Electronic excitation of gas-phase furan molecules by electron impact

Abstract

Experiments and ab-initio calculations of the differential and integral cross sections for the electronic excitation from the ground state 1 A 1 to the 3 B 2 and 3 A 1 states of gas-phase furan molecules, by low-energy electron impact, were performed. Experimental differential cross sections were measured at incident electron energies between 5 to 15 eV and for scattering angles from 10 • to 130 • . The calculated cross sections were obtained using the Schwinger multichannel method implemented with pseudopotentials. The influence of channel-coupling and polarization effects is investigated through the comparison between three different models of scattering calculations, each one considering a distinct channel-coupling scheme. The comparison of experimental and calculated cross sections for electronically inelastic electron scattering by C 4 H 4 O molecules is found to be mostly reasonable. The existing discrepancies in this combined theoretical and experimental study help to illustrate difficulties in readily establishing reliable electronic excitation cross sections of polyatomic molecules by low-energy electrons.PACS numbers: 34.80. Gs, 34.80.Bm I INTRODUCTIONElectron-induced breakage of chemical bonds through dissociative electron attachment, a process mediated by the formation of temporary anionic states, has been recognized as a very efficient mechanism leading to the production of permanent lesions on DNA chains in the form of single-and double-strand breaks [1][2][3][4][5]. Much of the latest ongoing work on this subject has been devoted to the study of inelastic processes, more specifically, those involving electronic and/or vibrational excitations of the DNA basic constituents by impact of lowenergy electrons [6][7][8][9][10][11]. In particular, the electronic excitation cross sections obtained in these studies revealed the presence of several core-excited resonances that appear at energies ranging typically from 5 eV to 10 eV. These findings are relevant because the formation of such short-lived negative ion states represents an alternative, and probably rather effective if compared to "one-particle" shape resonances, doorway for electron-induced damage to DNA. Thus, the determination of electronic excitation cross sections for biomolecules (such as the nitrogenous bases or the phosphate group) certainly represents a crucial step towards a deeper insight into the mechanisms of DNA damage by secondary electrons and establishes itself as a challenging task for experimentalists and theoreticians. On the experimental point of view, the difficulties for obtaining reliable cross sections are related to the resolution of the molecular spectra via energy loss assignments and widths of spectral features observed and also to the sensitivity of handling very low energy electrons in the electron spectrometer.From the theoretical perspective, there are also subtle aspects related to the description of such kind of processes, observed in applications involving relative small molecules with low-lying excited ...

Theoretical and experimental study on electron interactions with chlorobenzene: Shape resonances and differential cross sections

“…Opening inelastic channels, considered closed in our calculations, would allow flux loss from the elastic channel to the inelastic ones, lowering the elastic cross sections towards the experiment. 48 Regarding the calculated IAM-SCAR DCSs, a better agreement with the present experimental DCSs in the electron energy range above 20 eV is observed. At 50 eV, the experimental differential cross sections have been re-scaled at 30 • with the IAM-SCAR data, more suitable to represent the scattering situation at this energy.…”

The electron-furfural scattering dynamics for 63 energetically open electronic states

“…Let us now consider Figure 2, where we compare our band I and band II furfural ICSs, respectively, to corresponding electronic-state ICSs in furan from the work of da Costa et al 31 and Regeta and Allan. 32 Specifically, in Fig.…”

“…In this case, the band I electronicstates in furfural 4,7 might correspond to the excitation of the 3 B 2 electronic-state in furan. 31,32 That excitation is a (HOMO) → (LUMO) process, which is similar to the excitation of the 3 A ′ electronic-state (in band I) in furfural. 4 Specifically, the HOMOs of both species are dominated by the out-of-plane π-bonding of the furan ring and the LUMOs correspond to the π * -furan ring contribution.…”

“…Lastly, excitation of the 1 B 2 electronic-state of furan is a (HOMO) → (LUMO) process that is similar to that for the 1 A ′ state (in band II) in furfural, the singlet version of the ππ * transition in band I. The significant localisation of the contributing orbitals to the ring suggests that, to first order, the 3 B 2 ICS of da Costa et al 31 and Regeta and Allan 32 in furan can be compared to the present ICSs for band I in furfural, while the furan ( 3 A 1 + 1 B 2 ) integral cross sections of Regeta and Allan 32 can be compared to the present ICSs for band II in furfural. This comparison forms another rationale for our current investigation.…”

“…4 Specifically, the HOMOs of both species are dominated by the out-of-plane π-bonding of the furan ring and the LUMOs correspond to the π * -furan ring contribution. Additionally excitation of the 3 A 1 state in furan is a (HOMO − 1) → (LUMO) process 31,32 that corresponds to excitation of the 3 A ′ state (in band II) in furfural. 4 This transition is characterised by an excitation from the out-of-plane O(2p)/C(2p) ring p-bonding network to the π * -furan ring orbital.…”