Photon impact ionization

These distinguishing characteristics of chemical ionization mass spectrometry have been discussed previously, but as a result of recent studies (to be discussed in more detail later) we have come to recognize another characteristic aspect of the chemical ionization process when the high-pressure mass spectrometric technique is used. When chemical ionization is effected at a pressure in the mass spectrometer ionization chamber on the order of 1 Torr, the ions comprising the mass spectra are produced by collision processes, and after formation the ions undergo a number of collisions with molecules of reactant gas before they pass out of the ionization chamber. Thus, unlike the conditions which obtain in conventional (low-pressure) electron-impact and photon-impact ionization, the ions produced by chemical ionization are not isolated. That is to say, they are not formed... 

Electrons and photons do not impact molecules or atoms. They interact with them in ways that result in various electronic excitations, including ionization. For this reason it is recommended that the terms electron impact and photon impact be avoided. 

The ionization of ammonia clusters (i.e. multiphoton ionization,33,35,43,70,71 single photon ionization,72-74 electron impact ionization,75 etc.) mainly leads to formation of protonated clusters. For some years there has been a debate about the mechanism of formation of protonated clusters under resonance-enhanced multiphoton ionization conditions, especially regarding the possible alternative sequences of absorption, dissociation, and ionization. Two alternative mechanisms63,64,76,77 have been proposed absorption-ionization-dissociation (AID) and absorption-dissociation-ionization (ADI) mechanisms see Figure 5. 

Photoionization, as already pointed out, is characterized by a step function for ionization probabiUty versus energy. The change in mode of ionization is thus much more easily detectable than for electron impact which produces only changes of slope. The combination of photon impact ion sources with mass analysis has been a major advance in technique since it has allowed the direct study of formation and breakdown of excited ions. The first account of such an experiment was given by Hurzeler, Inghram and Morrison (1958) who employed the especially convenient Seya-Namioka type of monochromator, which had then just been described, in conjunction with a conventional magnetic sector mass... 

It is apparent that in both cases energy E is deposited in [AB+ +eej] and that, as in the case of excitation, the photon energy is analogous to the electron energy loss. However, since there are now two electrons sharing the excess energy in electron-impact ionization, it is necessary to use time correlation (coincidence techniques) for the simulation of photoionization... 

Utilizing ionization efficiency curves to determine relative populations of vibrationally excited states (as in the photoionization experiments) is a quite valid procedure in view of the long radiative lifetime that characterizes vibrational transitions within an electronic state (several milliseconds). However, use of any ionization efficiency curve (electron impact, photon impact, or photoelectron spectroscopic) to obtain relative populations of electronically excited states requires great care. A more direct experimental determination using a procedure such as the attenuation method is to be preferred. If the latter is not feasible, accurate knowledge of the lifetimes of the states is necessary for calculation of the fraction that has decayed within the time scale of the experiment. Accurate Franck -Condon factors for the transitions from these radiating states to the various lower vibronic states are also required for calculation of the modified distribution of internal states relevant to the experiment.991 102... 

P.J. Marchalant, K. Bartschat, R-matrix with pseudostates calculation for single and double ionization of helium by photon impact, Phys. Rev. A 56 (1997) 1697-1700. 

Hydrocarbon molecules are abundant constituents of planetary atmospheres and major compounds in combustible gas mixtures and in fusion edge plasmas [7-11]. Methane is the simplest of these hydrocarbon molecules. Acetylene, C2H2, is the simplest hydrocarbon molecule that contains 2 carbon atoms. Thus absolute total and partial photon [24-27] and electron [15,28-34] ionization cross-sections and nascent fragment ion energy distributions [19,20,28,36-40] have been studied extensively for these molecules. For the deuterated methane molecule electron impact ionization and dissociative ionization cross-sections were determined for the CD (x=l—4) molecule and radicals applying a fast neutral beam technique [41]. Electron impact total ionization cross-sections have been determined also theoretically applying the BEB (Binary-Encounter-Bethe) model [42], the DM (Deutsch-Mark) method [43] and the JK (Jain-Khare) method [44], Partial electron impact ionization cross-sections were calculated for methane [45,46] as well as total electron impact cross-sections for various CH radicals [47]. The dissocia-... 

Quite apart from thermolysis occurring before fragmentation, the temperature of the ion source may have a marked effect on the appearance of a mass spectrum. Comparison of mass spectra obtained with hot and cooled ion-sources and of spectra obtained by photon impact or field ionization show by the increased amount of fragmentation that a molecular ion possesses a greater excess of internal energy when formed in a hot, electron-impact source. Possible origins of this excess internal energy are collision with or radiation from surfaces. Some effects of hot and cold ion sources are discussed. 

PI is single photon ionization El electron impact ionization and TPI two photon ionization. 

On the other hand, Hansen et al. [28] measured A -x-ray intensity ratios for various elements following A -capture decay of radioactive nuclides and pointed out that the KP/Ka ratios by electron capture (EC) decay are considerably different from those by photon and electron impact ionization. Paic and Pecar [29] found that the Kp/Ka ratios for Ti, V, Cr, and Fe by EC are smaller by almost 10% than those by photoionization (PI), but no appreciable difference was observed for Cu and Zn. A similar excitation mode dependence was measured for Mn by Arndt et al. [30]. They stated that the reason for the difference is due to the excess 3d electron in EC and the large shakeoff probability in PI. Rao et al. [31] also observed smaller KP/Ka intensity ratios by EC for Mn and Fe. Since no appreciable difference was found for high-Z elements, they concluded that the difference observed for 3d elements can be ascribed to the chemical effect. It is usual that the chemical forms of the samples for EC measurements are different from those for PI. In order to elucidate the excitation mode dependence on the Kp/Ka ratios in 3d elements, it is necessary to perform theoretical calculations which takes into account the chemical effect as well as the difference in the electron configurations. 

In the case of ion impacts, the excitation processes are more complicated than those for the photon impacts, which are schematically shovm in Fig. 12. The K L , and K L" ionized states are mainly produced through the direct Coulomb potential acting between projectiles and orbital electrons on the target atom, where n 2. Then the single and double ionization cross sections due to the direct Coulomb potential, o kilo and can be calculated using the single... 

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