Femtosecond time-resolved photodissociation dynamics of methyl halide molecules on ultrathin gold films

Abstract

SummaryThe photodissociation of small organic molecules, namely methyl iodide, methyl bromide, and methyl chloride, adsorbed on a metal surface was investigated in real time by means of femtosecond-laser pump–probe mass spectrometry. A weakly interacting gold surface was employed as substrate because the intact adsorption of the methyl halide molecules was desired prior to photoexcitation. The gold surface was prepared as an ultrathin film on Mo(100). The molecular adsorption behavior was characterized by coverage dependent temperature programmed desorption spectroscopy. Submonolayer preparations were irradiated with UV light of 266 nm wavelength and the subsequently emerging methyl fragments were probed by photoionization and mass spectrometric detection. A strong dependence of the excitation mechanism and the light-induced dynamics on the type of molecule was observed. Possible photoexcitation mechanisms included direct photoexcitation to the dissociative A-band of the methyl halide molecules as well as the attachment of surface-emitted electrons with transient negative ion formation and subsequent molecular fragmentation. Both reaction pathways were energetically possible in the case of methyl iodide, yet, no methyl fragments were observed. As a likely explanation, the rapid quenching of the excited states prior to fragmentation is proposed. This quenching mechanism could be prevented by modification of the gold surface through pre-adsorption of iodine atoms. In contrast, the A-band of methyl bromide was not energetically directly accessible through 266 nm excitation. Nevertheless, the one-photon-induced dissociation was observed in the case of methyl bromide. This was interpreted as being due to a considerable energetic down-shift of the electronic A-band states of methyl bromide by about 1.5 eV through interaction with the gold substrate. Finally, for methyl chloride no photofragmentation could be detected at all.

Photoionization and photofragmentation in mass spectrometry with visible and UV lasers

“…For Au 3 + , which was re‐formed without fragmentation, a photocatalytic reaction cycle could be formulated in which methyl radicals are thermally “liberated”; this process is then followed by the light‐induced abstraction of the remaining bromine atoms. Later in that year, a third halomethane was added to the investigation (Vaida et al, ). The group published a study comparing methyl iodide, methyl bromide and methyl chloride.…”

Photoinduced triplet-state electron transfer of platinum porphyrin: a one-step direct method for sensing iodide with an unprecedented detection limit

“…[12][13][14] In addition to its environmental and biological importance, iodide is a key component of a redox couple oen used in solar-energyharvesting systems. 15 Various methods can be used for the determination of iodide, 16 including photoluminescence, [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] colorimetric detection, [33][34][35][36][37] time-resolved absorption techniques, 38 mass spectrometry, [39][40][41] chromatography, 42 Raman scattering 43,44 and electrochemical proling. 45,46 However, because of its large size and weakly basic nature, the binding capacity of iodide is the weakest among the halide ions.…”

Recent Advances in Luminescent Recognition and Chemosensing of Iodide in Water

“…[36] Furthermore, hazardous iodine radioactive isotopes ( 129 I/ 131 I) have been released into biosphere due to the human activities as Fukushima nuclear disaster, [37] which in aqueous media and physiological conditions are transformed into iodide salts bringing along pollution and public health problems, therefore, it is strongly desirable the development of reliable, selective and real-time analytical techniques for detecting and monitoring iodide concentration in drinking water, foods, environmental and biological samples. Standard analytic methods for the quantitative detection of I À include electrochemical techniques, [38] mass-spectrometry, [39] time-resolved absorption, [40] chromatography, electrophoresis, [41] and Raman scattering. [42] However, many of these methods require multistep sample preparation and/or sophisticated high-cost instrumentation.…”