Reduced and Exact Quantum Dynamics of the Vibrational Relaxation of a Molecular System Interacting with a Finite-Dimensional Bath

We investigate the vibrational relaxation of a Morse oscillator, nonlinearly coupled to a finite-dimensional bath of harmonic oscillators at zero temperature, using two different approaches: Reduced dynamics with the help of the Lindblad formalism of reduced density matrix theory in combination with Fermi\u2019s Golden Rule, and exact dynamics (within the chosen model) with the multiconfiguration time-dependent Hartree (MCTDH) method. Two different models have been constructed, the situation where the bath spectrum is exactly resonant with the anharmonic oscillator transition frequencies, and the case for which the subsystem is slightly off-resonant with the environment. At short times, reduced dynamics calculations describe the relaxation process qualitatively well but fail to reproduce recurrences observed with MCTDH for longer times. Lifetimes of all the vibrational levels of the Morse oscillator have been calculated, and both Lindblad and MCTDH results show the same dependence of the lifetimes on the initial vibrational state quantum number. A prediction, which should be generic for adsorbate systems is a striking, sharp increase of lifetimes of the subsystem vibrational levels close to the dissociation limit. This is contradictory with harmonic/linear extrapolation laws, which predict a monotonic decrease of the lifetime with initial vibrational quantum number

Effect of non-adiabatic coupling on the isotopic dependence of the photodissociation cross section of CO

Calculations of the photodissociation cross sections of CO from the ground X$^1\Sigma^+$ state into the coupled Rydberg B$^1\Sigma^+$ state and the valence dissociative D$^{'1}\Sigma^+$ state have been carried out using the smooth exterior complex scaling (SECS) method. Results are presented for all of the 6 isotopomers involving the 12C, 13C, 16O, 17O and 18O isotopes. The absorption profiles of the predissociating resonances induced by non-adiabatic coupling vary considerably from one isotopomer to another. It is confirmed that resonance widths obtained simply from a diagonalization of the complex Hamiltonian do not give a true indication of the absorption profile. A correct profile is only obtained when the sum over all resonant and non-resonant continuum states is performed. A brief discussion is given of the implications for isotopic fractionation in interstellar CO

Is the simplest chemical reaction really so simple?

Modern computational methods have become so powerful for predicting the outcome for the H + H(2) → H(2) + H bimolecular exchange reaction that it might seem further experiments are not needed. Nevertheless, experiments have led the way to cause theorists to look more deeply into this simplest of all chemical reactions. The findings are less simple