Photoion-pair formation in Cl 2

Abstract

The relative photoion yield curves of Cl + 2, Cl − and Cl + from Cl 2 (normalized to absolute cross sections) are presented, from threshold to 740 Å. A structured spectrum involving the formation of Cl +—Cl − ion pairs, and amounting to ≈ 1% of total ionization, extends from its threshold to ≈ 750 Å. The spectrum is divided into four distinct bands. The two lowest in energy must correlate with Cl +( 3P) + Cl −( 1S). The other two are inferred to have Cl +( 1D) and Cl +( 1S) asymptotes, respectively. The fine structure observed implies predissociation of photoexcited Rydberg states by ion-pair states. The Rydberg states can be assigned by their absolute energies, vibrational spacings and vibrational envelopes. The lowest-energy ion-pair band can be observed at least 0.16 eV below its thermochemical threshold at 300 K. This band has a pronounced temperature effect, which can be interpreted as an example of heterogeneous predissociation. The contribution to Cl + from dissociative ionization rapidly overwhelms the ion-pair component below 802.5 Å, the dissociative ionization threshold.