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Notes: We introduce a novel spectroscopic technique which utilizes a two-pulse sequence of femtosecond duration phase-locked optical laser pulses to resonantly excite vibronic transitions of a molecule. In contrast with other ultrafast pump–probe methods, in this experiment a definite optical phase angle between the pulses is maintained while varying the interpulse delay with interferometric precision. For the cases of in-phase, in-quadrature, and out-of-phase pulse pairs, respectively, the optical delay is controlled to positions that are integer, integer plus one quarter, and integer plus one half multiples of the wavelength of a selected Fourier component. In analogy with a double slit optical interference experiment, the two the two pulse experiments reported herein involve the preparation and quantum interference of two nuclear wave packet amplitudes state of a molecule.These experiments are designed to be sensitive to the total phase evolution of the wave packet prepared by the initial pulse. The direct determination of wave packet phase evolution is possible because phase locking effectively transforms the interferogram to a frame which is referenced to the optical carrier frequency, thereby eliminating the high (optical) frequency modulations. This has the effect of isolating the rovibrational molecular dynamics. The phase locking scheme is demonstrated for molecular iodine. The excited state population following the passage of both pulses is detected as the resultant two-beam dependent fluorescence emission from the B state. The observed signals have periodically recurring features that result from the vibrational dynamics of the molecule on the electronically excited potential energy surface. In addition, coherent interference effects cause the magnitude and sign of the periodic features to be strongly modulated. The two-pulse phase-locked interferograms are interpreted herein by use of a simple analytic model, by first order perturbation theory and by quantum mechanical wave packet calculations. We find the form of the interferogram to be determined by the ground state level from which the amplitude originates, the deviation from impulsive preparation of the wave packet due to nonzero pulse duration, the frequency and anharmonicity of the target vibrational levels in the B state, and the detuning of the phase-locked frequency from resonance. The dependence of the interferogram on the phase-locked frequency and phase angle is investigated in detail.

Notes: The isomerization dynamics of cis-stilbene in the first excited singlet state were studied by the technique of fluorescence upconversion. Lifetime measurements were made with subpicosecond resolution in 2-propanol, decanol, n-hexane, and tetradecane. The cis-stilbene fluorescence decay curves are single exponential in all solvents except for decanol, where they are adequately described by a double exponential. A weak viscosity dependence of the decay times is observed in both alcohols and alkanes. These results are discussed in terms of the Bagchi, Fleming, Oxtoby theory [J. Chem. Phys. 78, 7375 (1983)] for activationless electronic relaxation in solution, and the limitations of hydrodynamic models of microscopic friction. The fluorescence decay times of α,α' dideutero cis-stilbene (cis-stilbene-D2) in tetradecane are found to be approximately 20% greater than those of cis-stilbene. A picosecond component in the fluorescence anisotropy decay measurements made in 2-propanol suggest that we are directly measuring motion along the reaction coordinate.