G**) potential energy and electric dipole functions. The observed intensity distribution reflects the increasing local mode character of higher overtone wave functionswithin the normal coordinate subspace.

Raman spectroscopy and theoretical modeling of HCl vibrational frequency shifts in high pressure argon
View Description Hide DescriptionRaman vibrational frequencies of HCl in argon were measured at pressures up to 110 MPa. The mean frequency of the asymmetric Q‐branch is shown to accurately measure vibrational shifts through a density region where line shape changes due to motional narrowing render the peak maximum an inaccurate measure of pressure induced frequency shifts. A semiclassical, analytical expression utilizing Hutson’s HCl–Ar pair‐potentials is used to determine the derivative of the HCl vibrational frequency with respect to Ar density in the limit of zero density. The predictions are in reasonable agreement with experimental results, although the experimental frequency shifts are about 20% smaller (less redshifted) than theoretical predictions, which may represent the influence of multibody interactions. Experimental HCl Raman Q‐branch and S‐branch linewidths and peak shifts are compared qualitatively with previous R‐branch (IR absorption) results. Separation of the vibrational (Q‐branch) and rotational parts of the frequency shift suggest that the rotational contribution is positive (blueshifted) for all J values and approaches zero with increasing J.

Vibronic structure of the emission spectra from single vibronic levels of the S _{1} manifold in naphthalene: Theoretical simulation
View Description Hide DescriptionThe vibrational structure of electronic spectra of naphthalene is simulated by means of a perturbative calculation of the intensities, based on the vibronic basis set. The formalism is implemented to include the contribution of Herzberg–Teller induced activity for totally symmetric modes and to describe the interference between the latter and the allowed Franck–Condon intensity. Geometries, vibrational normal modes, and vibronic coupling parameters required to model the spectra are obtained by means of ab initio and semiempirical calculations. The structure of absorption and single vibronic level fluorescence spectra is reproduced in detail and consistently for all the spectra examined. It is shown that the intensity of modes ν_{8a } and ν_{5a } is strongly affected by interference effects, and that Dushinsky mode mixing of totally symmetric modes plays a major role in redistributing the intensity among the vibronic bands of the spectra.

High resolution spectroscopy of the He^{79}Br_{2} van der Waals molecule: An experimental and theoretical study
View Description Hide DescriptionThe structure, dissociationdynamics, and intermolecular potential energy surfaces of the He^{79}Br_{2} van der Waals molecule have been studied using high resolution, two color, pump–probe laser induced fluorescencespectroscopy and three dimensional quantum mechanical calculations. A conical nozzle produces higher centerline cluster densities than a standard nozzle, and allows data collection further downstream from the nozzle. This yields improved signal to noise ratios and lower Doppler widths. He^{79}Br_{2} is found to have a T‐shaped average geometry with He to Br_{2} center‐of‐mass distances of 3.98 Å and 4.11 Å for the X and B states, respectively, somewhat longer than previously reported. Spectra were also obtained for excitation to excited bending levels of the van der Waals coordinate. However, these spectra have yet to be rotationally assigned. Vibrational predissociation line widths for the B state of He^{79}Br_{2} have been measured for three new vibrational levels and range from 0.036 cm^{−1} for B, v′=8 to 0.062 cm^{−1} for B, v′=12.
These values are somewhat larger than was expected based on previous HeBr_{2} linewidth measurements for higher vibrational levels. Forms for the potential energy surface that have previously been used to simulate the spectra of HeCl_{2} have been applied to the HeBr_{2} data. For the HeBr_{2} X state, two potentials are tested. First, a slightly anisotropic, one center Morse–Spline–van der Waals potential with angle dependent parameters is used. Second a much more anisotropic potential obtained from ab initio calculations is tested. The more anisotropic potential produces a significantly better fit to the data. The B state potential is constructed using Morse atom–atom potentials for the short range part of the He–Br interaction. This simple potential is sufficient to fit the main excitation band, but does not yield a fit to spectra involving vibrationally excited van der Waals modes.

Far‐infrared spectrum of N_{2} and N_{2}‐noble gas mixtures near 80 K
View Description Hide DescriptionThe far‐infrared absorption spectra of gaseous nitrogen, and mixtures of nitrogen with the foreign gases argon and neon, have been measured at pressures near 1 atm and at temperatures near 78 and 89 K. Spectra were obtained over the wave number range 20–100 cm^{−1} using a Fourier transform spectrometer and a multiple reflection absorption cell of 52 m path length. They show a broad continuum associated with the pure rotational collision‐induced S branch of the N_{2} molecule plus structure attributed to transitions in dimers, not previously observed in this spectral region. In the case of N_{2}–Ar, there are strong similarities with the fundamental vibrational band under similar conditions of pressure and temperature as reported by McKellar [J. Chem. Phys. 88, 4190 (1988)]. The integrated absorption coefficient has been evaluated for the N_{2}–N_{2} rotational band; at 78 K it equals 3.1×10^{−31} cm^{5} sec^{−1}, a factor of 2 greater than typical values at temperatures above 100 K.

The infrared photodissociation spectra of Fe_{ n }(CH_{3}OH)_{ m } complexes and their deuterated analogs near 10μ
View Description Hide DescriptionThe infrared photodissociationspectra of Fe_{ n }(CH_{3}OH)_{ m }, Fe_{ n }(CH_{3}OD)_{ m }, Fe_{ n }(CD_{3}OH)_{ m }, and Fe_{ n }(CD_{3}OD)_{ m } (n=5–15; m=1,2) have been recorded from 920–1090 cm^{−1}. Analysis of the spectral features indicates that in all cases, molecularly adsorbed methanol is the absorbing chromophore, with no evidence of methoxy formation. The observed frequencies for the C–O stretching vibration and other vibrational modes are shifted ∼40 cm^{−1} to the red of the corresponding fundamentals of gas‐phase methanol. The spectral shifts and thus the magnitude of the cluster–methanol interaction are observed to be independent of iron cluster size over the size range studied, implying an insensitivity of the interaction to the underlying cluster structure.

Nonadiabatic effects in a method that combines classical and quantum mechanics
View Description Hide DescriptionWe have included nonadiabaticeffects in the calculation of the dynamical evolution of a system where a quantum particle in a double well is coupled to a classical oscillator. By performing an exponential resummation of the evolution operator we have included ‘‘polarization’’ effects (similar to the self‐energy corrections for an electron that moves in a polarizable medium) which lead to a renormalization of the energy of the quantum particle.

The reaction of manganese clusters and manganese cluster carbides with hydrogen: The Mn–CH_{3} bond energy
View Description Hide DescriptionManganeseclusters are formed by laser vaporization in an inert gas condensation source cooled to −160 °C. A pure manganese target is used as well as a manganese target containing 2 mol % carbon. The clusters are reacted with hydrogen both in the region of cluster growth and in a flow‐tube reactor (FTR) downstream of the cluster source. The reactions, both with hydrogen atoms in the cluster growth region and dissociative chemisorption of H_{2} in the FTR, result in hydrogen atoms bound to the clusters, except that for Mn_{15} and smaller clusters the H atoms are unstable against H_{2} desorption. Above Mn_{15} stable hydrides are formed, but the rate of reaction in the FTR varies considerably with cluster size. This abrupt change in the ability to bind hydrogen may reflect a significant change in the character of the bonding within the cluster, perhaps from van der Waals to metallic. Mn_{ n }C clusters readily react with hydrogen for n≳6 to form Mn_{ n }CH_{2}. Further reactivity generally follows the pattern of the bare clusters, suggesting that the two hydrogen atoms on Mn_{ n }CH_{2} are bound to the carbon. Under certain conditions a large intensity of Mn_{ n }CH_{3} species (n≥1) is seen. From these results and earlier reports of reactions of Mn with various hydrocarbons, an energy of 1.21±0.09 eV is determined for the Mn–CH_{3}bond.

Using laser pulse dynamics to probe the relaxation of an anisotropic velocity distribution of excited iodine
View Description Hide DescriptionThe pulse characteristics of a photolytic, gain‐switched iodine laser are a sensitive measure for relaxation processes that occur in the active medium. On a time scale shorter than a few microseconds this concerns the relaxation of the speed and the angular parts of the anisotropic velocity distribution which is produced by the photodissociation of the parent molecule CF_{3}I. A comprehensive rate equationmodel of the laser dynamics is fit to experimental data, to obtain time constants for these collision‐controlled relaxation processes for various buffer gases and pressures.

Dynamics of the photodissociation of triplet ketene
View Description Hide DescriptionCalculations of the microcanonical dissociation rate for vibrationally excited ketene on the first excited triplet surface (T _{1}) are presented. The calculations utilize the quantum reactive scattering methodology of absorbing boundary conditions (ABC) with a discrete variable representation (DVR) to obtain the cumulative reaction probability for dissociation over the barrier. Model 1‐ and 2‐degree of freedom potential energy surfaces for the T _{1}surface were obtained by fitting to the best available ab initio structures, energies, and frequencies. The dissociation rates in these reduced‐dimensionality calculations give good overall agreement with the experimentally measured rates, although the steplike features seen in the experiments are washed out by the tunneling through the narrow barrier predicted in the ab initio calculations. Further model calculations reveal that a barrier frequency of approximately 50–100i cm^{−1} is required to recover the step structure seen experimentally, which suggests that there is either another transition state region on the T _{1}surface farther out towards the product channel, or that there is surface‐hopping dynamics taking place between the T _{1} and S _{0} ketene potential energy surfaces, or that the ab initio barrier frequency is simply too large.

State‐resolved differential scattering in open‐shell collisions: Cl(^{2} P _{3/2})+HCl from high‐resolution infrared‐laser Dopplerimetry
View Description Hide DescriptionTime‐ and frequency‐resolved IR‐laser absorption methods are used to probe state‐resolved collisional energy transfer in open‐shell collisions of Cl(^{2} P _{3/2}) radicals with HCl(J) in the near single‐collision regime. Translationally ‘‘hot’’ Cl(^{2} P _{3/2}) radicals are formed by excimer laserphotolysis of Cl_{2}, which then collide with a room‐temperature distribution of HCl peaked at J≂3. Final‐state distributions of the HCl are monitored via transient absorption detection of a cw IR probe laser by the collisionally populated states (J=4, 5, 6,...,12). In previous work [J. Chem. Phys. 102, 7046 (1995)], these transient signals are used to extract absolute integralcollisional cross sections for state‐resolved rotational energy transfer into final J states. In the present study, high‐resolution IR Dopplerimetry with the single‐mode probe laser is used to measure translational distributions of the collisionally populated HCl(J) as a function of final J state. Analysis of these translational distributions leads to state‐resolved differential scattering cross sections for rotational energy transfer, which exhibit a strong propensity for forward scattering into all levels observed (J≤12). These results are compared with quasiclassical trajectory calculations(QCT) on a recently modified potential energy surface of Schatz and Gordon. The theoretical analysis is in good agreement with experiment, with the angular distribution dominated by forward scattering for most of the final HCl rotational states. However, for the very highest J states collisionally populated, the QCT calculations predict a shift from predominantly forward to more isotropic scattering that is not evident in the experimental results.

Spatially nonlocal fluctuation theory of rapid chemical reactions
View Description Hide DescriptionUsing ideas from statistical nonequilibrium thermodynamics we develop a spatially nonlocal theory of the influence of diffusion on rapid chemical reactions. The new approach generalizes the hydrodynamic (local) theory by including elementary events that influence both diffusion and reaction on molecular length scales and eliminates problems in the local theory that occur at short times. Examples of the nonlocal approach are given for several reaction schemes, including a simple reversible reactionA+B■C, and single species reactions of the form A+A→products. For low densities, the present theory is equivalent to the usual Smoluchowski method. In this regime, an intermediate version of the theory, including only nonlocal effects due to reaction, is compared to the full nonlocal theory and to the earlier hydrodynamic level theory and is shown to produce results for the time dependence of the radial distribution function that are similar at low density to the full theory. The application of the new approach at higher densities and to lower dimensions is described briefly.