Rapid passage induced population transfer and coherences in the 8 micron spectrum of nitrous oxide

Abstract

Rapid passage signals showing the effects of molecular alignment have been observed when low pressure samples of nitrous oxide are interrogated by radiation from a pulsed 7.84 µm quantum cascade laser. These effects occur when the sweep rate of the laser through a Doppler broadened absorption line is much faster than the collisional relaxation time, and when the power density of the linearly polarized laser radiation is sufficient to cause optical pumping. Using a laser pulse of duration 1.3 µs, the frequency sweeps approximately 90 GHz. The variation of the laser tuning rate during the laser pulse, from about100 MHz/ns at the beginning to about 20 MHz/ns at the end, allows the relationship between sweep rate and collisional damping to be investigated. It is shown, by comparing the experimental signals with those calculated by coupled Maxwell–Bloch equations, how the rapid passage effects in nitrous oxide are influenced by the number density, transition cross-section and reorientation lifetime.