Photothermal lensing effects are observed in the early expansion stages (X/D=1ndash;7) of a supersonic free jet and narrowPandRbranch rotational lines have been recorded at 100 MHz resolution for the ngr;2+5ngr;3band of acetylene. Rotational Raman loss spectra were also recorded and, from the collisionhyphen;broadened linewidths of these, it is estimated that about 100 collisions occur in the beam transit from pump to probe positions. This is sufficient to produce rotational and some vibrational relaxation in forming the lens. The photothermal intensities are consistent with a Boltzmann rotational distribution in the ground state and a rotational temperature of 70 K is deduced for 100 PSI expansions of 20percnt; C2H2in helium. Charachyphen; teristics of the photothermal signal in the jet are described and the time and spatial dependence of the signal is used to measure a flow velocity 625 m/s for the jet. This photothermal method may prove useful as a diagnostic probe of beam properties and as a spectroscopic measure of weak onehyphen; and twohyphen;photon transitions of molecules cooled in jets.
展开▼
