Mechanisms for the near-UV photodissociation of CH3I on D2O/Cu(110)

Abstract

The system of CH3I adsorbed on submonolayer, monolayer, and multilayer thin films of D2O on Cu(110) has been studied by measuring the time of flight (TOF) distributions of the desorbing CH3 fragments after photodissociation using linearly polarized λ = 248 nm light. For multilayer D2O films (2-120 ML), the photodissociation is dominated by neutral photodissociation via the "A-band" absorption of CH3I. The polarization and angle dependent variation in the observed TOF spectra of the CH3 photofragments find that dissociation is largely via the (3)Q0 excited state, but that also a contribution via the (1)Q1 excitation can be identified. The photodissociation results also indicate that the CH3I adsorbed on D2O forms close-packed islands at submonolayer coverages, with a mixture of C-I bond axis orientations. For monolayer and submonolayer quantities of D2O we have observed a contribution to CH3I photodissociation via dissociative electron attachment (DEA) by photoelectrons. The observed DEA is consistent with delocalized photoelectrons from the substrate causing the observed dissociation- we do not find evidence for an enhanced DEA mechanism via the temporary solvation of photoelectrons in localized states of the D2O ice.

Excitation and quenching mechanisms in the near-UV photodissociation of CH3Br and CH3Cl adsorbed on D2O or CH3OH on Cu(110)

“…These observations are consistent with our previous study of CH 3 I/D 2 O/Cu(110). 28 At the shorter 193 nm wavelength, we also observe neutral photodissociation of both CH 3 Br and CH 3 Cl, with apparent cross sections comparable to those for the gas-phase. That these neutral photodissociation TOF features are very well-defined indicates that the measured CH 3 photofragments are from 'CH 3 -up' oriented molecules that do not significantly interact with the surface during or post bond-cleavage.…”

Photodissociation of methyl iodide adsorbed on low-temperature amorphous ice surfaces

“…37 The slowest component had a translational energy of 0.4 eV, which is ∼1.9 eV less than the gas-phase value. The energy lost by these methyl fragments was most likely gained by the corresponding iodine fragments through collisions.…”

“…On a Cu(110) substrate, a DEA mechanism for dissociation of CH 3 I was indicated by a TOF feature that was present for CH 3 I adsorbed on < 1.0 ML D 2 O but not for CH 3 I on thicker ice films. 37 The present study involved 50 ML of ice adsorbed on polycrystalline zirconia, a wide band-gap (>5 eV) substrate. Since the bulk band gap of the zirconia was larger than the incident laser photon energy (4.28 or 4.77 eV) and the ice film was thick, dissociation of adsorbed CH 3 I by singlephoton-induced electron attachment was unlikely.…”

“…The angle-resolved measurements of Miller et al showed that fewer collisionally slowed methyl fragments were detected at off-normal angles. 37 Thus, methyl fragments oriented toward the surface are the most likely to lose energy due to collisions, and iodine fragments oriented away from the surface are the most likely to gain energy from rebounding methyl fragments.…”

“…In fact, a recent study of the methyl fragment produced by 248-nm photodissociation of CH 3 I on D 2 O aptly complements the present study of the iodine photofragment. 37 The possible mechanisms for the direct release of reactive halogen species involved in ozone depletion episodes at polar sunrise are not fully understood. Questions remain unanswered regarding the relative quantum yields of these reactive halogen species, the effects of solar radiation and ice morphology on the uptake and release of these halogens, and the possibility of enhanced catalytic processes on ice (snow) surfaces.…”