A Near-Threshold Shape Resonance in the Valence-Shell Photoabsorption of Linear Alkynes

Abstract

The room-temperature photoabsorption spectra of a number of linear alkynes with internal triple bonds (e.g., 2-butyne, 2-pentyne, and 2- and 3-hexyne) show similar resonances just above the lowest ionization threshold of the neutral molecules. These features result in a substantial enhancement of the photoabsorption cross sections relative to the cross sections of alkynes with terminal triple bonds (e.g., propyne, 1-butyne, 1-pentyne, ...). Based on earlier work on 2-butyne [ Xu et al., J. Chem. Phys. 2012, 136, 154303 ], these features are assigned to excitation from the neutral highest occupied molecular orbital (HOMO) to a shape resonance with g (l = 4) character and approximate π symmetry. This generic behavior results from the similarity of the HOMOs in all internal alkynes, as well as the similarity of the corresponding gπ virtual orbital in the continuum. Theoretical calculations of the absorption spectrum above the ionization threshold for the 2- and 3-alkynes show the presence of a shape resonance when the coupling between the two degenerate or nearly degenerate π channels is included, with a dominant contribution from l = 4. These calculations thus confirm the qualitative arguments for the importance of the l = 4 continuum near threshold for internal alkynes, which should also apply to other linear internal alkynes and alkynyl radicals. The 1-alkynes do not have such high partial waves present in the shape resonance. The lower l partial waves in these systems are consistent with the broader features observed in the corresponding spectra.

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).…”

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

“…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.…”

Resonance signatures in the body-frame valence photoionization of CF4

“…The importance of the inclusion of the coupling between nearly degenerate shape resonances of the same total symmetry for the accurate description of RFPADs found here is similar to the strong inter-channel coupling effects seen in the total photoionization cross sections in the resonant near-threshold valence ionization of the linear alkynes. 31 It is interesting to compare the case of 4t 2 ionization with that of 3t 2 ionization. The computed total cross sections for the latter are shown in Fig.…”