Revision of the second ionization energy of toluene

Abstract

Charge stripping (CS) of the molecular ion of toluene, C(7)H(8) (+)-->C(7)H(8) (2+)+e, is often used as a reference for the determination of second ionization energies in energy-resolved CS experiments. For calibration of the kinetic energy scale, a value of IE(C(7)H(8) (+))=(15.7+/-0.2) eV derived from the appearance energy of the toluene dication upon electron ionization has been accepted generally. Triggered by some recent discrepancies between CS measurements on the one hand and different experimental methods as well as theoretical predictions on the other, we have reinvestigated the photon-induced double ionization of toluene using synchrotron radiation. These photoionization measurements yield phenomenological appearance energies of AE(C(7)H(8) (+))=(8.81+/-0.03) eV for the monocation and AE(C(7)H(8) (2+))=(23.81+/-0.06) eV for the dication. The former is in good agreement with a much more precise spectroscopic value, IE(C(7)H(8))=(8.8276+/-0.0006) eV. Explicit consideration of the Franck-Condon envelopes associated with photoionization to the dication in conjunction with the application of the Wannier law leads to an adiabatic ionization energy IE(a)(C(7)H(8) (+))=(14.8+/-0.1) eV, which is as much as 0.9 eV lower than the previous value derived from electron ionization. Because in many previous CS measurements the transition C(7)H(8) (+)-->C(7)H(8) (2+)+e was used as a reference, the energetics of several gaseous dications might need some readjustment.

Dissociative double photoionization of N2 using synchrotron radiation: Appearance energy of the N2+ dication

“…Strictly speaking, Wannier power law is valid only for atoms, because for molecules the photoionization curve represents a superposition of ionizations to all vibrational states of the molecular ions and this causes the curve to rise very slowly at the onset. 32 In the case of double dissociative ionization leading to atomic fragments, the molecule-photon interaction vertically populates the molecular dication in a high lying excited state above the dissociation limit along which the molecule dissociates into fragmentation channels such as N 2+ + N. In this case, the population transfer from the bound state of N 2 into the repulsive continuum state occurs within a range of interatomic distances according to the FranckCondon factor and this causes a broadening of the energy dependence of the ion yield at threshold. As a consequence, single or double dissociative photoionization cross sections of molecules are expected to rise with a gentler slope compared to the corresponding photoionization in atomic systems ͑a rule of thumb would be as follows: single photoionization of a molecule goes linear with the photon energy and the double ionization goes quadratic͒.…”

Theory meets experiment: Gas-phase chemistry of coinage metals

“…As IE(Cu(H 2 O) + ) is much larger than D(H 2 O-Cu + ), the charge-stripping process has a low efficiency, but the sensitivity of the experiment is still perfectly sufficient for its detection and the exclusion of instrumental artifacts such as overlapping isobaric ions [57]. In this context, we note that at the time the charge-stripping study of Cu(H 2 O) + has been performed, the threshold of electron transfer in keV collisions was believed to be associated with the respective vertical transition, whereas experimental evidence meanwhile indicates that adiabatic thresh- olds are sampled in energy-resolved charge-stripping experiments [66].…”

“…tions [66]. Combined with an independent determination of either EA(M) or IE(M) via photodetachment or photoionization experiments, for example, the redox-chemistry of M −/0/+ can thus be assessed.…”

General trends in dissociations of medium-sized hydrocarbon dications

“…3). The regularity in the stabilities and fragmentation patterns can be most probably ascribed to the fact that dications with odd number of hydrogen atoms are open-shell species presumably with doublet ground state [23], whereas those with even number of hydrogen atoms are closed-shell species, most likely with singlet ground states [19,22,27,32]. The open-shell dications can therefore more easily lose a hydrogen radical, whereas the closed-shell dications preferentially eliminate an H 2 molecule.…”

“…Considering the second explanation presented above, let us refer to the known energetics for the hydrogen eliminations from neutral, singly, and doubly charged benzene (Scheme 1) [14][15][16][17][18][19][20]. Elimination of a hydrogen atom from neutral benzene requires almost 5 eV.…”