Sensitive detection of acrolein and acrylonitrile with a pulsed quantum-cascade laser

Abstract

We report on spectroscopic measurements of acrolein and acrylonitrile at atmospheric pressure using a pulsed distributed feedback quantum-cascade laser in combination with intra- and inter-pulse techniques and compare the results. The measurements were done in the frequency region around 957 cm⁻¹. In the inter-pulse technique, the laser is excited with short current pulses (5–10 ns), and the pulse amplitude is modulated with an external current ramp resulting in a ∼2.3 cm⁻¹ frequency scan. In the intra-pulse technique, a linear frequency down-chirp during the pulse is used for sweeping across the absorption line. Long current pulses up to 500 ns were used for these measurements which resulted in a spectral window of ∼2.2 cm⁻¹ during the down-chirp. These comparatively wide spectral windows facilitated the measurements of the relatively broad absorption lines (∼1 cm⁻¹) of acrolein and acrylonitrile. The use of a room-temperature mercury-cadmium-telluride detector resulted in a completely cryogen-free spectrometer. We demonstrate ppb level detection limits within a data acquisition time of ∼10 s with these methodologies.