Near threshold photodissociation of acetylene

Abstract

The photodissociation of jet-cooled HCCH molecules following excitation to their S1 state has been investigated further, at a number of wavelengths in the range 205–220 nm, using the H atom photofragment translational spectroscopy (PTS) technique. Analysis of the rovibrational structure evident in the total kinetic energy release (TKER) spectra so obtained confirms previous reports that the resulting C2H(X̃) fragments are formed in most (if not all) of the v2 bending vibrational levels permitted by energy conservation, and that there is a clear preference for populating those states in which the axial projection of this vibrational angular momentum is maximized (i.e., states with l=v2). The distribution of H atom recoil velocity vectors resulting from photolyses at the shorter excitation wavelengths (e.g., λphot=205.54 nm) shows bimodal rotational distributions, and a marked anisotropy—especially in the case of those H atoms that are formed in association with C2H(X̃) fragments carrying little rotational excitation. Two competing dissociations mechanisms have been identified. Our discussion of these observations is guided by the recent ab initio calculations of Cui and Morokuma [Chem. Phys. Lett. 272, 319 (1997)]. Channel I conforms to their proposal that the S1 molecules reach the H+C2H(X̃) asymptote as a result of sequential nonadiabatic couplings via the T3, T2, and T1 potential energy surfaces. The product energy disposal at the longest excitation wavelengths is rationalized in terms of the forces acting as the dissociating molecule traverses a late barrier in the C–H exit channel on the T1 surface, while the propensity for populating states with l=v2 reflects the importance of parent torsional motion in promoting the S1→T3, T3→T2, and T2→T1 surface couplings. The population of low rotational states with high recoil anisotropy at shorter excitation wavelengths is ascribed to channel II, involving a direct nonadiabatic transition from S1 to T1 for a structure with one near linear CCH angle. In contrast to channel I, there is no extensive torsional motion and the anisotropy of the initial excitation is retained through to fragmentation. Excitation of the ν1′ mode of HCCH enhances the branching to channel II.