Use Of A Free-Expansion Jet In Ultrahigh Resolution Raman Spectroscopy

Rotationally-resolved, ultra-high-resolution stimulated Raman spectra of CH4, CD4, and SF6 have been obtained in cw and pulsed free-expansion jets, using a simple and inexpensive nozzle-and-vacuum-chamber apparatus. Through variation of the nozzle orifice diameter, nozzle backing pressure, and axial location of the spectroscopic probe region it is possible to nearly independently control the spectral Doppler width, rotational temperature, and molecular density in the jet. Using relatively mild expansion conditions we have been able to cool a pure methane jet to a rotational temperature of 13K. We show here how this con-trol of temperature, independent of density, allows separation of otherwise unresolvable spectral features and unambiguous identification of band-heads, hot-bands, and other spec-tral features.