Kenneth G. Hay

Quantum cascade laser investigations of CH4 and C2H2 interconversion in hydrocarbon/H2 gas mixtures during microwave plasma enhanced chemical vapor deposition of diamond

CH4 and C2H2 molecules (and their interconversion) in hydrocarbon/rare gas/H2 gas mixtures in a microwave reactor used for plasma enhanced diamond chemical vapor deposition (CVD) have been investigated by line-of-sight infrared absorption spectroscopy in the wavenumber range of 1276.5−1273.1 cm−1 using a quantum cascade laser spectrometer. Parameters explored include process conditions [pressure, input power, source hydrocarbon, rare gas (Ar or Ne), input gas mixing ratio], height (z) above the substrate, and time (t) after addition of hydrocarbon to a pre-existing Ar/H2 plasma. The line integrated absorptions so obtained have been converted to species number densities by reference to the companion two-dimensional (r,z) modeling of the CVD reactor described in Mankelevich et al. [J. Appl. Phys. 104, 113304 (2008)]. The gas temperature distribution within the reactor ensures that the measured absorptions are dominated by CH4 and C2H2 molecules in the cool periphery of the reactor. Nonetheless, the measurem...

Quantum cascade laser spectroscopy: diagnostics to non-linear optics

In many examples of the use of mid-infrared quantum cascade (QC) lasers for gas detection or process monitoring, an assumption is made that their use is an obvious extension of tuneable diode laser spectroscopy. We wish to show that making such an assumption is not necessarily justified when the frequency sweep rate is rapid, as is down-chirped QC laser infrared radiation. This is demonstrated via a series of experiments designed to investigate the physics of the interaction of chirped infrared laser radiation with low pressure gases. The unusual signals, which characterise the rapid passage of the down-chirped radiation through a low pressure gas, are due to two main effects, the laser sweep rate, and the long path length of the refocusing cells used. The sweep rate of the laser frequency may be faster than the inter-molecular collision frequency, allowing the build up of a strong molecular alignment within the gas. The long optical path lengths in the refocusing absorption cells, used to facilitate sensitive detection of trace gases, allow the build up of a large macroscopic polarisation within the gas cell. We give examples of this behaviour in molecules with large transition dipole moments, ammonia and nitrous oxide, and with a very small one 18O12C16O. We also outline the use of Maxwell–Bloch calculations to investigate the origins of this behaviour, and hence to define operating conditions where the concentration of trace molecules may be determined.

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.

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).

An investigation of collisional processes in a Dicke narrowed transition of water vapor in the 7.8 μm spectral region by frequency down-chirped quantum cascade laser spectroscopy

Information about intermolecular potentials is usually obtained through the analysis of the absorption line shapes recorded in the frequency domain. This approach is also adopted to study the effects of motional narrowing and speed dependence of the pressure broadening coefficients. On the other hand, time domain measurements are directly related to molecular collisions and are therefore frequently employed to study molecular relaxation rates, as well as the effects of velocity changing collisions and the speed dependence of the absorption cross sections. 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. In the present work the frequency down-chirped radiation emitted by an intrapulsed quantum cascade laser operating near 7.8 mum is employed to investigate the collisional relaxation processes, and the collisional narrowing, in the 15(0,15)<--16(1,16) and 15(1,15)<--16(0,16) doublet in the water vapor nu(2) band. The effects of He, Ne, Ar, N(2), and CO(2) as collisional partners are investigated. The experimental results clearly indicate the dependence of the collisional cross sections upon the chirp rate. They also demonstrate that by using different chirp rates it is possible to gain information about the intermolecular processes driving the molecular collisions and the related energy transfer.

Delayed rapid passage and transient gain signals generated using a chirped 8 μm quantum cascade laser

Rapid passage signals have been observed in which a delayed switch from absorption to emission is observed as the optical density of acetylene in a long path length cell is increased. The delayed switch results in the generation of a large, narrow, transient gain signal. Using numerical solutions of the Maxwell–Bloch equations we have demonstrated that this effect is due to constructive interference between the incident laser field and the field generated by the response of the gaseous medium. This occurs owing to the minimal collisional damping at the low gas pressures within the gas cell, so that the chirp rate of the laser is faster than the collisional reorientation time of the molecules.

Collisional Effects On Quantum Cascade Laser Induced Molecular Alignment

Intra‐pulse quantum cascade laser spectrometers are able to produce both saturation and molecular alignment of a molecular gas. The resultant time domain spectra resemble those seen in coherent optical nutation experiments. In this paper we discuss the ways in which this technique may be used to investigate nitrous oxide‐foreign gas collisions, and also those involving a Dicke narrowed line of water. In both cases the chirp rate dependence of the pressure broadening, and its dependence on the collision partner is demonstrated.

Study of the Q branch structure of the 14N and 15N isotopologues of the ν4 band of ammonia using frequency chirped quantum cascade lasers.

Intrapulse quantum cascade (QC) laser spectrometers are able to produce both saturation and molecular alignment of a gas sample owing to the rapid sweep of the radiation through the absorption features. In the QC lasers used to study the (14)N and (15)N isotopologues of the ν4 band of ammonia centered near 1625 cm(-1), the variation of the chirp rate during the scan is very large, from ca. 85 to ca. 15 MHz ns(-1). In the rapid chirp zone the collisional interaction time of the laser radiation with the gas molecules is short, and large rapid passage effects are seen, whereas at the slow chirp end the line shape resembles that of a Doppler broadened line. The total scan range of the QC laser of ca. 10 cm(-1) is sufficient to allow the spectra of both isotopologues to be recorded and the rapid and slow interactions with the laser radiation to be seen. The rapid passage effects are enhanced by the use of an off axis Herriott cell with an effective path length of 62 m, which allows a buildup of polarization to occur. The effective resolution of the chirped QC laser is ca. 0.012 cm(-1) full width at half-maximum in the 1625 cm(-1) region. The results of these experiments are compared with those of other studies of the ν4 band of ammonia carried out using Fourier transform and Laser Stark spectroscopy. They also demonstrate the versatility of the down chirped QC laser for investigating collisional effects in low pressure gases using long absorbing path lengths.

Real-Time Diagnostics in Exhaust Plumes, Flames, and the Atmosphere Using an Intra-Pulse Quantum Cascade Laser Spectrometer

Quantitative measurements of real-time variations of the chemical composition of a jet engine plume, laminar flame and ambient atmosphere are demonstrated using 5 and 8 ?m intra-pulse quantum cascade laser spectrometers.

Chirped Quantum Cascade Laser Induced Transient Gain in Strongly Absorbing Molecular Gases

Using a mW power chirped pulse quantum cascade laser propagating in a 60 m pathlength Herriott cell, delayed rapid passage and transient gain signals have been observed in the 8 micron spectrum of acetylene.