Threshold electron impact excitation of Cl2

A high resoltuion electron impact threshold spectrocscopy technique was used to examine the excitation of Cl2 in the 2-14 eV region. This study complements previous photon absorption and emission measurements, because it is capable of detecting transitions which are optically forbidden. In the region up to 7.5 eV, broad dissociative structures are correlated with optically active valence states, although relative intensities in the threshold spectrum differ considerably and indicate a substantial contribution from the optically forbidden transition. At 7.46 eV a series of 5 equidistant sharp peaks is detected and intepreted as arising from the 2PIsub(g) Feshback resonance, which differs from the ground state positive ion Cl2+ by a pair of Rydberg electrons: (4ssigma)2. The decay channels responsible for the appearance of the resonance in a threshold spectrum are discussed and it is suggested that they include several valence states of the (2431) and (2341) configurations, whose potential energy curves cross the Cl2-, 2PIsub(g) curve in the region of energy at which the resonance state is formed. At higher incident electron energies and up to ionisation. Rydberg states predominate, starting with (2430) 4s 3sup(,)1PIsub(g) states detected for the first time. The absence of broad peaks above 8 eV and the irregular appearance of Rydberg bands is consistent with the strong Rydberg-valence configuration mixing proposed by Peyerimhoff and Buenker. Where our resolution permits comparison, good general agreement is found with recent synchroton radiation absorption measurements of optically allowed transitions. (orig.)