Multiplexed Chirped Pulse Quantum Cascade Laser Measurements Of Ammonia And Other Small Molecules

Spectrometers based on Quantum Cascade (QC) lasers can be run in either continuous or pulsed operation. Although the instrumentation based upon the most recent versions of continuously operating QC lasers can have higher resolution than chirped lasers, using chirped pulse QC lasers can give an advantage when rapid changes in gas composition occur. For example, when jet engines are being tested, a variety of temperature dependent effects on the trace gas concentrations of the plume may be observed. Most pulsed QC lasers are operated in the down chirped mode, in which the chirp rate slows during the pulse. In our spectrometer the changes in frequency are recorded using two Ge etalons, one with a free spectral range of 0.0495 cm$^{-1}$, and the other with a fringe spacing of 0.0195 cm$^{-1}$.They can also be deployed in multiplex schemes in which two or more down-chirped lasers are used. In this paper we wish to show examples of the use of multiplexed chirped pulse lasers to allow overlapping spectra to be recorded. The examples of multiplex methods used are taken partly from measurements of $^{14}$NH$_{3}$ and $^{15}$NH$_{3}$ in the region from 1630 to 1622 cm$^{-1}$, and partly from the use of other chirped pulse lasers operating in the 8 $\mu$m region. Among the effects seen are rapid passage effects caused by the rapid down-chirp, and the use of gases such as nitrogen to cause variation in the shape of the collisional broadened absorption lines

A Comparison Of The Methods Of Studying The Spectra Of The Ash2 Radical

The first studies of the $^{2}$A$_{1}$-$^{2}$B$_{1}$ electronic band system of the AsH$_{2}$ and AsD$_{2}$ radicals were made at Sheffield University in the period from1966 to 1968 by Dixon, Duxbury and Lamberton using flash photolysis of arsine and deuterated arsine. The bands have a complex rotational structure associated with that of an asymmetric rotor. Band centres of the 0,v$_{2}$,0-0,0,0 progression were identified for v$_{2}$'=0 tp v$_{2}$'=5, although only the structure of the bands from v$_{2}$'=1 to 3 was analysed in detail. After a long time interval in 1986 a low resolution emission spectrum of AsH$_{2}$ was recorded by NI et al. However, it was not until 2007 that He and Clouthier studied the electronic transition of jet-cooled AsH$_{2}$ using laser induced fluorescence and wavelength-resolved emission. Following on from this in 2009 Zhao and colleagues recorded absorption spectra of the AsH$_{2}$ radical by cavity ringdown spectroscopy. Finally in 2012 Grimminger and Clouthier recorded the equivalent transitions in AsD$_{2}$ and AsHD. They also carried out ab initio calculations. By comparing the recent spectroscopic results with those of Dixon et al, we wish to show the complementarity of the different methods for understanding the behaviour of AsH$_{2}$ and AsD$_{2}$ radicals

Effects Of Spin-orbit Coupling On The Spin-rotation Interaction In The Ash2 Radical

The occurence of predissociation in the electronic spectrum of AsH$_{2}$ is very dependent upon the magnitude of the spin-orbit coupling parameter of the central atom. Making use of Table 5.6 in "The Spectra and Dynamics of Diatomic Molecules, ELSEVIER" by H. Lefebvre-Brion and R.W. Field, it is possible to appreciate the rapid rate of increase of the spin-orbit constants associated with the heavy central atom in the di-hydrides NH$_{2}$, PH$_{2}$ and AsH$_{2}$. The spin-orbit constants range from 42.7 cm$^{-1}$ for NH$_{2}$, to 191.3 cm$^{-1}$ for PH$_{2}$, and 1178 cm$^{-1}$ for AsH$_{2}$. The effects of spin-orbit coupling may be seen in a plot of the separation of the central $^{R}$Q$_{0,9}$ and $^{P}$Q$_{1,N}$ sub-bands as the value of v$_{2}$' increases from 0 to 5. As the value of v$_{2}$' increases beyond 2 the spectrum becomes more and more fuzzy as the effects of predissociation become more obvious. This means that unlike the example of the behaviour of PH$_{2}$, where the vibronic level pattern can be followed below and above the barrier to linearity, in AsH$_{2}$ and AsD$_{2}$ the absorption spectrum becomes completely diffuse below the barrier to linearity in the A $^{2}$A$_{1}$ state. The change in the magnitude of the doublet splittings as v$_{2}$' increases may be seen in the plots of the doublet splittings showing the spin-uncoupling as a result of the increase of overall rotation. In the absorption spectrum of SbH$_{2}$, recorded in 1967 by T. Barrow in the Chemistry Department at Sheffield University, all the absorption features showed the effects of predissociation, consistent with a spin-orbit constant of 2834 cm$^{-1}$ for the central atom of SbH$_{2}$

Quantum cascade laser spectroscopy, trace gas analysis to non-linear optics

This paper reviews quantum cascade laser spectroscopy and its applications for trace gas sensing and non-linear optics. The development of pulsed lasers for these applications are discussed focusing on the intra-pulse method. The operation of the quantum cascade laser spectrometer is investigated experimentally and by computer solutions of Maxwell-bloch equations

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 (OCO)-O-18-C-12-O-16. 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

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 mu m 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 CO2 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

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

The ac Stark effect in nitric oxide induced by rapidly swept continuous wave quantum cascade lasers

A large ac Stark effect has been observed when nitric oxide, at low pressure in a long optical path (100 m) Herriot cell, is subjected to infrared radiation from a rapidly swept, continuous wave infrared quantum cascade laser. As the frequency sweep rate of the laser is increased, an emission signal induced by rapid passage occurs after the laser frequency has passed through the resonance of 1-0 R(11.5)(3/2/)molecular absorption line. At very high sweep rates a laser field-induced splitting of the absorptive part of the signal is observed, due to the ac Stark effect. This splitting is related to the Autler-Townes mixing of the e, f lambda doublet components of the 1-0 R(11.5)(3/2) transition, which lie under the Doppler broadened envelope