Ionization efficiency curve measurements

Indeed, by using soft El ionization, we have been able to unambiguously detect products from all five reaction pathways (2a)-(2e), determine their branching ratio and characterize their dynamics.34 Here we discuss some of the results that we have obtained on this reaction, which well exemplify the power of soft El ionization. First of all, from measurements of the El efficiency curves at various to/e ratios (15, 42, and 43), we have found that the parent ion at m/e = 43 (CH2CHO+, corresponding to one of the main reaction channels, the vinoxy radical,) is not stable, so measurements of angular and TOF distributions were carried out at m/e = 42. Incidentally, from the El ionization efficiency curve at m/e = 42 we have obtained some direct information on the IE of the vinoxy radical, for which no such information was available till now. The IE should be <11 eV. 

This expression reproduces the experimentally measured ionization efficiency curves surprisingly well, considering the simplicity of the model on which it is based. There is a discontinuity in the function at the maximum (when X = Xmax) but this affects only a small region of the ionization efficiency curve, and satisfactory values of the cross section are still obtained over this region. A great advantage of this method is that it is very simple to apply, depending on only three parameters the molecular polarizability volume, the ionization potential, and the maximum electron impact ionization cross section. These can be measured or calculated values (from the ab initio EM method described above, for example). 

MgF ionization-efficiency curves obtained from mass spectral observations on the MgFg (1,) and MgF -Al (2) systems are of the two-process type. Evidence (j, ) has been presented which shows that the low-energy portion (<13 ev) of the curve arises from the direct ionization (A) of MgF(g). These curves also show an inflection point at higher energies ( 14 eV) which has been interpreted (1 - ) as due to the onset of the dissociative ionization (B) of MgP (g). The appearance potentials, in eV, which have been observed for these two processes from measurements on various MgP systems are tabulated below. 

Delwiche (j ) has reported an ionization efficiency curve for SFg which was obtained by retarded potential difference (RPD) measurements. This curve reveals the existence of an excited state, lying 2177 cm above the ground state. We include this level and assume that the ground and excited states are both singlets since SFg has no unpaired electrons. 

MS Monoenergetic electron impact on PH2 (from benzylphosphane) was used, and the ionization efficiency curve was extrapolated to zero ion current. The sharp onset pointed to the adiabatic character of Ej. A measurement with conventional electron energy spread and the usual semilogarithmic method due to [6] yielded Ej = 9.96 eV [2]. 

Most of the discussion in this chapter has dealt with measurements and techniques unique to ion cyclotron resonance spectrometry. However, it is often useful to have ionization efficiency curves available for analyzing the reactions which may be taking place in a given chemical system, and most mass spectrometers can be used to obtain such curves. 

Junk and Svec also reported N4 formation to be proportional to the square of the pressure, but did not measure the appearance potential nor examine the ionization-efficiency curves. Comes reports that he obtained results for N4" formation in N2 in accord with the associative ionization reaction... 

The current produced by the cations in the collection chamber is measured at various applied potentials and yields an ionization efficiency curv e. The ener r at which cations are just formed is the appearance potential and corresponds to I. In practice, the electrons generally have an energy spread with a Boltzmann distribution and the determination of the appearance potential is rather difficult however, procedures have been developed that yield /-values reproducible to 0.1-0.2 e.v. or better (for summaries, see 4-6). 

These unsolved problems can be addressed by detailed CMB experiments, which use soft El ionization for product detection and possibly also very low collision energies. In fact, by measuring the El efficiency curves for the C3H products it may be possible to obtain some information on which isomer is actually formed, because the IE of C-C3H is expected to be... 

Photoionization studies of rare gas dimers have been carried out . Photoionization efficiency curves for the vdW molecules (Ar. .. Ar, Kr. .. Kr, Xe. .. Xe) were obtained by means of molecular beam techniques. The dissociation energies of the ions were calculated from the measured first ionization potentials (Table 6) and the dissociation energies of the parent vdW molecules. As the dissociation energy of vdW molecules is neghgible compared the dissociation energy of the rresponding cations, the latter energies (available from other sources) can be used to estimate first ionization potentials of vdW systems which have not yet been studied (values in parentheses in Table 6). 

In this chapter we focus attention on the efficiency of ionization, the ionization cross section, and consider some recent experimental measurements and theoretical studies of the ionization process. A sketch of electron impact ionization curves, the variation of the ionization cross section as a function of the electron energy, using CO as an example, are shown in Figure 1. The mass spectrum, collected at the electron energy corresponding to the maximum in the ionization cross section, is also shown, although there will be no further discussion of fragmentation in this... 

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