Vacuum ultraviolet excited state dynamics of the smallest ring, cyclopropane. II. Time-resolved photoelectron spectroscopy and ab initio dynamics.

The vacuum-ultraviolet photoinduced dynamics of cyclopropane (C3H6) were studied using time-resolved photoelectron spectroscopy (TRPES) in conjunction with ab initio quantum dynamics simulations. Following excitation at 160.8 nm, and subsequent probing via photoionization at 266.45 nm, the initially prepared wave packet is found to exhibit a fast decay (<100 fs) that is attributed to the rapid dissociation of C3H6 to ethylene (C2H4) and methylene (CH2). The photodissociation process proceeds via concerted ring opening and C-C bond cleavage in the excited state. Ab initio multiple spawning simulations indicate that ring-opening occurs prior to dissociation. The dynamics simulations were subsequently employed to simulate a TRPES spectrum, which was found to be in excellent agreement with the experimental result. On the basis of this agreement, the fitted time constants of 35 ± 20 and 57 ± 35 fs were assigned to prompt (i) dissociation on the lowest-lying excited state, prepared directly by the pump pulse, and (ii) non-adiabatic relaxation from higher-lying excited states that lead to delayed dissociation, respectively.