Volume 103, Issue 15, 15 October 1995
Index of content:

Mass‐resolved multiphoton ionization spectra of XeKr in the region of Xe* (6p,5d)
View Description Hide DescriptionSingle isotopomer (2+1) resonantly enhanced multiphoton ionization (REMPI) spectra of jet‐cooled XeKr have been recorded using time‐of‐flight (TOF) mass detection. Vibrational analyses for several transitions involving excited states that dissociate to Kr(^{1} S _{0})+Xe* (6p,5d) between 77 560 and 80 150 cm^{−1} are reported for the first time. Equilibrium bond lengths were established by Franck–Condon calculations, while excited state symmetries were inferred from excitation spectra recorded using circularly polarized light. A local vibrational perturbation evident in the dominant band system dissociating to Kr(^{1} S _{0})+Xe*5p ^{5}6p[5/2]_{2} has also been analyzed. The identity of the perturbing state is proposed, based in part, on predissociationspectra obtained by monitoring atomic fragments in TOF detection.

Interaction‐induced contributions to polarizability anisotropy relaxation in polar liquids
View Description Hide DescriptionWe use molecular dynamics simulation to investigate polarizability anistropy relaxation in two polar liquids, methanol and acetonitrile, which have similar dielectric constants at room temperature, but are very different at the molecular level. Interaction‐induced contribution to the polarizability is included using first‐order perturbation theory and separated into a component which projects along the sum of molecular polarizability anistropies and relaxes through collective reorientation and a ‘‘collision induced’’ component which relaxes through other mechanisms involving mainly translational motion. We find that interaction‐induced effects on the polarizabilityanisotropy time correlation are important on all relevant time scales, especially for the more polarizable acetonitrile. In methanol, even though most of the molecular polarizability is along the CO bond, we find that the OH bond dynamics make a substantial direct contribution to polarizabilityanisotropyrelaxation. We compare our results to the experimentally determined nuclear portion of the optical Kerr effect response and discuss their implications for the use of this response in solvation dynamics theories. We find that the short‐time optical Kerr response of acetonitrile is dominated by collision‐induced polarizabilitydynamics, while librational orientational dynamics is the main contributor for methanol.

The C _{1s } shakeup spectra of Buckminsterfullerene, acenaphthylene, and naphthalene, studied by high resolution x‐ray photoelectron spectroscopy and quantum mechanical calculations
View Description Hide DescriptionThe C_{1s } core photoelectron shakeup spectrum of fullerene has been studied experimentally and theoretically, as well as the corresponding spectra of the smaller model compounds acenaphthylene and naphthalene. It is found that many of the shakeup excitations of C_{60} can be classified in terms of excitations found in the model compounds. A population analysis of the orbitals involved reveals a systematic behavior, enabling a generalization to extended aromatic systems and to an infinite graphite layer.

Circumstellar carbon chain molecules: A density function theory study of C_{ n }O, n=3–9
View Description Hide DescriptionThe infrared vibrational frequencies and intensities of the C_{ n }O linear chains, n=3–9, in their electronic ground state is predicted at the Becke–Lee–Yang–Parr (BLYP) level of theory. The computational model is assessed in three steps: (i) comparison of calculated and experimental rotational parameters for the whole series; (ii) comparison of experimental and calculated infrared frequencies, intensities and isotopic shifts for C_{3}O (this molecule can be considered the prototype of the chains whose ground electronic state is ^{1}Σ^{+}); (iii) comparison of calculated and experimental infrared frequencies and intensities for C_{4}O (this molecule can be considered the prototype of the chains whose ground electronic state is ^{3}Σ^{−}). The excellent agreement between experimental and computational results allows the prediction of the infrared pattern to 20 cm^{−1} for the frequencies and a few percent for the relative intensities. Analysis of the infrared intensities in terms of local atomic oscillators within the chains shows that while for short chains the intensity arises from the motion of the two carbon atoms nearest to the oxygen, for C_{7}O and C_{9}O the intensity arises in conjunction with the motion of carbon atoms close to, but not at, the other end of the molecule. For these two molecules, the infrared intensity is therefore similar in nature to that of pure carbon chains.

Intermolecular vibrations of phenol⋅(H_{2}O)_{3} and d _{1}‐phenol⋅(D_{2}O)_{3} in the S _{0} and S _{1} states
View Description Hide DescriptionWe report a combined spectroscopic and theoretical investigation of the intermolecular vibrations of supersonic jet‐cooled phenol⋅(H_{2}O)_{3} and d _{1}‐phenol⋅(D_{2}O)_{3} in the S _{0} and S _{1} electronic states. Two‐color resonant two‐photon ionization combined with time‐of‐flight mass spectrometry and dispersed fluorescenceemission spectroscopy provided mass‐selective vibronic spectra of both isotopomers in both electronic states. In the S _{0} state, eleven low‐frequency intermolecular modes were observed for phenol⋅(H_{2}O)_{3}, and seven for the D isotopomer. For the S _{1} state, several intermolecular vibrational excitations were observed in addition to those previously reported. Ab initio calculations of the cyclic homodromic isomer of phenol⋅(H_{2}O)_{3} were performed at the Hartree–Fock level. Calculations for the eight possible conformers differing in the position of the ‘‘free’’ O–H bonds with respect to the almost planar H‐bonded ring predict that the ‘‘up–down–up–down’’ conformer is differentially most stable. The calculated structure, rotational constants, normal‐mode eigenvectors, and harmonic frequencies are reported. Combination of theory and experiment allowed an analysis and interpretation of the experimental S _{0} state vibrational frequencies and isotope shifts.

Two‐photon ionization of alkali‐halide clusters spectroscopy of excess‐electron excited states
View Description Hide DescriptionNa_{ n }F_{ n−1} clusters are produced in a laser vaporization source. They are ionized by one‐photon or resonant two‐photon ionization, and detected in a high resolution time of flightmass spectrometer. By scanning the ionization wavelength, it is possible to obtain ionization potentials and absorption spectra of these clusters, for n ranging from 2 to 42. The results we obtain show many discrepancies with the cuboid model, which has proven valid for small sizes. The possible origin of these disagreements is discussed.

Dipolar interactions in deuteron spin systems. I. Spin diffusion
View Description Hide DescriptionThe influence of dipolar interactions on the longitudianl relaxation of deuteron spin systems is investigated. Spin diffusion rates are evaluated, including approximate rates due to double quantum spin diffusion and three‐spin flip–flop transitions. It is shown that slow molecular rotations in supercooled liquids do not affect the spin diffusion rates significantly provided that the motional correlation times are below the average spin lattice relaxation timeT _{1} which becomes on the order of one second close to the glass transition temperature T _{ g }. However, the broad distribution of deuteron T _{1} values at T<T _{ g } results in a large effect of spin diffusion upon the long time decay of the longitudinal magnetization in T _{1}experiments. These effects are estimated in terms of a simple model in agreement with recent experiments. It is also shown that the initial decay determining the average rate 〈T ^{−1} _{1}〉 remains unaffected by spin diffusion. Finally, we show that small amplitude motions on a time scale of 10^{−6}–10^{−3} s may cause temperature dependent spin diffusion effects.

Dipolar interactions in deuteron spin systems. II. Transverse relaxation
View Description Hide DescriptionThe effect of the so‐called local field terms of the dipolar interactions on the transverse relaxation of deuteron spin systems is investigated for a variety of physical situations. It is shown that the response of the spin systems to arbitrary pulse sequences can be calculated in an operator formalism. For illustration, calculations of the signals following a quadrupolar echo sequence are performed, and expressions for the echo amplitude are given including that for a system of a deuteron coupled to an arbitrary number of spins. A simple model is formulated and the transverse relaxation rate in supercooled liquids is calculated for the whole range of motional correlation times of experimental interest.

Structure and IR‐spectrum calculations for small SF_{6} clusters
View Description Hide DescriptionA new second order perturbation approach for evaluating the splittings and shifts of the vibrational bands of homogeneous molecular clusters, consistently treating degenerate normal modes, is described. The Hamiltonian of the system comprises harmonic and anharmonic intramolecular vibration terms, and the intermolecular potential. The anharmonic intramolecular contributions and the intermolecular potential are treated as a perturbation. A new site‐site intermolecular potential model for SF_{6}, featuring exchange, dispersion,electrostatic and induction terms, is presented. The new potential, with the parameters adjusted according to the observed monomer transition dipole moment and reproducing the experimental temperature dependence of the second virial coefficient, is used to determine SF_{6} cluster structures up to the hexamer and, by means of the new line shift formalism, to calculate the corresponding IR‐spectra in the region of the ν_{3} vibrational mode (at 947.968 cm^{−1}). The contributions of the various potential terms to the frequency shifts are analyzed and the leading interaction mechanism is confirmed to be the electrostatic one (implicitly the resonant dipole‐dipole coupling). The theoretical spectra are shown to fairly describe the experimental evidence when considering only exchange, dispersion and electrostatic interactions. With the available atomic polarizabilities, induction seems to lead to a systematic redshift of the entire spectrum for all cluster sizes. The structure of the cluster vibrations is investigated in terms of the individual monomer vibrations and is correlated with the found geometrical cluster configurations.

Diffusive torsional dynamics of malachite green molecules in solid matrices probed by fluorescence decay
View Description Hide DescriptionThe torsional dynamics of phenyl rings of malachite green molecules in the excited state is studied in polymeric and monomeric glass matrices by measuring the fluorescence decay time as a function of temperature. It is shown that the phenyl rings rotate diffusively in solid polymers (polymethyl methacrylate and polyvinyl alcohol) quite rapidly even at low temperatures. To analyze the experimental results, we used the concept of microviscosity which controls the diffusive rotational motion of phenyl rings of malachite green molecules in solid matrices. By using the reaction‐rate theory, we show that a horizontal excited‐state potential surface rather than a downhill potential surface for the rotation of phenyl rings can more reasonably explain the rotational motion in polymers. If we assume that the potential is horizontal, the temperature dependence of the microviscosity can be described by Andrade equation with a definite activation energy which is known to be valid for many liquids over a wide range of temperatures. This implies that the microscopic dynamics of small molecular rotations in a solid polymer resembles the behavior in many liquids. By monitoring the fluorescence decay of malachite green molecules doped in ethanol monomeric glass during its phase transition, we show that the effects of phase transition are well represented in the fluorescence decay time. We then propose to use malachite green molecules as sensitive optical microprobes of local dynamics in various solid matrices and their phase transitions, etc.

The electronic spectroscopy of jet‐cooled m‐difluorobenzene
View Description Hide DescriptionThe S _{1}(^{1} B _{2})–S _{0}(^{1} A _{1}) electronic transition of m‐difluorobenzene cooled in a supersonic free jet has been investigated in detail by use of laser induced fluorescence excitation and dispersed fluorescence spectroscopy. By analysis of over 50 vibronic transitions we have assigned 16 new S _{1} vibrational frequencies, and have confirmed seven previous assignments. This advances the total number of known S _{1} fundamental vibrational frequencies to 23 out of the possible 30. The assignments of two S _{0} frequencies have also been somewhat modified. Two different types of symmetry‐forbidden transitions were identified: transitions involving one‐quantum changes in b _{2} modes were found; and transitions involving combinations of b _{1} vibrations were observed to be unusually active in the vibronic spectrum. The observation of these forbidden transitions in both the fluorescence excitation and dispersed fluorescence spectra are explored in terms of first‐ and second‐order vibronic coupling.

The microwave spectrum and structure of the methanol⋅SO_{2} complex
View Description Hide DescriptionThe rotational spectra of nine isotopomers of the methanol⋅sulfur dioxide van der Waals complex were observed with a pulsed molecular beamFourier transformmicrowave spectrometer. Each rotational transition is split into an A‐state (m=0) and an E‐state (m=±1) transition due to methyl top internal rotation effects. The A and E transitions show an additional inversion splitting ranging from a MHz to a few tens of MHz in seven of the isotopomers. The inversion splitting is absent in the two S^{16}O^{18}O isotopomers. The center frequencies of the inversion doublets were used in a simultaneous fit of both the A‐ and E‐state transitions, producing rotational constants which allowed a complete determination of the structure of the complex. Analysis of the moments of inertia indicate that the complex has a stacked structure. The center of mass distance between the two monomers is 3.08(5) Å. The effective torsional barrier height is V _{3}=128.6(1) cm^{−1} based on the assumption that the methyl group rotates against a heavy frame. The dipole moment is μ_{ T }=1.94(3) D. The inversion motion is discussed based on effects on the splitting associated with isotopic substitution and the transition dipole direction.

Time‐resolved diode laser spectroscopy of the ν_{6} band of propargyl produced by the UV photolysis of allene
View Description Hide DescriptionThe propargyl radical (CH_{2}C≡CH) produced by the ArF excimer laserphotolysis of allene was observed by time‐resolved infrared diode laser spectroscopy. More than one hundred and fifty absorption lines have been assigned to the ν_{6} (CH_{2}‐wagging) fundamental band of propargyl. Most of the absorption lines were observed as doublets due to the spin–rotation interaction in the ^{2} B _{1}ground electronic state. The rotational and spin–rotation interaction constants derived for the ground vibrational state are, A _{0}=9.608 47(36), B _{0}=0.317 674(24), C _{0}=0.307 098(24), ε_{ aa }=−0.017 58(95), and ε_{ bb }=−0.000 355(76) cm^{−1}, where the figures in parentheses are 2.5 times standard deviations to be attached to the last digit. The ν_{6} band origin is 687.176 03(62) cm^{−1}, consistent with the infrared spectrum observed in the argon matrix. Anomalously large vibrational changes in the Arotational constant and the Δ_{ K } centrifugal distortion constant are accounted for by the a‐type Coriolis interaction between the ν_{6} and ν_{10} states, where ν_{10} is the CH_{2}‐rocking vibration.

A multichannel quantum defect fit to the n*=6–8 core‐penetrating s∼p∼d supercomplexes of CaF
View Description Hide DescriptionThe spectrum of the core‐penetrating s, p, and dRydberg series of calcium monofluoride between n*=6 and 8 have been analyzed using multichannel quantum defect theory (MQDT). From least‐squares fits of the n*=6, 7, and 8 (−) Kronig symmetry p∼d supercomplexes we have determined B^{+} (the rotational constant of CaF^{+}) to be 0.3729 cm^{−1}, the mixing coefficients between the pπ and dπ series (the 0.98 Π series is 41% p and 59% d while the 0.35 Π series is 41% d and 59% p), and the eigenchannel (rotationless) quantum defects of the 0.98 Π, 0.35 Π, and 0.14 Δ series. A similar MQDT fit was attempted for the (+) Kronig symmetry s∼p∼d supercomplexes, but the mixing coefficients for the σ series could not be determined even though the mixing coefficients for the π series were as well determined as in the fit to the (−) symmetry series. The failure of the s∼p∼d model to fit the (+) symmetry series, particularly the σ series, is an indication that, for the (+) symmetry series, mixing of the sσ, pσ, and dσ series with the fσ series must be considered. Experiments are currently under way to observe and characterize the f σ, π, δ, γ series so that an s∼p∼d∼f supercomplex model can be developed for the (+) symmetry series.

Simple modeling of line‐mixing effects in IR bands. I. Linear molecules: Application to CO_{2}
View Description Hide DescriptionA simple approach is developed in order to model the influence of collisions on the shape of infrared absorption by linear molecules. It accounts for line‐mixing effects within, as well as between, the different branches (P,Q,R) of the band. It is based on use of the strong collision model, of a classical representation of rotational levels, and of the rigid rotor approximation. The absorption coefficient then has a very simple analytical expression; its wave number and pressure dependencies are computed by using eight parameters which depend on the considered vibrational transition, the temperature, and the nature of the perturber only. These quantities are band‐averaged values of the detailed spectroscopic and collisional parameters of the molecular system. Tests of the model are presented in the ν_{3} and 3ν_{3} bands of CO_{2} perturbed by He and Ar at elevated pressures. They demonstrate the accuracy of our approach in accounting for the effects of collisions on the spectral shape in a wide density range; indeed, the superposition of Lorentzian individual lines at low pressure, as well as the collapse (narrowing) of the band at very high pressure are satisfactory predicted.

Investigation of the effect of reagent CN rotational excitation on the dynamics of the CN+O_{2} reaction
View Description Hide DescriptionThe reaction of CN with O_{2} has been studied through a photolysis‐probe laser experiment in a cell at a total pressure of 70 mTorr. Rotationally hot CN reagent was prepared by 193 nm photolysis of BrCN. NCO(X̃ ^{2}Π) product in various vibronic levels was detected by laser fluorescence excitation in its Ã ^{2}Σ^{+}–X̃ ^{2}Π band system at variable delays after the photolysis laser. In order to monitor the CN collisional relaxation which is taking place simultaneously with the reaction, we have also derived the CN rotational state distribution as a function of the photolysis‐probe delay from laser fluorescence excitation spectra of the CN B ^{2}Σ^{+}–X ^{2}Σ^{+} (0,0) band. From these observations, we deduce that rotationally hot CN reacts more slowly than thermalized CN. Moreover, reaction of the former yields NCO product with greater bending vibronic excitation. These results are compared with previous dynamical studies of this reaction, carried out with crossed beams and in cells.

Selected ion flow drift tube studies of the reactions of Si^{+}(^{2}P) with HCl, H_{2}O, H_{2}S, and NH_{3}: Reactions which produce atomic hydrogen
View Description Hide DescriptionThe reaction rate coefficients, k, for the reactions of ground‐state Si^{+}(^{2}P) with HCl, H_{2}O, H_{2}S, and NH_{3}, have been measured as a function of reactant ion/reactant neutral center‐of‐mass kinetic energy, KE_{CM}, in a selected ion flow drift tube (SIFDT) apparatus, operated with helium at a temperature 298±2 K. The values k of the studied reactions have very pronounced, negative energy dependencies; the rate coefficients decrease by about 1 order of magnitude as KE_{CM} increase from near thermal values to ∼2 eV. The results are interpreted in terms of a simple model assuming the reactions to proceed via the formation of long‐lived complexes. These intermediate complexes decompose back to reactants or forward to products, the unimolecular decomposition rate coefficients for these reactions being k _{1} and k _{2}, respectively. It is found that a power law of the form k _{−1}/k _{2}=const(KE_{CM})^{ m } closely describes each reaction.

Vibrational population dynamics of the HgI photofragment in ethanol solution
View Description Hide DescriptionThe vibrational population dynamics of HgI fragments in ethanol solution, resulting from the 320 nm photolysis of HgI_{2}, are examined both experimentally and by a simulation. The experiments reveal an HgI population distribution which rapidly relaxes toward equilibrium. At the earliest times, the HgI exhibits vibrational coherent wave‐packet motion that dephases with a time constant of ca. 1 ps. These data are used to gain insight into the character of the solvated potential energy curves. The population relaxation was adequately reproduced by master equations which were formulated to incorporate the HgI anharmonicity and a solvent frequency dependent friction. This treatment characterizes the spontaneous vibrational relaxation timescale for the n″=1→0 transition to be ca. 3 ps, and is used to identify the relaxation rate constants for all other HgI level pairs. The simulations estimate that the initial excess energy of HgI is centered at n″≂10 which corresponds to a total excess energy of ca. 1050 cm^{−1}.

Quantum calculations of reaction probabilities for HO + CO→ H + CO_{2} and bound states of HOCO
View Description Hide DescriptionA time‐dependent (TD) quantum wavepacket calculation of reaction probabilities is reported for the reaction HO + CO → H + CO_{2} for total angular momentumJ=0. The dynamics calculation employs the potential‐averaged five‐dimensional model (PA5D) and is made possible by using a normalized angular quadrature scheme to minimize the requirement for computer memory. Reaction probabilities are obtained from the ground state as well as rotationally excited state in either one of the reactant diatoms. Strong resonances are found in the present study and calculated reaction probabilities are dominated by many narrow and overlapping resonances. These features are in qualitative agreement with several lower dimensional quantum dynamics studies. However, quantitative comparison of the present result with previously reported quantum calculations, including a recent planar four‐dimensional (4D) calculation of Goldfield et al., shows that our calculated reaction probabilities are much smaller than those found in reduced dimensionality calculations. We also found reaction probability to be more sensitive to the rotational motion of CO than of HO. In addition to reaction probabilities, the bound state calculation for the stable intermediate complex HOCO has also been carried out and energies of several low‐lying vibrational states are obtained. The potential energy surface (PES) of Schatz–Fitzcharles–Harding (SFH) is used in all the calculates presented in this paper.

Study of low‐lying electronic states of ozone by multireference Mo/ller–Plesset perturbation method
View Description Hide DescriptionThe geometry and relative energy of the seven low‐lying electronic states of ozone and the ground state of ozonide anion have been determined in C _{2v } symmetry by the complete active space self‐consistent field (CASSCF) and the multireference Mo/ller–Plesset perturbation (MRMP) methods. The results are compared with the photodetachment spectra of O^{−} _{3} observed recently by Arnold et al. The theoreticalelectron affinity of ozone is 1.965 eV, which is 0.14 eV below the experimental result of 2.103 eV. The calculated adiabatic excitation energies (assignment of Arnold et al. in parentheses) of ozone are ^{3} A _{2} 0.90 eV (1.18 eV), ^{3} B _{2}, 1.19 eV (1.30 eV), ^{3} B _{1}, 1.18 eV (1.45 eV), ^{1} A _{2}, 1.15 eV (∼1.6 eV), ^{1} B _{1}, 1.65 eV (2.05 eV), and ^{1} B _{2}, 3.77 eV (3.41 eV), respectively. Overall the present theory supports the assignment of Arnold et al. However, the simple considerations of geometry and energy are insufficient to determine a specific assignment of the ^{3} B _{2} and ^{3} B _{1} states. The dissociation energy of the ground state of ozone is computed to be 0.834 eV at the present level of theory. The present theory also predicts that none of the excited states lies below the ground state dissociation limit of O_{3}.