Absolute cross sections for electronic excitation of furan by electron impact

We measured differential cross sections for exciting the three lowest electronically excited states in furan, as functions both of electron energy and of scattering angle. Emphasis of the present work is on recording detailed excitation functions, revealing resonances in the excitation process. The cross section for the first triplet state has a shoulder in the first 2 eV above threshold, assigned to the high-energy tails of the shape resonances, followed by two peaks at 6.5 and 8.0 eV, assigned to two ${}^2(\pi ,{\pi }^{*2})$ core-excited shape resonances with substantial configuration mixing. The excitation mechanism thus closely follows that of the prototype case of ethene, except that as the number of double bonds doubles, the numbers of both shape and core-excited resonances also double. The cross section for the singlet states around 6 eV (primarily the $S_2{}^1{B}_2$ state) rises linearly with energy and attains very high values in the forward direction as expected for a dipole-allowed transition, but has a vertical onset at higher scattering angles, possibly due to a threshold core-excited ${}^2(\pi ,{\pi }^{*}3\overline{s})$ resonance responsible also for dissociative attachment. Elastic cross sections are also presented. Both elastic and inelastic absolute values compare favorably with existing measurements where available, except very near threshold. The results are compared to existing calculations.

Figure 1

Angular distributions of the elastic cross section. Continuous lines and circles show the present data. Squares show the experimental data of Khakoo et al. [4], and dashed lines show the calculated cross sections of da Costa et al. [3]. Horizontal arrows indicate which scale belongs to which spectrum.

Figure 2

Elastic differential cross sections. Present data are shown as continuous red (gray) lines and circles, the calculated cross sections of da Costa et al. [3] are shown as triangles, and the experimental data of Khakoo et al. [4] are shown as squares (average of their ${40}^{\circ }$ and ${50}^{\circ }$ data is compared to our ${45}^{\circ }$ data).

Figure 3

Representative energy-loss spectra emphasizing allowed (bottom spectrum) and forbidden (top four spectra) transitions. They were recorded at the constant residual energies indicated. The topmost spectrum, the difference of the $E_r=2.5$ eV, $\theta ={90}^{\circ }$ and the $E_r=20$ eV, $\theta =0^{\circ }$, enhances the forbidden transitions in the 5.5- to 7.5-eV energy range (the section assigned to the ${}^1({\pi }_3,3s)$ transition is emphasized by orange [light gray] color). Dissociative attachment spectra from Ref. [16] (p. 114, in arbitrary units) are reproduced under the bottom energy-loss spectrum to illustrate the coincidence in energy and the similarity of band profiles.

Figure 4

Representative energy-loss spectrum recorded at a constant incident energy.

Figure 5

Cross sections for electronic excitation at $0^{\circ }$.

Figure 6

Cross sections for electronic excitation at ${45}^{\circ }$. Dissociative attachment spectra of Bulliard [16] (p. 114; not in absolute units) are reproduced under the top spectrum.

Figure 7

Cross sections for electronic excitation at ${90}^{\circ }$. Squares show data from Ref. [3].

Figure 8

Cross sections for electronic excitation at ${135}^{\circ }$. Squares show ${130}^{\circ }$ data from Ref. [3].

Figure 9

Cross sections for electronic excitation at ${180}^{\circ }$.

Figure 10

Angular distributions of electronic excitation. Squares show experimental data from Ref. [3].

Figure 11

Integral cross sections for electronic excitation. Squares denote the experimental data and triangles the theoretical data of Ref. [3]. (Blue [gray] upward-pointing triangles model 1, and green [gray] downward-pointing triangles model 2 for $T_1$ and model 3 for $T_2$.)

Figure 12

Comparison of representative spectra of ethene (bottom, from Ref. [2]) and furan (top). Vibrational excitation cross sections reveal primarily shape resonances; electronic excitation cross sections reveal primarily core-excited shape resonances.

Figure 13

Configurations of ethene and its shape and core-excited resonances. Arrows indicate parentage relations, i.e., the excitation pathways.

Figure 14

Configurations of furan and its shape and core-excited resonances. Parentheses indicate that both configurations contribute to a given state. The curved green (gray) arrows at the bottom indicate parentage relations; i.e., they indicate for each resonance into which excited states of neutral furan it is expected to decay by formal removal of only one electron.