H. M. Rosenstock

Franck—Condon Factors for Polyatomic Molecules

General expressions are derived for calculating Franck—Condon factors for most transitions of polyatomic molecules (excluding transitions between linear and bent configurations) in the harmonic oscillator approximation. The derivation employs the method of generating functions and also linear transformation of normal mode coordinates between initial and final states. Input data required in the general case are geometries, frequencies, and vibrational force fields for the initial and final states. Explicit algebraic expressions are presented for the transition between linear, symmetric, triatomic initial and final states. The results are used to show that fragmentation cannot result from direct vertical ionization to the ground electronic states of CS2+ and CO2+ or to the excited 2IIu state of CO2+. An attempt was made to fit the experimental photoionization data for C2H2 and C2D2. This resulted in an estimated increase of 0.05 A in the equilibrium C–C bond distance and no change in the C–H bond distance i...

Experimental and Theoretical Studies of Photoionization‐Efficiency Curves for C2H2 and C2D2

Photoionization‐efficiency curves for C2H2 and C2D2 have been remeasured in the wavelength region from onset of ionization to 600 A, using a gold‐coated grating and the Hopfield continuum as a photon source. The curve shapes of the molecule ions near threshold are compared with calculations of the Franck—Condon factors. Nearly exact agreement between experiment and calculation is obtained when taking into account hot bands. A general discussion is given for the shape of the molecule‐ion curve and for that of the fragment ions, C2H+ and C2D+. The observed onsets of ionization for the latter ions are 17.22— and 17.34 eV, respectively.

Interpretation of photoionization thresholds:quasiequilibrium theory and the fragmentation of benzene

Abstract Quasiequilibrium theory has been applied to the parent ion fragmentation of benzene. Assuming uniform excitation energy transfer in the fragmentation threshold region and applicability of the step-function photoionization threshold law, it is possible to calculate fragment photoionization threshold curves in good agreement with experiment. It is concluded that the unimolecular decomposition occurs via two independent pairs of competing reactions. One pair of reactions leads to the formation of C 6 H 5 + and C 6 H 4 + , and involves the benzene ion ground state. The other pair of reactions, leading to C 4 H 4 + and C 3 H 3 + involves the first excited state of C 6 H 6 + lying 2.25 eV above the ground state, or an open chain isomer having a similar heat of formation. At threshold the C 6 H 5 + ion has a phenyl ion structure, the C 3 H 3 + ion a cyclopropenyl ion structure and the C 4 H 4 + ion may have a cyclobutene ion structure. A heat of formation of ∼100 kcal/mol is derived for the benzyne molecule, in good agreement with semi-empirical estimates. Kinetic shift effects on the fragmentation thresholds are found to be important. Some difficulties are encountered in the comparison of relative abundances of parent ion metastable transitions to the relative abundances of the daughter ions near threshold. The calculated energy dependence of the unimolecular rate of formation of C 4 H 4 + ion is in good agreement with experiment. However, the weak energy dependence of the rate of formation of C 6 H 5 + found experimentally cannot be explained. Suggestions for further work are outlined.

Photoelectron–photoion coincidence study of the bromobenzene ion

The technique of variable time photoelectron–photoion coincidence mass spectrometry has been applied to the fragmentation of bromobenzene ion producing a phenyl ion. A detailed analysis of the variation of the breakdown curve with parent ion residence time was performed. The results lead to ΔH °f0 (phenyl ion)=270 kcal/mole in close agreement with recalculated results from an earlier study on chlorobenzene. This, combined with other photoionization results leads to ΔH °f0 (phenyl radical)=83±3 kcal/mole, slightly higher than the value 80.9±2 kcal/mole obtained from neutral kinetics. The analysis leads to a rate‐energy dependence for the fragmentation process and an equivalent 1000 K Arrhenius pre‐exponential factor of ∼9.4×1014 sec−1, which may be compared to the value 2×1015 sec−1 for the analogous neutral process. The possible contribution of spin orbit splitting is discussed.

On the mechanisms of C6H6 ionization and fragmentation

Abstract Photoionization and electron monochromator studies have been carried out on benzene, 1,5-hexadiyne, 2,4-hexadiyne, pyridine and some C4H4 isomers to develop new information concerning the details of benzene ion fragmentation. Present results, together with those of other workers, indicate that a number of C6H6+ ion isomers have energies comparable to that of the first excited state of the benzene ion. These could act as intermediate states in skeletal fragmentation of the benzene ion and account for the observed small kinetic shift. Results on vinyl acetylene and butatriene indicate that these ion structures are not formed at the fragmentation threshold in benzene. Results on 1,5-hexadiyne and pyridine indicate the formation of a new C4H4+ ion structure, more stable than the two linear isomers. The evidence for lack of competition between hydrogen loss and skeletal fragmentation in the benzene ion is reviewed. Photoionization of benzene reveals autoionization contributions to parent ion and fragment ion production; assignments of the autoionizing levels are given. The photoionization behavior of 1,5-hexadiyne suggests that this also fragments via two pairs of non-competing reactions, as postulated for benzene.

Kinetic shift in chlorobenzene ion fragmentation and the heat of formation of the phenyl ion

The fragmentation of chlorobenzene ion has been studied by photoelectron–photoion coincidence techniques. By varying the residence time it is possible to obtain breakdown curves as a function of residence time. The parent–daughter transition region shifts to lower energies as the residence time is increased (kinetic shift). The shift is of the order of 0.4 eV in going from 0.7 to 8.9 μs. A systematic analysis of the breakdown curves and residence time effects has been carried out using quasiequilibrium theory. The experimental results and analysis lead to ΔHf00  (phenyl ion)  =275±1 kcal/mol (1151±4 kJ/mol. The systematic analysis shows that this experiment leads to a quite accurate rate‐energy curve in the range of 104–106 s−1. The sensitivity of the QET model has been studied, and the limitations to the determination of activated complex parameters is critically discussed. The parameters obtained in this work are rather similar to those of an analogous neutral process, i.e., thermal decomposition of bro...

The measurement of ionization and appearance potentials

Abstract Measurement techniques and methods of data interpretation leading to ionization potentials, appearance potentials and heats of formation of gaseous positive ions are summarized and critically discussed. Special attention is paid to defining the accuracies, problems and limitations of the techniques and interpretations. References have been selected and cited which discuss these matters in more detail.

Direct Observation of the Decomposition of Multiply Charged Ions into Singly Charged Fragments

A new technique has been developed for the direct observation of the decomposition of multiply charged polyatomic ions. Positive ions or ion pairs and ejected electrons, formed in a uniform electrostatic field by a 1‐keV electron beam, are accelerated in opposite directions to multiplier detectors. The masses of the positive ions are determined by measuring the time interval between electron and ion pulses by delayed coincidence. The formation of positive ion pairs is demonstrated and their masses determined by electron—ion—ion delayed coincidence techniques. The effects of initial kinetic energy on mass‐spectrum peak shapes and travel time correlations between members of an ion pair are discussed. Both of these effects are experimentally observed. The technique has been applied to the study of ionization and fragmentation of CO2, CF4, CH4, and C3H6. Numerous decompositions leading to the formation of positive ion pairs are observed. Significant fractions of particular high‐kinetic‐energy fragment ions ar...

Initial Preparation and Metastable Transitions in Mass Spectra

By variation of initial preparation of the reacting species, the detection of nonequilibrium kinetic effects is discussed for unimolecular ionic decomposition processes occurring in the mass spectrometer. A particular example, dependence of the rates of competing metastable transitions of hexyl ions on mode of preparation is studied in detail. The hexyl ions were prepared by electron impact ionization and decomposition of a variety of normal alkanes, n‐hexyl bromide and di‐n‐hexyl ether. At low electron energies (30 eV) the ratio of the competing metastable transitions is constant within experimental error. This supports the applicability of the quasiequilibrium theory of mass spectra to such processes. At higher electron energies there is a slight dependence of the ratio on parent ion mass.

Photoionization Mass Spectrometry of NO

The photoionization efficiency curve of the NO molecule has been measured in the range 1360–600 A, and the autoionization structure compared with the absorption spectrum. A number of plausible vibrational progressions are suggested in the 1130–900 A range.