Journal of Physical Chemistry A, Vol.112, No.3, 412-418, 2008
Electronic spectroscopy and photodissociation dynamics of the 1-hydroxyethyl radical CH3CHOH
The electronic spectroscopy and photodissociation dynamics of the CH3CHOH radical in the region 19 400 - 37 000 cm(-1) (515-270 nm) were studied in a molecular beam using resonance-enhanced multiphoton ionization (REMPI), photofragment yield spectroscopy, and time-of-flight (TOF) spectra of H and D fragments. The onset of the transition to the Rydberg 3s state, the lowest excited state, is estimated at 19 600 +/- 100 cm-1. The 3s state dissociates fast, and no REMPI spectrum is observed. The origin band of the transition to the 3p(z) state, identified by 2 + 2 REMPI, lies at 32 360 +/- 70 cm(-1), and a vibrational progression in the C-O stretch is assigned. When exciting CH3CHOH near the onset of the unstructured absorption to the 3s state, only one peak is observed in the center-of-mass (c.m.) translational energy (E-t) distribution obtained by monitoring H photofragments. The measured recoil anisotropy parameter beta = -0.7 +/- 0.1 is typical of a perpendicular transition. The O-H bond energy is determined to be D-0 = 1.1 eV +/- 0.1 eV. At excitation energies >31 200 cm(-1) (3.87 eV) a second, low E-t peak appears in the c.m. E-t distribution with beta approximate to 0. Its relative intensity increases with excitation energy, but its beta value does not change. In contrast, the beta value of the higher E-t peak becomes monotonically less negative at higher excitation energies, decreasing to -0.2 +/- 0.1 at 35 460 cm(-1). By comparison of the TOF distributions of the isotopologs CH3CHOH, CH3CHOD, and CD3CHOH, it is concluded that two major product channels dominate the photodissociation, one leading to acetaldehyde and the other to vinyl alcohol (enol) products. There is no indication of isomerization to ethoxy. It, appears that separate conical intersections lead to the observed channels, and the dynamics at the conical intersection and the exit channel deposit much of the available energy into internal energy of the products.