Browsing by Author "Sun, Zhigang"

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    Analysis of femtosecond stimulated Raman spectroscopy of excited-state evolution in bacteriorhodopsin
    (2010) Niu, Kai; Zhao, Bin; Sun, Zhigang; Lee, Soo-Y
    The dispersive lines observed in time-resolved femtosecond stimulated Raman spectroscopy (FSRS), using a pair of 809 nm, 3 ps Raman pump, and 840-960 nm ultrashort probe pulse, for the first 500 fs photoisomerization dynamics in the excited state of bacteriorhodopsin, BR* (S-1), created by a prior 500 nm, 35 fs actinic pump pulse, have previously been attributed to Raman initiated by nonlinear emission (RINE). We used four-wave mixing energy level diagrams to describe the FSRS process, which include RINE as a subset, and a 29-mode harmonic oscillator model for BR568 in the calculations. Our calculations showed that FSRS of BR* effectively occurs from the ground vibrational state of each of the observed 800-1800 cm(-1) modes of S-1. The lifetime on S-1 determines the linewidth and decay of the dispersive lines, and is estimated to be similar to 600 fs, comparable to the stimulated emission decay time. The FSRS dipole couplings are from the ground vibrational state of S-1 to high energy vibrational states on BR (S-0), and we place a fast decay lifetime of similar to 100 fs on S-0 which can be attributed to the correlation function from the many unobserved low frequency modes. The FSRS dispersive lines are shown to be due to the inverse Raman scattering term with vertical bar 0 >< 1 vertical bar vibrational coherence on the S-1 surface, and are not due to RINE with vibrational coherence on the S-0 surface. Our calculations show that the RINE process gives rise to broad featureless spectra. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3330818]
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    Extraction of state-to-state reactive scattering attributes from wave packet in reactant Jacobi coordinates
    (2010) Sun, Zhigang; Guo, Hua; Zhang, Dong H
    The S-matrix for a scattering system provides the most detailed information about the dynamics. In this work, we discuss the calculation of S-matrix elements for the A+BC -> AB+C, AC+B type reaction. Two methods for extracting S-matrix elements from a single wave packet in reactant Jacobi coordinates are reviewed and compared. Both methods are capable of extracting the state-to-state attributes for both product channels from a single wave packet propagation. It is shown through the examples of H+HD, Cl+H-2, and H+HCl reactions that such reactant coordinate based methods are easy to implement, numerically efficient, and accurate. Additional efficiency can be gained by the use of a L-shaped grid with two-dimensional fast Fourier transform. (C) 2010 American Institute of Physics. [doi:10.1063/1.3328109]