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

Vibronically excited 2-methylallyl radical @CH2C(CH3)CH2# at 4.6–5.6 eV has been studied by 111 and 212 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 212 REMPI spectrum of 2-methylallyl radical at 38 000– 40 700 cm21 is identified as B ˜ (1 2A1)ˆX ˜ (1 2A2) transition, i.e., the excitation of a nonbonding electron to the 3s Rydberg state (3sˆn). 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 cm21 by 111 REMPI spectroscopy and assigned to C˜ (1 2B2)ˆ˜X(1 2A2) and E ˜ (2 2A2)ˆX ˜ (1 2A2). Much broader 111 REMPI signals at 41 000– 43 5 00 cm21 with HWHM of ;80 cm21 for each vibronic band could be due to D˜ (2 2B2)ˆ˜X(1 2A2) and/or F ˜ (3 2B2)ˆX ˜ (1 2A2) via an intensity borrowing from C˜ (1 2B2) ˆ˜X(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.