Jet-cooled infrared spectra of molecules and complexes with a cw mode-hop-free external-cavity QCL and a distributed-feedback QCL

Abstract

The coupling of quantum cascade lasers (QCLs) with an off-axis cavity enhanced absorption (CEA) spectrometer and an astigmatic multiple pass absorption (MPA) spectrometer are described in this paper. A continuous wave (cw) liquid nitrogen cooled distributed-feedback QCL at 5.7 μm and a cw room temperature mode-hop-free external-cavity QCL at 6.1 μm were employed as the light source. For the CEA spectrometer, the effects of mirror size and laser scan rate were evaluated. For the MPA spectrometer, a pair of astigmatic mirrors with a 55 cm mirror distance was aligned to the 366-pass configuration. The jet-cooled samples were generated using a homemade pulsed slit jet nozzle assembly. Two LabVIEW programs were written to automate and synchronize the timing of the laser scan, the pulsed slit jet molecular expansion, and the data acquisition. Infrared spectra of jet-cooled methyl lactate and the Ar–H2O complex and room temperature N2O and NH3 samples were measured using both the rapid scan and the wavelength modulation methods to evaluate the sensitivity and resolution of the CEA and MPA spectrometers. The combination of the MPA spectrometer with the external-cavity QCL using the rapid scan method was found to be the best suited combination to measure high resolution jet-cooled infrared spectra.