Effect of Unimolecular Decay Kinetics on the Interpretation of Appearance Potentials

Abstract

The interpretation of appearance potential data on diatomic molecules should take account of possible effects caused by predissociation, emission of light and autoionization. In the case of complex polyatomic molecules, the kinetics of predissociation and the internal thermal energy of the molecules become especially important. The intensities of the parent, fragment, and metastable ions produced by photoionization of n-propylamine, n-propanol, and methyl ethyl ketone are studied as a function of photon energy. The excess kinetic energies of the fragment ions are found to be negligibly small. The data are interpreted in terms of Rosenstock's quasi-equilibrium theory of unimolecular decomposition and indicate that the theory is qualitatively correct for the dissociative processes investigated. However, the theory is shown to be quantitatively inadequate at least in the energy range near threshold. In this region the rate constant for dissociation varies much more rapidly with energy than the theory predicts. Some of the assumptions of the theory are examined and compared to deductions from the data. The meaning of appearance potential data is examined in the light of these results. The effects of both the kinetics of dissociation and of internal thermal energy on ionization efficiency curves are significant. Most of the methods used to determine appearance potentials tend to minimize these effects and there is probably some cancellation of errors. A new method for the determination of appearance potentials is described. Experimental methods which can yield more detailed information concerning dissociation processes of complex molecular ions are suggested.

Charge exchange mass spectra of some C5H10 isomers

“…Presumably charge exchange of N2+' with COS resulted largely in formation of S+' (AE = 13.6 eV (28)) which reacts with COS by reaction [2], as previously reported (29).…”

“…The relatively low yield of C3H5+ is to be expected in light of the small yield of molecular ions with internal energies >6 eV where this fragment dominates the breakdown graph. The onset of formation of C3H5+ in the breakdown graphs is considerably higher than the calculated critical energy, most likely a reflection of a kinetic shift (2,41). Although the present study of C5H,, isomers was designed primarily to assess the properties of the charge exchange reagent systems developed rather than to solve significant problems of ion chemistry, some comment on the fragmentation of the different isomers appears desirable.…”

“…~l t h~u g h (6)(7)(8) have been concerned with the question of qualitative kinetic information can be obtained by distinguishing among isomeric molecules from varying the energy of the impacting electrons, their charge exchange mass spectra. quantitative data, in the form of a breakdown With the advent of high pressure chemical ionigraph, can be obtained only by determining the zation mass spectrometry, a number of groups have normalized second derivatives of the ion signals as used charge exchange ionization either separately, a function of the ionizing electron energy (2). or in conjunction with proton transfer ionization in The breakdown graph, i.e., the fractional ion analytical applications (9)(10)(11)(12)(13)(14).…”

Modelling the Dissociation Dynamics and Threshold Photoelectron Spectra of Small Halogenated Molecules

“…63 The true E 0 is then obtained by modelling the rates using the unimolecular rate theories described in the preceding section. 73 A competitive shift arises when another fragmentation channel, with comparable rates to the existing reactions, also becomes accessible at similar energies. The onset of this new channel is not observed at the dissociation threshold, but at a higher energy, only when it is of a similar rate with the fastest reaction.…”

The isolation of 11-oxotetrahydrorhombifoline from Ormosia coutinhoi and a study of its mass spectrum

“…T h e appearance of a strong metastable peak a t m/e 87. 6 this ion arises directly from the molecular ion (calculated value 87.5), and since the most intense peak in the spectrum of 11-dz is also a t m/e 152, the ion CII has lost ring A and is almost certainly the analogue of the ion CI in the spectrum of I. The fragmentation sequence leading to ion A (m/e 221) and subsequently to ions a t m/e 193, 178, and 150 (ion B), which was the predominant pathway in the spectrum of I , is of considerably reduced importance in the spectrum of 11.…”