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The state selective photodissociation of acetylene, C2H2/C2D2, was studied in the wavelength range 121.2–132.2 nm by high resolution Rydberg atom time-of-flight measurements on the atomic fragment, H/D. In the wavelength region studied members of all four Rydberg series and the highly excited Ẽ valence state were state selectively excited using tunable vacuum-ultraviolet laser radiation. The lifetime of the excited states which were studied varied from 58 fs to more than 2 ps. Formation of the ethynyl radical in its X̃ electronic ground state and its first electronically excited à state is observed with practically no indication of B̃ state fragments. Two decay channels with different dissociation dynamics were also observed. In both channels the observed decay dynamics depended strongly on the excited state of the parent molecule. Further there are major differences between these two dissociation pathways with respect to the measured internal energy and angular distributions. In one channel the dissociation is dominated by dynamical effects and the C2H fragments are formed with a high degree of vibrational excitation. In contrast to this in the second channel a smooth internal energy distribution is observed indicating that the fragment quantum state distribution is spread over a considerable range of the available phase space. Moreover, this second channel can be fit with a phase space model constrained only by conservation of energy and angular momentum. This is further evidence for the randomization of internal energy during the dissociation process.

The photodissociation of jet-cooled HCCH molecules following excitation to their S 1 state has been investigated further, at a number of wavelengths in the range 205-220 nm, using the H atom photofragment translational spectroscopy ͑PTS͒ technique. Analysis of the rovibrational structure evident in the total kinetic energy release ͑TKER͒ spectra so obtained confirms previous reports that the resulting C 2 H(X) fragments are formed in most ͑if not all͒ of the v 2 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 ϭv 2 ͒. The distribution of H atom recoil velocity vectors resulting from photolyses at the shorter excitation wavelengths ͑e.g., phot ϭ205.54 nm͒ shows bimodal rotational distributions, and a marked anisotropy-especially in the case of those H atoms that are formed in association with C 2 H(X) 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 ͑1997͔͒. Channel I conforms to their proposal that the S 1 molecules reach the HϩC 2 H(X) asymptote as a result of sequential nonadiabatic couplings via the T 3 , T 2 , and T 1 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 T 1 surface, while the propensity for populating states with lϭv 2 reflects the importance of parent torsional motion in promoting the S 1 →T 3 , T 3 →T 2 , and T 2 →T 1 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 S 1 to T 1 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 1 Ј mode of HCCH enhances the branching to channel II.