Photoinduced dissociative electron attachment of CH3Br on Pt(111): The role of the local work function

Abstract

The photoinduced dissociative electron attachment of CH3Br on Pt(111) has been examined by measuring CH3 photofragment translational energy distributions from CH3Br adsorbed on Xe multilayers deposited upon CH3Br covered Pt(111). Subvacuum level, photoexcited substrate electrons produced by 308 nm laser irradiation were found to propagate through the Xe layer where attachment to CH3Br led to fragmentation. A simple model for surface dissociative electron attachment, DEA, was found to quantitatively predict the observed fragmentation dynamics for the CH3Br/Xe/CH3Br/Pt(111) system. The role of the local work function in surface DEA processes was explored by examining the dependence of the photofragment yield upon the thickness of the intermediate Xe layer and the fractional coverage of the Pt(111) bound first layer of CH3Br. Three dimensional variation of the local work function above the substrate was required to account for the observed DEA dynamics.

Femtosecond‐laser photoemission and photodesorption from magnesia supported gold clusters

“…6: (i) The excitation mechanism generally observed in methyl halide molecules that are adsorbed on metal surfaces consists in the attachment of hot electrons, which have been excited from the metal valence band states, to the molecules leading to the formation of transient anions. However, these anion states are repulsive and hence the excitation leads to dissociation of the molecules liberating methyl radicals and halide ions (dissociative electron attachment, DEA [15,16]; see also excitation mechanism (1) in Fig. 6).…”

“…Whereas on insulating substrates like LiF [10,11] and MgO [12] direct photodissociation was found to prevail, dissociative electron attachment was discussed as predominant mechanism for methyl fragment generation on the surfaces of metals like aluminum [13], platinum [14][15][16], nickel [17], copper [11], and silver [18]. On the surface of the semiconductor GaAs the mechanism leading to dissociation could be tuned through the appropriate doping [19,20].…”

Photochemical Exploration of Reaction Dynamics at Catalytic Metal Surfaces: From Ballistics to Statistics

“…Microscopic reversibility implies that an entrance channel barrier should be surmounted in the reverse reaction, methane dissociative adsorption. In the photochemical hydrogenation reaction, methyl radicals were produced with translational energies of up to 2 eV, 33 and the reactive trajectories may have sampled a portion of the reactive potential somewhat differently than in a thermal reaction. As a consequence, it was instructive to examine the thermal hydrogenation, CH 3 (ad) + H(ad) f CH 4 (g), by angle resolved thermal programmed reaction (TPR), where detailed balance should apply.…”

Photodissociation dynamics of molecules on surfaces.

“…This is contrary to w hat we would expect the top adsorbate layer com pletely covers th e underlayers. The decrease of hot electrons through increasing layers of adsorbed molecules has been noted in previous studies [37]. We would expect th a t as additional layers are added, th a t charge transfer would become a less effective process and more of the signal would result from direct photodissociation.…”

“…[37,17], and th e electron affinity of brom ine EAsr-is 3.36 eV [37,26]. Using a range of 0 to 4 quanta for the vibrational energy of th e CHz fragm ent and using equation 4.8 we find for gas phase CHzBv, th a t Tavaii ranges from 0.89 eV for v = 0 to 0.59 eV for v=4.…”