The Core and Valence Level Spectrum of Acetylene

Acetylene, C2H2, is isoelectronic with N2 and is the simplest triple-bonded hydrocarbon. It has the same ordering of the HF MO hole levels as N2, as shown in Fig. 49, but there are some important differences because the nuclear charge is more spread out in C2H2 and the potential on the carbon sites is less attractive. Therefore, the l ru level in C2H2 

Quasi-Particle Properties of Hole Levels in Molecules 

The recent experimental core level spectrum by Cavell and Allison129 in Fig. 50 is very similar to that of N2 (Fig. 45), with a weak Is 3(1 jAr) satellite with 2% relative intensity at 7.2 eV above the main line, a strong 1 s 3(1 n 2 ri) satellite with 10% relative intensity at 12.2 eV, followed by a prominent discrete and continuous satellite spectrum at higher energies. The total relaxation shift is 16 eV, the MO HF ljs level lies well above the low-lying nn satellites and the limited range of the shake-up and shake-off spectrum shown in Fig. 50 only accounts for about one third of the relaxation shift (through Eq. (22)). 

Dealing with a case of nearly degenerate 20g and 2ct 3(12b 1 Jrg) configurations one may ask what kind of spacial correlations are involved. In Sects. 7.1 and 7.2 we discussed the breaking of the gerade-ungerade symmetry of the molecule and dynamic localization of the hole, i.e. hopping of a localized hole between the nuclei. That discussion was based 

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