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.

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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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The heterogeneity of the decay of the fluorescence of transparinaric acid in single-component lipid bilayers at temperatures above their gel/liquid crystalline phase transition is shown to be due to the presence of regions of higher local density and higher acyl chain order than the predominant fluid regions. This conclusion is based on selective excitation behavior and the observation of time-resolved fluorescence anisotropies that increase at long times. The fractional amplitude of the long lifetime component of the fluorescence shows a temperature variation that conforms to conventional descriptions of critical behavior. The critical exponent extracted from this variation is 1.1, close to the value of 1.0 that describes ultrasonic data. We therefore conclude that liquid crystalline lipid bilayers exhibit critical behavior with significant density and order fluctuations. This behavior must be taken into account in the interpretation of fluorescence and other spectroscopic measurements of the properties of bilayers.

Fluorescence upconversion experiments with subpicosecond resolution are described. The time dependent fluorescence spectra of several probe molecules In polar solvents are measured and used to construct the Stokes shift correlation function C(I). The correlation functions decay nonexponentially in contrast to the predictions of simple Debye/Onsager continuum theory. At high values of the static dielectric onstants the solv~tion times are much slo~er th.an predicted from standard continuum theory. Generalizations of continuum theory to mclude non-spherical shapes, non~Debye ?l~lectrIc response and saturation effects are discussed. The experimental data are qualitatively In accord With predictions based on a frequency and position dependent dielectric response.

An analysis is presented of the complex anisotropy behavior of trans-parinaric acid in single component DEPC lipid bilayers. It is shown that a model involving two species with distinct lifetime and motional behavior is required, and is adequate, to explain the observed data. In particular, the observed increase in the anisotropy at long times demonstrates the presence of a species with a long fluorescence lifetime that has a high anisotropy. The time dependence of the anisotropy for these two environments is treated using both a purely mathematical sum of exponentials and a constrained fit based on an approximate solution of the anisotropic diffusion problem. In this latter model the anisotropy is described in terms of the second and fourth rank order parameters, (P2) and (P4), and a single dynamical parameter, D1, the perpendicular diffusion coefficient for this uniaxial probe. The parameters of both models are accurately determined from the fits to the data when two environments coexist and an association is made between lifetime components and distinct rotational sites. The values of the parameters obtained demonstrate the "solid-like" and "fluidlike" nature of these two coexisting environments.