Threshold ion-pair production spectroscopy (TIPPS) of H2 and D2

Abstract

The threshold ion-pair production spectra at the J″=0 and J″=1 thresholds of H₂ and J″=0, 1 and 2 thresholds of D₂ obtained with single photon excitation are presented. The ion-pair yield spectra of H₂ and D₂ over these energy ranges demonstrate strong resonant enhancement, parts of which dominate the TIPPS signals, permitting the assignment of the lower states of these resonances. From those thresholds with weak resonant enhancement (the J″=0 threshold of H₂ and the J″=1 threshold of D₂) a very small direct contribution to ion-pair production can be observed. The behaviour of the TIPPS spectra taken with different applied discrimination fields is understood by modeling the field ionization behaviour of a MATI spectrum of H₂, containing both the similarly resonantly enhanced v⁺=8 S(0) ionization threshold and the non-resonantly enhanced S(1) ionization threshold. From the H₂ J″=1 and D₂ J″=0 TIPPS spectra the energetic field-free thresholds of the H₂ and D₂ ion-pair limits were determined to be 139714.8±1.0 cm⁻¹ and 140370.2 cm⁻¹±1.0 cm⁻¹, respectively.