Contrasting mechanisms for photodissociation of methyl halides adsorbed on thin films of C₆H₆ and C₆F₆

Abstract

Photodissociation of CH3X on C6F6 thin films on copper finds that dissociation by photoelectrons is entirely quenched. Different behaviour is observed for these molecules on C6H6 films, including a CT-complex pathway for CH3I on thick C6H6 films.

Adsorbate dissociation due to heteromolecular electronic energy transfer from fluorobenzene thin films

“…The TPD spectra observed for the sequence from mFBz through pFBz were similar in terms of general features to those observed for benzene 7,18,19 and hFBz. 7,26 Submonolayer quantities of the fluorobenzenes were observed to desorb from the Cu(100) substrate with a peak near 210 K (ranging from 200 K to 220 K).…”

“…The TPD spectra observed for the sequence from mFBz through pFBz were similar in terms of general features to those observed for benzene 7,18,19 and hFBz. 7,26 Submonolayer quantities of the fluorobenzenes were observed to desorb from the Cu(100) substrate with a peak near 210 K (ranging from 200 K to 220 K). As the dose was increased, the monolayer desorption peak broadened toward lower temperatures, characteristic of repulsive intermolecular interactions.…”

“…4 In the astrophysical context, non-thermal photodesorption of H 2 O and benzene molecules from mixed benzene:H 2 O ices due to near-UV absorption by the benzene has been demonstrated. 5,6 The present study expands upon an earlier work 7 in which an EET process was identified by the dynamics seen with time-of-flight spectroscopy in which contribution to the photodissociation of CH 3 I adsorbed on benzene thin films was observed, but absent for CH 3 I on hexafluorobenzene thin films. Here we study CH 3 photofragment kinetic energies from the UV photodissociation of CH 3 I adsorbed on series of different fluorobenzene thin films and report systematic changes in the observed photodissociation dynamics.…”

“…2(g)), new peaks appear at 47 ms and 60 ms (peaks ''A'' and ''B'') with the first between the I and I* peaks and the second appearing at a later time. 7 In contrast to the I and I* peaks, which are most prominent when p-polarized light is used, these additional peaks are of similar intensity for both p-and s-polarized incident light. The TOF spectra obtained using p-polarized light are composed of both the neutral CH 3 I photodissociation channel (I and I*) features, plus the new dissociation channel that has an intensity and peak location that substantially overlaps with the I and I* features.…”

“…To convert the cross sections to absolute values, the measured reference cross section for CH 3 I on hFBz is B8.0 Â 10 À19 cm 2 , similar to what was reported using 3ML hFBz films. 7 The relative depletion cross sections for CH 3 I on the various 10ML thin films are shown in Table 1. In comparison to the reference hFBz thin film, that from pFBz is very similar.…”