Absorption Spectrum and Cross Sections of the Allyl Radical Measured Using Cavity Ring-Down Spectroscopy:  The à ← X Band

Cavity ring-down spectroscopy (CRDS) was used to measure the electronic absorption spectrum of the allyl radical (CH2CHCH2) between 370 and 420 nm at 297 K. The allyl radical was produced from the 193 nm excimer laser photolysis of allylic precursors. Optimized geometries in the ground (X̃) and first excited (Ã) states and adiabatic and vertical excitation energies in the à ← X̃ transition were calculated by ab initio molecular orbital calculations at CASSCF level of theory. The equilibrium structure of the à state was found in a nonplanar C2 geometry with CH2 twisted groups. The removal rate of the allyl radical associated with self-reactions by its absorption in the à ← X̃ transition was probed by CRDS. The absorption cross section of the allyl radical at 402.9 nm was determined to be (2.0 ± 0.4) × 10-19 cm2 molecule-1 through analysis of time-dependent absorption traces.