Transient spectroscopy 0,0 Transition

Radhakrishnan HS, Ahn C, Van Hoeymissen J, Dross F, Cowem N, Van Nieuwenhuysen K, Gordon I, Mertens R, Poortmans J (2012) Gettering of transition metals by porous silicon in epitaxial silicon solar cells. Phys Status Solid (a). doi 10.1002/pssa.201200232,209 1866-1871 Rohatgi A, Davis JR, Hopkins RH, McMullin PG (1983) A study of grown-in impurities in silicon by deep-level transient spectroscopy. Solid State Electron 26 1039 Schindler R (1994) The art of living with defects in silicon gettering and passivation. Solid State Phenom 37 343... 

Throughout the 20th century it was a cornerstone of mechanistic analysis that we could never "see" a transition state. By definition, a transition state is not a stable point on a potential energy surface. Its lifetime is comparable to or less than that of a vibration, so how could we ever hope to see a transition state Well, it turns out that vibrational times are on the order of ps (10" s) convince yourself of this by converting a typical IR stretch (3000-1000 cm ) to a time. What if we could do transient spectroscopy on a time scale that is faster than ps, namely in the femtosecond (10 s) time domain What would we see ... 

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If tlie pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112 and 113] can be perfomied. The second type of experiment, dynamic absorption spectroscopy [57, 114. 115. 116. 117. 118. 119. 120. 121 and 122], can be perfomied if the pump and probe pulses are short compared to tlie period of the vibrational modes that are coupled to the electronic transition. 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

Multidimensional and heteronuclear NMR techniques have revolutionised the use of NMR spectroscopy for the structure determination of organic molecules from small to complex. Multidimensional NMR also allows observation of forbidden multiple-quantum transitions and probing of slow dynamic processes, such as chemical exchange, cross-relaxation, transient Over-hauser effects, and spin-diffusion in solids. 

Most importantly, the careful kinetic analysis of the rise and decay of the transient species in equation (69) shows that the decarboxylation of Ph2C(OH)CO occurs within a few picoseconds (kc c = (2-8) x 1011 s-1). The observation of such ultrafast (decarboxylation) rate constants, which nearly approach those of barrier-free unimolecular reactions, suggests that the advances in time-resolved spectroscopy can be exploited to probe the transition state for C—C bond cleavages via charge-transfer photolysis. 

The purpose of this article is to review the results of transient low pressure studies of carbon monoxide oxidation over transition metal substrates. Particular emphasis is given to the use of in-situ electron spectroscopy, flash desorption, modulated beam and titration techniques. The strengths and weaknesses of these will be assessed with regard to kinetic insight and quantification. An attempt will be made to identify questions that are ripe for investigation. Although not limited to it, the presentation emphasizes our own work. A very recent review of the carbon monoxide oxidation reaction C l) will be useful to readers who are interested in a more comprehensive view. 

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