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

Abstract

Intrapulse quantum cascade laser spectrometers are able to produce both saturation and molecular alignment of the gas sample. This is due to the rapid sweep of the radiation through the absorption features. The intrapulse time domain spectra closely resemble those recorded in coherent optical nutation experiments. In the present paper, the frequency down-chirped technique is employed to investigate the nitrous oxide-foreign gas collisions. We have demonstrated that the measurements may be characterized by the induced polarization dominated and collision dominated measurement limits. The first of these is directly related to the time dependence of the long range collision cross sections. Among the collisional partners considered, carbon dioxide shows a very unusual behavior of rapid polarization damping, resulting in the production of symmetrical line shapes at very low gas buffer pressures. In the collision dominated regime, the pressure broadening parameters, which we have derived, are comparable at slow chirp rates, with those derived from other experimental methods. By comparing the pressure broadening coefficients of Ar, N(2), and CO(2) with those of He, making use of the chirp rate independence of the pressure broadening by helium, we have shown that at higher chirp rates there is clear evidence of the chirp-rate dependence of the pressure broadening parameters of N(2) and CO(2).

Interleaved electro-optic dual comb generation to expand bandwidth and scan rate for molecular spectroscopy and dynamics studies near 1.6 µm

“…Furthermore, chirp times from a few μs down to a few tens of ns enable sensitive background-free detection of the free induction decays. Similar to that demonstrated in the mid-IR using cw [50] and pulsed quantum cascade lasers (QCL) [51,52,53,54], here we show that AWG driven EO-DOFCs can reach the chirp rates necessary to extend these MW/THz techniques to broad bandwidths in the near-IR [55,56,21] and beyond.…”

Sub-Doppler spectra of infrared hyperfine transitions of nitric oxide using a pulse modulated quantum cascade laser: Rapid passage, free induction decay, and the ac Stark effect

“…As our effective laser linewidth is expected to be similar to that achieved by Remillard et al 16 in their Lamb Dip experiment, about 4 MHz, we would not expect to see resolved spectra. In order to resolve this conundrum we will make use of several comparators available, the analysis of the role of FID and rapid passage in analysis of the patterns in the N 2 O spectrum, 7,9 the role of the ac Stark effect (Autler-Townes effect) 3 in determining the effective hyperfine splitting, and the difference in the signals recorded using the short and long pathlength cells. First, the analysis of the signals recorded using nitrous oxide 7,9 showed that rapid-passage effects, with scan rates much faster than the collision frequency, were the most effective in inducing large oscillatory signals when the scan direction was towards line centre, as it is in the present examples.…”

“…Within a pulse of duration 1.5 μsec the laser frequency can scan over 90 GHz (∼3 cm −1 ) to lower frequencies. [7][8][9] This is known as a frequency downchirp. As the duration of the interaction time of the laser radiation with an absorbing molecule may be very short, between 2 and 6 ns, the interaction time is shorter than the time between molecular collisions.…”

“…The rapid passage line shape also becomes much more asymmetric at high gas pressure, and the emissive part may have an amplitude which is two or three times larger than the power emitted by the QC laser at that frequency. 7,9 In the chirped pulse QC laser experiments on nitrous oxide 9 it was discovered that the oscillatory signals generated by the rapid sweep show effects due to both rapid passage, as well as free induction decay (FID).…”

“…The present experiments, based on the use of a rapid (nsec) pulse-modulated (chirped) QC laser, were motivated both by some experiments using the frequency down chirp of long pulse QC lasers to study rapid passage behaviour in nitrous oxide, and acetylene, [7][8][9] experiments to measure the ac Stark effect in NO using a rapidly swept, cw 5 μm QC laser, 13 and also some observations of infrared FID and optical nutation signals made at the Pacific Northwest National Laboratory (PNNL) 14 using a pulse-modulated cw QC laser operating near 1255 cm −1 .…”