Infrared predissociation spectrum of the H+3 ion

We have observed an infrared spectrum of the H+3 ion containing nearly 27 000 lines which span only 222 cm-1 from 872 to 1094 cm-1. A beam of H+3 ions at a potential of from 1.2 to 10.5 kV is aligned to be collinear with an infrared laser beam from a carbon dioxide cw laser. Photodissociation occurs to produce fragment H+ ions which are separated from the parent H+3 ions using an electrostatic analyzer. Doppler tuning is accomplished by scanning the H+3 ion beam potential and resonance lines corresponding to an increase in fragment H+ ion current are detected by means of a velocity modulation technique. The observed linewidths range from 3 to 60 MHz, with additional broader lines also being detected by chopping the laser beam. We believe that each resonance line arises from predissociation of H+3 to form H2 and H+. Pseudo-low resolution spectra constructed by computer convolution of the experimental data show well defined peaks which correspond closely in transition frequency to j = 3--5 rotational transitions of H2 in its v = 0, 1, 2, and 3 vibrational levels. It is therefore suggested that the H+3 ions studied by our technique are best regarded as H2xxxH+ complexes in which the vibrational and rotational states of the H2 are largely preserved. We believe that many of the observed resonance lines arise from H+3 ions with up to 2 or 3 eV internal energy above the lowest dissociation limit, and consequently that many metastable levels with a wide range of lifetimes exist