Unstable periodic orbits, recurrences, and diffuse vibrational structures in the photodissociation of water near 128 nm.

The photodissociation of H2O in the second absorption band (X→B) is investigated in a completely time-dependent approach. The Schrödinger equation is solved by a time-dependent close-coupling method expanding the two-dimensional wave packet in terms of free rotor states. The vibrational degree of freedom of the OH fragment is fixed and only motion on the B-state potential-energy surface is considered. The calculated absorption spectrum exhibits a long progression of diffuse structures, ΔE∼0.1 eV, in very good agreement with the experimental spectrum. The structure is readily explained in terms of a recurrence of the autocorrelation function after about 40 fs. The recurrence, in turn, is attributed to special indirect trajectories which on the average perform one oscillation within the deep potential well before they dissociate into products H+OH. These trajections are ‘‘guided’’ by so-called unstable periodic orbits which persist to energies high above the H+OH(2 Σ) threshold. The existence of unstable periodic orbits leading to a recurrence of the autocorrelation function gives, for the first time, a consistent explanation of the diffuse structure in the absorption spectrum of H2 O in the second band. [ABSTRACT FROM AUTHOR]