Dissociative photoionization mechanism of methanol isotopologues (CH3OH, CD3OH, CH3OD and CD3OD) by iPEPICO: energetics, statistical and non-statistical kinetics and isotope effects

Abstract

The dissociative photoionization of energy selected methanol isotopologue (CH(3)OH, CD(3)OH, CH(3)OD and CD(3)OD) cations was investigated using imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy. The first dissociation is an H/D-atom loss from the carbon, also confirmed by partial deuteration. Somewhat above 12 eV, a parallel H(2)-loss channel weakly asserts itself. At photon energies above 15 eV, in a consecutive hydrogen molecule loss to the first H-atom loss, the formation of CHO(+)/CDO(+) dominates as opposed to COH(+)/COD(+) formation. We see little evidence for H-atom scrambling in these processes. In the photon energy range corresponding to the B[combining tilde] and C[combining tilde] ion states, a hydroxyl radical loss appears yielding CH(3)(+)/CD(3)(+). Based on the branching ratios, statistical considerations and ab initio calculations, this process is confirmed to take place on the first electronically excited Ã(2)A' ion state. Uncharacteristically, internal conversion is outcompeted by unimolecular dissociation due to the apparently weak Renner-Teller-like coupling between the X[combining tilde] and the à ion states. The experimental 0 K appearance energies of the ions CH(2)OH(+), CD(2)OH(+), CH(2)OD(+) and CD(2)OD(+) are measured to be 11.646 ± 0.003 eV, 11.739 ± 0.003 eV, 11.642 ± 0.003 eV and 11.737 ± 0.003 eV, respectively. The E(0)(CH(2)OH(+)) = 11.6454 ± 0.0017 eV was obtained based on the independently measured isotopologue results and calculated zero point effects. The 0 K heat of formation of CH(2)OH(+), protonated formaldehyde, was determined to be 717.7 ± 0.7 kJ mol(-1). This yields a 0 K heat of formation of CH(2)OH of -11.1 ± 0.9 kJ mol(-1) and an experimental 298 K proton affinity of formaldehyde of 711.6 ± 0.8 kJ mol(-1). The reverse barrier to homonuclear H(2)-loss from CH(3)OH(+) is determined to be 36 kJ mol(-1), whereas for heteronuclear H(2)-loss from CH(2)OH(+) it is found to be 210 kJ mol(-1).

Coherent vibrations in methanol cation probed by periodic H3+ ejection after double ionization

“…1c, in which a distinct peak appears at~26.3 THz, corresponding to the period of~38.0 fs. This long-lasting oscillation can only be ascribed to the motion of the vibrational wave packet on the electronic ground state of CH 3 OH + because even the lowest electronically excited state of CH 3 OH + is located 0.5 eV above the dissociation threshold for CH 2 OH + +H 15,16 and the wave packet prepared on these electronically excited states is expected to decay into the dissociation continuum.…”

Vibrational and electronic excitations in fluorinated ethene cations from the ground up

“…This was shown to be the case in F-atom loss in C 2 F 4 + , 9 and probably applies in CH 3 -loss from CH 3 OH + . 41 The nature of the nonstatistical fluorine atom loss from singly to triply fluorinated ethene cations has long been misunderstood. [10][11][12][13] Contrary to F-loss from C 2 F 4 + , detailed kinetic energy release studies have shown that F-loss from C 2 H 3 F + and 1,1-C 2 H 2 F 2 + is, in part, an impulsive process.…”

Wave packet bifurcation in ultrafast hydrogen migration in CH3OH+ by pump-probe coincidence momentum imaging with few-cycle laser pulses

“…Therefore, it is plausible that the excited states, from which CH 2 + can eventually be generated after the migration, is higher lying electronically excited states such as theB andC states. It is then possible in the course of the migration that the wave packet initially prepared in theB and/orC states of CH 3 OH + is transferred to the electronic ground state of migrated CH 2 OH 2 + via internal conversion in a similar manner to that proceeding through the conical intersection associated with the motion of the hydrogen atom in the hydroxyl group of CH 3 OH [12,13].…”

“…It was argued in Refs. [12,13] that CH 3 OH + prepared in the secondB and/or thirdC electronic states decays rapidly into theà state via internal conversion and that the dissociation proceeds on the potential energy surface (PES) of theà state. Therefore, it is highly probable in the present study that the wave packet is prepared in the vibrationally highly excited state of theà state of CH 3 OH + by a pump laser pulse.…”