RESUMEN
Using light energy to drive chemical reactions on semiconductor surfaces is the basis for technological applications ranging from the removal of organic pollutants to the generation of renewable solar fuels, yet our understanding of the mechanisms has been hindered by the multistep nature of the process and the wide range of time scales over which it occurs (femtoseconds to seconds). In this work, we use ultrafast laser pump-probe techniques to follow the time evolution of substrate-induced photooxidation of acetone on a titania surface. A UV light at 260 nm initiates carrier-induced fragmentation of adsorbed acetone on a TiO2(110) surface that was pretreated with oxygen. The photoreaction results in the ejection of methyl radicals into the gas-phase that are detected by the probe pulse via resonant multiphoton ionization. The time evolution of the methyl radicals leaving the surface exhibits ultrafast rise times, 300-700 fs, followed by a more gradual rise that plateaus by 10 ps, with faster rates at a low acetone coverage. These results are interpreted in terms of a time-dependent rate expression and a mechanism in which the fragmentation of the acetone surface species is driven by interactions with nonequilibrium, "hot" holes.
RESUMEN
Time- and Angle-resolved photoelectron spectroscopy from surfaces can be used to record the dynamics of electrons and holes in condensed matter on ultrafast time scales. However, ultrafast photoemission experiments using extreme-ultraviolet (XUV) light have previously been limited by either space-charge effects, low photon flux, or limited tuning range. In this article, we describe XUV photoelectron spectroscopy experiments with up to 5 nA of average sample current using a tunable cavity-enhanced high-harmonic source operating at 88 MHz repetition rate. The source delivers >1011 photons/s in isolated harmonics to the sample over a broad photon energy range from 18 to 37 eV with a spot size of 58 × 100 µm2. From photoelectron spectroscopy data, we place conservative upper limits on the XUV pulse duration and photon energy bandwidth of 93 fs and 65 meV, respectively. The high photocurrent, lack of strong space charge distortions of the photoelectron spectra, and excellent isolation of individual harmonic orders allow us to observe laser-induced modifications of the photoelectron spectra at the 10-4 level, enabling time-resolved XUV photoemission experiments in a qualitatively new regime.
RESUMEN
The photooxidation of ethanol and 2-propanol was studied under UHV conditions on a single crystal TiO2(110) surface using a combination of temperature programmed desorption (TPD) and pump-probe laser ionization techniques. Previous studies of these reactions have shown that the first step involves photocatalytic dehydrogenation to either an acetaldehyde or acetone intermediate. In this work, we show that when adsorbed alcohols are irradiated with UV light in the presence of molecular oxygen, methyl radicals are ejected from the surface. Furthermore, it is shown that these radicals possess kinetic energy distributions which are remarkably similar to those measured for the photooxidation of acetaldehyde and acetone. This result suggests that methyl radicals are produced during a second photocatalytic step which involves photooxidation of the aldehyde/ketone intermediates.
RESUMEN
A new technique for studying surface photochemistry has been developed using an ion imaging time-of-flight mass spectrometer in conjunction with a fast camera capable of multimass imaging. This technique, called pixel imaging mass spectrometry (PImMS), has been applied to the study of butanone photooxidation on TiO2(110). In agreement with previous studies of this system, it was observed that the main photooxidation pathway for butanone involves ejection of an ethyl radical into vacuum which, as confirmed by our imaging experiment, undergoes fragmentation after ionization in the mass spectrometer. This proof-of-principle experiment illustrates the usefulness and applicability of PImMS technology to problems of interest within the surface science community.
RESUMEN
State-resolved laser techniques were used to study the internal state distributions of gas phase methyl radicals which are produced during the photooxidation of acetone on TiO2(110). This approach was used as a means of understanding the nature of the bimodal kinetic energy distributions for these radicals. Specifically, we investigated the population of the ν2 "umbrella mode" which has been shown to be important in similar photodissociation reactions where methyl radicals are liberated. We observed that for methyl radicals undergoing prompt dissociation (EK = 0.15 eV), the vibrational population in the umbrella mode is quite cold and can be characterized by a Tvib = 151 ± 15 K. Methyl radicals in this channel were also characterized by a rotational energy distribution of Trot = 325 ± 25 K which is comparable to the gas phase value obtained by acetone photolysis. State-resolved energy distributions also show that methyl radicals which are vibrationally excited have an overall kinetic energy distribution which is ~35 meV less than those which are in their vibrational ground state. This value is comparable to, but not exactly in agreement with, the known vibrational spacing of the ν2 mode and suggests that vibrationally excited methyl radicals have less energy available for translation.