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The double photoionization of HI molecules has been investigated using vacuum ultraviolet synchrotron radiation in the energy range between 27 and 35 eV. The product ions have been detected by the use of time-of-flight mass spectrometry and the threshold energy for HI2+ and H++I+ formation has been determined. These results have been interpreted by the use of a theoretical model which has been previously applied by us to HBr2+ and HCl2+. On the basis of the reliability of such a model, an assessment of the systematic trends of the bond features along the HX2+ (X=F, Cl, Br, I) homologous series is given in this paper. In particular, the increase of the stability of these dications, in their lowest electronic states, when going towards the heavier molecules, has been rationalized considering the systematic variation of the charge transfer coupling between the H–X2+ and the H+–X+ states. [ABSTRACT FROM AUTHOR]

The double photoionization of HCl molecules by synchrotron radiation has been studied in the energy range between 30 and 50 eV. The HCl2+ and Cl2+ product ions have been detected by a photoelectron–photoion-coincidence technique, while the H++Cl+ formation, which follows the double ionization of HCl, has been studied by photoelectron–photoion–photoion coincidence. The photon energy threshold for the production of HCl2+ ions has been found to be 35.4±0.6 eV, while for the dissociative channel leading to H++Cl+, it has been measured a threshold at 36.4±0.6 eV and a change in the slope of the cross-section energy dependence at 38.7±0.7 eV. The production of H+Cl2+ occurs with a threshold photon energy of 42.8±1.1 eV. These results appear to be in a good agreement with previous data by different experimental techniques and recent theoretical calculations performed by our laboratory. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]