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 pulse experiments reported herein involve the preparation and quantum interference of two nuclear wave packet amplitudes in an excited electronic 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.
In this paper we focus on the relation between the propensity of a molecular crystal to undergo chemical reactions after impact or shock and energy transfer rates. When a crystal receives a shock, low-frequency lattice vibrations (called phonons) are excited. Typical phonon frequencies are 0-200 cm-'. This energy must then be converted to bond stretch frequencies (1000-2000 cm-') before bond breaking can occur. W e derive a simple formula for the total energy transfer rate into a given vibron band in terms of the density of vibrational states and the vibron-phonon coupling. We are able to estimate the phonon upconversion rate in widely varying energetic materials such as TATB, HMX, and Pb styphnate by examining existing inelastic neutron scattering data. We find that the estimated energy transfer rates in pure unreacted material are several times greater for the sensitive explosives studied than the insensitive explosives.The initiation of a crystal upon shock is a complicated process undoubtedly depending on a host of material properties. Upon shock, low-frequency lattice vibrations called acoustic phonons are primarily excited. Before a detonation wave can begin, bonds must break. Therefore, the initial energy in the acoustic phonons must somehow be deposited into bond-stretching modes. Acoustic phonons have frequencies less than 100 cm-', whereas bond stretches have frequencies greater than 1000 cm-'. It is clear, then, that acoustic phonon energy must be upconverted to higher vibrations before detonation can occur. '-' Highfrequency vibrations in a molecular crystal are called vibrons. Dlott and Fayer9.10 have studied multiphonon upconversion associated with shock-induced chemistry. They derived a master equation for phonon and vibron temperatures. The master equation depends on the phonon-vibron energy transfer rate, which was estimated from experimental results on anthracene. Kim and Dlott11,12 have studied multiphonon up-pumping in naphthalene through molecular dynamics and the master equation approach. There has been much work on deriving theoretical expressions for phonon lifetimes in anharmonic solid^.'^-'^ Califano et al. compared the theoretical expressions to the results of molecular dynamics calculations and experiment~.'~-'' Holian19 has conducted molecular dynamics simulations of vibrational energy transfer in diatomic fluids, with the aim of understanding shock-induced chemistry.In the present paper, we address the question of how the phonon-vibron energy transfer rate differs in a wide range of energetic materials. Our ultimate goal is to understand how phonon dynamics controls chemical reactivity. As mentioned above, a chemical reaction is the end result of a long chain of events. Phonon energy must be upconverted to higher frequencies (vibrons), and then vibron energy must be localized in a particular molecule. Since defects in solids (often micron-sized voids) produce localized regions of high temperature, we expect that a treatment of defects (defects which lead to enhanced ignition are call...
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