High-resolution vacuum-ultraviolet photoabsorption spectra of 1-butyne and 2-butyne

Abstract

The absolute photoabsorption cross sections of 1- and 2-butyne have been recorded at high resolution by using the vacuum-ultraviolet Fourier-Transform spectrometer at the SOLEIL Synchrotron. Both spectra show more resolved structure than previously observed, especially in the case of 2-butyne. In this work, we assess the potential importance of Rydberg states with higher values of orbital angular momentum, l, than are typically observed in photoabsorption experiments from ground state molecules. We show how the character of the highest occupied molecular orbitals in 1- and 2-butyne suggests the potential importance of transitions to such high-l (l = 3 and 4) Rydberg states. Furthermore, we use theoretical calculations of the partial wave composition of the absorption cross section just above the ionization threshold and the principle of continuity of oscillator strength through an ionization threshold to support this conclusion. The new absolute photoabsorption cross sections are discussed in light of these arguments, and the results are consistent with the expectations. This type of argument should be valuable for assessing the potential importance of different Rydberg series when sufficiently accurate direct quantum chemical calculations are difficult, for example, in the n ≥ 5 manifolds of excited states of larger molecules.

Low-Temperature Oxidation of Ethylene by Ozone in a Jet-Stirred Reactor

“…For the quantification of these peroxide species that are typically identified as low-temperature oxidation intermediates, routines to convert mass spectra into mole fraction profiles were used that were developed by Moshammer et al Cross sections were used from Dodson et al for H 2 O 2 and from Moshammer et al for CH 3 OOH. The cross section of C 2 H 5 OOH was calculated using the codes developed by Lucchese and co-workers, − as described earlier . The results are shown in Figure in which a low-temperature oxidation regime and the negative-temperature coefficient regime are clearly visible in the profile of H 2 O 2 (similar to Figure for the ethylene).…”

“…The mole fraction of hydroxy-acetaldehyde, as shown in Figure b, is based on a photoionization cross section of 6.1 Mb at 10.5 eV, which was determined using the routines from Lucchese and co-workers. − Besides the importance of this intermediate in the low-temperature region, also non-negligible amounts are seen in the intermediate temperature range.…”

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

“…A more detailed analysis of the electronic autoionization structure will require more extensive theoretical calculations of the excited states of the 2-butyn-1-yl radical and cation. As in earlier studies of propyne and 2-butyne, the calculation of the partial photoionization cross sections to the X̃ + 1 A′, ã + 3 A″, and à + 1 A″ states and, more specifically, the partial wave decompositions of those cross sections could provide insight into which Rydberg series are expected to dominate to each ionization threshold. Such calculations may also serve to reveal which channels, and thus which Rydberg series, interact most strongly.…”

“…It is also possible that such calculations will reveal shape resonances in photoionization from the singly occupied molecular orbital (SOMO). For 2-butyne, the photoionization cross section shows a very intense maximum just above threshold that corresponds to a shape resonance. , Although there appears to be a broad maximum centered at about ∼10.5 eV in the 2-butyn-1-yl spectrum of Figure , it is nowhere nearly as dramatic as in the 2-butyne spectrum. Given the dominant dπ character of the SOMO, shape resonances with fσ and fπ character may be most relevant for the 2-butyn-1-yl radical.…”

Quantification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Elusive Intermediates during Low-Temperature Oxidation of Dimethyl Ether

“…We considered two theoretical approaches for calculating absolute photoionization cross sections. In the first approach, the Franck–Condon overlap envelope, S , was included and σ was approximated The transition moment D was calculated using single channel frozen-core Hartree–Fock (FCHF) theory , and the ePolyScat codes of Lucchese et al, which have been used elsewhere − to calculate photoionization cross sections for a variety of systems. In the FCHF calculations, the geometries of the neutral species were optimized using M06-2X/cc-pVTZ, and their orbital energies were calculated using restricted HF/aug-cc-pVTZ.…”