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 fur the first time. The radicals were prod uced 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 correspo nds 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 assign ed at 25 491 +/- 10 cm(-1) (392.3 +/- 0.1 nm). For 3-pentoxy, 15 vibronic b ands were labeled in three progressions. The vibrational intervals in each progression are of consistent anharmonic tendency. On the basis of our assi gnments of the C-O stretch progression, the transition origin T-0, the C-O stretching vibration frequency nu (C-O), and the anharmonic constant chi (C -O) are derived to be 26 437 +/- 10, 578 +/- 6, and -17 +/- 2 cm(-1), respe ctively. The initial vibrational intervals of two other unknown modes are 5 96 +/- 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 fluor escence (LIF) excitation spectra of many C-2-C-5 alkoxy radicals is tentati vely explained as arising from the opening of a predissociation channel at relatively low excitation energies. Results of efforts to obtain LIF excita tion spectra for 10 additional large alkoxy radicals are reported.