Abstract
We simulate the femtosecond laser induced desorption dynamics of a diatomic molecule from a metal surface by including the effect of the electron and phonon excitations created by the laser pulse. Following previous models, the laser induced surface excitation is treated through the two temperature model, while the multidimensional dynamics of the molecule is described by a classical Langevin equation, in which the friction and random forces account for the action of the heated electrons. In this work, we propose the additional use of the generalized Langevin oscillator model to also include the effect of the energy exchange between the molecule and the heated surface lattice in the desorption dynamics. The model is applied to study the laser induced desorption of O$_2$ from the Ag(110) surface, making use of a six-dimensional potential energy surface calculated within density functional theory. Our results reveal the importance of the phonon mediated process and show that, depending on the value of the electronic density in the surroundings of the molecule adsorption site, its inclusion can significantly enhance or reduce the desorption probabilities.
Highlights
Laser-driven photochemistry has proven to be a useful tool for promoting reactions at surfaces or even as a way to open new reaction channels not accessible by thermal activation [1,2,3,4,5]
The laser-induced surface excitation is treated through the two temperature model, while the multidimensional dynamics of the molecule is described by a classical Langevin equation, in which the friction and random forces account for the action of the heated electrons
One usually distinguishes between desorption induced by electronic transitions (DIET) and desorption induced by multiple electronic transitions (DIMET) [6]
Summary
Laser-driven photochemistry has proven to be a useful tool for promoting reactions at surfaces or even as a way to open new reaction channels not accessible by thermal activation [1,2,3,4,5]. Desorption on metals can be induced either by directly exciting the molecule (IR photons) or it can be substrate mediated (UV/Vis/near IR photons). DIET is practically realized by using continuous wave or nanosecond-pulse lasers with low intensity, resulting in small desorption yields that increase linearly with laser fluence. In DIET on metals, the adsorbate captures a hot electron and forms a short lived excited state (negative ion resonance). DIMET, which is the subject of the present study, is realized by intense femtosecond laser pulses. Such pulses are short in comparison to typical relaxation times of adsorbate excited states and, they can produce multiple excitations of an adsorbate that lead to desorption. DIMET results in relatively large desorption yields that increase superlinearly with laser fluence [1]
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