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

The threshold ion-pair production spectra at the J " = 0 and J " = 1 thresholds of H-2 and J " = 0, 1 and 2 thresholds of D-2 obtained with single photon excitation are presented. The ion-pair yield spectra of H-2 and D-2 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 H-2 and the J " = 1 threshold of D-2) 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 H-2, containing both the similarly resonantly enhanced upsilon(+) = 8 S(0) ionization threshold and the non-resonantly enhanced S(1) ionization threshold. From the H-2 J " = 1 and D-2 J " = 0 TIPPS spectra the energetic field-free thresholds of the H-2 and D-2 ion-pair limits were determined to be 139714.8 +/- 1.0 cm(-1) and 140370.2 cm(-1) +/- 1.0 cm(-1), respectively.