Volume 104, Issue 2, 08 January 1996
Index of content:

Magnetic circular dichroism of CD_{3}I in the vacuum ultraviolet
View Description Hide DescriptionThe magnetic circular dichroism(MCD) of CD_{3}I is reported in the 48 700–60 100 cm^{−1} region. This region embraces the 5p → 6smanifold, much of the 5p →6 pmanifold, and perhaps some members of the 5p → 5dRydbergmanifold of states. The results in the 5p →6 smanifold are confirmatory of the same work in CH_{3}I [Scott et al., J. Chem. Phys. 68, 4678 (1978)]; some questions occur concerning the conferral of absorption intensity on the highly forbidden 5p →6 s[1] state of these molecules. The MCD of part of the 5p →6 pmanifold of CD_{3}I is also reported. Five possible 5 p→6 p origins have been securely identified and a sixth has been tentatively assigned. Apart from one of these, that at 58 930 cm^{−1}, the other five origins assigned here match none of those found by MPI spectroscopy [Dobber, Buma, and de Lange, J. Chem. Phys. 99, 836 (1993)], suggesting a degree of complementarity of both data sets. Relative magnetic moments for all six observed states in the 5p → 6p region have been determined: the 58 930 cm^{−1} origin has been assigned as a ^{1}Σ^{+} state, possibly an interloper from the 5dmanifold, and the 59 630 cm^{−1} origin has been assigned as a ^{1}Π_{1} state. The 5p → 6pmanifold of CH_{3}I is also discussed and assignments, which are largely vested in correlations of energies, absorption intensities, and MCD specifics with the corresponding excitations of CD_{3}I, are given.

Deuteron spin diffusion and spin lattice relaxation in amorphous solids
View Description Hide DescriptionThe effect of spin diffusion on the decay of longitudinal magnetization in deuterated glassy systems is investigated. The decay of longitudinal magnetization was measured for different dilutions of amorphous deuterated toluene in protonated toluene. For increasing dilution the magnetization was found to decay more slowly, indicating the importance of spin diffusion also in deuteron spin systems. A strong correlation between the apparent width of the distribution of spin lattice relaxation times and the absolute value of the average spin lattice relaxation time was found. This correlation can be explained in terms of spin diffusion. A simple model is able to reproduce the main features of the observed phenomena. The initial decay rates are shown to be independent of spin diffusion. Conclusions concerning the frequency dependencies of observed average spin lattice relaxation times and width parameters are drawn.

Statistical properties of molecular spectra and molecular dynamics: Analysis of their correspondence in NO_{2} and C_{2}H^{+} _{4}
View Description Hide DescriptionCorrespondence between the spectral statistics and classical dynamics of NO_{2} and C_{2}H^{+} _{4} is investigated. Particular focus is given to the role of the lower of the two coupled adiabatic surfaces on the dynamics and level statistics, which is found to yield the main contribution to the level statistics of NO_{2} and C_{2}H^{+} _{4}. The specific features of the potential surfaces characterizing each molecule that contribute to their spectral and dynamical properties are identified. Classical dynamics simulations run on the lower adiabatic surface correspond closely with results of the statistical analysis of the energy levels on this surface: The correspondence sought between the phase space fraction that is chaotic, q _{cl}, and parameter q determined by the level statistics, is confirmed in our analysis. The classical analysis moreover reveals qualitatively distinct trends in the relation between q _{cl} and the total energy of each molecule, the cause of which is investigated in detail. Classical simulations are also carried out on the coupled‐surface potential using the Meyer–Miller analog Hamiltonian, from which comparisons with level statistics of the corresponding Hamiltonian are drawn. The effect of surface coupling is to enhance q by about 10% for NO_{2} and by about 30% for the less chaotic C_{2}H^{+} _{4}.

Size‐consistent quasiparticle representation of nonlinear optical susceptibilities in many‐electron systems
View Description Hide DescriptionThe optical response of a many‐electron system is calculated by mapping it onto a coupled set of classical oscillators representing the electron–hole pair components of the reduced single‐ electron‐density matrix. This classical representation is rigorously established using a Poisson bracket relation. Expressions for the nonlinear optical susceptibilities obtained using a Green’s‐function solution of the oscillatorequations of motion are used to analyze the size scaling of the off‐resonant response and the resonant structure of the response.

Pure vibrational spectroscopy of S _{0} formaldehyde by dispersed fluorescence
View Description Hide DescriptionDispersed fluorescence spectra from the 0_{00} rotational level of 4^{0}, 4^{1}, 5^{1}, and 3^{1}4^{1} S _{1} formaldehyde (H_{2}CO) have been recorded. From these spectra, 198 new vibrational states have been assigned with energies up to 12 500 cm^{−1}, and their positions have been determined to within an uncertainty of 1 cm^{−1}. The assignment of vibrational lines to specific vibrational states becomes increasingly difficult at the higher energy regions of the spectra (≳9000 cm^{−1}) due to extensive state mixing. Harmonic and first‐order anharmonic vibrational constants were extracted from fits to these vibrational states. For states with highest zero‐order coefficient squared greater than 35%, the standard deviation of the spectroscopic fit is 6.9 cm^{−1}. For states which are lower energy (<9500 cm^{−1}) and relatively pure (zero‐order coefficient squared greater than 0.75 or largest in a given normal mode combination), the standard deviation is 1.7 cm^{−1}. Good agreement with ab initio vibrational constants calculated by Martin et al. [J. Mol. Spectrosc. 160, 105 (1993)] is achieved, except in cases where all observed states contributing to the determination of a particular constant are significantly mixed. These deviations are readily explained by a consideration of anharmonic vibrational interactions that occur among specific combinations of normal modes. The average mean deviation between all experimentally determined energies and a recent theoretical calculation by Burleigh et al. [J. Chem. Phys. 104, 480 (1996)] is 2.6 cm^{−1}.

An accurate quartic force field for formaldehyde
View Description Hide DescriptionAn accurate quartic force field for formaldehyde is obtained by refining the ab initio quartic force field of Martin, Lee, and Taylor [J. Mol. Spectrosc. 160, 105 (1993)]. The refinement was achieved by iteratively fitting a subset of the coefficients of a Taylor‐series expansion of the potential‐energy surface to 138 of the observed transition frequencies, many of which were obtained by Bouwens et al. [J. Chem. Phys. 104, 460 (1994)] using dispersed fluorescence spectroscopy. We fit the vibrational energies (≤7600 cm^{−1}) for 138 states with an absolute mean deviation of 1.5 cm^{−1}.

Fermi resonances and local modes in stibine, SbH_{3}: A Fourier interferometric and laser photoacoustic study of the overtone spectrum
View Description Hide DescriptionThe third stretching overtone region of a natural sample of stibine, SbH_{3}, has been studied with high resolution infrared spectroscopy and the fifth and the sixth overtone region with Ti:Sapphire ring laser intracavity photoacoustic spectroscopy. The third overtone consists of a local mode pair of bands (400A _{1}/E) which have been rotationally assigned both for ^{121}SbH_{3} and ^{123}SbH_{3} with a vibration‐rotation model based on rectilinear normal coordinates. The vibrational dependencies of the model parameters are explained well with a simple block diagonal vibrational model. An extension of the standard vibration‐rotation model is used to show that the upper state rotational energy level structures of both isotopic species are close to the rotational structure of an asymmetric rotor. High resolution laser spectrum of the fifth overtone consisting of a local mode pair of bands (600A _{1}/E) shows severe perturbations in the upper state rotational structure. The (510A _{1}/E) and (700A _{1}/E) bands have been recorded with low resolution. All experimentally known vibration‐rotation band origins of ^{121}SbH_{3} have been reproduced well with a curvilinear internal valence coordinate system based Fermi resonance local mode model. The potential energy surface obtained agrees well with recent ab initio results.

Cascading amplified spontaneous emission of NO Rydberg states
View Description Hide DescriptionAmplified spontaneous emission(ASE) from single rotational levels of the Rydberg states (3dσ,π)H ^{2}Σ^{+}, H′ ^{2}Π(v=1), (4pσ)M ^{2}Σ^{+}(v=0), (4pπ)K ^{2}Π(v=1), (4dσ,π)O ^{2}Σ^{+}, O′ ^{2}Π(v=0), 4f(v=1), and (5sσ)S ^{2}Σ^{+}(v=0) of nitric oxide, populated through optical–optical double resonance excitation has been observed. The ultraviolet laser is fixed to a specific rotational component of the A ^{2}Σ^{+}(v′=0 and 1)←X ^{2}Π_{3/2}(v″=0) transitions of NO. When the second visible laser was introduced collinearly and the frequency was resonant to the transitions from the A ^{2}Σ^{+} state to the higher Rydberg states, the highly directional ASE in the near infrared region was found to be generated along the excitation laser beams.ASE excitation spectra exhibit an excellent signal‐to‐noise ratio, demonstrating the usefulness of the laser‐induced ASE as a novel spectroscopic technique for excited states of molecules. Dispersed ASEspectra revealed the cascading radiative decay from the initially populated levels down to the A ^{2}Σ^{+} state. The rotational dependence of the ASE pattern as well as the effect of a perturbation between Rydberg and valence states were briefly discussed.

Rotational diffusion measurements of suspended colloidal particles using two‐dimensional exchange nuclear magnetic resonance
View Description Hide DescriptionWe present here an experimental and theoretical study of the application of two‐dimensional exchange nuclear magnetic resonance spectroscopy(NMR) to the investigation of the rotational diffusion of colloidal particles. The theoretical discussion includes the nature of the NMR frequency time‐correlation function where the NMR interaction is represented by the chemical shiftanisotropy (CSA). Time‐correlation functions for the isotropic rotational diffusion of a suspension of colloidal particles containing single and multiple sites are derived in addition to time‐correlation functions for the rotational diffusion of a suspension of symmetric top particles containing an isotropic distribution of a single CSA interaction. Simulations of two‐dimensional exchange spectra for particles undergoing isotropic rotational diffusion are presented. We performed two‐dimensional exchange NMR experiments on a colloidalsuspension of spherical poly(methyl methacrylate) (PMMA) particles which were synthesized with a 20% enrichment in ^{13}C at the carbonyl site. Rotational diffusion time‐correlation functions determined from the experimental exchange spectra are consistent with the composition of the colloidalsuspension. Detailed explanations of the syntheses of the enriched methyl ^{13}C‐(carbonyl)‐methacrylate monomer and the small quantities of 20% enriched ^{13}C‐(carbonyl)‐poly(methyl methacrylate) microspheres used for this study are presented.

Zero‐kinetic‐energy pulsed‐field ionization spectroscopy of the a ^{1}Δ state of SH^{+} (SD^{+})
View Description Hide DescriptionThe results of a zero‐kinetic‐energy pulsed‐field ionization study on the a ^{1}Δ (v ^{+}=0) excited ionic state of SH^{+} (SD^{+}) obtained via two‐photon excitation of the [a ^{1}Δ]3dπ ^{2}Φ (v′=0) Rydberg state and subsequent one‐photon pulsed‐field ionization are reported. Accurate ionization energies as well as rotational constants are obtained. A detailed comparison between the rotational branching ratios resulting from the pulsed‐field ionization process and those of a direct ionization process is made. The results elucidate the dynamics of the high‐nRydberg states involved in pulsed‐field ionization of SH (SD).

Validity of a hybrid quantum/classical approach in photodissociation/recombination of I_{2} in rare gas matrices
View Description Hide DescriptionIn this work, we examine the validity of a hybrid quantum/classical method used to study the photodissociation/recombination dynamics of I_{2}(A) in rare gas matrices. Our simplified model includes a I_{2} molecule embedded in a linear chain of rare gas atoms (Kr or Xe). The aggregate is partitioned into a quantum system and a classical bath and their dynamics are coupled self‐consistently within the Ehrenfest framework. Two partitioning schemes are used. The first scheme treats the I–I coordinate quantum mechanically and the rare gas coordinates classically. The second and more reliable scheme includes in the quantum system both the I–I mode and the symmetric motion of the two nearest rare gas atoms. Both models show substantial energy transfer from the dissociating iodine to the solvent, followed by coherent vibrational motion in the recombined I_{2}. It is found that the one‐dimensional quantum/classical scheme is consistent with its higher dimensional counterpart, although the latter shows much faster dephasing.

Anisotropic dynamical effects on two‐dimensional potential energy surface reactions: Bond breaking electron transfer reactions
View Description Hide DescriptionThe effect of solvent and intramolecular dynamics on the rate of a bond breaking electron transferreaction is investigated. The reaction takes place on a two‐dimensional potential energy surface with one coordinate the solvent’s polarization and the other the breaking bond’s displacement. The dynamics are governed by overdamped spatial diffusion along the polarization coordinate and by energy diffusion along the bond coordinate. A scheme is presented that treats the transition from rate control by the equilibrium rate constantk _{ r } (as evaluated by, e.g., a Golden Rule calculation) to dynamical control, where the rate is controlled by diffusion on the surface, with rate constantk _{ d }, that accounts for the different character of the dynamics in the two directions. The overall rate constant has the form appropriate to a consecutive reaction mechanism: k ^{−1}=k ^{−1} _{ d }+k ^{−1} _{ r }. The k _{ d }rate constant is analyzed numerically and the results compared with a number of approximation schemes. A method of analysis is developed for situations where one dynamics is fast/slow compared with the other. Which time controls k _{ d } depends strongly on the relation between the fast diffusion rate and k _{ r }.

Short‐range approach to an A+BC collision complex with a contribution of an ionic state A ^{+}+BC ^{−}: Application to A=H
View Description Hide DescriptionAn effective quasi‐one‐electron asymptotic technique is developed for treating the electronic part of A+BC molecular interaction. The theory proceeds from a model of the motion of an electron in the field of A ^{+} and BC. The interaction between the electron and the ion A ^{+} is handled by a Green function technique while the interaction of the electron with BC is modeled by a short‐range approach. The theory provides the electronic wave functions,potential surfaces, and transitiondipole moments as functions of three nuclear coordinates specifying the geometrical configuration of the A+BC system. Calculations of potential surfaces are carried out for the case in which A is a hydrogen atom. The main advantage of the theory resides in the fact that the full structure of the potential surfaces and that of transitiondipole moments are expressed in terms of a few physical parameters. This feature, along with the ease of computation, turns the theory into a convenient tool for solving the electronic part of molecular dynamics problems for many laser assisted collisions and reactions. The theory significantly extends the method proposed earlier [V. S. Dubov, J. Chem. Phys. 97, 7342 (1992)].

Photodissociation of HOCl: A model for the prediction of the OH Λ‐doublet and Cl spin–orbit product state distributions
View Description Hide DescriptionHOCl is a closed shell system in its ground electronic state. In common with many molecules it is photodissociated to give open shell fragments. In this paper the photodissociation process HOCl(X ^{1} A ^{′})+hν→HOCl(2 ^{1} A ^{′})→OH+Cl is investigated. A model is proposed to predict the OH product spin–orbit and Λ‐doublet state distributions as well as the distribution of the Cl atom spin–orbit states. The model considers both the electronic structure of the system and the nuclear motion. Predictions are made not only of the individual fragment state distributions and of their dependence on total initial angular momentum and photon frequency, but also of the degree of correlation between the production of the different states of the two open shell fragments. The computed results agree with the available experiments, in particular the current theory is the first to agree with experimental findings in predicting a smoothly varying OH rotational state distribution. It is demonstrated that the model reproduces the frequency dependence of the absorption line shape as computed using more exact procedures. In the simple treatment presented here rotational transitions during the dissociation process are ignored, as is the detailed coupling of the electronic states in the asymptotic region. Some of the remaining disagreements between the current predictions and the experimental results are ascribed to these approximations in the model. It is hoped that the predictions of the correlated product state distributions will stimulate coincidence experiments on the photodissociation process.

Analysis of the zero‐point energy problem in classical trajectory simulations
View Description Hide DescriptionWe examine methods for dealing with the flow of zero‐point energy in classical trajectory simulations and identify some of the problems associated with their use. Fundamental issues which must be considered, both in assessing the extent of the zero‐point energy problem and in the development of useful remedies, are discussed.

Bistability in an uncatalyzed bromate oscillator in a continuously fed stirred tank reactor
View Description Hide DescriptionUncatalyzed gallic acidoscillating system has been investigated in a continuously fed stirred tank reactor (CSTR). In the [Bromate]_{0}–[Bromide]_{0} concentration space, a region has been located where a bistability is observed between an oscillatory branch and a flow branch. To our knowledge this is the first evidence of bistability in an uncatalyzed bromate oscillator. Some observations have been explained in terms of the skeleton mechanism proposed in the past.

Classical simulation of a cage effect in the dissociation of I_{2}Rg_{ n } clusters (Rg = Ar,Kr,Xe; n⩽5)
View Description Hide DescriptionThe optical dissociation of I_{2} can be markedly suppressed, if the I_{2} molecule is weakly bound to one or more rare‐gas (Rg) atoms (cage effect). A classical simulation of this process gives a fast disappearance of the cage effect and of the fluorescence intensity, as soon as the optical excitation energyE _{ω} exceeds the dissociation energy of I_{2} by a certain amount of energy, which is controlled by the binding energy of I–Rg_{ n } in the ground state. For lower E _{ω} the van der Waals potential between I_{2} and Rg_{ n } is strong enough in the asymptotic region to prevent the separating iodine atoms from dissociating. The oscillating I atoms can then transfer part of their vibrational energy to the Rg motion till the rare‐gas atoms are evaporated. This mechanism gives a fluorescencespectrum of I_{2} with a cutoff at higher vibrational energies—smeared out only by the thermal excitation of the ground‐state complex. The dependence of the spectrum on temperature and potential parameters has been investigated. At high excitation energiesE _{ω} a spectrum with an isolated peak can occur, if the van der Waals binding energy is increased or if more than one rare‐gas atom is bound to I_{2}. Other mechanisms which could result in a cage effect at higher E _{ω} require a hard collision between an I and a rare‐gas atom immediately after excitation. This is possible at high temperatures or for a linear conformation I–I–Rg. For an extended range of photon energies the simulations gave high yields of I–Rg_{ n } fragments from a I_{2}–Rg_{ n } beam.

Laser fluorescence excitation spectroscopy of BNe electronic states correlating with the excited valence B(2s2p ^{2} ^{2} D) atomic asymptote
View Description Hide DescriptionThe laser fluorescence excitation spectrum of the BNe van der Waals complex, in the vicinity of the B atom 2s2p ^{2} ^{2} D←2s ^{2}2p ^{2} P transition at 208.9 nm, is reported. A total of six partially resolved molecular bands, as well as a broad, unstructured feature to the blue of these bands, have been observed. Three BNe electronic states, denoted as C ^{2}Δ, D ^{2}Π, and E ^{2}Σ^{+}, correlate with the B(2s2p ^{2} ^{2} D)+Ne atomic asymptote, and the observed bands are assigned as (v′,0) progressions of the C ^{2}Δ−X ^{2}Π_{1/2} and D ^{2}Π–X ^{2}Π_{1/2} band systems. Rotational analysis of the C–X bands has been carried out, and spectroscopic constants characterizing the upper and lower states determined. The onset of the continuous excitation is assigned as the energy to reach the B(2s2p ^{2} ^{2} D)+Ne atomic asymptote. Identification of this threshold has allowed the determination of dissociation energies of the X, C, and D states. The observation of banded features in this wavelength range contrasts sharply with the continuous free←bound excitation in the B ^{2}Σ^{+}–X ^{2}Π_{1/2} transition, because of the purely repulsive B(2s ^{2}3s ^{2} S)–Ne interaction [X. Yang, E. Hwang, P. J. Dagdigian, M. Yang, and M. H. Alexander, J. Chem. Phys. 103, 2779 (1995)]. The differences in the binding energies of the BNe electronic states are discussed in terms of their expected electronic structures.

Pump–dump coherent control with partially coherent laser pulses
View Description Hide DescriptionThe theory of coherent control of photodissociation with partially coherent laser pulses is developed and applied to the pump–dump control scenario of a collinear model of DH_{2}. The coherence characteristics of the pump pulse are shown to be crucial for maintaining control over the product yield, whereas the coherence properties of the dump pulse are only of secondary importance. Control is shown to survive for partially coherent laser pulses, but only for a range of incoherence which precludes control with typical nanosecond pulsed dye lasers.

Multicenter molecular integrals of spherical Gaussian functions by Fourier transform convolution theorem
View Description Hide DescriptionThe two‐electron four‐center integral of the homogeneous solid spherical harmonic Gaussian‐type functions (GTF’s), r ^{2n+l } Y _{ lm }(r̂)exp(−αr ^{2}), has been evaluated analytically by decomposing it into a linear combination of two‐center integrals through coincidence of centers. The two‐electron two‐center integrals are integrated analytically through the Fourier transformation convolution theorem. A compact integration formula is obtained for a general two‐electron irregular solid spherical harmonic operator [4π/(2L+1)]^{1/2} Y _{ LM } (r̂ _{12})/r ^{(L+1)} _{12}. This formula is applied to evaluate two‐center integrals of the Coulomb repulsion, the spin–other–orbit interaction and the spin–spin interaction by letting L=0, 1, and 2, respectively. The integration results are in terms of the spherical Laguerre GTF’s, L _{ n′} ^{ l′+1/2}(σR ^{2})R ^{ l′} Y _{ l′m′} (̂)exp(−σR ^{2}), of the relative nuclear coordinate plus one error‐type F‐function term. One‐electron multicenter integrals have also been evaluated through Fourier transformation convolution theorem. These are the integrals of two and three center overlap, three‐center multipole moments r ^{ LY } _{ LM }(r̂) (e.g., L=1 and 2 for dipole and quadrupole moment, respectively), and three‐center irregular solid spherical harmonics Y _{ LM }(r̂)/r ^{(L+1)} (e.g., L=0, 1, and 2 for nuclear attraction, spin–orbit interaction, and electron spin–nuclear–spin interaction, respectively). All of the integral results are given explicitly in terms of the relative nuclear coordinates and the Gaussian exponential parameters. Explicit expressions of derivatives can be easily obtained to generate force constants, to optimize geometries and to optimize Gaussian exponential parameters.