Three-photon photodissociation of adsorbed halomethanes by excimer lasers: A general mechanism

Multiple photon excitation of adsorbed toluene by KrF laser

“…Moreover, the kinetic energy of the desorbed CH 3 radical determined from the temporal peak position of the mass spectrometric signal was as low as 30 meV. This is consistent with the mechanism of the (2-1) PD, where a molecule finds itself in a repulsive state with potential energy close to the bond dissociation energy [15].…”

“…The molecule photoexcited to a repulsive state can release from the multilayer of physisorbed molecules. Both the dissociation and intact ejection of physisorbed CH 3 I molecules from the multilayer as onephoton and three-photon processes have been discovered [15]. These experimental data support this mechanism.…”

“…The excitation to higher energies in the continuum reduces the chance of dissociation as more repulsive states can be converted into bound ones due to avoiding crossing [17]. The three-photon dissociative processes in adsorbed toluene are most likely driven by the mechanism of (2-1) photodissociation (PD) [15]. This mechanism includes the following steps: Two-photon excitation of the adsorbed molecule in the electronic continuum, fast vibrational relaxation, and molecular transition caused by the stimulated emission of the third photon.…”

“…D 0 is the bond dissociation energy [19]. To realize the (2-1) PD, the bond dissociation energy must be lower than the laser photon energy (hν = 5 eV) [15]. As a part of the excitation energy (ΔE) of the molecule is lost in its vibrational relaxation, then possible dissociative processes are further limited by the condition: D 0 < hν − ΔE.…”

“…The laser-modified surface influenced both the laser field acting on the adsorbed molecules and the dynamics of their excitation. Our previous study dealt with adsorbed halomethanes irradiated with excimer lasers [13][14][15]. We found that the three-photon dissociation of halomethanes prevailed over the one-photon process at laser intensities higher than 1 MW cm −2 if the first photon excited a repulsive state.…”