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Title: A vacuum ultraviolet laser pulsed field ionization-photoion study of methane (CH 4): Determination of the appearance energy of methylium from methane with unprecedented precision and the resulting impact on the bond dissociation energies of CH 4 and CH 4 +

Abstract

Here, 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 (AE 0) for the formation of methylium, CH 3 +, from methane, CH 4, as AE 0 (CH 3 +/CH 4) = 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 CH 4 + ion and the fragment CH 3 + 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 AE 0 (CH 3 +/CH 4) threshold ensues because of higher PFI detection efficiency for fragment CH 3 + than for parent CH 4 +. This, in turn, is a consequence of the underlying high- n Rydberg dissociation mechanism for the dissociative photoionization of CH 4, which was proposed in previousmore » synchrotron-based VUV-PFI-PE and VUV-PFI-PEPICO studies of CH 4. The present highly accurate 0 K dissociative ionization threshold for CH 4 can 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 D 0 (H-CH 3) = 432.463 ± 0.027 kJ/mol and D 0(H-CH 3 +) = 164.701 ± 0.038 kJ/mol.« less

Authors:
ORCiD logo [1];  [1];  [2]; ORCiD logo [3]; ORCiD logo [1]
  1. Univ. of California, Davis, CA (United States)
  2. Argonne National Lab. (ANL), Argonne, IL (United States)
  3. Argonne National Lab. (ANL), Argonne, IL (United States); Univ. of Chicago, Chicago, IL (United States)
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22), Chemical Sciences, Geosciences, and Biosciences Division; National Science Foundation (NSF)
OSTI Identifier:
1366503
Grant/Contract Number:
AC02-06CH11357
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Physical Chemistry Chemical Physics. PCCP (Print)
Additional Journal Information:
Journal Name: Physical Chemistry Chemical Physics. PCCP (Print); Journal Volume: 19; Journal Issue: 14; Journal ID: ISSN 1463-9076
Publisher:
Royal Society of Chemistry
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; active thermochemical tables; dissociative ionization; photoionization; thermochemistry

Citation Formats

Chang, Yih -Chung, Xiong, Bo, Bross, David H., Ruscic, Branko, and Ng, C. Y. 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 CH4 and CH4+. United States: N. p., 2017. Web. doi:10.1039/C6CP08200A.
Chang, Yih -Chung, Xiong, Bo, Bross, David H., Ruscic, Branko, & Ng, C. Y. 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 CH4 and CH4+. United States. doi:10.1039/C6CP08200A.
Chang, Yih -Chung, Xiong, Bo, Bross, David H., Ruscic, Branko, and Ng, C. Y. Mon . "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 CH4 and CH4+". United States. doi:10.1039/C6CP08200A. https://www.osti.gov/servlets/purl/1366503.
@article{osti_1366503,
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 CH4 and CH4+},
author = {Chang, Yih -Chung and Xiong, Bo and Bross, David H. and Ruscic, Branko and Ng, C. Y.},
abstractNote = {Here, 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 CH4 can 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 and D0(H-CH3+) = 164.701 ± 0.038 kJ/mol.},
doi = {10.1039/C6CP08200A},
journal = {Physical Chemistry Chemical Physics. PCCP (Print)},
number = 14,
volume = 19,
place = {United States},
year = {Mon Mar 27 00:00:00 EDT 2017},
month = {Mon Mar 27 00:00:00 EDT 2017}
}

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  • We have examined the dissociative photoionization reaction N{sub 2}+h{nu}{yields}N{sup +}+N+e{sup -} near its threshold using the pulsed field-ionization photoelectron-photoion coincidence (PFI-PEPICO) time-of-flight (TOF) method. By examining the kinetic-energy release based on the simulation of the N{sup +} PFI-PEPICO TOF peak profile as a function of vacuum ultraviolet photon energy and by analyzing the breakdown curves of N{sup +} and N{sub 2}{sup +}, we have determined the 0-K threshold or appearance energy (AE) of this reaction to be 24.2884{+-}0.0010 eV. Using this 0-K AE, together with known ionization energies of N and N{sub 2}, results in more precise values for themore » 0-K bond dissociation energies of N-N (9.7543{+-}0.0010 eV) and N-N{sup +} (8.7076{+-}0.0010 eV) and the 0-K heats of formation for N (112.469{+-}0.012 kcal/mol) and N{sup +} (447.634{+-}0.012 kcal/mol)« less
  • The formation of methyl cation (CH{sub 3}{sup +}) from methane (CH{sub 4}) has been investigated in high resolution using the newly perfected pulsed field ionization photoelectron{endash}photoion coincidence (PFI-PEPICO) scheme. The PFI-PEPICO data reveal that fragmentation of CH{sub 4} in high-{ital n} Rydberg states occurs at energies above the dissociation threshold prior to pulsed field ionization. The crossover point of the breakdown curves is found to depend strongly on the Stark field in the ion source and thus traditional simulation procedures based on such a feature for ion dissociation energy determination are not appropriate in PFI-PEPICO studies. We show that formore » a prompt dissociation process, the disappearance energy of the parent molecule provides an accurate measure of the 0 K ion dissociation threshold, as that for CH{sub 3}{sup +} from CH{sub 4} is 14.323{plus_minus}0.001 eV. {copyright} {ital 1999 American Institute of Physics.}« less
  • We have developed an ion-molecule reaction apparatus for state-selected absolute total cross section measurements by implementing a high-resolution molecular beam vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) ion source to a double-quadrupole double-octopole ion-guide mass spectrometer. Using the total cross section measurement of the state-selected N{sub 2}{sup +}(v{sup +}, N{sup +}) + Ar charge transfer (CT) reaction as an example, we describe in detail the design of the VUV laser PFI-PI ion source used, which has made possible the preparation of reactant N{sub 2}{sup +}(X {sup 2}{Sigma}{sub g}{sup +}, v{sup +}= 0-2, N{sup +}= 0-9) PFI-PIs with high quantummore » state purity, high intensity, and high kinetic energy resolution. The PFI-PIs and prompt ions produced in the ion source are shown to have different kinetic energies, allowing the clean rejection of prompt ions from the PFI-PI beam by applying a retarding potential barrier upstream of the PFI-PI source. By optimizing the width and amplitude of the pulsed electric fields employed to the VUV-PFI-PI source, we show that the reactant N{sub 2}{sup +} PFI-PI beam can be formed with a laboratory kinetic energy resolution of {Delta}E{sub lab}={+-} 50 meV. As a result, the total cross section measurement can be conducted at center-of-mass kinetic energies (E{sub cm}'s) down to thermal energies. Absolute total rovibrationally selected cross sections {sigma}(v{sup +}= 0-2, N{sup +}= 0-9) for the N{sub 2}{sup +}(X {sup 2}{Sigma}{sub g}{sup +}; v{sup +}= 0-2, N{sup +}= 0-9) + Ar CT reaction have been measured in the E{sub cm} range of 0.04-10.0 eV, revealing strong vibrational enhancements and E{sub cm}-dependencies of {sigma}(v{sup +}= 0-2, N{sup +}= 0-9). The thermochemical threshold at E{sub cm}= 0.179 eV for the formation of Ar{sup +} from N{sub 2}{sup +}(X; v{sup +}= 0, N{sup +}) + Ar was observed by the measured {sigma}(v{sup +}= 0), confirming the narrow {Delta}E{sub cm} spread achieved in the present study. The {sigma}(v{sup +}= 0-2; N{sup +}) values obtained here are compared with previous experimental and theoretical results. The theoretical predictions calculated based on the Landau-Zener-Stueckelberg formulism are found to be in fair agreement with the present measured {sigma}(v{sup +}= 1 or 2; N{sup +}). Taking into account of the experimental uncertainties, the measured {sigma}(v{sup +}= 1 or 2, N{sup +}) for N{sup +}= 0-9 at E{sub cm}= 0.04-10.0 eV are found to be independent of N{sup +}.« less
  • By employing an electric field pulsing scheme for vacuum ultraviolet laser pulsed field ionization-photoion (PFI-PI) measurements, we have been able to prepare a rovibrationally selected PFI-PI beam of N{sub 2}{sup +}(v{sup +}= 1, N{sup +}) with not only high intensity and high quantum state purity, but also high kinetic energy resolution, allowing absolute total cross sections [{sigma}(v{sup +}= 1, N{sup +})] for the N{sub 2}{sup +}(X; v{sup +}= 1, N{sup +}) + Ar, N{sup +}= 0-8 charge transfer reaction to be measured at center-of-mass collision energies (E{sub cm}) down to thermal energies. The {sigma}(v{sup +}= 1, N{sup +}= 0-8) valuesmore » determined at E{sub cm}= 0.04-10.00 eV are in good agreement with the theoretical predictions based on the Landau-Zener-Stueckelberg formulism. Taking into account the experimental uncertainties, the {sigma}(v{sup +}= 1, N{sup +}), N{sup +}= 0-8, measured at E{sub cm}= 1.56 eV are found to be independent of N{sup +}.« less
  • The vacuum ultraviolet (VUV) pulsed field ionization photoelectron (PFI-PE) spectra for CH{sub 3}SH and CH{sub 3}CH{sub 2}SH have been obtained near their ionization thresholds. Using a semiempirical simulation scheme, we have obtained satisfactory fits to fine structures resolved in the VUV-PFI-PE spectra, yielding accurate ionization energies of 76thinsp256.3{plus_minus}2.9thinspcm{sup {minus}1} (9.454thinsp58{plus_minus}0.000thinsp36thinspeV) and 74thinsp948.7{plus_minus}2.9thinspcm{sup {minus}1} (9.292thinsp46{plus_minus}0.000thinsp36thinspeV) for CH{sub 3}SH and CH{sub 3}CH{sub 2}SH, respectively. {copyright} {ital 1998 American Institute of Physics.}