Photon impact

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. 

This model is based on the calculation of probability p of x-ray photon impact on the j-kind particle, referred to fixed size class ... 

In agreement with this expectation Sjogren (16) found that when bombarding C02 with Ne+ ions (RE 21.6 e.v.) of low velocity and at low pressure, vanishing fractions of 0+ ions were obtained. This result indicates that when using electron or photon impact, O + (4S) is formed at 19.1 e.v. after preionizing a highly excited triplet state of neutral C02. 

If the detection screen D is constructed so that the locations of individual photon impacts can be observed (with an array of scintillation counters, for example), then two features become apparent. The first is that only whole photons are detected each photon strikes the screen D at only one location. The second is that the interference pattern is slowly built up as the cumulative effect of very many individual photon impacts. The behavior of any particular photon is unpredictable it strikes the screen at a random location. The density of the impacts at each point on the screen D gives the interference fringes. Looking at it the other way around, the interference pattern is the probability distribution of the location of the photon impacts. 

Liquid scintillation counters are highly efficient for low CL intensities and consist of two photon-counting channels provided with a variable discriminator. The sample is placed between these two detectors to ensure a high optical efficiency. The discriminator is adjusted to allow photon impacts to be transmitted and small background noise pulses to be rejected. As disadvantages they suffer from saturation errors and provide nonlinear relationships between the CL intensity and the total counts. 

H. Hurzeler, M. G. Inghram, and J. D. Morrison. Photon Impact Studies of Molecules Using a Mass Spectrometer. J. Chem. Phys., 28(1958) 76-82. 

It could be argued that by showing what can be done with one kind of monoenergetic excitation, the main contribution of photon impact... 

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... 

In many cases the photon can be represented by the two alternative models of a plane wave and a particle-like wavepacket. This should also apply to interference phenomena with individual photons [21]. For a given point at the screen of an experiment with two apertures, the resulting interference pattern obtained from individual photon impacts could thus be interpreted in two alternative ways ... 

These questions appear to be understandable in terms of both photon models. The wavepacket axisymmetric model has, however, an advantage of being more reconcilable with the dot-shaped marks finally formed by an individual photon impact on the screen of an interference experiment. If the photon would have been a plane wave just before the impact, it would then have to convert itself during the flight into a wavepacket of small radial dimensions, and this becomes a less understandable behavior from a simple physical point of view. Then it is also difficult to conceive how a single photon with angular momentum (spin) could be a plane wave, without spin and with the energy hv spread over an infinite volume. Moreover, with the plane-wave concept, each individual photon would be expected to create a continuous but weak interference pattern that is spread all over the screen, and not a pattern of dot-shaped impacts. 

Ions produced by photon impact analysis of photoionization efficiency curves and comparison with ground-state cross section. 

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. 

On the grounds of very general symmetry arguments the triple differential cross section for two-electron emission following photon impact can be represented as... 

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. 

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... 

Recently Konnen et al. (1974a) observed ion production from collisions between sputtered halogen atoms and organic molecules such as aniline. No reactive ionization was found. The positive organic fragment ion spectra resemble those obtained by electron and photon impact. 

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