Femtosecond Photodesorption of Small Molecules from Surfaces: A Theoretical Investigation from First Principles

Abstract

A microscopic model for the excitation and relaxation processes in photochemistry at surfaces is developed. Our study is based on ab initio calculations and the surrogate Hamiltonian method treating surface electron-hole pairs as a bath of two-level systems. Desorption probabilities and velocities in the experimentally observed range are obtained. The excited state lifetime is calculated, and a dependence of observables on pulse length is predicted. DOI: 10.1103/PhysRevLett.90.117601 PACS numbers: 79.20.La, 68.43.Tj Surface photochemistry occurs in many instances, including photocatalysis, atmospheric chemistry, and photochemical reactions at interstellar dust particles. Despite its ubiquitous nature, a microscropic understanding of the underlying basic processes remains a great challenge. In order to obtain a detailed mechanistic picture, the complexity of the studied systems and processes has to be reduced significantly. The simplest photochemical phenomenon on surfaces is laser-induced desorption of small molecules. Two-state models such as the Menzel-GomerRedhead or Antoniewicz scenarios [1] have proven useful to qualitatively understand the mechanism of DIET (desorption induced by electronic transitions). In both models, the excitation from the electronic ground to an excited state and the relaxation back to the ground state are modeled as vertical transitions. While significant progress has recently been made concerning reliable potential energy surfaces for DIET dynamics [2,3], excitation and relaxation mechanisms have been modeled only semiphenomenologically [4,5] assuming a separation of time scales of excitation, excited state nuclear dynamics, and quenching. Since electronic relaxation occurs on the time scale of femtoseconds [5], this assumption breaks down with the introduction of femtosecond laser pulses.Electronic relaxation is caused by the interaction of the excited adsorbate-substrate complex with electron-hole pairs in the surface [5]. In this Letter, we develop a microscopic model for this interaction assuming that electron-hole pairs can be described as a bath of twolevel systems (TLS). The surrogate Hamiltonian approach [6] is employed in which for a finite time the infinitely many TLS are approximated by a finite number. The finite time interval for which converged results can be obtained depends on the interaction strength and the number of TLS. This means that convergence is controllable and no further assumptions are needed. The surrogate Hamiltonian approach represents one possibility to treat a quantum system interacting with its environment.Since it does not rely on the approximation of weak coupling between system and environment and a separation of time scales of system dynamics and relaxation, it is particularly well suited to describe ultrafast charge transfer events in condensed phase [7]. In the present study, the construction of the bath is tailored to model electron-hole pairs in insulator and semiconductor surfaces. A particularly well studied example where quenching by ...

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