We have observed an infrared spectrum of the H+3ion containing nearly 27thinsp;000 lines which span only 222 cmminus;1from 872 to 1094 cmminus;1. A beam of H+3ions 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+3ions using an electrostatic analyzer. Doppler tuning is accomplished by scanning the H+3ion 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+3to form H2and H+. Pseudohyphen;low resolution spectra constructed by computer convolution of the experimental data show well defined peaks which correspond closely in transition frequency toj=3ndash;5 rotational transitions of H2in itsv=0, 1, 2, and 3 vibrational levels. It is therefore suggested that the H+3ions studied by our technique are best regarded as H2sdot;sdot;sdot;H+complexes in which the vibrational and rotational states of the H2are largely preserved. We believe that many of the observed resonance lines arise from H+3ions 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. The vibrationhyphen;rotation levels of the H2sdot;sdot;sdot;H+system are discussed in terms of the theoretical models which have been developed for van der Waals complexes and semiquantitative calculations using anabinitioH2sdot;sdot;sdot;H+interaction potential are described. Measurements of the H+centerhyphen;ofhyphen;mass kinetic energy associated with individual resonance lines are described; they provide information about the energy of the predissociating H+3level relative to its H2+H+dissociation channel. Many of the resonance lines are associated with a relatively small energy release (10ndash;500 cmminus;1), but energy releases of over 3500 cmminus;1are also observed, which must arise from transitions between pairs of levels, both of which lie well above the lowest dissociation limit. This large energy release is almost certainly due to vibrational predissociation, while the smaller energy releases are associated either with rotational predissociation or tunnelling through a centrifugal barrier. Preliminary observations of similarly complex predissociation spectra of D+3, D2H+, and DH+2have been made. A striking result is that spectra of D2H+detected by monitoring either H+or D+photofragment ions are different. The results described have important implications for studies of reactive scattering processes and for our understanding of the potential energy surfaces for polyatomic molecules.
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