Time-Resolved Mass Spectrometric Studies

The rapid formation of molecular iodine following the flash photolytic dissociation of CHgl has been observed by time-resolved mass spectrometry.60 This has been attributed to the reaction (20) rather than to slow termolecular recombination. The experimental difficulties associated with sampling by this technique have been discussed by Meyer.61 This reaction is further discussed in Section IX.D on reaction of I(52Py2) with alkyl iodides. 

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Chuang, T.J., Russia, I. (1984) Time-resolved Mass-spectrometric Study on Infrared Laser Photodesorption of Ammonia from Cu(lOO). Phys. Rev. Lett. 52 2045. 

Partial pressures of different species present in the vapour phase over Th02, UO2, UC, were studied at very high temperature (>3000 K), by transient heating of the sample with a 532 nm laser pulse of ca. 8 ns duration with time-resolved mass-spectrometric detection. The surface temperature was varied by changing the laser incident power on the sample. For Th02, the authors give partial pressures of Th(g), ThO(g), Th02(g) and 0(g) at 5146 and 6253 K. Since in this type of experiment, it is difficult to establish reliably the relevant temperatures, these data have not been used in the analysis. Never-... 

Mass spectrometric studies are not limited to the investigation of stable intermediates they have also been carried out on reaction transition states. The ultrafast studies by Zewail, for example, are nominally mass spectrometric based, where photoionization is used to detect reactive species on exceedingly short (femtosecond) time scales.Time resolved studies provide insight into the rates of unimo-lecular reactions, but do not provide direct thermochemical insight. 

Temperature-programmed vacuum pyrolysis in combination with time-resolved soft ionization mass spectrometry allows principally to distinguish between two devolatilization steps of coal which are related to the mobile and non-mobile phase, respectively. The mass spectrometric detection of almost exclusively molecular ions of the thermally extracted or degraded coal products enables one to study the change of molecular weight distribution as a function of devolatilization temperature. Moreover, major coal components can be identified which are released at distinct temperature intervals. 

Mass spectra of short-lived, unstable episelenides were obtained by taking advantage of a mass spectrometric technique developed for the time-resolved detection of transient intermediates in flash-photolyzed systems <66JA4277>. Detection does not depend on the electronic absorption characteristics of the transient, and, in combination with kinetic absorption spectroscopy, the technique assumes great flexibility. The apparatus consists, essentially, of a photolysis cell attached to a small leak into the ion source of an Atlas CH4 mass spectrometer. Selected mass peaks can be studied with a response time of a few milliseconds, and thereafter at times limited by bleeding of the photolyzed mixture into the ion chamber. Typical photolytic flash energies were 480 calories, passed into a reaction volume of 5 ml. 

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