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Dissociative electron attachment to CS 16 data where the reference molecule was CS 2 present. The maximum variation between measurements was a factor of two for the S − bands and three for C − . The uncertainty in the average cross section values presented here is expected to be an order of magnitude at most. The cross sections determined here are presented in Table 1. The CS cross sections are presented with two significant figures because the uncertainties in their relative intensities are significantly smaller than the absolute uncertainties. There are two main sources of uncertainty for these measurements; first, the variation in the composition of the gas sample due to changing conditions in the microwave source. Positive mass spectra cannot be recorded at exactly the same time as the negative ion spectra, but the changeover between recording positive and negative ion spectra is kept as short as possible to minimize this source of uncertainty. Furthermore, measuring times were also short to minimize any effects due to changes in the discharge over time. The second main source of uncertainty is the low intensity of the reference signal of S − from CS 2 in the electron attachment spectrum as the number density of CS 2 is low and the electron attachment cross section of CS 2 is rather small, 3.7 × 10 −3 Å 2 [21]. The cross section for the S − peak at 1.8 eV from S 2 O estimated here, 0.09 Å 2 , is within an order of magnitude of the value reported previously, 0.3 Å 2 , [19] from different experiments with the same apparatus where SO 2 and He were passed through the microwave discharge to generate SO, S 2 O, SO 2 and S 2 O 2 . There were additional uncertainties in the determination of this S 2 O cross section. It was necessary to use CS as the reference molecule in one dataset where there was not sufficient signal from CS 2 . It is expected that the previously reported value of 0.3 Å 2 is more reliable than the present one. The quality of the data for the weaker O − signal from S 2 O at 5.4 eV only allowed the estimation of an upper limit to its cross section. 3.6. Dissociation Dynamics It interesting to compare the dissociative electron attachment to CS observed here with previous measurements for CO [42, 43, 44], which is, of course, valence-isoelectronic with CS. The dissociative electron attachment spectra of CO and CS are remarkably similar. O − is observed with a sharp onset at the thermodynamic threshold for O − formation from CO at 9.63 eV, in the same way that S − is observed from CS at its thermodynamic threshold. A second smaller O − signal from CO appears at 10.88 eV, the threshold for formation of O − + excited C ( 1 D), with another sharp onset, which is analogous to the second smaller S − peak observed here from formation of S − + C ( 1 D). In the case of CO the second O − peak is largely obscured by the high energy tail of the first O − band except in experiments where the kinetic energy of the fragment O − ion is selected [42]. It is interesting to note that the two S − peaks observed here from CS are observed without kinetic energy selection of the fragment ion. As well as the similarity between O − from CO and S − from CS there is also some similarity between the formation of C − from CS and CO. A C − peak is observed for
Dissociative electron attachment to CS 17 both CO and CS in between the two peaks of O − and S − . Above 10.2 eV C − is produced from CO with a cross section 3300 times smaller than for the O − peak above 9.63 eV [44]. C − production from CS shows a maximum at ∼ 6.40 eV with a cross section about ten times smaller than the intensity of the first S − peak, see Figure 11. In contrast to CS where the C − has an onset at the thermodynamic threshold, however, the formation of C − from CO is delayed by about 0.37 eV [45]. Dissociative electron attachment to CS can be assigned as follows: CS ( 1 Σ + ) + e − (5.43 eV) → S − ( 2 P) + C ( 3 P) (8) CS ( 1 Σ + ) + e − (6.70 eV) → S − ( 2 P) + C ( 1 D) (9) CS ( 1 Σ + ) + e − (6.40 eV) → C − ( 4 S) + S ( 3 P) (10) It is not straightforward to determine the nature of the electron attachment resonance or resonances that lead to dissociation of CS. Each of the three dissociative electron attachment bands observed here appears to be cut off at low energy by the threshold for formation of the ion. Therefore, the positions of the maxima of the electron atttachment resonances responsible for dissociation, are not known as they are likely to be below the thermodynamic thresholds. GAMESS RHF STO-3G calculations of CS, CS 2 , OCS and CO performed here indicate that the lowest unoccupied orbital of each of these molecules is π ∗ . CO, CS 2 and OCS all show 2 Π shape resonances at between 1 and 2 eV in electron scattering measurements [46, 47, 23], and in the case of OCS in the dissociative electron attachment spectrum [34], which can be assigned to electron attachment into these π ∗ orbitals. Therefore, it is expected that the lowest lying electron attachment resonance of CS is 2 Π with attachment of the free electron into the lowest π ∗ orbital. It is sometimes possible to find a linear relationship between the calculated unoccupied molecular orbital energies and the experimentally observed positions of electron attachment resonances for a family of related molecules [48, 49]. Such linear relationships can be used to predict the positions of electron attachment resonances from calculated unoccupied molecular orbital energies [50]. Here, for example, the the lowest 2 Π resonance of CS could be predicted from the energy of the lowest π ∗ orbital of CS using a linear relationship between the lowest 2 Π resonances and the lowest π ∗ unoccupied orbitals of CS 2 , OCS and CO. There is not, however, a simple linear relationship between the energies of the lowest 2 Π resonances and π ∗ orbital energies calculated with GAMESS of CO, CS 2 and OCS. A sensible linear relationship may not even be found with only CS 2 and OCS. Thus, it has not been possible here to predict the energy of the first 2 Π resonance of CS. Calculations of the integral elastic scattering cross sections for electron collisions with CS indicate the presence of broad 2 Π and 2 ∆ resonances around 7 eV [16]. These resonances could possibly play a role in the electron attachment observed here, but it is expected to be very unlikely that the first 2 Π resonance of CS would be as high as 7 eV. The lowest 2 Π is more likely to be in the range of 0 to 3 eV given that those of CS 2 , OCS and CO are between 1 and 2 eV. It may be that some of the dissociative electron attachment observed here above 5 eV is
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