Written to be the definitive text on the rotational spectroscopy of diatomic molecules, this book develops the theory behind the energy levels of diatomic molecules and then summarises the many experimental methods used to study their spectra in the gaseous state. After a general introduction, the methods used to separate nuclear and electronic motions are described. Brown and Carrington then show how the fundamental Dirac and Breit equations may be developed to provide comprehensive descriptions of the kinetic and potential energy terms which govern the behaviour of the electrons. One chapter is devoted solely to angular momentum theory and another describes the development of the so-called effective Hamiltonians used to analyse and understand the experimental spectra of diatomic molecules. The remainder of the book concentrates on experimental methods. This book will be of interest to graduate students and researchers interested in the rotational spectroscopy of diatomic molecules.

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.

We have observed and assigned seven microwave and millimeter wave lines arising from rovibronic components of the A 2 ⌺ g ϩ ←X 2 ⌺ u ϩ electronic spectrum of the He 2 ϩ ion; this is the first observation of any spectrum of the homonuclear 4 He 4 He ϩ species. The vibration-rotation levels involved in our observations all lie within 8 cm Ϫ1 of the lowest degenerate He͑ 1 S͒ϩHe ϩ ͑ 2 S͒ dissociation limits for both electronic states. We use an ion beam technique in which weakly bound levels dissociate in an applied electric field to produce He ϩ fragments. These fragments are separated from all other ions with an electrostatic kinetic energy analyzer, and microwave transitions are detected as changes in the He ϩ fragment current arising from resonant population transfer. Four of the transitions are detected using a single microwave frequency; the remaining three are measured by means of a microwave-microwave double resonance method. The assignment of the spectrum is achieved by means of ab initio electronic structure calculations, made within the Born-Oppenheimer approximation. The agreement between experiment and theory is excellent and leads to an accurate characterization of the He•••He ϩ 2 ⌺ g ϩ charge/induced-dipole state.

The infrared predissociation spectrum of the H3+ ion has attracted considerable attention; theoretical models have been developed which account for many of the observed features and which make further predictions. This paper describes the results of experiments designed to test these predictions. The spectrum is recorded by bringing a mass-selected H3+ ion beam into parallel or antiparallel coincidence with a cw carbon dioxide infrared laser beam. In the earlier work, 27 000 lines were observed over the range 874–1094 cm−1, each line being recorded by detecting H+ fragment ions produced by predissociation. The spectrum varied according to the H+ kinetic energy window selected, and it was proved that many of the lines arise from metastable states of H3+ lying above the H2+H+ dissociation limit. The spectrum showed no immediately recognizable pattern, but low resolution convolutions revealed the existence of a coarse-grained structure of four main peaks. The isotopic species H2D+ and D2H+ showed similarly complex spectra which, however, differed depending on whether H+ or D+ fragments were detected. The most important conclusion from subsequent theoretical models is that the metastable states involved are in a region of classical chaos and hence cannot be simply assigned in terms of vibrational modes. However, the coarse-grained spectrum is associated with the remnants of a periodic orbit in which quasilinear H3+ undergoes a large amplitude bending motion. Rotational angular momentum barriers lead to trapping of these essentially regular states, which are embedded in a classically chaotic manifold. Semiclassical trajectory studies and three-dimensional quantum mechanical calculations are consistent with each other. Our present experimental methods are described and questions concerning reproducibility are addressed. We describe new measurements over the range 964–992 cm−1 spanning the position of one of the peaks observed earlier in the convoluted spectrum. The H3+ spectrum is recorded for a series of different H+ kinetic energy windows and the results are summarized in bar charts. Convolutions of the data recorded for H+ ions with very small center-of-mass kinetic energies are consistent with the earlier results and with theoretical predictions, but also reveal additional structure. Convolutions for large H+ kinetic energies (≥500 cm−1) reveal less evidence of characteristic structure. Measurements over the region 1025–1045 cm−1 are also described; they are only for very small H+ kinetic energy release, but the linewidths are also tabulated. Most of the metastable states of H3+ predissociate predominantly through a single channel, but examples of multiple dissociation channels have also been recorded. Direct measurements of some predissociation lifetimes are presented. Selected regions of the spectra of D2H+ and H2D+, measured by recording H+ and D+ fragments separately with kinetic energy windows from 0 to 3000 cm−1, are described. The results are in excellent agreement with theoretical predictions, as also are measurements of the background spontaneous predissociation.

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