Resonance-enhanced multiphoton ionization combined with electronic ground state depletion spectroscopy of jet-cooled allyl radicals (C(3)H(5)) provides vibronic spectra of the 3s and 3p Rydberg states. Analysis of the vibronic structure following two-photon excitation of rovibrationally cold allyl radicals reveals transitions to the 3p(z) ((2)A(1)) Rydberg state with an electronic origin at 42230 cm(-1). More than 40 transitions to vibrational levels in the partially overlapping spectra of the 3p(y) ((2)B(2)) Rydberg state and the 3s ((2)A(1)) Rydberg state are identified and reassigned on the basis of predictions from ab initio calculations and results and simulations of pulsed-field-ionization zero-kinetic-energy photoelectron spectra obtained recently using resonant multiphoton excitation via selected vibrational levels of these two Rydberg states (J. Chem. Phys. 2009, 131, 014304). Depletion spectroscopy reveals that the transition to the short-lived 3p(x) ((2)B(1)) Rydberg state in vicinity of three-state same symmetry conical intersections predicted theoretically carries most of the oscillator strength of these coupled 3s and 3p Rydberg states. The results allow for the first time to experimentally derive the energetic ordering of the 3p Rydberg states of the allyl radical.

Direct ab initio molecular dynamics using the trajectory surface hopping method with Tully's fewest switches simulates the photodissociation dynamics of ethyl radical, C(2)H(5), following electronic excitation to the A-state. Nonadiabatic dissociation dominates and produces ground state ethylene and fast hydrogen atoms with an anisotropic angular distribution. Surface hopping also generates hot ground state ethyl radicals followed ultimately by unimolecular dissociation to C(2)H(4)+H. The calculated excited state lifetime and the product recoil energy distribution obtained from an ensemble of trajectories are consistent with previous experiments and suggest that a strictly nonadiabatic mechanism can account for nonradiative decay. This process is in competition with adiabatic dissociation producing electronically excited state ethylene and H, a dissociation channel that has not yet been experimentally observed. The branching ratio between adiabatic and nonadiabatic dissociation pathways depends sensitively on the quality of the potential energy surfaces. At the multireference configuration interaction with singles and doubles level of theory, 15% of all trajectories dissociate adiabatically.

How does one identify order in complex dynamical systems? A Born-Oppenheimer molecular dynamics simulation of the dissociation of ethyl radical, C(2)H(5), produces an ensemble of classical trajectories which are decomposed in the time-frequency domain using wavelets. A time-dependent scalar metric, the normalized instantaneous orbital complexity, is constructed and shown to correlate not only to the more conventional Lyapunov exponents but also to the dissociation time for an individual trajectory. The analysis of the ensemble of trajectories confirms that the long-lived trajectories are associated with a low degree of ergodicity. While the analysis of molecular dissociation dynamics is the narrow focus of the present work, the method is more general for discovery and identification of ordered regimes within large sets of chaotic data.

Direct classical trajectory calculations for ethyl radical, C2H5, at the HCTH147@6-31 +G**/6-31G** level of theory support the experimental observation that the dissociation of highly excited ethyl radicals to ethylene and and a hydrogen atom can occur much more slowly than predicted by statistical rate theories. Only 78% of the trajectories of ethyl radicals prepared in a microcanonical ensemble with 120-kcal/mol excitation energy above the zero-point energy and zero total angular momentum dissociate to form C2H4 + H. The remaining hot ground-state ethyl radicals have a lifetime of >>2 ps, during which a time-frequency analysis finds them trapped for extended periods of time in long-lived quasiperiodic trajectories.

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