Time-Resolved Detection of the CF₃ Photofragment Using Chirped QCL Radiation

Abstract

This paper demonstrates how a quantum cascade laser (QCL) in its intrapulse mode can provide a simple method for probing the products of a photolysis event. The system studied is the 266 nm photodissociation of CF3I with the CF3 fragments subsequently detected using radiation at approximately 1253 cm(-1) generated by a pulsed QCL. The tuning range provided by the frequency down-chirp of the QCL operated in its intrapulse mode allows a approximately 1 cm(-1) segment of the CF3 nu3 band to be measured following each photolysis laser pulse. Identification of features within this spectral region allows the CF3 ( v = 0) number density to be calculated as a function of pump-probe delay, and consequently the processes which populate and deplete this quantum state may be examined. Rate constants for the population cascade from higher vibrational levels into the v = 0 state, k 1, and for the recombination of the CF3 radicals to form C2F6, k2, are measured. The returned values of k1 = (2.3 +/- 0.34) x 10(-12) cm(3) molecule(-1) s(-1) and k2 = (3.9 +/- 0.34) x 10(-12) cm(3) molecule(-1) s(-1) are found to be in good agreement with reported literature values.

Towards simultaneous calibration-free and ultra-fast sensing of temperature and species in the intrapulse mode

“…Preservation of time resolution is reconciled with good spectral resolution of 0.013 cm -1 ; the latter of which is possible through reasonable chirp rates, and therefore less likely to encounter the RP effect at pressures [13,14] seen in combustion environments. This novel sensor is also easy with regards to implementation and data interpretation, is compact and uses only a single laser.…”

“…Thus far, applications of this technique to monitor molecular species (e.g. NH 3 , •CF 3 , CO, COS) comprise only a scant number of concentration measurements [7,10,12] and rapid-passage (RP) effect studies [11,13,14].…”

Spectral characteristics of quantum-cascade laser operating at 106 μm wavelength for a seed application in laser-produced-plasma extreme UV source

“…The temperature excursions of the optical active region were reported to be as much as 40°C [11], resulting in a significant downchirp. This mechanism of rapid frequency scanning explains the advantage of QCLs for high-speed gas spectroscopy applications [3,5,12]. This effect is also very important for the seeding of pulsed molecular lasers, so the measurement of the self-chirp rate was also of interest in this work.…”

“…The QCLs of sufficient reliability and performance became commercially available since early 2000 and in a matter of a few years have transformed the field of gas sensing and detection, opening a way for versatile gas sensing devices, which operate reliably with high sensitivity measured in [parts per billion (10 9 ) "ppb"] and with unprecedented analysis speeds [3][4][5]. As a result of the demand for the gas spectroscopy in the mid-IR, the presently available QCL chips cover a broad range of wavelengths.…”

Quantum Cascade Laser Absorption Spectroscopy as a Plasma Diagnostic Tool: An Overview

“…These S ( T ) values are individually corrected by a factor which includes all deviations from the linear Beer-Lambert law. In special cases where no line strength data or absorption cross sections are known but molecular constants for the species of interest are available, a spectral simulation can be applied to retrieve S as demonstrated by Hancock et al for the CF 3 [57]. This approach is particularly useful for transient molecules.…”