Infrared predissociation spectrum of the H+3 ion

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Abstract
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 H2⋅⋅⋅H+ 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. The vibration‐rotation levels of the H2⋅⋅⋅H+ system are discussed in terms of the theoretical models which have been developed for van der Waals complexes and semiquantitative calculations using an ab initio H2⋅⋅⋅H+ interaction potential are described. Measurements of the H+ center‐of‐mass kinetic energy associated with individual resonance lines are described; they provide information about the energy of the predissociating H+3 level relative to its H2+H+ dissociation channel. Many of the resonance lines are associated with a relatively small energy release (10–500 cm−1), but energy releases of over 3500 cm−1 are 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+2 have 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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