Absorption spectrum and cross sections of the allyl radical measured usingcavity ring-down spectroscopy: The (A)over-tilde <- (X)over-tilde band

Cavity ring-down spectroscopy (CRDS) was used to measure the electronic abs orption 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 photoly sis of allylic precursors. Optimized geometries in the ground ((X) over til de) and first excited ((A) over tilde) states and adiabatic and vertical ex citation energies in the (A) over tilde <-- (X) over tilde transition were calculated by ab initio molecular orbital calculations at CASSCF level of t heory. The equilibrium structure of the (A) over tilde state was found in a nonplanar C-2 geometry with CH2 twisted groups. The removal rate of the al lyl radical associated with self-reactions by its absorption in the (A) ove r tilde <-- (X) over tilde transition was probed by CRDS, The absorption cr oss section of the allyl radical at 402.9 nm was determined to be (2.0 +/- 0.4) x 10-(19) cm(2) molecule(-1) through analysis of time-dependent absorp tion traces.