Dissipative Quantum Dynamics with Many Coupled Molecular States: Photodesorption from Metal Surfaces

Photofragmentation of Tetranitromethane: Spin-Unrestricted Time-Dependent Excited-State Molecular Dynamics

In this study, the photofragmentation dynamics of tetranitromethane (TNM) is explored by a spin-unrestricted time-dependent excited-state molecular dynamics (u-TDESMD) algorithm based on Rabi oscillations and principles similar to trajectory surface hopping, with a midintensity field approximation. The leading order process is represented by the molecule undergoing cyclic excitations and de-excitations. During excitation cycles, the nuclear kinetic energy is accumulated to overcome the dissociation barriers in the reactant and a sequence of intermediates. The dissociation pathway includes the ejection of NO₂ groups followed by the formation of NO and CO. The simulated mass spectra at the ab initio level, based on the bond length in possible fragments, are extracted from simulation trajectories. The recently developed methodology has the potential to model and monitor photoreactions with open-shell intermediates and radicals.

Density Matrix Treatment of the Nonmarkovian Dissipative Dynamics of Adsorbates on Metal Surfaces

A density matrix treatment is presented for the vibrational relaxation of the frustrated translational mode of a molecule adsorbed on a metal surface. The system is modeled as a vibrating adsorbate oscillator coupled to a bath of harmonic oscillators representing either phonons or electronic density fluctuations. The integrodifferential equations for time evolution of the density matrix including a (nonmarkovian) delayed dissipation are solved using a generalized Runge-Kutta scheme. The equations are also solved in the instantaneous dissipation and the Markov limits, to ascertain their validity. Numerical results are presented for Na/Cu, CO/Cu, and CO/Pt systems. The population of an initially excited state is given over time for varying temperatures and shows that memory effects are needed in a proper description valid even at short times. Calculations of populations for different coupling strengths between the adsorbate species and the substrate metal surface indicate that a weaker coupling leads to increased oscillation amplitudes and longer relaxation times. The time evolution of quantum coherence is also described.