Final state distributions of methyl radical desorption from ketone photooxidation on TiO2(110)

Abstract

In this work, we report on product energy distributions for methyl radicals produced by UV photooxidation of a set of structurally related carbonyl molecules, R(CO)CH(3) (R = H, CH(3), C(2)H(5), C(6)H(5)), adsorbed on a TiO(2)(110) surface. Specifically, laser pump-probe techniques were used to measure the translational energy distributions of methyl radicals resulting from α-carbon bond cleavage induced by photoexcited charge carriers at the TiO(2) surface. Photoreaction requires the presence of co-adsorbed oxygen and/or background oxygen during UV laser (pump) exposure, which is consistent with the formation of a photoactive oxygen complex, i.e., η(2)-bonded diolate species (R(COO)CH(3)). The methyl translational energy distributions were found to be bimodal for all molecules studied, with "slow" and "fast" dissociation channels. The "fast" methyl channel is attributed to prompt fragmentation of the diolate species following charge transfer at the TiO(2) surface. The average translational energies of the "fast" methyl channels are found to vary with R-substituent and correlate with the mass of the remaining surface fragments, RCO(x) (x =1 or 2). By comparison, the average energies of the "slow" methyl channels do not show any obvious correlation with R-substituent. The apparent correlation of the "fast" methyl translation energies with surface fragment mass is consistent with a simple two-body fragmentation event isolated on the diolate molecule with little coupling to the surface. These results also suggest that the total available energy for methyl fragmentation does not vary significantly with changes in R-substituent and is representative of exit barriers leading to "fast" methyl fragments.

Final State Distributions of the Radical Photoproducts from the UV Photooxidation of 2-Butanone on TiO2(110)

“…Mass 15 only appears at short pump−probe delays consistent with the arrival time distribution of methyl radical photoproducts determined in an earlier study. 22 We also note that mass 26 (C 2 H 2 + ) and a small feature near mass 30 are also enhanced at shorter pump−probe delays. The latter suggests that their velocity distributions are more similar to the methyl product rather than the other C 2 H x + (mass 27−29) fragments whose relative intensities are comparable at both 25 and 60 μs pump−probe delays (see Figure 1).…”

“…The translational energies for the overall distribution, ⟨E t (total)⟩, and that for the "slow" and fast" channels, ⟨E t (slow)⟩ and ⟨E t (fast)⟩, are given in Table 2 along with those for the methyl radical loss channel determined in an earlier study. 22 The overall average translational energy obtained from the experimental data is small, ⟨E t ⟩ = 51 meV, which reflects the dominant contribution of the "slow" ethyl fragmentation channel, ⟨E t (slow)⟩ = 24 meV, relative to the "fast" channel, ⟨E t (fast)⟩ = 98 meV. Hence, the primary ethyl fragmentation process leads to photoproducts with low kinetic energies.…”

“…The methyl data were obtained using identical O 2 /butanone coverage (dosed at 100 K) and running conditions (200 K; 5 × 10 −8 Torr). 22 The low yield of methyl (b) translational energy distribution. "•" are the experimental data, the dotted curves correspond to the fitted distributions for the "slow" (red) and "fast" (blue) distributions, and the full curve (dark red) is the sum of the "slow" and "fast" distributions.…”

Exploring surface photoreaction dynamics using pixel imaging mass spectrometry (PImMS)

“…25 Signal due to methyl radical is significantly larger at the shorter laser delay consistent with the higher average kinetic energy for the methyl radical ( E t (total) = 0.195 eV) compared to ethyl radical ( E t (total) = 0.051 eV) produced by 2-butanone photooxidation. 31 In addition to showing the product TOF-MS in Figs. 3(a) and 3(b), we have also included "4D" representations (which are functions of time, x pixel, y pixel, and signal intensity) of the data as recorded by the camera.…”

Photooxidation of ethanol and 2-propanol on TiO2(110): evidence for methyl radical ejection

“…On the other hand, recent work on ketone photooxidation has shown that acetaldehyde and acetone (along with co-adsorbed oxygen) photooxidize via methyl radical ejection. [40][41][42][43] In this paper, we have investigated the photooxidation of ethanol and 2-propanol under ultrahigh vacuum (UHV) conditions using a combination of temperature programmed desorption (TPD) measurements and time-of-flight mass spectrometry (TOF-MS) combined with state-resolved laser ionization of desorbing photoproducts. We present results which show that when these adsorbed alcohols are irradiated with ultraviolet light in the presence of oxygen at low temperature, methyl radicals are ejected into vacuum.…”