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

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 mm!. The infrared frequency bandwidth was determined to be 9065 MHz from measurements of Doppler broadened OCS transitions at 5 mm. The minimum detectable per pass fractional absorption is 1 ppm. We observe a frequency dependent ringdown cavity transmission of 65 ppm due to spatial variations of the mirror reflectivity. The y6 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*10 molec/cm and the minimum detectable density is 1*10 molec/cm. We expect this spectrometer to be a powerful tool for the study of transient species formed in molecular beams. © 2002 American Institute of Physics. @DOI: 10.1063/1.1461825#