DETERMINATION OF THE ROTATIONAL AND VIBRATIONAL-CONSTANTS OF PHOTOLYTICALLY GENERATED CF RADICALS BY REMPI MEASUREMENTS

The detailed spectroscopic study of transient free CF radicals with th e resonance enhanced multiphoton ionization (REMPI) method is the aim of this paper. The CF radicals were generated from the gaseous (50-200 Pa) halocarbons CCl3F, CCl2F2, CClF3, 1,2-C2Cl2F4, C2ClF3, and C6F6 b y a combined infrared and UV/VIS laser photolysis. Because of the smal l absorption of the halocarbon molecules in the dye laser range used ( 328-485 nm) a preceding production of precursors (capable of absorbing dye laser photons) was necessary, which was achieved by IR multiphoto n dissociation of the parent molecules with a TEA CO2 laser pulse. In a subsequent absorption of UV/VIS dye laser photons the precursors wer e photolyzed yielding CF radicals. These could be spectroscopically in vestigated by their REMPI signals in a (2 + 1)-photon absorption proce ss in the focussed radiation of the same dye laser pulse. About 20 vib rational bands were measured, rotationally resolved, and were assigned to a two-photon 3p pi D (2) Pi(r) <--<-- X (2) Pi(r) resonant transit ion followed by one-photon absorption for direct ionization of CF. Com pared to the cold CF radicals, commonly extracted from a flow reactor, the laser-photolytically generated CF radicals were found to be vibra tionally and rotationally extremely excited (v '' less than or equal t o 12, J less than or equal to 80.5). Using the wide rotational contour of the vibrational bands we could determine with greater precision th an before the electronic, vibrational, and rotational constants T-e, w (e), w(e) x(e), w(e)y(e), A(v), B-v, and D-v, respectively, for the Ry dberg state 3p pi D (2) Pi(r) (v' = 0-6) and the electronic ground sta te X (2) Pi(r) (v '' = 0-12). Based on these constants we calculated t he Morse and the Dunham potential curves for both states. A detailed s tudy of missing and shifted rovibronic lines of the D state yielded fi rst information on the vibrational levels v '' = 3, 4, and 5 of the B (2) Delta(r) valence state, which cannot be measured directly by REMPI . Additionally, we measured for the first time in the UV wavelength ra nge a (1 + 1)-photon REMPI spectrum belonging to the vibrational bands v' = 0 <-- v '' = 0 and v' = 1 <-- v '' = 0 of the A (2) Sigma(+) <-- X (2) Pi(r) transition. Even in the range of high rotational quantum numbers J we found no indication of Lambda-type doubling of the rovibr onic states within a resolution of 0.2 cm(-1). (C) 1996 Academic Press , Inc.