Energy partitioning in the femtosecond-laser-induced associativeD2desorption from Ru(0001)

Abstract

Energy transfer to different degrees of freedom during the femtosecond-laser-induced recombinative desorption of ${\mathrm{D}}_{2}$ from a deuterium-covered Ru(0001) surface $({\mathrm{D}}_{\mathrm{ads}}+{\mathrm{D}}_{\mathrm{ads}}∕\mathrm{Ru}\ensuremath{\rightarrow}{\mathrm{D}}_{2,\text{gas}}+\mathrm{Ru})$ has been investigated. $(1+{1}^{\ensuremath{'}})$-resonance-enhanced multiphoton photoionization (REMPI) and time-of-flight (TOF) measurements are utilized to provide information on the internal and external (translational) energy content, respectively. Rovibrational population distributions of the reaction product are detected via various $B\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{u}^{+}\ensuremath{\leftarrow}X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+}$ Lyman bands using tunable vacuum ultraviolet laser radiation in the resonant excitation step. Rotational quantum state populations in the vibrational ground state and the first excited state are measured yielding average rotational energies of $⟨{E}_{\mathrm{rot}}⟩∕{k}_{\mathrm{B}}=800$ and $1500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively, for an absorbed laser fluence $⟨F⟩$ of $85\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$. In addition, a mean vibrational energy of the desorbing molecules is extracted which amounts to $⟨{E}_{\mathrm{vib}}⟩∕{k}_{\mathrm{B}}=1200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Extensive TOF measurements enable complete energy balancing with $⟨{E}_{\mathrm{trans}}⟩∕2{k}_{\mathrm{B}}=2500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $⟨F⟩=85\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$ and underline the nonthermal and unequal energy partitioning between the different degrees of freedom within the reaction product. The effects of multidimensional electronic friction between substrate and adsorbate layer and peculiarities of the potential energy landscape governing the ${\mathrm{D}}_{2}$ recombination are discussed.