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Chemical Physics Letters

Pressure dependence of degenerate four-wave mixing in NO and NO2: effects of population and thermal gratings

R. Fantoni, L. De Dominicis, M. Giorgi, Richard B. Williams

Published Sep 1, 1996• 14 citations
Publisher
DOI
10.1016/0009-2614(96)00796-8

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Physical Review A•May 1, 1994•7 citations
We present a combined theoretical and experimental study on the effects of different population, orientation, and alignment relaxation rates in resonant four-wave mixing (RFWM). Signal generation in RFWM can be viewed as the formation of and scattering from laser-induced population, orientation, and alignment gratings. We show that the relative contributions from the upper-state and lower-state population, orientation, and alignment gratings to the observed output signal can be changed by varying the polarizations of the three input fields. A theory is developed to account for these changes in collisional environments where the three multipole moments of the total angular momentum distribution, i.e., the population, the orientation, and the alignment, relax unequally. This theory is applied to the OH radical in an atmospheric-pressure H2/O2/He flame for which we have measured the line profiles using high-resolution degenerate and nearly degenerate four-wave mixing. We find that orientation and alignment gratings relax more rapidly than population gratings for low rotational levels of OH in the presence of He but at essentially the same rate for high rotational levels. A discussion is presented of the importance of this effect in the interpretation of RFWM experiments.
We report detailed measurements of transition line shapes of degenerate four-wave mixing in the A2Σ+(υ′ = 0) ← X2Π(υ″ = 0) band of nitric oxide. We compared spectra obtained under conditions in which either population or thermal gratings were significant. Under conditions in which population gratings were significant, spectral lines were narrower and had intensities that varied more rapidly with the absorption B coefficient than in spectra dominated by thermal-grating scattering. In both cases we found excellent agreement with calculations that used perturbation theory to describe the population-grating contributions and the squared absorption spectra to describe the thermal-grating contributions. Our results show that quantitative prediction of degenerate four-wave mixing spectra requires relatively detailed knowledge of the ratio of contributions from the two mechanisms.
Degenerate four-wave mixing in dense sodium vapor under conditions of strong pressure-induced Zeeman pumping is investigated. Thermal diffusion and its interplay with the Zeeman light shift are found to be responsible for the generation of a grating mechanism that is not sensitive to thermal washout. The effect can be observed experimentally after eliminating the influence of radiation trapping by the use of nitrogen as a buffer gas. Under these conditions large nonlinearities are produced which also influence the propagation of the signal wave via collision-aided circular birefringence.