tokens: light reflections. To further reduce the negative effects of the background filament radiation, its spectrum was recorded under high vacuum conditions (5 × 10 −8 mbar) and subtracted from measured molecular spectra. The electron energy was calibrated by measurement of the cross section of the (0, 0) band of (C 3 Π u –B 3 Π g ) at 337 nm (Zubek 1994 ; Országh et al. 2012 ) and the He i (1s2p 3 –1s4d 3 D 1,2,3 ) 447.14 nm emission line (NIST 2015 ). The (C 3 Π u –B 3 Π g )(0, 0) cross section exhibits a pronounced peak at 14.1 eV. The cross section for the He i (1s2p 3 –1s4d 3 D 1,2,3 ) transition exhibits a step at 23.736 eV. The electron energy calibration was performed in the mixture of C 2 H 2 :N 2 :He 1:1:1 (established by an absolute pressure gauge) to avoid problems with different charging effects of the electron monochromator electrodes in different gases. The difference between energy scales calibrated by nitrogen and helium was less than 0.015 eV. Two different measurement modes were used in the present study: (i) fluorescence spectra induced by monoenergetic electrons at fixed electron energy, and (ii) PECs—the intensity of photons of a specific wavelength detected as a function of the electron energy, sometimes referred to as the relative excitation function. PECs are proportional to a dissociative excitation-emission cross section (DEE CS) and can be calibrated to its absolute values. Additionally, PEC curves can be used for determination of the apparent thresholds in PEC, which are related to the thresholds for the dissociative excitation reactions (Stano et al. 2003 ). 3. Results and Discussion 3.1. Spectrum We have recorded the electron-induced UV–VIS spectrum of acetylene in the spectral range from 190 to 700 nm. The spectral range 190–257 nm was recorded at an electron energy of 70 eV and low resolution of the optical monochromator (2 nm FWHM), and the rest was recorded at 50 eV and higher resolution of the optical monochromator (0.4 nm FWHM). The spectral range 305–530 nm is presented in Figures 2 (a) and (b). All the spectra presented in this paper were corrected for the spectral response of the apparatus. The intensities in the spectra were calibrated to absolute values according to the known emission cross sections for the dissociative excitation-emission channel, which we will give more details on later. The spectra exhibit lines and bands corresponding to dissociative excitation of acetylene by electron impact (Beenaker et al. 1975 ; Tsuji et al. 1975 ). Figure 2. Acetylene spectrum in the wavelength region of (a) 310–420 nm and (b) 420–660 nm at electron energy 50 eV with slits set to 100 μ m (optical resolution ∼0.4 nm). Due to the lower sensitivity of the optical system in the region 190–257 nm, it was necessary to decrease the resolution of the optical monochromator (using 500 μ m for both slits). We were able to identify two C i lines at 193.1 nm (C i (2s 2 2p3s 1 P 0 –2s 2 2p 2 1 D)) and 248.4 nm (C i (2s 2
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