Threshold ion-pair production spectroscopy (TIPPS) of H-2 and D-2

The threshold ion-pair production spectra at the J " = 0 and J " = 1 thresh olds of H-2 and J " = 0, 1 and 2 thresholds of D-2 obtained with single pho ton excitation are presented. The ion-pair yield spectra of H-2 and D-2 ove r these energy ranges demonstrate strong resonant enhancement, parts of whi ch dominate the TIPPS signals, permitting the assignment of the lower state s 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 sma ll direct contribution to ion-pair production can be observed. The behaviou r of the TIPPS spectra taken with different applied discrimination fields i s 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 thr eshold. From the H-2 J " = 1 and D-2 J " = 0 TIPPS spectra the energetic fi eld-free thresholds of the H-2 and D-2 ion-pair limits were determined to b e 139714.8 +/- 1.0 cm(-1) and 140370.2 cm(-1) +/- 1.0 cm(-1), respectively.