Degenerate four-wave mixing (DFWM) spectroscopy is shown to provide a facile means for probing angular momentum (or rotational) anisotropy in nonequilibrated ensembles of gaseous molecules, with judicious selection of experimental conditions permitting quantitative determination of population distributions and Zeeman coherences for magnetic sublevels of the target species. A theoretical description of the nonlinear response induced under such circumstances is obtained by incorporating a state multipole expansion of the zero-order density operator into a perturbative (weak-field) treatment for the DFWM interaction. Aside from allowing the effects of incident field polarizations and phase-matching geometries to be considered in detail, this compact spherical tensor formalism provides guidelines for the extraction of spatial information from rovibronically resolved spectral data. Furthermore, these analyses have identified unusual polarization schemes that lead to signal generation only in the presence of rotational anisotropy, thereby suggesting a new class of four-wave mixing measurements that permit the selective detection of molecular orientation and alignment.

Rotation-tunneling analysis of the origin band in the tropolone π*←π absorption system Semiclassical molecular dynamics simulations of excited state double-proton transfer in 7-azaindole dimers

A state-multipole expansion of the zero-order density operator has been used to derive analytical signal expressions for near-resonant degenerate four-wave mixing spectroscopy performed on systems that possess angular momentum anisotropy. The predictions of this formalism are found to be in good agreement with experimental optical–optical double resonance measurements involving aligned intermediate states.

The stimulated emission pumping (SEP) process has been detected by means of a newly developed zero-background scheme based upon the Doppler-free technique of degenerate four-wave mixing (DFWM) spectroscopy. Rotationally resolved SEP-DFWM spectra recorded for the (02000) vibrational level of CS2 X̃ 1∑+g exhibit signal-to-noise ratios approaching 1000:1.

The effects of finite-bandwidth excitation are incorporated into the theoretical treatment of degenerate four-wave mixing (DFWM) spectroscopy by employing a general description of applied electromagnetic fields, thereby permitting the transient nature of typical laser sources to be taken explicitly into account. A computationally efficient perturbative (weak-field) formalism is developed by exploiting exponentially-decaying wave forms as a flexible basis for expanding the temporal envelope functions of incident quasimonochromatic pulses. This approach has the distinct advantage of yielding time-domain response integrals that can be evaluated analytically since they are almost as simple in form as their monochromatic counterparts. The resulting frequency-domain expressions for the induced DFWM signal polarization reflect the pronounced influence of finite-bandwidth excitation without needing to cobble such behavior into phenomenological rates for population decay and coherence dissipation. Spectral line shapes are examined for both stationary and nonstationary ensembles of isolated (gas-phase) target molecules, with the latter analyses demonstrating the systematic degradation of spatial discrimination introduced by nonmonochromatic implementations of the sub-Doppler (phase-conjugate) four-wave mixing scheme. In keeping with recent experimental findings, the precipitous drop in signal intensity accompanying the action of collisional dephasing processes is predicted to be tempered greatly by the use of short-duration excitation pulses.