Determination of the Rotational and Vibrational Constants of Photolytically Generated CF Radicals by REMPI Measurements

Abstract

The detailed spectroscopic study of transient free CF radicals with the 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 C6F6by a combined infrared and UV/VIS laser photolysis. Because of the small 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 multiphoton dissociation of the parent molecules with a TEA CO2laser pulse. In a subsequent absorption of UV/VIS dye laser photons the precursors were photolyzed yielding CF radicals. These could be spectroscopically investigated by their REMPI signals in a (2 + 1)-photon absorption process in the focussed radiation of the same dye laser pulse. About 20 vibrational bands were measured, rotationally resolved, and were assigned to a two-photon 3pπD2Πr←←X2Πrresonant transition followed by one-photon absorption for direct ionization of CF. Compared to the cold CF radicals, commonly extracted from a flow reactor, the laser-photolytically generated CF radicals were found to be vibrationally and rotationally extremely excited (v″ ≤ 12,J≤ 80.5). Using the wide rotational contour of the vibrational bands we could determine with greater precision than before the electronic, vibrational, and rotational constantsTe, ωe, ωexe, ωeye,Av,Bv, andDv, respectively, for the Rydberg state 3pπD2Πr(v′ = 0—6) and the electronic ground stateX2Πr(v″ = 0—12). Based on these constants we calculated the Morse and the Dunham potential curves for both states. A detailed study of missing and shifted rovibronic lines of theDstate yielded first information on the vibrational levelsv′ = 3, 4, and 5 of theB2Δrvalence state, which cannot be measured directly by REMPI. Additionally, we measured for the first time in the UV wavelength range a (1 + 1)-photon REMPI spectrum belonging to the vibrational bandsv′ = 0 ←v″ = 0 andv′ = 1 ←v″ = 0 of theA2Σ+←X2Πrtransition. Even in the range of high rotational quantum numbersJwe found no indication of Λ-type doubling of the rovibronic states within a resolution of 0.2 cm−1.