Chirp dependence of wave packet motion in oxazine 1.
ABSTRACT The motion of vibrational wave packets in the system oxazine 1 in methanol is investigated by spectrally resolved transient absorption spectroscopy. The spectral properties of the probe pulse from 600 to 700 nm were chosen to cover the overlap region where ground-state bleach and stimulated emission signals are detected. The spectral phase of the pump pulse was manipulated by a liquid crystal display based pulse-shaping setup. Chirped excitation pulses of negative and positive chirp can be used to excite vibrational modes predominantly in the ground or excited state, respectively. To distinguish the observed wave packets in oxazine 1 moving in the ground or excited state, spectrally resolved transient absorption experiments are performed for various values of the linear chirp of the pump pulses. The amplitudes of the wave packet motion show an asymmetric behavior with an optimum signal for a negative chirp of -0.75 +/- 0.2 fs/nm, which indicates that predominantly ground-state wave packets are observed.
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ABSTRACT: The ring-opening reaction of a trifluorinated indolylfulgide has been studied as a function of temperature and optical pre-excitation where it was found that reaction times decreased as temperature increased from 10.3 ps at 12 degrees C to 7.6 ps at 60 degrees C. Simultaneously, the quantum yields for the ring-opening reaction grew from 3.1% (12 degrees C) to 5.0% (60 degrees C). When the reaction was started from a non-equilibrium state generated by a directly preceding ring-closure process, the ring-opening reaction became faster and the quantum efficiency increased by more than a factor of three. Analysis of the experimental results points to mode-specific photochemistry in that the promoting, photochemically active modes of the photoreaction are efficiently excited by the directly preceding ring-closure reaction.Physical Chemistry Chemical Physics 08/2009; 11(25):5019-27. · 3.83 Impact Factor
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ABSTRACT: We calculated the dynamics of nuclear wave packets in coupled electron-vibration systems and their nonlinear optical responses. We found that the quantized nature of the vibrational modes is observed in pump-probe spectra particularly in weakly interacting electron-vibration systems such as cyanine dye molecules. Calculated results based on a harmonic potential model and molecular orbital calculations are compared with experimental results, and we also found that the material parameters regarding the geometrical structure of potential energy surfaces are directly determined by accurate measurement of time-resolved spectra.The Journal of Chemical Physics 12/2007; 127(19):194304. · 3.16 Impact Factor
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ABSTRACT: The well-documented propensity of negatively-chirped pulses to enhance resonant impulsive Raman scattering has been rationalized in terms of a one pulse pump-dump sequence which "follows" the evolution of the excited molecules and dumps them back at highly displaced configurations. The aim of this study was to extend the understanding of this effect to molecules with many displaced vibrational modes in the presence of condensed surroundings. In particular, to define an optimally chirped pulse, to investigate what exactly it "follows" and to discover how this depends on the molecule under study. To this end, linear chirp effects on vibrational coherences in poly-atomics are investigated experimentally and theoretically. Chirped pump-impulsive probe experiments are reported for Sulforhodamine-B ("Kiton Red"), Betaine-30 and Oxazine-1 in ethanol solutions with <10 fs resolution. Numerical simulations, including numerous displaced modes and electronic dephasing, are conducted to reproduce experimental results. Through semi-quantitative reproduction of experimental results in all three systems we show that the effect of group velocity dispersion (GVD) on the buildup of ground state wave-packets depends on the pulse spectrum, on the displacements of vibrational modes upon excitation, on the detuning of the excitation pulses from resonance, and on electronic dephasing rates. Akin to scenarios described for frequency-domain resonance Raman, within the small-displacement regime each mode responds to excitation chirp independently and the optimal GVD is mode-specific. Highly-displaced modes entangle the dynamics of excitation in different modes, requiring a multi-dimensional description of the response. Rapid photochemistry and ultrafast electronic dephasing narrow the window of opportunity for coherent manipulations, leading to a reduced and similar optimal chirp for different modes. Finally, non-intuitive coherent aspects of chirp "following" are predicted in the small-displacement and slow-dephasing regime, which remain to be observed in experiment.Physical Chemistry Chemical Physics 03/2010; 12(9):2149-63. · 3.83 Impact Factor