Quantum kinetic theory of vibrational heating and bond breaking by hot electrons

A quantum kinetic theory of surface bond-breaking induced by laser-excited hot electrons is presented within a vibrational-heating mechanism, in which the fast electron-vibration energy transfer leads to heating and breaking of a local bond on the femtosecond scale. The theory consists of two related parts. The first part discusses generally the validity of the Pauli master equation in describing the dynamics of vibrational heating and reaction. Starting from the Liouville-equation formalism of the density matrix, a general kinetic equation is derived for the reduced density distribution of a bond coupled with a hot-electron bath. In the eigenstates representation, the diagonal approximation of this equation corresponds to the generalized Pauli master equation, while the off-diagonal part describes the quantum coherence effect beyond the master equation. For a bond with no initial coherence and weak electron-vibration coupling, the effect of the off-diagonal elements is negligible. In the second part, the inelastic transition rates needed in the kinetic equations are derived both perturbatively and nonperturbatively in a resonance model of electron-vibration coupling. The general theory is applied to two examples: (i) C-O heating on Cu surface, where we compare the difference between quantum and classical descriptions, and demonstrate the importance of a quantum-mechanical model; and (ii) desorption of O-2 from Pt(lll) is studied as a model bond-breaking process, and the results are compared with recent experimental data.