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Volume 103, Issue 14, 08 October 1995

Emission spectroscopy of the predissociative excited state dynamics of acrolein, acrylic acid, and acryloyl chloride at 199 nm
View Description Hide DescriptionThe emission spectroscopy of acrolein (C_{3}H_{4}O), acrylic acid (C_{2}H_{3}COOH), and acryloyl chloride (C_{2}H_{3}COCl) excited at 199 nm elucidates the dominant electronic character of the excited state reached by the optical transition at this wavelength. Progressions in the C=C and C=O stretching overtones and various combination bands suggest the antibonding orbital has mixed π*(C=C)/π*(C=O) character. We interpret the results in conjunction with ab initio calculations at the configuration interaction singles level to identify the influence of resonance in the excited state of these conjugated molecules. The results on acrylic acid are of particular interest as excitation in this absorption band produces the HOCO intermediate of the OH+CO→H+CO_{2}reaction that is important in combustion.

Radiation damping in high resolution liquid NMR: A simulation study
View Description Hide DescriptionRadiation damping generates surprising new features in two‐dimensional and spin–echo experiments. The theoretical interpretation of the unexpected results has led to some controversy. The purpose of the present paper is to show, by computer simulations, that the experimental results can be completely understood by describing radiation damping through a classical magnetic field caused by the current induced in the sample coil(resonantly enhanced by the high quality factor of the tuned circuit), and acting back on the spins. We examine a variety of two‐dimensional and ‘‘multiple quantum’’ experiments in homogeneous fields, and spin‐echo experiments in a fixed field gradient (with and without molecular diffusion).

The 2 ^{3}Π_{ g } and 3 ^{3}Π_{ g } states of ^{7}Li_{2}: Optical–optical double resonance spectroscopy and ab initio calculations
View Description Hide DescriptionThe (2p+2p) 2 ^{3}Π_{ g } and (2s+3p) 3 ^{3}Π_{ g } states of ^{7}Li_{2} have been studied both experimentally and theoretically. Vibrational levels v=0–41 of the 2 ^{3}Π_{ g } state and v=6–10 of the 3 ^{3}Π_{ g } state have been observed by perturbation facilitated optical–optical double resonance (PFOODR) spectroscopy. Our ab initio calculations show that the 2 ^{3}Π_{ g } state, although dissociating into 2p+2p atomic limit, is a Rydberg state and strongly mixed with the (2s+3p) 3 ^{3}Π_{ g } and (2s+3d) 4 ^{3}Π_{ g }Rydberg states. Our theoretical calculations show good agreement with our experimental results.

Spectral signatures of critical transition structures derived from temperature‐dependent IR spectra
View Description Hide DescriptionWe describe a modified configuration of the temperature drift (TD) technique for recording infrared spectra that arise from vibrationally excited states. TD is a differential measurement that records decrements in IR absorptions due to modest increases in temperature. In particular, it permits identification of oscillators that are coupled to large amplitude, low frequency motions associated with transition structures, such as transitions to free rotations about single bonds that are hindered at ambient temperatures. Selected portions of the spectra of two species were recorded to demonstrate the feasibility of locating spectral features of critical transition structures.

The resonance Raman spectrum of cyclobutene
View Description Hide DescriptionThe vibronic pattern of the resonance Raman spectrum of cyclobutene is simulated by ab initio molecular orbital and by density functional theory calculations. Both Franck–Condon and Herzberg–Teller contributions are included in the analysis of the spectrum. The results suggest an initial dynamics of cyclobutene in the S _{1}excited state in which the molecule attempts a cis–transisomerization of the ethylene moiety.

Laser‐induced fluorescence spectroscopy of CCH (X̃ ^{2}Σ^{+}) in vibrationally excited levels up to 4500 cm^{−1}
View Description Hide DescriptionVibrationally excited levels of the CCH radical in the X̃ ^{2}Σ^{+} state up to 4500 cm^{−1} above the ground vibrational level have been studied both at high pressure and in a supersonic jet by laser‐induced fluorescence(LIF). The CCH radical was produced by photolyzing acetylene with 193 nm laser light. Nineteen UV bands are rotationally analyzed and assigned as transitions of parallel type from X̃(0,υ^{ l } _{2},υ_{3}), (υ_{2}=0–10, υ_{3}=0–2, l=0–3) to four K sublevels (K=0–3) of a common vibrational level, T, of an upper electronic state most likely B̃ ^{2} A′. Among them, bands at 37 010 and 36 075 cm^{−1} were previously reported [J. Chem. Phys. 98, 6690 (1993)]; an improved spin–orbit constant of X̃(0,7^{1},0) was determined in this work. Vibrational energies of the l=0 and 1 levels were determined in a simultaneous analysis of UV and IR bands. Assignment of a vibrational level T as the common upper state of the observed LIF bands permits us to determine spectroscopic parameters of the l=2 and 3 levels of the X̃ state, which are reported for the first time. The geometry of level T at 39 157.41 cm^{−1} is accordingly deduced and agrees with the ab initio prediction of the B̃ state. Our vibrational assignments of the X̃ state, particularly those of bending progressions, are supported by observations of the l=2 and 3 levels.

Overtone spectroscopy in nitrous oxide
View Description Hide DescriptionThe near infrared and visible absorptionspectrum of nitrous oxide^{14}N_{2} ^{16}O has been recorded by Fourier transform absorption spectroscopy, between 6500 and 11 000 cm^{−1}, and by Intracavity Laser Absorption Spectroscopy, between 11 700 and 15 000 cm^{−1}. Nineteen new bands are observed and, altogether, 34 cold and 10 hot bands are rotationally analyzed. The related upper term values, vibrational assignments, and principal rotational constants, as well as the relative band intensities are quantitatively discussed in terms of the formation of vibrational clusters, on the basis of the effective Hamiltonian developed by J. L. Teffo, V. I. Perevalov and O. M. Lyulin [J. Mol. Spectrosc. 168, 390 (1994)].

Theory of coherent transient spectroscopy in molecular aggregates: The effects of interacting excitons
View Description Hide DescriptionUsing a density matrix formalism we derive a general expression for the resonant response of an ensemble of molecular aggregates to two optical pulses. The aggregates, which can be of arbitrary geometry, are described by a Frenkel exciton Hamiltonian including exciton–exciton interactions. A hierarchy of resonant coherent transient signals is generated having wave vectors s k _{1}+(1−s)k _{2}, where k _{ i } is the wave vector of the ith pulse and s is an integer. General expressions for the differential pump–probe absorptionspectrumD(ω;t _{ D }) (s=0) and the photon echo I _{echo}(t _{ D }) (s=−1) are presented for arbitrary pulse intensities. Applications are made to a linear chain of interactingFrenkel excitons with nearest neighbor exciton–exciton interactionA and exciton transfer J. Analytical expressions for D(ω;t _{ D }) and I _{echo}(t _{ D }) are obtained which are valid to third order in the aggregate‐field interaction. The influence of the optically active red (blue) biexciton which detaches from the bottom (top) of the two‐exciton band when A<−2‖J‖ (A≳2‖J‖) is investigated. Biexcitons appear as extra peaks in D(ω;t _{ D }) and as oscillations in I _{echo}(t _{ D }). Finite size effects are responsible for quantum beat oscillations in D(ω;t _{ D }) and long time oscillations in the echo decay. Quantum beats arise predominantly from the difference between the lowest one‐exciton frequencies. Our theory successfully describes the salient spectral features in the pump–probe spectra of J‐aggregates.

Induction of optical transitions through complexation within Hg–rare gas van der Waals systems
View Description Hide DescriptionThe high repulsive states of HgAr and HgNe van der Waals complexes, correlating with Hg 6s6d atomic states have been investigated by double resonance spectroscopy, through the first excited stateA ^{3}0^{+} and B ^{3}1 of the complexes. The repulsive potentials have been fitted through numerical Franck–Condon simulations. They have been characterized by perturbative calculation as quasi‐pure 6dΣ potentials in Hund’s case a. The strong Hg–rare gas electrostatic interaction potential overruns the spin–orbit interaction at distances shorter than 7 Å. These observed repulsive states are mostly of Ω=1 character correlating with ^{3} D _{3} at infinite distances. The contribution from the potential of Ω=0^{−} symmetry correlating with ^{1} D _{2} is of minor importance. Therefore, the absorption in the repulsive states of the complex arises mostly from proximity induced absorption in an optically forbidden transition ^{3} P _{1}→^{3} D _{3}. A perturbative model accounts well for the bound free absorption intensities experimentally observed.

High resolution electronic spectroscopy of MgCH_{3}
View Description Hide DescriptionThe MgCH_{3} radical was produced by a laser ablation/photolysis technique in a cold supersonic free‐jet expansion and probed by laser induced fluorescence. Rotationally resolved spectra for both spin‐orbit components of the Ã ^{2} E←X̃ ^{2} A _{1} electronic transition have been recorded. The analysis of these spectra yields the rotational constants of MgCH_{3} and therefrom a structure for the radical is proposed. A comparison is made among a series of alkyl organometallic radicals.

Spectroscopy and intramolecular dynamics via molecular vibrogram analysis
View Description Hide DescriptionMolecular spectra have traditionally been interpreted in the frequency domain and more recently in the time domain. For simple absorption and emission spectra these are the Fourier transform of one another. In this paper we illustrate that it can be useful to examine spectra in a compromise representation which balances uncertainties in time and energy. In this paper we give the basic ideas and several examples which reveal dynamical effects hidden in either energy or time representations. The resulting ‘‘vibrogram’’ is available from experimental spectra or theoretical calculations.

Complex angular momentum analysis of resonance scattering in the Cl+HCl→ClH+Cl reaction
View Description Hide DescriptionResonance effects in the differential cross sections of the Cl+HCl(v,j)→ClH(v′,j′)+Cl reaction are analyzed using Regge pole and complex angular momentum (CAM) techniques. This is the first detailed application of CAM theory to reactive molecular scattering. The rovibrational transitions studied are v=1, j=5→v′=0, j′=15, and v=1, j=5→v′=1, j′=5 at total energies E=0.66, 0.68, 0.70 eV. The CAM theory expresses the scattering amplitude as a background subamplitude plus a pole subamplitude. The uniform (and nonuniform) semiclassical evaluation of the background subamplitude is discussed. It is necessary to include explicitly the resonance Regge pole in the semiclassical theory because it has a small imaginary part. We derive a new generic semiclassical formula, involving the complementary error function for the resonance angular scattering. The position and residue of the resonance Regge pole at each E are extracted numerically from scattering matrix elements calculated by the centrifugal sudden hyperspherical (CSH) quantum scattering method. There is good agreement between the semiclassical CAM and CSH angular distributions. However, the latter involve summing a partial wave (PW) series with a large number of numerically significant terms—as a result the PW computations provide no physical insight. We also show that a simple semiclassical optical model becomes inaccurate when the rotational period of the ClHCl complex is comparable to the resonance lifetime. We derive a new ‘‘sticky’’ optical model which allows for rotation of the complex. All our calculations use the Bondi–Connor–Manz–Römelt semiempiricalpotential energy surface.

A classical path/forced rotor theory of state‐to‐state rotational energy transfer
View Description Hide DescriptionThe problem of rotational energy transfer (RET) is examined from a point of view intermediate between the current approaches based on empirical models or numerical solution of the coupled scattering equations. A semiclassical perspective is taken in which a classically described collision with an atom exerts a time dependent torque on the rotor and the resulting forced rotor dynamics is addressed quantum mechanically. By treating the anharmonicity in the rotational energy levels as a perturbation, a simple approximate expression is found for the inelastic transition probability. It reveals a marked difference between the distribution of final rotational states incurred from an individual collision trajectory as compared to trajectory averaged measures of RET, such as cross sections. The theory is applied to the scattering of Li_{2}(A ^{1}Σ^{+} _{ u }) by Ne, Ar, Xe;Na_{2}(A ^{1}Σ^{+} _{ u }) by He and Ne; N^{+} _{2}(X ^{2}Σ^{+} _{ g }) by He, and CN(X ^{2}Σ^{+}) by He. Its predictions compare well with those from a fully quantum mechanical description of rigid rotor scattering and with experiment. The insight into the energy transfer dynamics gained from the semiclassical approach is used to examine the assumptions underlying empirical models of rotational energy transfer.

Phase space distribution function formulation of the method of reactive flux: Memory friction
View Description Hide DescriptionThe Grote–Hynes transmission coefficient for the rate of barrier crossing in the presence of memory friction is rederived here using the method of reactive flux. By combining the methodology developed in an earlier paper [D. J. Tannor and D. Kohen, J. Chem. Phys. 100, 4932 (1994)] with the non‐Markovian Fokker–Planck equation of Adelman [S. Adelman, J. Chem. Phys. 64, 124 (1976)] we are able to obtain not only the asymptotic rate constant but the behavior of the rate constant at all times. The salient features of the time dependent rate constant,k(t), are interpreted in terms of the time evolution of the representative distribution functions that originate at the top of the barrier. The short time behavior of the rate constant is very different in the dynamic and static limits, with close analogies to the stochastic theory of spectral line shapes. The dependence of the ‘‘plateau time’’—the time for the rate constant to reach its steady state value—on the memory kernel is explored numerically, and analytical expressions are obtained in the dynamic and static limits.

Overcoming the zero‐point dilemma in quasiclassical trajectories: (He,H^{+} _{2}) as a test case
View Description Hide DescriptionWe present a new technique for circumventing the problem of the zero‐point leak in classical trajectories by extending the action‐billiard approach of de Aguiar and Ozorio de Almeida [Nonlinearity 5, 523 (1992)]. In addition to demonstrating its utility in a model problem, we examine the application of various methods of overcoming the zero‐point leak in the case of collinear He+H^{+} _{2} collisions. We also show that not neglecting leaky trajectories gives, on an average, good agreement with quantal results for collinear as well as 3‐dimensional collisions.

Three‐color triple resonance spectroscopy of highly excited ng Rydberg states of NO: Decay dynamics of high‐l Rydberg states
View Description Hide DescriptionWith three‐color triple resonance excitation, rotational‐state‐resolved laser multiphonon ionization spectra of highly excited ngRydberg states (n=11–67, v=0 and 1) of NO have been observed. Determining the term values of the observed states, we have found that those values are in good agreement with the theoretical values calculated on the basis of the long range force model. Through detailed analysis of the spectral intensity distributions, we have proved, for the first time, the theoretical prediction that even in gRydberg states there are efficient vibrational and rotational autoionization processes much faster than the radiative decay. In addition, we have also found the presence of non‐negligible predissociation processes competing with the autoionization processes, roughly estimating both predissociation and autoionization rates.

Excitons in solid argon
View Description Hide DescriptionA model for consistently describing the entire Ar exciton series is presented. Making use of a reliable approximation of the band structure in the whole first Brillouin zone, as provided by appropriately chosen Slater–Koster parameters, is an essential part of our approach. Its dominating effect on the final results is demonstrated by performing comparative studies based on several simpler band models. In fact, the energy defect of the n=1 exciton can to a large extent be understood as a consequence of the large‐k variation of the bands. Our model, furthermore, makes allowance for the spin–orbit coupling of the valence bands, the exchange interaction and the dielectric function of the electron‐hole potential. Without using any free parameter, the calculated Ar exciton series is in excellent agreement with the experimental binding energies.

Vibrational frequencies of the HF dimer from the coupled cluster method including all single and double excitations plus perturbative connected triple excitations
View Description Hide DescriptionDissociation energies, equilibrium structures, equilibrium dipole moments, harmonic vibrational frequencies, infrared (IR) intensities, and zero‐point vibrational energies (ZPVEs) are presented for the hydrogen fluoride monomer and dimer at the coupled cluster with single and double excitations plus perturbative connected triple excitations [CCSD(T)] level of theory with a series of large basis sets. Discussion focuses on the results from the largest basis set, triple zeta plus double polarization and one set of higher angular momentum functions [TZ2P(f,d)]. The structural parameters and dissociation energy agree well with the recent theoretical values reported in a high level ab initio study by Peterson and Dunning [J. Chem. Phys. 102, 2032 (1995)] and with experimental results. Here, the harmonic vibrational frequencies and IR intensities for the dimer are also predicted. The predicted vibrational frequency shifts relative to the monomer for the HF and DF stretching coordinates in the dimer are close to the experimental estimates.

A novel method to calculate eigenfunctions and eigenvalues in a given energy range
View Description Hide DescriptionA new method to calculate eigenfunctions and eigenvalues in a given energy range is proposed, which can therefore be applied to highly excited states of electronic and/or vibrational states of a molecule. The spectral components of a wave packet that lie outside the energy range are projected out through the time evolution; that is, the packet is screened onto the energy range. If the range includes only a single root, the corresponding eigenfunction is screened first, and the eigenvalue follows as its expectation value. For a case where there is more than a single root, several methods can be figured out. One typical and effective procedure is to construct local basis functions in terms of the aforementioned energy screened wave packets to represent the Hamiltonian in them and to diagonalize it. The concept to construct a local basis was originally developed by Neuhauser [J. Chem. Phys. 93, 2611 (1990)]. The present method performs it in a more efficient and theoretically satisfactory way.

Molecular orbital coefficients and transition dipoles of real polyenes
View Description Hide DescriptionA simple four configuration model that quantitatively reproduces all of the 1^{1} B _{ u } and 2^{1} A _{ g } state 0‐0 energies that have been measured in high resolution spectroscopicexperiments has been described previously. This model has been useful for explaining trends in the electronic properties of series of unsubstituted and substituted linear polyenes. While this model led to analytical expressions for the excitation energies, there were no closed form expressions for other quantities, such as the transition dipoles, that depend explicitly on the coefficients of the one‐electron molecular orbitals. This paper derives exact expressions for the one‐electron molecular orbital coefficients for an alternating chain as well as exact expressions for the transitiondipole moments. This facilitates a detailed examination of the dependence of the transitiondipole moments on the polyene length and alternation parameter which leads to an expression that accurately describes the dependence of the transitiondipole moments on these chain parameters. The application of these expressions to an analytical analysis of nonlinear response in linear polyenes will be the subject of a subsequent paper.