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.

Bragg diffraction of laser light from crystalline aqueous colloids of polystyrene spheres is examined to determine crystal structure, orientation, and elasticity. A new technique using Kossel rings is described which simultaneously measures structure, lattice spacings, and crystallite orientation. The monodisperse polystyrene sphere latex dispersions crystallize into large single crystals, which, depending on sphere concentration, are either face-centered or body-centered cubic. The interparticle spacings in the crystals are many times larger than the sphere diameter (0.109 μm). The use of tunable lasers to easily determine crystal structure is described, and the technique is further illustrated by the experimental determination of the bulk modulus. The bulk modulus is a macroscopic physical constant which can be used to monitor intersphere potentials and the screening of the particle charges by electrolytes in the solution. Data are presented which suggest that crystallite orientation occurs with the closest packed sphere layers parallel to the sample cell quartz walls.

The recently developed technique of time-resolved spectroscopy with phase-locked optical pulse pairs is further explored with additional experimental data and more detailed comparison to theory. This spectroscopic method is sensitive to the overall phase evolution of an optically prepared nuclear wave packet. The phase locking scheme, demonstrated for the B←X transition of gas phase molecular iodine, is extended through the use of in-quadrature locked pulses and by examination of the dispersed fluorescence signal. The excited state population following the interaction with both pulses is detected as the resultant two-field-dependent fluorescence emission from the B state. The observed signals have periodically recurring features that result from rovibrational wave packet dynamics of the molecule on the excited state electronic potential energy curve. Quantum interference effects cause the magnitude and sign of the periodic features to be strongly modulated. The two-pulse phase-locked interferograms are interpreted with first order time-dependent perturbation theory. Excellent agreement is found between the experimental interferograms and those calculated from literature values of the parameters governing the electronic, vibrational and rotational structure of I2. A relationship between the phase-locked interferograms and the time-dependent linear susceptibility is obtained. The in-phase and in-quadrature phase-locked interferograms together provide a complete record of the optical free induction decay. Thus by combining the in-phase and in-quadrature data, we obtain the contributions to both the absorptive and dispersive linear susceptibilities arising from transitions within the pulse spectrum.

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The effects of coherent interference between resonant and nonresonant signals and the effects of absorption on multiresonant four-wave mixing spectra are investigated both theoretically and experimentally in azu-lene-doped naphthalene crystals at 2 K. Both effects can strongly alter line shapes and intensities. Line splittings and negative spectral features are demonstrated. Coherent interference, although measurable in this system, is weak. Absorption, however, plays an important role, and its effects on phase matching, peak shapes, and peak intensities are predicted theoretically and are confirmed by the experimental measurements. A new expression for the four-wave mixing is derived in terms of sample absorption coefficients and absorption cross sections, and general conditions for maximum efficiency are determined.

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This paper develops the theoretical framework for the analysis of relative peak intensities in fully resonant, three laser, coherent four-wave mixing spectra of molecular vibrational and vibronic levels. For a Franck-Condon system, the relative vibronic peak intensities are shown to scale as the square of the absorption spectrum for all resonances not associated with the normal mode selected by the fixed vibrational resonance. A large enhancement of vibronic resonances involving the selected mode is predicted for typical potential well offsets. Higher order mode coupling causes deviations from these predictions by allowing other coupled resonances to be enhanced as well. Vibrational peak intensities are similarly related to the emission spectrum. Pentacene in benzoic acid mixed crystals at 2 K are used to experimentally study the mode mixing effects. It is shown that important coupling occurs between the 747.7 and 790.8 cm-I vibronic modes of the pentacene SI electronic state. Two possible mechanisms for this coupling are suggested. This work shows that the relative intensities of vibrational and vibronic resonances in fully resonant four-wave mixing spectra can serve as a useful probe of mode coupling.

In this paper, we demonstrate the feasibility of a new family of mode-selective spectroscopies based on multiresonant nonlinear mixing. Three tunable lasers are used for fully resonant four-wave mixing so the output signal depends on the simultaneous contribution from three resonances. Mode selection Is accomplished by tuning the lasers to a particular vibrational resonance. Vibrational spectra are obtained by scanning the second resonance.The resulting spectrum shows selective enhancements of all the modes that are related to the mode selected by the first resonance. In this paper, we demonstrate the capabilities of this approach for enhancing features that are very weak In conventional spectroscopies and for separating features that are spectrally overlapped.