Electron-impact spectroscopy excitation

A further technique exists for the determination of triplet energy levels. This technique, called electron impact spectroscopy, involves the use of inelastic scattering of low-energy electrons by collision with molecules. The inelastic collisions of the electrons with the molecules result in transfer of the electron energy to the molecule and the consequent excitation of the latter. Unlike electronic excitation by photons, excitation by electron impact is subject to no spin selection rule. Thus transitions that are spin and/or orbitally forbidden for photon excitation are totally allowed for electron impact excitation. 

The lower triplet state corresponds to the 3a (3a ) transition of the tt,x type. This band could underlie the 385-to 200-nm band since the Franck-Condon accessible region may be quite high in energy. Excited electronic states of ketene have been recently studied by electron impact spectroscopy (87). 

The reactivity of furan is most often associated with triplet states. Calculations (INDO) of charge distribution agree with reactivities actually found, and triplet cr-complexes are suggested as key intermediates.11 Low-lying triplet states have been detected in furan by variable-angle electron-impact spectroscopy (singlet -> triplet transitions are most intense at 3.99 and 5.22 6V).12 Excited stated may be quenched by furan without any indication ofexciplex formation.13... 

The UV absorption spectrum of carbon dioxide has been widely studied both experimentally [20, 22, 27, 116, 146] and theoretically [18, 21, 24,147-150] (see also Sect. 1.2.2). A huge effort has been lavished on identiiying the excited states involved in the electrrMiic transitions. To this end, electron impact spectroscopy has proved to be an additional useful diagnostic tool [151-153]. 

Electron-impact energy-loss spectroscopy (EELS) differs from other electron spectroscopies in that it is possible to observe transitions to states below the first ionization edge electronic transitions to excited states of the neutral, vibrational and even rotational transitions can be observed. This is a consequence of the detected electrons not originating in the sample. Conversely, there is a problem when electron impact induces an ionizing transition. For each such event there are two outgoing electrons. To precisely account for the energy deposited in the target, the two electrons must be measured in coincidence. 

Figure Bl.25.12. Excitation mechanisms in electron energy loss spectroscopy for a simple adsorbate system Dipole scattering excites only the vibration perpendicular to the surface (v ) in which a dipole moment nonnal to the surface changes the electron wave is reflected by the surface into the specular direction. Impact scattering excites also the bending mode v- in which the atom moves parallel to the surface electrons are scattered over a wide range of angles. The EELS spectra show the higlily intense elastic peak and the relatively weak loss peaks. Off-specular loss peaks are in general one to two orders of magnitude weaker than specular loss peaks.

This example demonstrates some of the dangers which beset the use of mass spectroscopy to determine bond enthalpies in systems of this kind. In general, if the excitation energy and excess kinetic energy of the ions produced by electron impact... 

A rapid reaction kinetic technique (time scale = 10-1000 ps) that typically uses a Van de Graff accelerator or a microwave linear electron accelerator to promptly generate a pulse of electrons at sufficient power levels for excitation and ionization of target substances by electron impact. The technique is the direct radiation chemical analog of flash photolysis and the ensuing kinetic measurements are accomplished optically by IR/visible/UV adsorption spectroscopy or by fluorescence spectroscopy. 

A brief review and reassessment of data on the photophysics of benzene has been presented by Pereira. Evidence for the l E2g valence state has been obtained by u.v. two-photon spectroscopy.Slow electron impact excites fluorescence in thin films of benzene at 77 K as well as emission from isomers." The fluorescence yields and quenching by chloroform of alkyl-benzenes and 1-methylnaphthalene after excitation into Si, Sz, and S3 states and after photoionization have been measured. The channel-three process has been reconsidered in terms of the effects of local modes and Morse oscillator potentials. Excited-state dipole moments of some monosubstituted benzenes have been estimated from solvent effects on electronic absorption spectra, Structural imperfections influence the photochemistry of durene in crystals at low temperatures. Relaxation time studies on excited oxido-substituted p-oligophenylenes have been made by fluorescence depolarization... 

Selection rules are the real key to spectroscopy. Without them, transitions between so many levels would be possible that reconstructing energy level diagrams from observational data would become an almost insuperable task. However, since selection rules may hinder accessing certain excited states, it is also necessary to know experimental means of violating them, so that one can, where necessary, reach otherwise inaccessible states. Thus, in a spectrum in which only singlet states can be photo-excited, it may prove useful that electron impact excitation can violate the... 

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