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The effects of rotational excitation on the isomerization dynamics of a model triatomic molecule are analyzed via a quantum-mechanical analysis. The model is chosen to represent approximately hydrogen cyanide undergoing a 1,2 hydrogen shift isomerization. A new method is developed which combines elements of the discrete variable representation–distributed Gaussian basis-function method with a set of parameter-dependent zeroth-order basis functions. Our results indicate that molecular rotation has a significant effect on unimolecular reaction probability, particularly when changes in the rotational quantum number produce or eliminate critical Fermi resonance interactions. [ABSTRACT FROM AUTHOR]

The preparation of highly vibrationally excited states in polyatomic molecules is analyzed by quantum mechanically modeling the vibrational quasicontinuum. Our model includes 10 000 coupled states in an energy range of 200 cm-1, grouped into ten ‘‘tiers’’ according to their first-order couplings to other states. Our results reveal that state preparation in the quasicontinuum is dominated by the presence of ‘‘intramolecular vibrational relaxation (IVR) resonance’’ states embedded in the quasicontinuum. This hypothesis is confirmed by calculations using the method of optical potentials of the resonance positions and widths. These IVR resonance states correspond to high-energy vibrational motions which are only partially localized and which relax slowly into the dense state background, and which are possibly analogous to unstable classical periodic orbits. [ABSTRACT FROM AUTHOR]

Conventional quantum mechanical studies of molecular vibrations usually neglect the effects of rotation. When a rotating molecule has high enough energy to dissociate, this approximation is generally inaccurate. In this study, the effect of molecular rotation on vibrational predissociation is investigated for a model collinear H–C–C molecule. The vibrational–rotational interaction in this simple model is solely centrifugal, which introduces additional coupling of the two stretching modes. For a large range of angular momenta J, vibrational predissociation resonance widths (which are proportional to unimolecular state-specific reaction rates) are computed quantum mechanically using the complex coordinate method. The resonance widths are found to have substantial dependence on J. The results of corresponding classical trajectory studies show that the J-dependence of the resonance widths is a purely quantum effect. The quantum results are verified by a comparison of the complex coordinate method with the R-matrix propagation method. The resonance widths given by the two procedures are found to be in very good agreement. [ABSTRACT FROM AUTHOR]

The classical dynamics of mode–mode vibrational energy transfer are investigated for a coupled Morse oscillator system by explicitly integrating Hamiltonian’s equations in action–angle coordinates. This method permit the identification of specific oscillator phase relationships which dictate the extent and timing of large-scale, nonperiodic energy transfer; we term this ‘‘impulsive energy transfer.’’ In previous classical trajectory studies, such impulsive events have been related to the dynamics of isomerization immediately prior to reaction. A qualitative understanding of the required phase relationships for impulsive energy transfer is gained via the Chirikov hindered rotor analysis, usually applied to resonant energy flow. The implications of the model are illustrated in a three mode isomerization system. [ABSTRACT FROM AUTHOR]

An accurate two mode model of HCN is used to demonstrate a technique for and the utility of using optimized linear combinations of bond modes to describe molecular vibrational states. While normal modes are exact at zero energy and bond modes describe dissociations conveniently, neither set of coordinates describe accurately the motions of most of the spectroscopically accessible molecular vibrational states of HCN. We present a newly developed method of describing molecular vibrations with an optimized set of linear combinations of bond modes. The Hamiltonian is canonically transformed to these coordinates using the transformation method. Subsequent diagonalization of the rotated Hamiltonian matrix gives the unaltered energies and a much improved description of the molecular vibrations as measured by the projection of the eigenstates onto the zero order states. This technique improves spectroscopic assignments and simplifies visualization of the quantum vibrational motions of molecules. This is illustrated by resolving an apparent intensity anomaly in the overtone spectrum of HCN. [ABSTRACT FROM AUTHOR]

The isomerization dynamics of a model of vibrationally excited trans-diimide are reported. Energy transfer into the torsional reaction coordinate does not proceed through the resonance processes described in a companion paper. Rather, a Coriolis coupling of the NH stretches to the torsion dominates the reaction when appropriate relative phase relationships are satisfied. Large amounts of energy can be transferred in this process; thus isomerization is observed in as few as three NH stretching periods. Furthermore, the phase relationships appropriate for isomerization are very restrictive, so that a surprisingly well defined average behavior exists for several vibrational periods both prior to and after the reactive event. We have identified a specific dynamic mechanism responsible for trans/cis isomerization and for trapping the reactive trajectories in the cis-configuration. Despite the complexity of this six degrees of freedom model system, the reactive dynamics can be described in a simple manner. [ABSTRACT FROM AUTHOR]

The classical dynamics of vibrationally excited trans-diimide (N2H2) were studied, focusing on the interaction of the torsion with the local NNH bends and NH stretches. Energy flow to and from the torsion is facilitated by nonlinear resonances between the NH stretch and a bend-torsion combination mode, as has been found for other types of vibrational motions. The significant frequency ratios for such combination resonances have been predicted by a simple, analytic model and verified using a local harmonic oscillator potential surface. Several of these resonances were observed for trans-diimide. Since the dominant couplings are primarily kinetic, these resonances should also be important in the torsional vibrational dynamics of other molecules. [ABSTRACT FROM AUTHOR]

The transition state absorption spectrum and the dynamics of the excited state are calculated for a one-dimensional reactive system with the basic features of the reaction K+NaCl→KCl+Na. We have propagated a Gaussian minimum wave packet on two Eckart potential curves coupled by an intense laser field, where the bound states supported by the upper potential well are exactly solved. By applying time-dependent perturbation theory, the continuum–bound transitions are calculated, and a bound excited state is obtained when the wave packet reaches the interactive region. The transition probabilities show a strong dependence on laser frequency which is well explained by Franck–Condon calculations. By contrast, a Landau–Zener model is quantitatively incorrect. By studying the excited wave packet dynamics and the transition probability spectra at different times, we conclude that the excitation to the upper potential surface is not localized to crossings of the dressed potential curves. [ABSTRACT FROM AUTHOR]

A theoretical proposal for preparation of chemically interesting coherent superpositions of molecular eigenstates using more than one laser was recently presented [T. A. Holme and J. S. Hutchinson, Chem. Phys. Lett. 124, 181 (1986)]. In this paper, the proposed coherent two-color excitation process is applied to the separation and selective control of excitations of singlet and triplet manifolds. The method is applied to diatomic CS, using experimentally derived vibronic states and spin-orbit couplings, and realistic laser intensities. We show that frequency tuning of two lasers can yield control of branching ratios between different excited triplet manifolds. We also generalize the previous two-laser proposal to multicolor excitations, and illustrate the extension with a three-laser highly selective excitation of a singlet vibronic state. [ABSTRACT FROM AUTHOR]

The detailed processes, including state preparation, intramolecular energy transfer, and unimolecular reaction, are studied for the dissociation of a model molecule by tuning an intense laser to transitions in the vicinity of local mode overtone states. Photochemical yields are calculated quantum mechanically for a range of frequencies and pulse times, by combining the method of complex coordinates with the time-dependent interaction representation. Yields for long pulse times are dominated by the absorption cross section at each frequency, which is in turn dominated by the transition moment for the eigenstate to which the laser is tuned. As a consequence, we observe the result that more reactive states actually can produce smaller photochemical yields. With shorter pulses (0.1 to 10 ps), the yield is a sensitive function of both frequency and pulse time. By mapping out the photochemical yield as a function of both pulse time and excitation frequency, the details of the photodecay process are revealed. [ABSTRACT FROM AUTHOR]

The excitation dynamics of a polyatomic system are presented in terms of both classical and quantum dynamics. These studies show that classical mechanics is qualitatively inaccurate in its description of the overtone excitation process. The quantum excitation is therefore an example of dynamic tunneling, which like most tunneling processes is slow. In a polyatomic molecule, intramolecular vibrational relaxation occurs much faster than does excitation, by several orders of magnitude. The disparity of time scales results in the excitation being very eigenstate specific in the small to medium molecule limit. We discuss the importance of these observations to photochemical experiments (possible preparation of long-lived localized excitations) and to theoretical interpretations of molecular wave functions (possible interference effects in the molecular state function). [ABSTRACT FROM AUTHOR]

A realistic model is developed to illustrate vibrational energy flow into a reactive coordinate. The isomerization of C≡N–H to H–C≡N is enhanced nonstatistically through overtone excitations of the H–N bond. The mechanism determined for this theoretical model is dominated by resonant energy transfer from the anharmonically suppressed H–N mode into a combination of the C≡N plus the bending reaction coordinate. Implications of this classical prototype to mode specific chemistry, in particular the prediction of possible quantum tunnelling enhancement, are discussed in terms of experimentally observable systems. [ABSTRACT FROM AUTHOR]

The high energy vibrational eigenstates of cyanoacetylene are calculated with variational and perturbation theory, using a new empirical potential energy function. A detailed analysis of the eigenstates reveals that the HC bond in HCCCN is not a good local mode, even as a zeroth-order description, due predominantly to the linear geometry of the molecule. A unique anharmonic normal mode description is developed and applied to calculate accurately the eigenenergy spectrum with second-order perturbation theory. An indirect coupling scheme is presented which accounts for mixing of nearly degenerate but uncoupled high energy zeroth-order states. These results are used to account for spectral congestion in the experimental high energy spectrum of cyanoacetylene. [ABSTRACT FROM AUTHOR]