High Spectral and Temporal Resolution Spectroscopy: Study of Atmospherically Relevant Molecules in Supersonic Beams using a QCL Dual-Comb Spectrometer

High resolution infrared spectroscopy is a valuable tool for applications ranging from exploring fundamental physical principles to a better understanding of our atmosphere. High-resolution techniques based on Fourier transform infrared (FTIR) or tunable laser spectroscopy have provided high quality infrared spectra with spectral resolution in the MHz range. A great challenge for existing techniques which remains is to obtain high spectral and high temporal resolution simultaneously. This is important in applications such as plasma diagnostics, gas phase chemical kinetics, and combustion diagnostics.

In this webinar, we show how quantum cascade laser (QCL) dual-comb spectroscopy can be employed to measure broadband (> 60 cm⁻¹) spectra with MHz spectral resolution and microsecond temporal resolution. We demonstrate the application of this technique to the measurement of the infrared spectra of CF₄ and CHCl₂F, two atmospherically relevant molecules, in pulsed, skimmed supersonic beams. The low rotational temperature of the beams and the narrow expansion cone after the skimmer made it possible to record the spectra of cold samples with high resolution. The spectra obtained in this study cover the range from 1200 cm⁻¹ to 1290 cm⁻¹. The narrowest recorded spectral lines have a full width at half maximum of 15 MHz (~0.0005 cm⁻¹): the temporal resolution is better than 4 ms. The results of this study demonstrate the potential of QCL dual-comb spectroscopy for broadband acquisition of spectra at high spectral and temporal resolution and high sensitivity in the mid-infrared range.  We will illustrate the power of this method through comparison of our results with previous results for these molecules obtained using Fourier Transform Infrared (FTIR) and diode laser spectroscopy of seeded continuous and pulsed supersonic jets.