Coincident velocity map image reconstruction illustrated by the single-photon valence photoionisation of CF₃SF₅

Abstract

Velocity map imaging offers high energy resolution and collection efficiency of the steady flux of photoelectrons and ions in continuous photoionisation experiments. In this proof-of-principle work, we show by the photoionisation of trifluoromethyl sulphur pentafluoride, CFSF, that the four-dimensional problem of reconstructing coincident velocity map images of electrons and ions of certain mass can be addressed by separating the energy distribution from the angular anisotropy. The energy spectrum is predominantly determined by the radial distribution of the image, whereas laboratory frame angular anisotropies are revealed based on the radial distribution of the image multiplied with a 2nd-degree Legendre polynomial. The reconstruction yields the energy correlation between the photoion and the photoelectron characteristic of the photoelectron spectrum and the kinetic energy release. The angular anisotropy β-parameter maps of the photoelectrons and photoions are also obtained as 2D functions of the electron and ion kinetic energies. For photoionisation of CFSF, the energy correlation reveals suprastatistical kinetic energy release (KER) in CF production in the ground cationic X[combining tilde] state, but statistical KER in the excited à and B[combining tilde] state bands. Although the photoelectron distribution is isotropic, the photoion anisotropy in the energy range of the X[combining tilde] state speaks for prompt dissociation after preferential ionisation of CFSF molecules aligned with the polarisation vector of the synchrotron radiation. The angular dependence of the photoionisation cross section is confirmed by ab initio calculations for vertical ionisation.

Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate

“…Fragmentation processes involving a sizable reverse barrier involve suprastatistical kinetic energy release. While there are experimental − and theoretical approaches to address this quantitatively, we do not expect them to reveal substantially more accurate energetics than offered by quantum chemical calculations for vanillin. Consequently, we only included the three primary fragmentation pathways as well as the sequential dissociations from the H-loss fragment, i.e., H, CO, CH 3 , H + CO, and H + CO + CH 3 loss, in the statistical model to derive experimental 0 K appearance energies, E 0 , for the four decomposition channels.…”

Valence Photoionization of Thymine: Ionization Energies, Vibrational Structure, and Fragmentation Pathways from the Slow to the Ultrafast

“…As observed, for example, for CF 3 SF 5 , a constant fraction of excess energy may be released as translational energy over a broad energy range in impulsive dissociations . Although this assumption may not hold as strongly for thymine, because the parent ion is not formed at a repulsive section of the potential energy surface, we adapted the statistical model assuming that the product energy distribution is statistical in all degrees of freedom except the reaction coordinate, in which a constant fraction of the excess energy is deterministically released.…”

“…[57] As observed, for example, forC F 3 SF 5 ,aconstantf raction of excesse nergy may be released as translational energy over a broad energy range in impulsive dissociations. [58] Although this assumption may not hold as strongly for thymine, because the parenti on is not formed at ar epulsive section of the potential energy surface, we adapted the statistical model assuming that the product energy distribution is statistical in all degrees of freedom except the reaction coordinate, in which aconstant fraction of the excess energy is deterministically released. Whereas the threshold to m/z 55 fragment ion formationw as modeled to be 11.95 eV,t he product energy wasc alculated to be 11.15 AE 0.10 eV.I ft he kinetic energy release factor (KERF) is constant,i ti sp ossible to account for this by removing one translational degree of freedom from the statistical calculation of the product energy distribution and rescaling the breakdown diagram in the photon energy range of Hl oss, that is, at hv > 12.99 eV,according to Equation (1).…”

Dissociative photoionization of 1,3‐dioxolane: We need six channels to fit the elephant

“…The question whether the ergodic hypothesis holds and the dissociation is governed by statistical theory is all the more pertinent when the breakdown curves exhibit unusual trends, as is certainly the case for 1,3‐dioxolane (see below). In nonstatistical dissociations, a nuclear 40 or an electronic 41,42 degree of freedom is decoupled from the rest, which may markedly change the dissociation kinetics and the resulting internal energy distributions 43 . Rate constants for nonstatistical channels may increase significantly and statistically minor channels may become dominant.…”