One- and two-photon excitation vibronic spectra of 2-methylallyl radical at 4.6–5.6 eV

Abstract

Vibronically excited 2-methylallyl radical [CH2C(CH3)CH2] at 4.6–5.6 eV has been studied by 1+1 and 2+2 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The 2-methylallyl radicals were produced by the flash pyrolysis of 3-bromo-2-methylpropene in a supersonic-jet expansion. The 2+2 REMPI spectrum of 2-methylallyl radical at 38 000–40 700 cm−1 is identified as B̃(1 2A1)←X̃(1 2A2) transition, i.e., the excitation of a nonbonding electron to the 3s Rydberg state (3s←n). Seven lowest-lying electronic states with excitation energy below 6 eV have been calculated in an MRCI level. Two new electronic bands have been observed at 38 500–41 000 cm−1 by 1+1 REMPI spectroscopy and assigned to C̃(1 2B2)←X̃(1 2A2) and Ẽ(2 2A2)←X̃(1 2A2). Much broader 1+1 REMPI signals at 41 000–43 5 00 cm−1 with HWHM of ∼80 cm−1 for each vibronic band could be due to D̃(2 2B2)←X̃(1 2A2) and/or F̃(3 2B2)←X̃(1 2A2) via an intensity borrowing from C̃(1 2B2)←X̃(1 2A2). Taking the computed geometries and vibrations of the ground- and excited electronic states, Franck–Condon factors (FCFs) have been calculated. Combining the FCFs with calculated excitation energies and oscillator strengths of the six electronic states at 4–6 eV, predicted spectral patterns have been used to assist spectroscopic analysis for the observed vibronic spectra of 2-methylallyl radical.