title: A vacuum ultraviolet laser pulsed field ionization-photoion study of methane (CH4): determination of the appearance energy of methylium from methane with unprecedented precision and the resulting impact on the bond dissociation energies of CH4and CH4.
abstract: We report on the successful implementation of a high-resolution vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) detection method for the study of unimolecular dissociation of quantum-state- or energy-selected molecular ions. As a test case, we have determined the 0 K appearance energy (AE0) for the formation of methylium, CH3+, from methane, CH4, as AE0(CH3+/CH4) = 14.32271 ± 0.00013 eV. This value has a significantly smaller error limit, but is otherwise consistent with previous laboratory and/or synchrotron-based studies of this dissociative photoionization onset. Furthermore, the sum of the VUV laser PFI-PI spectra obtained for the parent CH4+ion and the fragment CH3+ions of methane is found to agree with the earlier VUV pulsed field ionization-photoelectron (VUV-PFI-PE) spectrum of methane, providing unambiguous validation of the previous interpretation that the sharp VUV-PFI-PE step observed at the AE0(CH3+/CH4) threshold ensues because of higher PFI detection efficiency for fragment CH3+than for parent CH4+. This, in turn, is a consequence of the underlying high-n Rydberg dissociation mechanism for the dissociative photoionization of CH4, which was proposed in previous synchrotron-based VUV-PFI-PE and VUV-PFI-PEPICO studies of CH4. The present highly accurate 0 K dissociative ionization threshold for CH4can be utilized to derive accurate values for the bond dissociation energies of methane and methane cation. For methane, the straightforward application of sequential thermochemistry via the positive ion cycle leads to some ambiguity because of two competing VUV-PFI-PE literature values for the ionization energy of methyl radical. The ambiguity is successfully resolved by applying the Active Thermochemical Tables (ATcT) approach, resulting in D0(H-CH3) = 432.463 ± 0.027 kJ mol-1and D0(H-CH3+) = 164.701 ± 0.038 kJ mol-1.
authors.affiliation: Department of Chemistry, University of California, Davis, California 95616, USA. cyng@ucdavis.edu.
authors.forename: Yih-Chung
authors.identifier: http://orcid.org/0000-0002-3763-1864
authors.initials: YC
authors.lastname: Chang
authors.affiliation: Department of Chemistry, University of California, Davis, California 95616, USA. cyng@ucdavis.edu.
authors.forename: Bo
authors.initials: B
authors.lastname: Xiong
authors.affiliation: Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: David H
authors.initials: DH
authors.lastname: Bross
authors.affiliation: Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA and Computation Institute, The University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Branko
authors.identifier: http://orcid.org/0000-0002-4372-6990
authors.initials: B
authors.lastname: Ruscic
authors.affiliation: Department of Chemistry, University of California, Davis, California 95616, USA. cyng@ucdavis.edu.
authors.forename: C Y
authors.identifier: http://orcid.org/0000-0003-4425-5307
authors.initials: CY
authors.lastname: Ng