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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, (CH 3 OH) n H + . However, the stable dimer parent ion (CH 3 OH) 2 + is readily detected below the dissociation threshold to yield the dominant CH 3 OH 2 + 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 (CH 3 OH) 2 + parent ion dissociates via five parallel channels at low internal energies. The loss of both CH 2 OH and CH 3 O 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 CH 3 (H)OH + ⋯CH 2 OH ion, the precursor to the CH 2 OH-, H 2 O-, and CH 3 -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 CH 3 loss. The roaming pathways are quickly outcompeted by hydrogen bond breaking to yield CH 3 OH 2 + , which explains the dominance of the protonated methanol fragment ion in the mass spectrum.
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Orientational Analysis of Monolayers at Low Surface Concentrations Due to an Increased Signal-to-Noise Ratio (S/N) Using Broadband Sum Frequency Generation Vibrational Spectroscopy
Sum frequency generation (SFG) * Equal contributors. spectroscopy was used to deduce the orientation of the terminal methyl (CH3) group of self-assembled monolayers (SAMs) at the air–solid and air–liquid interfaces at surface concentrations as low as 1% protonated molecules in the presence of 99% deuterated molecules. The SFG spectra of octadecanethiol (ODT) and deuterated octadecanethiol (d37ODT) SAMs on gold were used for analysis at the air–solid interface. However, the eicosanoic acid (EA) and deuterated EA (d39EA) SAMs on the water were analyzed at the air–liquid interface. The tilt angle of the terminal CH3group was estimated to be ∼39 ° for a SAM of 1% ODT : 99% d37ODT, whereas the tilt angle of the terminal CH3group of the 1% EA : 99% d39EA monolayer was estimated to be ∼32 °. The reliability of the orientational analysis at low concentrations was validated by testing the sensitivity of the SFG spectroscopy. A signal-to-noise (S/N) ratio of ∼60 and ∼45 was obtained for the CH3symmetric stretch (SS) of 1% ODT : 99% d37ODT and 1% EA : 99% d39EA, respectively. The estimated increase in S/N ratio values, as a measure of the sensitivity of the SFG spectroscopy, verified the capacity to acquire the SFG spectra at low concentrations of interfacial molecules under ambient conditions. Overall, the orientational analysis of CH3SS vibrational mode was feasible at low concentrations of protonated molecules due to increased S/N ratio. In support, the improved S/N ratio on varying incident power density of the visible beam was also experimentally demonstrated.
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- Award ID(s):
- 1705817
- PAR ID:
- 10546960
- Publisher / Repository:
- SAGE Publications
- Date Published:
- Journal Name:
- Applied Spectroscopy
- Volume:
- 73
- Issue:
- 10
- ISSN:
- 0003-7028
- Format(s):
- Medium: X Size: p. 1146-1159
- Size(s):
- p. 1146-1159
- Sponsoring Org:
- National Science Foundation
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