Volume 97, Issue 1, 01 July 1992
Index of content:

The scattering of evanescent light waves from oriented, tethered rod polymers
View Description Hide DescriptionThe theory of Gao and Rice is used to investigate the scattering of evanescent light waves by rod polymers tethered to a flat interface. Calculations of the structure factor are performed for a distribution of rod orientations about the normal to the interface and for rods tilted from the normal. The structure factor is found to vary significantly with this tilt angle and with the width of the orientational distribution as well.

Infrared spectra of matrix isolated ClCO and a b i n i t i o calculation
View Description Hide DescriptionCl atoms and CO molecules were trapped in a solid Ar matrix at 16 K. The formation of ClCO was followed via its IR spectrum. The vibrational frequencies of ^{3} ^{5}Cl^{1} ^{2}C^{1} ^{6}O were observed at 1876.7, 570.1, and 334.6 cm^{−} ^{1}. Experiments with different CO isotopomers were performed in order to confirm the assignment of the absorptions and to characterize the force field. The results extend and correct earlier data [M. E. Jacox and D. E. Milligan, J. Chem. Phys. 4 3, 866 (1965)]. With the help of quantum chemical calculations (UHF, MP2), the optimized ClCO bond angle was obtained to be 129.2°. The computed bond lengths were determined to be 1.159 Å for C–O and 1.807 Å for C–Cl. Based on force constant calculations, the C–Cl bond in ClCO was found to be extremely weak compared to that in Cl_{2}CO.

Molecular theory of transition energy correlations for pairs of chromophores in liquids or glasses
View Description Hide DescriptionThe absorptionspectrum of an optical transition of a dilute solute in a glassy or liquid solvent is usually inhomogeneously broadened. In a concentrated solution, the question arises as to whether or not the transition energy distributions of nearby solutes are correlated. Such correlation has important implications for coherent or incoherent transport and optical dephasing experiments. We present a molecular theory of this correlation. For a simple model of Lennard‐Jones solutes in a Lennard‐Jones liquid solvent, we compare our theory to Monte Carlo simulations, finding reasonable agreement. For a model with longer range solute–solvent interactions, where the excited state solute is ionized, the theory predicts very significant correlation effects. This suggests that for more realistic models with dipolar interactions, significant correlation effects will also be present.

High‐resolution stimulated Brillouin gain spectroscopy of liquid carbon disulfide: Dependence of the linewidths and peak gain on scattering angle
View Description Hide DescriptionWe have used narrow linewidth continuous‐wave lasers to obtain high‐resolution stimulated Brillouin gain (SBG) spectra of liquid carbon disulfide at various scattering angles. For back‐scattering angles, the linewidths of the SBG peaks reflect the natural lifetimes of the stimulated acoustic waves, whereas for forward‐scattering angles the linewidths are dominated by laser jitter and by the uncertainty in the scatteringwave vector that results from focusing the laser beams in the interaction region. The peak gain for the forward‐scattering SBG spectra is an order of magnitude smaller than that observed at a back‐scattering angle of 178°. The dependence of the peak gain on scattering angle can be accounted for quantitatively by considering the angle dependencies of the Brillouin frequency, linewidth, and laser beam crossing efficiency.

Polarization dependence of the ac Stark effect in multiphoton transitions of diatomic molecules
View Description Hide DescriptionThe (2+2) resonance‐enhancedmultiphoton ionization (REMPI) of N_{2} via the a ^{1}Π_{ g }(v=1,J) levels shows a strong dependence on the polarization of the laser beam causing this process. This behavior is attributed to the acStark effect produced by the near resonance of the N_{2} o _{3} ^{1}Π_{ u }(v=0,J) levels with the sum of the first three photons. The multiphoton transitions are broadened and asymmetric in appearance; one level is even split. The line profiles change markedly as the polarization of the laser beam is varied from linear to circular. A general theory is presented for the acStark effect in a diatomic molecule undergoing a multiphoton transition. When the sum of the photon energies is resonant with an allowed transition, a splitting of the line is observed (Autler–Townes effect). Off resonance, the magnetic sublevels are shifted by different amounts, causing the line profile to be broadened and distorted. This theoretical treatment is able to explain in a satisfactory manner the observed behavior of (2+2) REMPI of N_{2} via the a ^{1}Π_{ g }–X ^{1}Σ^{+} _{ g } transition and the two‐photon laser‐induced fluorescence of CO via the A ^{1}Π–X ^{1}Σ^{+} transition.

Properties of the combination harmonic in spectra of primary electron spin echo envelope modulation of orientationally selected disordered systems. Application to aqua‐oxovanadium complexes
View Description Hide DescriptionThe properties of the sum combination (ν_{α}+ν_{β}) harmonic in the primary electron spin echo envelope modulation for systems with large g factor and hyperfine interaction anisotropy are investigated. The influence of the hyperfine interaction (HFI) parameters in the weak HFI limit on the magnitude of the shift of the combination harmonic away from the double Zeeman frequency of the nucleus are examined together with the quadrupole splitting of this line for a deuterium nucleus. The theoretical results are used for the analysis of the electron spin echo envelope modulation spectra of oxovanadium‐aqua complexes with H_{2}O and D_{2}O in frozen solution, which yield the HFI parameters of the ligand water protons. The structural information based on these HFI data are discussed in comparison with previous single crystal and powder electron nuclear double resonance results.

Calculation of the rotational Raman spectrum of H_{2} dissolved in water
View Description Hide DescriptionWe have carried out molecular dynamics simulations on the system of a hydrogen molecule dissolved in water or in heavy water in an attempt to understand the experimental results of Taylor and Strauss [J. Chem. Phys. 9 0, 768 (1989)] for the rotational Raman spectrum of H_{2}. By extracting from the simulation the time‐varying potential felt by the H_{2} molecule, we have computed linewidths in various theoretical limits: static, adiabatic, and Redfield. A start has been made at the nonadiabatic calculation. H_{2}/water is a system of a quantum rotor in a classical bath, in which there is little separation of time scales. The calculations do produce the correct absolute magnitude of the linewidths without adjustable parameters. However, the details of the observed variation of the widths with J and with the change of H_{2}O to D_{2}O are not reproduced and are discussed.

Solvation effects on the electronic structure of 4‐N, N‐dimethylaminobenzonitrile: Mixing of the local ππ* and charge‐transfer states
View Description Hide DescriptionThe effect of polar solvents acetonitrile and water on the electronic excited states of 4‐N, N‐dimethylaminobenzonitrile (DMABN) is studied through the optical spectroscopy of small clusters of DMABN/solvent. The clusters are created in a supersonic jet expansion. The results of mass resolved excitation spectroscopy (MRES), fluorescence excitation (FE), dispersed emission (DE), and photodepletion studies demonstrate that the solvent molecule can bind to DMABN at two distinct sites for the one‐to‐one cluster. Both DMABN (H_{2}O)_{1} clusters generate small blue shifts for the S _{1}←S _{0} cluster transition and evidence low‐energy vibronic structure nearly identical to that found for the bare molecule. The DMABN (CH_{3}CN)_{1} clusters behave quite differently. One cluster geometry induces a small blue shift of the S _{1}←S _{0} electronic transition with little change in its vibronic structure and intensity pattern. We suggest this binding site involves the cyano end of the DMABN molecule. The second cluster geometry induces a large red shift (∼1000 cm^{−} ^{1}) and significant broadening (>10^{3} cm^{−} ^{1}) of the lowest‐energy transition. This red shifted transition is associated with a charge‐transfer transition within the DMABN molecule lowered in energy due to the acetonitrile coordination with the DMABN aromatic ring. The lowering of the charge‐transfer state in DMABN (CH_{3}CN)_{ n }, n=1,...,5 clusters is supported by the following data: long wavelength emission from clusters with broad red shifted absorption; distinct lifetimes for emission at 350 nm (4.6 ns) and 400 nm (6.0 ns); broad red shifted absorption for one geometry of the DMABN (CH_{3}CN)_{1} cluster. These results support the idea that the charge‐transfer transition in DMABN is stabilized by short‐range dipole–dipole interactions between DMABN and polar nonhydrogen bonding solvents.

The grating decomposition method: A new approach for understanding polarization‐selective transient grating experiments. I. Theory
View Description Hide DescriptionIn this paper and the following Paper (II) we introduce a new method of viewing transient holographic grating experiments in which the gratings are formed by laser beams of orthogonal linear or circular polarizations (or one of each). In this paper, we show that the two traditional methods of modeling these gratings, electric‐field pictures and diagrammatic perturbation theory, may be augmented. We demonstrate that any grating can be decomposed into component intensity gratings that are related to the polarizations in its electric‐field picture. Each of these component gratings may be analyzed separately (with or without diagrammatic perturbation theory), facilitating the incorporation of secondary effects (such as transport and heat deposition) into the grating calculation. The grating decomposition method (GDM) illuminates spatial structure that is not evident in standard perturbative calculations; it also provides a physical description that makes qualitative insights more readily obtainable, while at the same time making the electric‐field approach rigorous and quantitative. Furthermore, the GDM reduces the complexity of many diagrammatic perturbation theory calculations. We also introduce effective two‐interaction matrix elements (ETIMEs), which can be used to greatly simplify perturbative grating calculations. We show that ETIMEs, when considered in conjunction with the symmetry properties of the third‐order susceptibility (χ^{(3)}), can often be used to prove that some of the component gratings in a decomposition do not contribute to the signal and therefore need not be considered. In II, we apply this theory to two grating problems.

The grating decomposition method: A new approach for understanding polarization‐selective transient grating experiments. II. Applications
View Description Hide DescriptionWe apply the theory developed in Paper I to two transient grating problems that present difficulties in interpretation and/or calculation. The first application is general, and illustrates the ability of the grating decomposition method (GDM) to facilitate calculations and to provide intuition and insight in complex orientational grating experiments: we apply the GDM to nuclear optical Kerr effect (OKE) polarizationgratings. We show that the circularly polarized component gratings of the polarization‐grating decomposition do not contribute to the signal, and that the OKE polarizationgrating can therefore be viewed as the sum of two gratings with orthogonal net molecular alignments. We also use the GDM and this system to explain why polarizationgratings can rotate the polarization of the probe beam. The second example is a detailed application of the GDM to an experiment in which the data cannot be fully interpreted using standard diagrammatic perturbation methods: picosecond transient gratings on the D lines of gas‐phase sodium atoms. We use the GDM and effective two‐interaction matrix elements to greatly simplify this problem. We show why, in atmospheric‐pressure experiments, Na intensity‐grating decays are dominated by excited‐state quenching, whereas Na polarization‐grating decays are not. We show that the polarization‐grating decays are dominated by Nadiffusion and are influenced by scattering among the ground‐state magnetic sublevels, but are unaffected by excited‐state decay. We further show why the envelopes of polarization decays do not match the corresponding intensity‐grating decays at large fringe spacings in low‐pressure Na cells.

Three‐dimensional doorway‐state theory for analyses of absorption bands of many‐oscillator systems
View Description Hide DescriptionA three‐dimensional doorway‐state theory is worked out for the purpose of analyzing absorption bands of many‐oscillator systems, such as the amide‐I infrared (ir) bands of globular proteins. An appropriate basis set of the doorway‐state subspace is defined, so that the complication in interpretation of computational results, which may arise from the interference among the transition dipoles of the basis states, is avoided. As an example of application of the formulation, calculations of the doorway states are given for the ir band of a model system consisting of 400 oscillators. It is shown that the doorway‐state formalism provides band assignments based on the origin of ir intensity, even in the case where the molecule has an irregular structure or irregular force constants and the molecular‐eigenstate picture is not appropriate to describe vibrational motions responsible for the ir intensity. Possibility of applying this formalism to problems in other energy regions (such as electronic absorption of molecular aggregates) is also pointed out.

Application of the three‐dimensional doorway‐state theory to analyses of the amide‐I infrared bands of globular proteins
View Description Hide DescriptionThe three‐dimensional doorway‐state theory is applied to globular proteins to analyze the structure‐spectrum correlation of the amide‐I infrared (ir) bands in detail. The following three examples are presented. (1) The doorway states of the frequency region around 1630 cm^{−1} of flavodoxin and carboxypeptidase A are calculated. It is shown that the ir intensity in this frequency region largely originates from the peptide groups in the central zones of β sheets. (2) Calculations on α‐lactalbumin and lysozyme clarify the vibrational motions giving rise to the difference in the amide‐I band envelopes of these two proteins. (3) Contributions of the A‐ and E _{1}‐modelike vibrational motions of α helices to the amide‐I band envelopes are analyzed. It is shown that the E _{1}‐modelike motions have large contributions to the ir intensities of the bands in the 1640–1630 cm^{−1} region, solving the question as to the origin of such bands observed for highly helical proteins. It is also demonstrated that the frequency splitting between the A‐ and E _{1}‐modelike motions depends strongly on the helix length. These three examples reveal the importance of the detailed analyses of vibrational dynamics that gives rise to characteristic amide‐I band envelopes.

An electron–nuclear double resonance study of the lowest triplet state of pyrimidine
View Description Hide DescriptionOptically‐detected electron–nuclear double resonance was employed to measure the entire hyperfine and quadrupoletensors of the two nitrogen nuclei and the hyperfine tensors of the four hydrogen nuclei in the lowest triplet state T _{0} of pyrimidine‐h _{4} as a guest in a single crystal of benzene‐d _{6}. The electron‐spin‐density distribution and the molecular geometry in the lowest triplet state have been determined as well as the orientation of the molecule in the benzene crystal. It is found that upon excitation into T _{0}, pyrimidine remains a planar molecule of approximately C _{2v } symmetry; the C–N–C angles, which are 115.5° in the ground state, and the angle between the directions of the nonbonding nitrogen orbitals both become >120°. Nearly 60% of the total electron‐spin density is located at the two nitrogen atoms, the two nonbonding orbitals each carrying a spin density of 0.21, and the π orbitals a spin density of 0.08. We conclude that the lowest triplet state of pyrimidine is appropriately described in terms of an nπ* excitation; we find no evidence for vibronic coupling of this state with higher lying ^{3}ππ* states.

An electron–nuclear double resonance study of the lowest triplet state of pyrazine
View Description Hide DescriptionThe hyperfine and quadrupoletensors of the two nitrogen atoms and the hyperfinetensors of the four hydrogen atoms of pyrazine in the lowest triplet state T _{0} are obtained from optically‐detected electron–nuclear double resonance(ENDOR) experiments on pyrazine‐h _{4} in a single crystal of benzene‐d _{6} at 1.2 K. Analysis of these tensors shows that pyrazine is in good approximation a planar molecule of D _{2h } symmetry in the lowest triplet state. The in‐plane structure changes significantly upon excitation into T _{0}. An increase of the C–N–C angles is observed and a reduction of the C–N bond lengths. From the hyperfinetensors the distribution of the electron‐spin density is derived. The nitrogen nonbonding orbitals together carry 36% of the total spin density and it is shown that the lowest triplet state of pyrazine is appropriately described in terms of a ^{3} nπ* excitation. The results of the optically‐detected ENDOR experiments give no indication of vibronic coupling of the lowest ^{3} nπ* state with nearby ^{3}ππ* states.

Resonant coherent anti‐Stokes and Stokes Raman scattering spectra of molecular impurities at low temperatures
View Description Hide DescriptionIn the present paper, the results of the measurements of steady‐state resonant coherent anti‐Stokes Raman scattering(CARS) and coherent Stokes Raman scattering (CSRS) spectra of a tetracene impurity in the monocrystals of anthracene and a glassy matrix of polystyrene at low temperatures are presented. The inhomogeneous widths of the pure electronic lines of the samples differ over two orders of magnitude. In agreement with theory, it is shown that the widths of the spectral lines corresponding to the vibrations in both the ground and the excited electronic state of tetracene are determined by the relatively small inhomogeneous variation of the Raman frequencies and not by the much larger inhomogeneous variation of pure electronic or vibronic transition frequencies. The CARS excitation profiles reveal a shift with respect to the absorption band and hole–shape peculiarities that could be explained as the result of interference of different light waves in the sample. In the methodological part of the paper, some restrictions to the experimental possibilities determined by the resonant character of excitation have been outlined.

Optical–optical double resonance multiphoton ionization spectroscopy of ammonia‐d _{3}. II. Jahn–Teller effect and related Fermi resonance of the B̃ ^{1} E‘ state
View Description Hide DescriptionBy selecting a number of resonantÃ 2^{1} rotational lines in the optical–optical double resonance multiphoton ionizationspectroscopy (OODR‐MPI) of ammonia‐d _{3}, we have obtained rotationally resolved spectra of the ND_{3} B̃←Ã 2^{1}←X̃ 0^{0} transitions in the range of 550–730 nm, in which not only were we able to reanalyze rotationally the previously studied bands [J. Opt. Soc. Am. B 7, 1884(1990)], but also to assign rotationally and vibronically some novel spectra pertaining to the ν_{1}, ν_{3}, ν_{4}, and ν_{3}+ν_{4} excitations. Based on the symmetry assignment and the relationship of the vibronic coupling level energies between the single and dual mode cases, we have found that the Jahn–Teller splitting of the ND_{3} B̃ state invokes a novel type of the Fermi resonance, t h e n o n a d i a b a t i c F e r m i r e s o n a n c e. By taking account of this Fermi resonance, the modified vibronic coupling parameters of the ν_{3} and ν_{4} modes were calculated to be λ_{3}=0.047 and λ_{4}=0.023, which agrees well with the previous theoretical expectation that the ammonia B̃ state is subject to a mild dynamic Jahn–Teller effect. At present, four pairs of the nonadiabatic Fermi resonance have been discovered among the ND_{3}nonadiabatic levels, which can be separated into two types—one with the same selection rules as a conventional Fermi resonance, i.e., B̃ 3^{1}(1/2)–B̃ 4^{2}(5/2), B̃ 1^{1}–B̃ 4^{2}(1/2), and B̃ 3^{1}(3/2)–B̃ 4^{2}(3/2), the last of which was studied quantitatively with a coupling parameter K=114, and the other with novel selection rules, i.e., B̃ 3^{1}(1/2)–B̃ 1^{1}.

Core x‐ray photoelectron shake‐up states of model molecules for polyaniline
View Description Hide DescriptionThe shake‐up features of the N _{1s } and C _{1s } core‐level x‐ray photoelectron spectra of model molecules of polyaniline are studied theoretically using the intermediate neglect of differential overlap method in combination with configuration interaction. The calculations are performed on N,N’‐diphenyl‐1,4‐phenylenediamine and N,N’‐diphenyl‐1,4‐benzo‐quinondiimine, which are model molecules for fully reduced and fully oxidizedpolyaniline, respectively. The core‐level spectra of these two molecules are also studied experimentally by means of x‐ray photoelectron spectroscopy. Experimental and theoretical results are found to be in excellent agreement, which allows for a detailed interpretation of the observed shake‐up spectra. Core ionization of imine nitrogens is shown to be accompanied by large shake‐up intensity. Our studies of model molecules are compared with recently published core‐level spectra of polyaniline at various oxidation levels. The increase in imine nitrogen content upon oxidation of the polymer give rise to an increase in the N _{1s } shake‐up intensity, a result which is in agreement with what we find for the model molecules.

Theoretical interpretation of acetone–HF infrared spectrum in the gas phase
View Description Hide DescriptionThe substructure of the infrared spectrum for acetone‐HF hydrogen‐bonded complex has been reconstructed for the first time using a b i n i t i o calculations at the self‐consistent‐field approximation. Vibrational calculations of the ν_{FH} and ν_{F⋅⋅⋅O} transitions taking into account the mechanical anharmonicity effects have been made using a variational method. The main shoulders of the infrared spectrum are interpreted in terms of combination transitions and hot combination transitions.

Beam‐gas study of bismuth–fluorine reactions using laser‐induced fluorescence of BiF
View Description Hide DescriptionRelative rotational and vibrational populations have been extracted by computer simulation of laser excited fluorescence spectra of BiF (X0^{+}) formed in a thermal molecular beam of bismuth reacting with molecular fluorine under single‐collision conditions. The observed rotational distribution is colder than the prior distribution calculated assuming three product fragments. The observed vibrational distribution is also cold, indicating that comparatively little energy is available to the ground state molecules probed. Since the dependence of the detected BiF (X0^{+}) product on the beamtime of flight points toward Bi_{2} as the major beam reactant, the product energy distributions presented here suggest a mechanism in which BiF (X0^{+}), Bi (^{4} S), and F (^{2} P) are formed in an end‐to‐end attachment of Bi_{2} and F_{2}. There were no indications that BiF (X0^{+}) can be readily formed by reaction of atomic bismuth.

Influence of the spatial distribution of reactive centers on diffusion controlled reactions
View Description Hide DescriptionThe influence of the spatial distribution of fixed reactive centers on the diffusion controlled kinetics of reagent particles is investigated on the basis of a mean‐field method for the reaction rate. The reaction kinetics are analyzed in systems where the reactive centers are randomly distributed on the sphere, a line, an array of lines, and other geometrical structures imbedded in two and three dimensions. Finite‐size effects influence the time dependence of the reaction rate on different time scales as a result of the competition for the diffusing particles by the fixed reactive centers.