Vacuum ultraviolet photoionization cross section of the hydroxyl radical

Abstract

The absolute photoionization spectrum of the hydroxyl (OH) radical from 12.513 to 14.213 eV was measured by multiplexed photoionization mass spectrometry with time-resolved radical kinetics. Tunable vacuum ultraviolet (VUV) synchrotron radiation was generated at the Advanced Light Source. OH radicals were generated from the reaction of O(D) + HO in a flow reactor in He at 8 Torr. The initial O(D) concentration, where the atom was formed by pulsed laser photolysis of ozone, was determined from the measured depletion of a known concentration of ozone. Concentrations of OH and O(P) were obtained by fitting observed time traces with a kinetics model constructed with literature rate coefficients. The absolute cross section of OH was determined to be σ(13.436 eV) = 3.2 ± 1.0 Mb and σ(14.193 eV) = 4.7 ± 1.6 Mb relative to the known cross section for O(P) at 14.193 eV. The absolute photoionization spectrum was obtained by recording a spectrum at a resolution of 8 meV (50 meV steps) and scaling to the single-energy cross sections. We computed the absolute VUV photoionization spectrum of OH and O(P) using equation-of-motion coupled-cluster Dyson orbitals and a Coulomb photoelectron wave function and found good agreement with the observed absolute photoionization spectra.

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.…”

“…In both cases, cross-sections were measured at photon energies which are above the first AIE but below the second AIE. In the first study (146), OH was produced via the O( The information summarised in sections 1, 2 and 3 suggest that it would be very beneficial to carry out angle resolved PE and CIS studies with PEPICO. A recent study of the OH radical by PEPICO (45), using synchrotron radiation as the photon source, illustrates the advantages of combining information from these methods to obtain enhanced information on the ionic states accessed and the ionization processes.…”

“…1 D) + H2O reaction in a flow reactor, where O( 1 D) was obtained by photodissociation of O3. 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.…”

“…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.…”

Optimization of UV-C light and lactic acid combined treatment in decontamination of sliced Brazilian dry-cured loin: Salmonella Typhimurium inactivation and physicochemical quality

“…2c and d). This effect was obtained, possibly due to the UV light in vacuum forming hydroxyl radical (Dodson et al, 2018). The recombination among formed substances results in H 2 O 2 .…”