Valence-Shell Photoionization of C₄H₅: The 2-Butyn-1-yl Radical

Abstract

We present new high-resolution data on the photoionization of the 2-butyn-1-yl radical (CH3CC–•CH2) formed by H atom abstraction from 2-butyne by F atoms. The spectra were recorded from 7.7 to 11 eV by using double-imaging, photoelectron–photoion coincidence spectroscopy, which allows the unambiguous correlation of photoelectron data and the mass of the species. The photoionization spectrum shows significant resonant autoionizing structure converging to excited states of the C4H5 + cation, similar to what is observed in the closely related propargyl radical (HCC–•CH2). The threshold photoelectron spectrum, obtained with a resolution of 17 meV, is also reported. This spectrum is consistent with previous measurements of the first photoionization band but has been extended to higher energy to allow the observation of bands corresponding to excited electronic states of the ion. A refined value of the adiabatic ionization energy is extracted: IE­(C4H5) = 7.93 ± 0.01 eV. A determination of the absolute photoionization cross section of the 2-butyn-1-yl radical at 9.7 eV is also reported: σion(C4H5) = 6.1 ± 1.8 Mb.

Photoionization studies of reactive intermediates using synchrotron radiation

“…Nevertheless, a weak vibrational component was observed at (11.170 ± 0.002) eV consistent with the PFI-ZEKE value for the first AIE (145). In the case of the OH radical, there have been two measurements of the photoionization cross-section at vuv photon energies, derived from a synchrotron source (146,147). In both cases, cross-sections were measured at photon energies which are above the first AIE but below the second AIE.…”

“…Partial pressures of OH were determined by fitting the observed time traces of the OH + signal, produced by vuv ionization of OH, with a kinetics model constructed using available rate coefficients.Photoionization cross-sections values were reported at 13.435 and 14.193 eV(146). In the second study(147), which is simpler and more direct, OH was produced from the F + H2O reaction, where fluorine atoms were produced by microwave discharge of a flowing F2/He mixture, and the OH absolute cross-section was measured at 13.8 eV. This was achieved by measuring the photon intensities of H2O + , OH + and O + at 13.8 eV with the microwave discharge on and off.…”

“…These examples highlight the point that the available experimental photoionization cross-sections of reactive intermediates have been measured relative to a suitable reference (or references, O and H2O in the case of OH) with a known cross-section, such as a reagent or another reactive intermediate. For example, in the measurement of the absolute photoionization cross-section of the 2-butyn-1-yl radical (CH3-C≡C-CH2), at a photon energy just above its first ionization energy (150), the radical was obtained from the F + 2butyne reaction and its cross-section was determined using the known cross-section of 2-butyne.Also, where comparison can be made between available calculated and experimental crosssections, the agreement is poor(146,147,150), highlighting the need for improved methods to calculate cross-sections which include resonant excitation processes, particularly at low photon energies just above the first ionization threshold, as well as more cross-section measurements on reactive intermediates.…”

VUV photoionization dynamics of the C60 buckminsterfullerene: 2D-matrix photoelectron spectroscopy in an astrophysical context

“…The total energy resolution is in fact a convolution of the electron bandwidth that is used to obtain the TPES and the photon resolution. This issue has been discussed previously 58,59 and we have found that the 50 meV electron bandwidth translates into a resolution of 13 meV. Due to spectral congestion, the width of the peaks in the TPES well exceed the total energy resolution which is the convoluted value of the electron resolution and photon resolution.…”

“…Such energy calibrations using higher harmonic ionizations of the contents of the molecular beam have been performed previously and discussed in detail elsewhere. 58,59 In order to correct for possible drifts in the C 60 neutral density, i.e. the relative intensities of the bands between the scans, a ''fast'' scan was performed over the entire energy range with 0.1 eV step sizes with the same acquisition time per point as in the aforementioned scans which had significantly smaller step sizes.…”