The spectroscopy and photodissociation dynamics of the vinoxy ͑CH 2 CHO͒ radical B(2 AЉ) ←X(2 AЉ) transition have been investigated by fast beam photofragment translational spectroscopy. We show conclusively that excitation to the B state is followed by predissociation, even for the origin transition. Two photodissociation channels are observed: ͑1͒ CH 3 ϩCO, and ͑2͒ HϩCH 2 CO, with a branching ratio of Ϸ1:4. The form of the translational energy distributions imply a significant exit barrier to formation of CH 3 ϩCO, and a considerably smaller barrier for HϩCH 2 CO formation. Dissociation ultimately proceeds by internal conversion to the ground electronic state; the internal conversion rate appears to be significantly enhanced by a curve crossing with either the Ã(2 AЈ) or C(2 AЈ) states. Ab initio calculations of critical points on the global potential energy surfaces aid in determining the dissociation mechanism. We present a simple model for dissociation over a barrier, the statistical adiabatic impulsive model, which satisfactorily reproduces the translational energy distributions.

The photodissociation spectroscopy and dynamics of the HCCO radical have been investigated using fast radical beam photofragment translational spectroscopy. An electronic band with origin at 33 424 cm Ϫ1 has been identified. This band exhibits rotational resolution near the band origin, but the well-defined rovibronic structure is homogeneously broadened at higher photon energies. Based on the rotational structure this band is assigned to the B 2 ⌸←X 2 AЉ transition. Photofragment translational energy and angular distributions were obtained at several excitation energies. At excitation energies close to the origin, the excited, spin-forbidden CH(a 4 ⌺ Ϫ )ϩCO channel dominates, while the ground state CH(X 2 ⌸)ϩCO channel is the major channel at higher photon energies. The translational energy distributions provide evidence of competition between intersystem crossing and internal conversion dissociation mechanisms, with some evidence for nonstatistical dynamics in the CH(X 2 ⌸)ϩCO channel. This work yields an improved heat of formation for HCCO, ⌬H f ,298 0 ϭ1.83Ϯ0.03 eV.

The photodissociation spectroscopy and dynamics of the CH 3 S and CD 3 S radicals have been investigated using fast radical beam photofragment spectroscopy of the à 2 A 1 ←X 2 E electronic band (T 0 Х26 400 cm Ϫ1) and an unstructured band near 45 600 cm Ϫ1. At all energies, only one major channel, CH 3 (X 2 A 2 Љ)ϩS(3 P j), was observed. Photofragment yield spectra for the à 2 A 1 ←X 2 E electronic band show resolved vibrational progressions extending well beyond those seen in laser-induced fluorescence studies of this band. Photofragment translational energy distributions yield the S(3 P j) fine-structure distribution for each vibrational level of the CH 3 product. Photofragment angular distributions were found to be highly anisotropic ͑␤ϭϪ0.2 to Ϫ1.0Ϯ0.1͒ with increasing anisotropy at higher photon energies. The results yield a refined heat of formation for CH 3 S ͑1.346Ϯ0.018 eV͒ as well as the mechanism by which the à 2 A 1 state is predissociated. Results at 45 600 cm Ϫ1 imply that dissociation occurs on the repulsive B 2 A 2 state.