Abstract
Degenerate four-wave mixing (DFWM) has emerged in recent years as a potentially powerful technique for quantitative measurement of minor species in gas-phase media.1,2 However, a complicated collision-rate dependence of the DFWM signal is observed when lasers with pulse lengths of several nanoseconds are used.3,5 On the other hand, picosecond lasers possess pulse widths shorter than the typical collisional time in atmospheric gas-phase media, and use of such lasers for DFWM measurements6 could reduce the sensitivity of the technique to the collision rate.2,7 While pulsed lasers are usually utilized to perform gas-phase DFWM measurements, DFWM analytical models are generally limited to the assumption of continuous wave (cw) laser inputs. These cw models have been applied successfully to modeling gas-phase DFWM with nanosecond-pulse lasers, where the pulse length (~10 nsec) is much longer than the typical collisional time (~100 psec) in atmospheric-pressure flames. In this paper, we use direct numerical integration (DNI) of the density matrix to investigate theoretically the use of lasers with pulse lengths (τL) much shorter than characteristic collision times (τC) for quantitative DFWM spectroscopy in gas-phase media. Both purely homogeneously broadened resonances and resonances that are both collision- and Doppler-broadened are considered.
© 1996 Optical Society of America
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