Speaker: Loïse Attal, Universität Potsdam; Université Paris-Saclay, CNRS

Addressing the dynamics of molecular systems coupled to an environment is a challenging task, especially when considering finite-size environments that can be affected by their interactions with the smaller system. In such cases, the usual open quantum system methods and approximations might fail as they assume the environment (or bath) to be infinite, always at thermodynamical equilibrium, and unperturbed by the system. In particular, they do not take into account the fact that finite environments can be heated by the excitation of the system and evolve out of equilibrium. Such situations may occur when studying, e.g., internal relaxation inside a large molecule, small molecules trapped in fullerenes or other finite clusters, or in contact with nano-scale devices.

In this context, we have developed a new theoretical model based on a system-bath approach where we consider a one-dimensional system (e.g. one vibrational mode) interacting with a large but finite harmonic bath (~100-1000 modes). The system and its coupling to individual bath modes are treated as rigorously as possible but the bath part of the Hamiltonian is simplified, with its modes being replaced by a single ladder of effective bath states that describe the energy stored inside the bath. This model allows us to treat large bats, to study the relaxation dynamics of the system at finite temperature and to analyze the response of the bath to the system’s excitation.

We present this Effective Bath State (EBS) model and benchmark results obtained on a model system where an O-H stretching mode interacts with a bath of 40 to 600 harmonic oscillators. We also discuss infrared spectra obtained for the phenylacetylene molecule and compare our results with experiments. Finally, a recent extension of the model to a two-dimensional system, and its application to CO adsorbed on a NaCl(100) surface will be presented.