Photo-double-ionization of ethylene and acetylene near threshold

Abstract

We present kinematically complete measurements of the photo double ionization of ethylene (double CC bond) and acetylene (triple CC bond) hydrocarbons just above the double ionization threshold. We discuss the results in terms of the coincident kinetic energy of the photo electrons and the nuclear kinetic energy release of the recoiling ions. We have incorporated quantum chemistry calculations to interpret which of the electronic states of the dication have been populated and trace the various subsequent fragmentation channels. We suggest pathways that involve the electronic ground and excited states of the precursor ethylene dication and explore the strong influence of the conical intersections between the different electronic states. The nondissociative ionization yield is small in ethylene and high in acetylene when compared with the dissociative ionization channels.The reason for such a striking difference is explained in part on the basis of a propensity rule which influences the population of states in the photo double ionization of a centrosymmetric closed shell molecule by favoring singlet ungerade and triplet gerade final states. This propensity rule and the calculated potential energy surfaces clarify a picture of the dynamics leading to the observed dication dissociation products.

Hydrogen and fluorine migration in photo-double-ionization of 1,1-difluoroethylene (1,1-C2H2F2Gaire1, Bocharova2, Sturm3 et al. 2014Phys. Rev. ASelf Cite41130View full textAdd to dashboardCite

“…2 ) with a doubleionization potential of about 31 eV, which is very similar to the PDI of C 2 H 4 and C 2 H 2 comprising a threshold energy of 30.3 and 33.2 eV, respectively [11]. In all PDI channels of 1,1-C 2 H 2 F 2 , larger photon energy leads to a long tail in threshold energy distributions, a result of higher-lying states being accessible at higher photon energy.…”

“…Fragmentation of the dication results in DI channels; otherwise, we detect the NDI channel. The kinetic energy release (KER), which is the sum of the kinetic energies of the nuclear fragments in the center-of-mass frame of the molecule, and the E sum of the photoelectrons are used to track down the most likely fragmentation pathways on the PES [11]. Note that the KER marks the energy difference between the points on the PES where the dissociation begins and the asymptotic limit of the corresponding final state on the same PES or another one if transition(s) occurred.…”

“…For 60-eV photon energy, the shoulderlike distribution suggests that at least three different states contribute to this dissociation pathway. This fluorine-migration channel seemingly requires less energy than the prompt C=C double-bond breaking which follows next and peaks at around 35 eV, i.e., at almost the same value as the C=C bond cleavage in the PDI of C 2 H 4 [11]. In the PDI of C 2 H 4 , however, the double-bond breaking goes along with the molecular hydrogen elimination channel (H + 2 + C 2 H + 2 ) and both are preceded by the deprotonation channel (H + + C 2 H + 3 ), while in the PDI of C 2 H 2 the symmetric breakup (CH + + CH + ) peaks at 39 eV and is preceded by the deprotonation (H + + C 2 H + ) starting at 34.75 eV and the asymmetric breakup (C + + CH + 2 ) at 35.25 eV [11].…”

“…corresponding to 5% to 20% of the ion momentum for the respective breakup channels) as employed in our earlier work on ethylene (C 2 H 4 ) and acetylene (C 2 H 2 ) [11]. The overall branching ratio for each of these channels is presented in Table I and also in Fig.…”

“…Simple hydrocarbon molecules are an ideal testbed for a series of studies with increasing complexity. Here, we choose the 1,1-difluoroethylene (1,1-C 2 H 2 F 2 , H H >C=C< F F ) molecule as a followup to our recent investigation of acetylene (C 2 H 2 ) and ethylene (C 2 H 4 ) [11]. The latter molecules are simple centrosymmetric closed-shell hydrocarbon molecules.…”