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Notes: 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.

Notes: We have observed and measured seven microwave transitions in D2+, which are vibration–rotation components of the 2pσu–1sσg electronic band system. We also report two microwave rotational components involving the 1sσ v=21 level of HD+, which lies close to the first dissociation limit. Our experiments use ion beam techniques in which state selection is achieved by electric field dissociation. The techniques and theory of electric field dissociation are discussed, and fragment ion kinetic energy spectra described. Ab initio calculations of the transition frequencies for both D2+ and HD+ are in excellent agreement with experiment, and the hyperfine structure of the rotational transitions in HD+ confirms previous demonstrations of the increasing asymmetry in the electron distribution as the dissociation limit is approached. The electronic transition in D2+ which involves the highest bound level of the ground state shows an unexpected hyperfine splitting, which is interpreted in terms of g/u symmetry breaking by the deuteron Fermi contact interaction.

Notes: We have measured and interpreted a microwave spectrum of the HeAr+ ion in which all of the observed energy levels lie within 8 cm−1 of the lowest dissociation limit, He(1S)+Ar+(2P3/2). We use an ion beam technique in which the HeAr+ ions are formed by electron impact, accelerated to kilovolt potentials, and mass-analyzed. After passage through an appropriate section of waveguide, the ions enter an electric field lens in which state-selective fragmentation occurs; the Ar+ ions produced in the lens are separated from all other ions by means of an electrostatic analyser and detected with an electron multiplier. Microwave transitions induced in the waveguide section result in population transfer which produces detected changes in the electric field-induced Ar+ fragment current. Many transitions have also been observed by a microwave–microwave double resonance technique. We have observed 68 lines spanning the frequency range 6–170 GHz; no immediately recognizable pattern is apparent. We have measured the Zeeman splitting produced by a small axial magnetic field for almost every line, which enables us to determine the values of the total angular momentum J involved in each transition, and also effective g factors for the two levels involved. We are therefore able to construct a purely experimental pattern of 37 levels lying within 8 cm−1 of the dissociation limit. The data are treated first by means of a conventional effective Hamiltonian in a case (c) basis, which allows electronic and vibrational quantum numbers to be assigned to most of the levels; the assignments are approximate, however, because very strong rotational-electronic coupling undermines the Born–Oppenheimer approximation.A more complete theoretical treatment is then presented, using the coupled-channel method in a case (e) representation to calculate the energy levels without making the Born–Oppenheimer approximation. The microwave transition frequencies and g-factors are fitted, together with earlier ultraviolet spectra, to provide a new interaction potential (designated MAL1) for He interacting with Ar+(2P3/2 and 2P1/2). The MAL1 potential is substantially more accurate than previous potentials, especially in the long-range region and for the A1 2Π3/2 state, which had not been observed before. An important new feature of the MAL1 potential is that the long-range C6 coefficient is strongly anisotropic, so that the different electronic curves have substantially different C6 coefficients. © 1995 American Institute of Physics.

Notes: We have observed and assigned seven microwave and millimeter wave lines arising from rovibronic components of the A 2Σ+g←X 2Σ+u electronic spectrum of the He+2 ion; this is the first observation of any spectrum of the homonuclear 4He4He+ species. The vibration-rotation levels involved in our observations all lie within 8 cm−1 of the lowest degenerate He(1S)+He+(2S) 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+ 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+ 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+ 2Σ+g charge/induced-dipole state. © 1995 American Institute of Physics.

Notes: We have observed a microwave spectrum of the HeKr+ ion in which all of the observed levels lie within a few cm−1 of either the first or second dissociation limit. We use an ion beam technique in which HeKr+ ions, formed by electron impact, are mass analyzed. Passage of the ion beam through an electric field lens results in selective fragmentation of energy levels lying close to dissociation. Kr+ ions formed in the lens are separated from all other ions by means of an electrostatic analyzer, and are detected with an electron multiplier. Microwave radiation induces transitions which result in population transfer and produce detected changes in the electric field-induced Kr+ fragment ion current. Additional transitions have been detected by a microwave–microwave double resonance method, and we have also made extensive use of the Zeeman effects produced by small applied coaxial magnetic fields to identify the J quantum numbers of the levels involved. Coupled channel calculations of the bound states of the He(centered ellipsis)Kr+ ion are carried out, fully including all the couplings between different electronic states correlating with He+Kr+ (2P3/2 and 2P1/2). The calculations allow the spectra to be assigned to pure rotational transitions involving levels in the X, A1, and A2 states that lie within 2.5 cm−1 of the dissociation limits. Because of a systematic near degeneracy between vibrational levels in the X and A1 states, the long-range He(centered ellipsis)Kr+ ion provides a very good example of Hund's case (e) in the form introduced by Mulliken, in which there are no projection quantum numbers onto the interatomic axis. Mulliken's case (e) is rather different from the Rydberg case (e) described by Lefebvre–Brion, and this is the first time that Mulliken's case (e) has been observed. The spectra allow the interaction potential for He(centered ellipsis)Kr+ to be determined accurately, for the first time, by least-squares fitting of potential parameters to the experimental line frequencies and g factors. The resulting interaction potential (designated MAL1) is compared with that previously determined for He(centered ellipsis)Ar+: the He(centered ellipsis)Kr+ potential is significantly shallower, because the long-range ion-induced dipole C4 coefficient is the same for the two systems but the larger Kr+ ion prevents the He atom approaching as close. © 1996 American Institute of Physics.

Notes: This chapter describes my research career, spanning the period from 1955 to 2000. My initial PhD work at the University of Southampton was concerned with the electronic structure and spectra of transition metal complexes and included studies of the electronic spin resonance (ESR) spectra of magnetically dilute single crystals. After a year at the University of Minnesota, I went to Cambridge University and for the next six years studied the ESR spectra of liquid phase organic free radicals. I commenced work on the microwave magnetic resonance (MMR) spectra of gaseous free radicals in 1965, and this work continued until 1975. I moved from Cambridge to Southampton in 1967. In 1975 I turned to the study of gas phase molecular ions, using ion beam methods. In the earlier years of this period I concentrated on simple fundamental species like H+2, HD+, and H+3. In the later years until my retirement in 1999, I concentrated on the observation and analysis of microwave spectra involving energy levels lying very close to a dissociation asymptote. DEDICATION This chapter is dedicated to the memory of Harry E. Radford, who died while it was being written. Harry was a quiet and shy man, who often worked alone and never indulged in self-promotion. So far as I know, he was never awarded any medals or prizes, nor elected to any academies or learned societies. Nevertheless he was an experimentalist of the highest originality and quality, a theorist of true intellectual depth, and a remarkable pioneer in many of the techniques of studying free radicals that are now commonplace.