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

Abstract

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 polarised laser radiation is sufficient to cause optical pumping. Using a laser pulse of duration 1.3 microseconds, 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. We show, by comparing the experimental signals with those calculated by coupled MaxwellBloch equations, how the rapid passage effects in nitrous oxide are influenced by the number density, transition cross-section and reorientation lifetime.

Supersonic free-jet quantum cascade laser measurements of ν4 for CF335Cl and CF337Cl and FTS measurements from 400 to 1260cm−1

“…It is not an electronic time constant issue, but appears to be a real swept-gain seen when the laser frequency is slewed at a sufficient rate over strongly absorbing lines with optical densities $1 and K divisible by three. Duxbury and Langford have interpreted these effects as a swept gain effect that occurs if the laser frequency is quickly slewed so that adiabatic rapid passage (ARP) can occur leading to excited state populations in select velocity sub-groups that can support coherent re-emission [38]. Their Maxwell-Bloch propagation models appear to reconcile nicely with their data taken with more intense pulsed QCLs.…”

Time dependent measurements of nitrous oxide and carbon dioxide collisional relaxation processes by a frequency down-chirped quantum cascade laser: Rapid passage signals and the time dependence of collisional processes

“…4 The recorded time domain spectra are analogous to the optical nutation signals observed in the coherent transient experiments, and similarly the effects observed experimentally can modeled by solving the coupled Maxwell-Bloch equations. [1][2][3][4] In the previous paper, 5 on a Dicke-narrowed doublet of the 2 band of water, and in a survey paper, 6 we have shown that the time dependence of the collisional broadening may be investigated by the use of the frequency down-chirped electromagnetic radiation emitted by a long pulse duration QCL.…”

“…Over the past 7 years, the frequency down-chirped radiation emitted by a quantum cascade laser ͑QCL͒, used in the intrapulse 1 method, has been demonstrated to produce both saturation 2 and also molecular alignment 3 and transient gain. 4 The recorded time domain spectra are analogous to the optical nutation signals observed in the coherent transient experiments, and similarly the effects observed experimentally can modeled by solving the coupled Maxwell-Bloch equations.…”

“…The resulting rapid passage signals observed in pure nitrous oxide, and with nitrogen as a collision partner, have been previously described and analyzed. 3 One of the most interesting collision partners we have studied, carbon dioxide, is a linear molecule of nearly the same mass and similar rotational constants as nitrous oxide. However, it shows a completely different behavior from the other collision partners, inducing very efficient broadening of the absorption lines of nitrous oxide and causing symmetrical line shapes to be observed at low partial pressures of carbon dioxide.…”

Non-symmetrical line broadening effects using short-pulse QCL spectrometers as determined with sub-nanosecond time-resolution

“…While a theoretical description for the non-linear absorption phenomena based on optical Bloch equations exists [25,26,28], the origin of distorted line shapes in the short-pulse mode is not yet completely understood [9]. The problem is often empirically minimised by finding a compromise between a reasonable signal-to-noise ratio (SNR) and a narrow spectral width [12].…”