High resolution pulsed infrared cavity ringdown spectroscopy: Application to laser ablated carbon clusters

Abstract

We report the design and performance of a tunable, pulsed high resolution mid infrared cavity ringdown spectrometer. Stimulated Raman scattering in H2/D2 is used to downconvert the output of a SLM Alexandrite ring laser (720–800 nm) to the mid infrared (3–8 μm). The infrared frequency bandwidth was determined to be 90±5 MHz from measurements of Doppler broadened OCS transitions at 5 μm. The minimum detectable per pass fractional absorption is 1 ppm. We observe a frequency dependent ringdown cavity transmission of ±5 ppm due to spatial variations of the mirror reflectivity. The υ6 band of linear C9 formed by laser ablation of graphite in a He molecular beam was measured, showing a factor of 2 improvement in sensitivity relative to previous IR diode laser experiments. Based on calculated IR intensities, the number density of C9 in the molecular beam is 1.3*1011 molec/cm3 and the minimum detectable density is 1*109 molec/cm3. We expect this spectrometer to be a powerful tool for the study of transient species formed in molecular beams.