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Dissociative electron attachment to CS 12 four different sets of pressure conditions. The intensity of the reference molecules in the positive and negative ion spectra, however, was sometimes rather low, which introduced some uncertainty into some of the ratios calculated. Care has been taken to ensure that fragment ions, such as CS + from CS 2 , do not contribute to the intensity of parent ions, such as CS + from CS, by checking data with electron energies below fragmentation thresholds. The observed attachment bands to be assigned were S − at 1.8 eV, S − at 5.43 eV, S − at ∼ 6.70 eV, O − at ∼ 5.4 eV and C − at ∼ 6.40 eV. The relative intensities of three peaks, S − at 5.43 eV, S − at ∼ 6.70 eV and C − at ∼ 6.40 eV, remained constant as the inlet conditions varied suggesting that they were due to dissociative electron attachment to the same molecule. By contrast, the intensity of S − at 1.8 eV varies strongly compared to the intensities of the peaks at 5.43, ∼ 6.70 and ∼ 6.40 eV as can be seen in Figures 10. At high CS 2 inlet gas pressure the S − at 1.8 eV intensity is relatively low, whilst at low CS 2 pressure this peak is high. Application of equation 4 leads to the conclusion that the S − attachment band at 1.8 eV is most likely due to dissociative electron attachment to S 2 O. The position of the maximum of this electron attachment band observed here, 1.8 ± 0.3 eV is in agreement with the previously reported value of 1.6 eV for formation of S − from S 2 O [19] within the experimental uncertainties. No other dissociative electron attachment bands were observed for S 2 O in that earlier measurement [19]. The small amount of O − detected at low pressures at ∼ 5.4 eV, which cannot be due to electron attachment to H 2 O [35], is, however, tentatively assigned here to S 2 O as well. In the earlier measurements S 2 O was generated from SO 2 and strong dissociative electron attachment to residual SO 2 in the gas sample between 4 and 6 eV would have masked this new electron attachment band of S 2 O at 5.4 eV, a possibility acknowledged in the earlier work. 2500 2000 (a) S - 1000 (b) S - C - Counts 1500 1000 500 C - 1 Counts 100 10 0 5 5.5 6 6.5 7 Electron Energy (eV) 5 5.5 6 6.5 7 Electron Energy (eV) Figure 11. Dissociative electron attachment bands assigned to CS Application of equation 4 to the remaining three peaks, S − at 5.43 eV, C − at ∼ 6.40 eV and S − at ∼ 6.70 eV, does not lead to an unambiguous assignment. Both CS and C 3 S 2 could be responsible for these three bands. The lack of larger C n S − m fragments
Dissociative electron attachment to CS 13 in the spectra favours assignment to CS over C 3 S 2 . Similarly, it would perhaps be unlikely that the C − ion would be so prominent in dissociative electron attachment to C 3 S 2 because of the molecular rearrangement required for dissociation to give C − . It is also more likely these three electron attachment bands are due to CS because their positions agree well with calculated thermodynamic threshold for formation of C − and S − from CS as discussed in section 3.4. Therefore, these three electron attachment bands are assigned to the unstable CS molecule. The integrated intensities of the bands assigned to CS are shown in Figure 11; spectrum (a) has a linear vertical axis and (b) is logarithmic. 3.4. Dissociative electron attachment thresholds The appearance energy AE(B − ) of a fragment anion B − in dissociative electron attachment to a molecule AB can be estimated using the equation AE(B − ) = D(A−B) − EA(B) + E ∗ (5) where D(A-B) is the dissociation energy of the chemical bond that is broken, EA(B) is the electron affinity of the fragment B and E* is the excess energy above the thermodynamic limit. Below it is assumed that E* is zero. In figure 11 the S − band at 5.43 eV appears to have an assymmetric peakshape, particularly compared to the CS 2 bands in figure 4. If it is assumed that there is a vertical onset for S − formation at its thermodynamic threshold on the low energy side of this band then the energy resolution of the electron beam may be calculated from the rise of the S − signal. The electron energy resolution calculated in this way is 150 meV, which is better than the resolution determined with SF 6 , 250 meV. The resolution determined from the right hand side of the peak is 300 meV. Therefore, it is concluded that the S − band at 5.43 eV is cut off at low energy by the threshold for formation of S − . The threshold for formation of S − determined experimentally from the position of half maximum on the S − rise is 5.26 ± 0.15 eV. The electron energy resolution calculated from this low energy rise of the C − band at ∼ 6.40 eV is also 150 meV, which suggests that this band starts at the thermodynamic threshold for C − production. The experimental threshold determined from the half maximum position of the C − signal rise is 6.21 ± 0.15 eV. The thresholds for formation of S − and C − from C 3 S 2 and CS have been calculated with equation 5. The bond energy of the C-S bond can be calculated from the heats of formation of CS (280.33 kJ/mol), S (276.98 kJ/mol) and C (716.67 kJ/mol) [25] leading to D(C-S)= 7.39 eV. A literature value is D(C-S)=7.379 ± 0.025 eV [36]. The electron affinities of S and C are 2.077 eV and 1.262 eV respectively [25]. The thermodynamic threshold for the reaction CS + e − → C + S − lies then at 5.302 ± 0.025 eV, which is in excellent agreement with the experimentally determined threshold of 5.26 ± 0.15 eV. The calculated threshold for the production of the carbon anion C − is 6.117 ± 0.025 eV, which agrees with the experimental value, 6.21 ± 0.15 eV, within the experimental uncertainty.
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