Clusters vibrational relaxation

Janda, K.C. and Bieler, C.R. (1990). Rotational rainbows, quantum interference, intramolecular vibrational relaxation and chemical reactions All in rare gas-halogen molecules, in Atomic and Molecular Clusters, ed. E.R. Bernstein (Elsevier, Amsterdam). 

The most important conclusions of these dynamical studies is that van der Waals clusters behave in a statistical manner and that IVR/VP kinetics are given by standard vibrational relaxation theories (Beswick and Jortner 1981 Jortner et al. 1988 Lin 1980 Mukamel and Jortner 1977) and unimolecular dissociation theories (Forst 1973 Gilbert and Smith 1990 Kelley and Bernstein 1986 Levine and Bernstein 1987 Pritchard 1984 Robinson and Holbrook 1972 Steinfeld et al. 1989). One can even arrive at a prediction for final chromophore product state distributions based on low energy chromophore modes. If rIVR tvp [4EA(Ar)i], a statistical distribution of cluster states is not achieved and vibrational population of the cluster does not reflect an internal equilibrium distribution of vibrational energy between vdW and chromophore states. If tvp rIVR, and internal vibrational equilibrium between the vibrational modes is established, and the relative intensities of the Ar = 0 torsional sequence bands of the bare chromophore following IVR/VP can be accurately calculated. A statisticsl sequential IVR/VP model readily explains the data set (i.e., rates, intensities, final product state distributions) for these clusters. 

Dr. Rohlfing s research interests include the experimental characterization of transient molecules relevant to combustion processes, linear and nonlinear laser spectroscopies, trace detection of pollutants, molecular beam and mass spectrometric studies of carbon and metal clusters, and vibrational relaxation dynamics. He is the author of approximately 50 peer-reviewed articles, holds membership in the American Chemical Society and the American Physical Society, and is a fellow of the American Association for the Advancement of Science. 

First, do dynamical correlations exist in processes involving multiple saddles, such as structural changes of macromolecules in clusters and proteins In the conventional theory, it is supposed that consecutive processes of going over saddles take place independent of one another. In other words, the system loses its memory of the past immediately, since the vibrational relaxation within a well is assumed to be much faster than the escape from it and multistep processes are conventionally assumed to be Markov processes. To the contrary, when the characteristic time scale of IVR is comparable to that of the reaction, the system can keep dynamical correlations as it goes over successive saddles. 

Pump-probe experiment is an efficient approach to detect the ultrafast processes of molecules, clusters, and dense media. The dynamics of population and coherence of the system can be theoretically described using density matrix method. In this chapter, for ultrafast processes, we choose to investigate the effect of conical intersection (Cl) on internal conversion (IC) and the theory and numerical calculations of intramolecular vibrational relaxation (IVR). Since the 1970s, the theories of vibrational relaxation have been widely studied [1-7], Until recently, the quantum chemical calculations of anharmonic coefficients of potential-energy surfaces (PESs) have become available [8-10]. In this chapter, we shall use the water dimer (H20)2 and aniline as examples to demonstrate how to apply the adiabatic approximation to calculate the rates of vibrational relaxation. 

It should be noted that our attempt to calculate vibrational relaxation for clusters and complex systems should be regarded as a preliminary attempt because the anharmonic potential function, themselves, are approximate and their performance should be carefully examined by calculating IR spectra in addition to vibrational relaxation. 

Vibrational relaxation is a sensitive probe of local stmcture and dynamics [5]. Vibrational lifetimes and absorption spectra of the asymmetric CO stretching mode (-1990 cm ) of W(CO)6 in siq)ercritical CO2 are reported as functions of solvent density and temperature [6]. Close to the critical temperature, the observables are density-independent over a 2-fold range of density. A cluster model can explain the data if small fixed-size solute-solvent clusters are formed in the range of densities around the critical density. If the size, and therefore the properties, e.g., local density and spectrum of fluctuations, are density-independent then the observables also become density-independent. Such a stmcture may form if there is a liquid-like... 

However, as a result of the loss of parent ions during the diffraction exposure by these inelastic processes, each diffraction exposure is limited to ca 10-20 s after which the trap is reloaded with ions. To summarize, each diffraction cycle is composed of ion loading, mass selection, vibrational relaxation, annealing, e"-beam exposure, ion ejection, and a second identical sequence to record a detector background. In a typical experimental run over ca 4-5 h, the diffraction pattern is collected for ca 300-400 cycles representing an average over ca 3-4 x 10 mass-selected clusters. 

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