From Energetics to Intracluster Chemistry: Valence Photoionization of Trifluoromethylsulfur Pentafluoride (CF3SF5) by Double Velocity Map Imaging

Branching ratios in the dissociative photoionization of iodomethane by photoelectron photoion coincidence

Iodomethane yields ten fragment ions after valence photoionization, in part by multiple dissociation pathways for each, thanks to a plethora of electronic states available in the parent ion as well as in the fragments. The comprehensive breakdown diagram from 11 eV to the double ionization onset, i.e., 26.7 eV, is recorded at high resolution using double imaging photoelectron photoion coincidence spectroscopy with synchrotron vacuum ultraviolet radiation. Based on fragment ion groupings, the changing branching ratios between these groups and between fragment ions within each group, as well as ancillary thermochemistry, we provide an overview of the dissociation pathways at play. Statistical and impulsive dissociations are identified using kinetic energy release analysis. Finally, a newly observed regime change is discussed in double ionization, whereby coincident H⁺ + I⁺ formation dominates over a 4 eV photon energy range, outcompeting the normally prevailing CH₃⁺ + I⁺ channel.

Isomer-Dependent Threshold Photoelectron Spectroscopy and Dissociative Photoionization Mechanism of Anisaldehyde

We studied the threshold photoionization and dissociative ionization of para-, meta-, and ortho-anisaldehyde by photoelectron photoion coincidence spectroscopy in the 8.20-19.00 eV photon energy range. Vertical ionization energies by equation of motion-ionization potential-coupled cluster singles and doubles (EOM-IP-CCSD) calculations reproduce the photoelectron spectral features in all three isomers. The dissociative photoionization (DPI) pathways of para- and meta-anisaldehyde are similar and differ markedly from those of ortho-anisaldehyde. In the para and meta isomers, the lowest-energy DPI channel corresponds to hydrogen atom loss to form the C₈H₇O₂⁺ fragment at m/z 135, which undergoes sequential dissociation processes at higher energies, such as carbon monoxide loss to C₇H₇O⁺ (m/z 107) and further, sequential CH₃, CH₂O, and CH₂CO losses to produce C₆H₄O⁺ (m/z 92), C₆H₅⁺ (m/z 77), and C₅H₅⁺ (m/z 65), respectively. Carbon monoxide loss from the parent ions, yielding C₇H₈O⁺ (m/z 108), is a subordinate dissociation channel parallel to H atom loss. At higher energies, it also gives rise to sequential formaldehyde (CH₂O) loss to produce C₆H₆⁺ (m/z 78). In the ortho-anisaldehyde cation, the vicinity of the aldehyde and methoxy groups opens up low-energy hydrogen-transfer processes, which allow for seven fragmentation channels to compete effectively with the H- and CO-loss channels. Thus, the fragmentation mechanism changes considerably, thanks to the steric interaction of the substituents. Functional group interactions, in particular H transfer pathways, must therefore be considered when predicting the isomer-specific unimolecular decomposition mechanism of cationic and neutral species, as well as mass spectra for isomers.

A plethora of isomerization processes and hydrogen scrambling in the fragmentation of the methanol dimer cation: a PEPICO study

The valence photoionization of light and deuterated methanol dimers was studied by imaging photoelectron photoion coincidence spectroscopy in the 10.00-10.35 eV photon energy range. Methanol clusters were generated in a rich methanol beam in nitrogen after expansion into vacuum. They generally photoionize dissociatively to protonated methanol cluster cations, (CH₃OH)_nH⁺. However, the stable dimer parent ion (CH₃OH)₂⁺ is readily detected below the dissociation threshold to yield the dominant CH₃OH₂⁺ fragment ion. In addition to protonated methanol, we could also detect the water- and methyl-loss fragment ions of the methanol dimer cation for the first time. These newly revealed fragmentation channels are slow and cannot compete with protonated methanol cation formation at higher internal energies. In fact, the water- and methyl-loss fragment ions appear together and disappear at a ca. 150 meV higher energy in the breakdown diagram. Experiments with selectively deuterated methanol samples showed H scrambling involving two hydroxyl and one methyl hydrogens prior to protonated methanol formation. These insights guided the potential energy surface exploration to rationalize the dissociative photoionization mechanism. The potential energy surface was further validated by a statistical model including isotope effects to fit the experiment for the light and the perdeuterated methanol dimers simultaneously. The (CH₃OH)₂⁺ parent ion dissociates via five parallel channels at low internal energies. The loss of both CH₂OH and CH₃O neutral fragments leads to protonated methanol. However, the latter, direct dissociation channel is energetically forbidden at low energies. Instead, an isomerization transition state is followed by proton transfer from a methyl group, which leads to the CH₃(H)OH⁺⋯CH₂OH ion, the precursor to the CH₂OH-, H₂O-, and CH₃-loss fragments after further isomerization steps, in part by a roaming mechanism. Water loss yields the ethanol cation, and two paths are proposed to account for m/z 49 fragment ions after CH₃ loss. The roaming pathways are quickly outcompeted by hydrogen bond breaking to yield CH₃OH₂⁺, which explains the dominance of the protonated methanol fragment ion in the mass spectrum.