Observation of fluorescence excitation spectra of tert-pentoxy and 3-pentoxy radicals

Fluorescence excitation spectra of tert-pentoxy [CH₃CH₂C(O·)(CH₃)₂] and 3-pentoxy radicals were observed for the first time. The radicals were produced by laser photolysis of the corresponding pentyl nitrites at 355 nm and studied in the wavelength range 345-400 nm. For tert-pentoxy, 12 vibronic bands were labeled in three progressions. One dominant progression corresponds to the C-O stretch mode with initial vibrational interval 551 ± 10 cm^-1. Two other unknown mode progressions have vibrational intervals of 587 ± 10 and 631 ± 10 cm^-1. The transition origin was tentatively assigned at 25 491 ± 10 cm^-1 (392.3 ± 0.1 nm). For 3-pentoxy, 15 vibronic bands were labeled in three progressions. The vibrational intervals in each progression are of consistent anharmonic tendency. On the basis of our assignments of the C-O stretch progression, the transition origin T₀, the C-O stretching vibration frequency v_C-O, and the anharmonic constant Χ_C-O are derived to be 26 437 ± 10, 578 ± 6, and -17 ± 2 cm^-1, respectively. The initial vibrational intervals of two other unknown modes are 596 ± 10 and 590 ± 10 cm^-1. The fluorescence lifetime of 3-pentoxy was determined to be about 150 ns. The short extent of the laser-induced fluorescence (LIF) excitation spectra of many C₂^- C₅ alkoxy radicals is tentatively explained as arising from the opening of a predissociation channel at relatively low excitation energies. Results of efforts to obtain LIF excitation spectra for 10 additional large alkoxy radicals are reported.