Three electronic band systems, D[12B2] -X[l2A2], C[2'Bl] -X[l2A2]. and B[12Al] -X[lZA2], are found and analyzedfor both C3H5 and C3D5 between 238 and 250 nm by resonant 1+1 multiphoton ionization. Partially-resolved rotational structure is simulated and fit to determine the upper-state vibronic symmetry of each band. Assignment of each band was then done using symmetry, isotope shift, and normal-mode information. The appearance of nominally forbidden bands in the B[ 12Al] -X[ 12A2] system is rationalized by the proposed nonplanarity of the upper state of the transition. The resulting double-well potential causes large inversion doubling of B-state levels, which explains anomalies in both the 1+1 and 2+2 MPI spectra of allyl radical.We report the rotational and vibrational analysis, and assignments, for three overlapping electronic hnds of the C3H5 and C3D5 radicals, observed by resonant 1+1 multiphoton ionization (MPI) in the 234-250-nm region. An analysis of the origin band of the C[22Bl] -X[12A2] system was the subject of a preliminary report.2 In this work, we complete the analysis and assignments of all the large bands between 238 and 250 nm. Allyl radical, C3H5, is the simplest hydrocarbon *-radical and is the prototype for extended conjugated systems with an odd number of electrons. Pedagogically, the structure, thermochemistry, and excited-state manifold of the allyl radical are often used as examples for molecular orbital treatments, from simple Hiickel to ab initio, of delocalized systems. It is surprising, in this context, that there is not much more spectrosoopic data for this fundamental molecule. Structural parameters, e.g., bond lengths and angles, have been extracted from ESR3 and gas-phase electron diffraction4 studies of the radical. Vibrational frequencies have been measured in matrix-isolation infrared5v6 and gas-phase resonance Raman experiments.' An absorption spectrum around 404 nm was reported by Currie and Ramsay,* following flash photolysis of allyl precursors, and assigned to the lowest energy valence excitation between the bonding and nonbonding r-orbitals. Subsequently, a more intense band system, near 224 nm, was found by Callear and Leeg in a similar experiment. The integrated absorption cross section of the latter band system was measured by Nakashima and Yoshihara.lo While resolvable vibrational structure was observed in each of these studies, no vibronic assignments were made because the spacings and intensities were irregular. One further excited state, which we have designated the B[ f2Al] state, has been ~b s e r v e d~' .~~ by 2+2 resonant MPI and assigned to the 3s Rydberg state forbidden in onephoton absorption. The origin of that band system was reported to be 40056.8 cm-', equivalent to a onsphoton wavelength of 249.65 nm, and several vibrational intervals in that excited state were extracted from the spectrum." Molecular orbital calculations for allyl radical have also been reported.13Our preliminary work2 on the electronic spectrum of allyl radical identified the sharp f...
