Experimental
The pulsed supersonic free jets were prepared by a pure acetylene gas. The collimated VUV beam was introduced into the chamber and intersected the free jet at right angles, 15 mm downstream from the orifice of the pulsed valve. When the IR-VUV double resonance experiment was performed, the collimated IR beam was counter-propagated co-linearly with the IR beam.
The VUV laser light was generated by a four-wave difference mixing (2ω1−ω2) technique in a Kr gas (15−70 Torr), using two dye lasers pumped by a XeCl excimer laser. The wavelength of ω1, the second harmonic of the dye laser in a BBO crystal, was 212.56 nm which was two-photon resonant with Kr 5p[1/2]0, and the wavelength of ω2 was tuned from 500 to 580 nm by the other dye laser resulting in the VUV wavelength of 135−130 nm. The ω1 and ω2 laser beams were separated from the VUV laser beam by a 30° lithium fluoride prism.
For the IR laser generation, we used an OPO laser pumped by an injection-seeded Nd:YAG laser. The OPA signal and idler radiation beams were separated by a 45° anhydrous-quartz prism. The wavelengths of second OPA signal and/or idler radiation were locked at a rotational line of the vibrational band of C2H2 by monitoring photo-acoustic signal. We also monitored the wavelength of the frequency-doubled NRO signal radiation (~400 nm) by using the wavemeter, in order to eliminate of the wavelength uncertainty due to the pressure broadening of photo-acoustic spectrum.
