Photoionization of Alkanes. Dissociation of Excited Molecular Ions
Authors: Steiner Bruce , Giese Clayton F. , Inghram Mark G. 
Published: January 2005

This report describes studies of photoionization and subsequent dissociation of all saturated paraffins from C2 to C6, plus n-heptane and n-octane, as a function of photon energy. The studies include data on all processes which occur below a photon energy of 11.9 ev. The instrument used was a mass spectrometer, combined with a Seya-Namioka monochromator. The direct experimental data are presented as parent, fragment, and metastable ionization efficiency curves. For each molecule the family of curves obtained by dividing the derivatives of each normalized ionization efficiency curve by the derivative of the total ionization are presented. These derived curves facilitate the test of theories of dissociation kinetics.

From a study of the total ionization curves, it is concluded that the adiabatic ionization transition is not accessible within the Franck-Condon region and that it is therefore not possible to determine the ionization potentials of these molecules by ordinary impact experiments.

The influence of stored thermal energy was studied by determining the ionization efficiency curves (except for metastables) at two temperatures, 300° and 415°K, and comparing the shifts in the curves with the differences in average stored energy. An analysis of transit times in the mass spectrometer was made which (in addition to providing information necessary to the understanding of the curve shapes and metastable intensities) suggests the existence of ``missing metastables,'' i.e., ions which dissociate in the regions of acceleration (fast metastables) or deflection (slow metastables) and hence are spread out in the mass spectrum so that they do not contribute appreciably to a mass peak. The experimental results confirm the existence of these missing metastables, although the intensity attributed to them is statistically rather uncertain. Changing the ion drawing-out potential in the ionization chamber permitted variation of the extraction time by a factor of 3.3 and the effect of this variation upon several fragment ionization efficiency curves was determined.

The experimental results have been compared with the predictions of the statistical theory of dissociation kinetics, a theory which has been widely used in the interpretation of mass spectra. This theory in simplified form involves three parameters, the threshold energy for dissociation E0, a frequency ν, and the number of oscillators n. The results of the comparison may be summarized as follows.

(1) The effects of temperature cannot be reconciled with the theory unless n is made a free parameter which takes on a variety of values less than or equal to the total number of oscillators.

(2) The statistical theory requires, as an energy randomization mechanism, the presence of many closely spaced electronic states. The experimental evidence is that these states do not exist.

(3) The statistical theory predicts the energy excess needed to give a rate constant such that dissociation will occur within the ionization chamber. Comparison with experimental results shows that this excess energy, calculated with reasonable values for E0 and ν and with n equal to the total number of oscillators, is much too large. Reducing n removes the difficulty.

(4) The shapes of the fragment and metastable curves, and the metastable intensity, may be calculated from the theory. The tails of the predicted curves are much too large, as is the predicted metastable intensity. Agreement may be obtained by using physically implausible values for E0 and/or ν, or by using reasonable values for E0 and ν, and reduced values of n. The reduced values of n do not agree with those obtained in (1).

(5) The shifts in fragment curves resulting from changing the extraction time require either unreasonable values for E0 and/or ν, or a reduced n. The reduced n which is required does not agree with that determined from (1) or (4).

(6) The intensity of the missing metastables, though statistically rather uncertain, apparently cannot be reconciled with the statistical theory regardless of what values are taken for E0, ν, and n.

(7) In many cases, the dissociation processes leading to different fragments compete (i.e., maintain approximately a constant intensity ratio) over a range of photon energy. Again, one cannot obtain this behavior from the statistical theory with any choice of E0, ν, and n which are consistent with the other data. Individually, these comparisons perhaps are not conclusive, but it is believed that collectively they constitute a definite failure of the statistical theory in its simple form.

The lack of a quantitative understanding of the excess energies demanded in the kinetics of dissociation prevents the use of the data for determination of reliable bond dissociation energies. Nevertheless, tentative values have been determined, and these are tabulated and discussed.