The potential energy curves of the two lowest 2Σ+ (X,B) and the two lowest 2Π (A,C) electronic states of the AlF+ and AlCl+ molecular ions have been calculated using highly correlated multireference configuration interaction (MR-CI) wave functions. It is found that both 2Σ+ states and the C 2Πr state are bound, whereas the A 2Πi state is repulsive. Electronic transition moment functions for all six pairs of states of both ions have also been calculated and used for evaluation of the radiative transition probabilities between bound vibrational states. The calculated charge distributions show that the X and C states are dominated by Coulombic attraction between a doubly charged positive aluminum and the singly charged negative halogen ion. The results provide a new assignment of the photoelectron spectra of the neutral AlF. They are also in good agreement with the recently observed optical B–X emission spectra of both molecular ions and C–X bands of AlCl+. The absence of C–X emission of AlF+ is most likely due to predissociation of the C 2Πr state by the repulsive A 2Πi state.

Optical spectra of ten AX+ ions (A=B, Al, Ga, In; X=F, Cl, Br) have been observed in the visible and near UV; a total of 18 band systems were newly discovered. The emission was produced by chemiluminescent reactions A++X2 at low (2–10 eVCM) kinetic energy in a beam-gas arrangement. A position-sensitive photon counting detector with large surface area and very low dark count rate was employed, the resolution was mostly 5–50 Å FWHM. Three types of band systems were observed: (1) For all AX+ combinations except BCl+ and BBr+, a very broad quasicontinuum with undulatory structure appears. On the basis of electronic state correlation arguments, photoelectron data, some ab initio calculations and, in one case, a known emission spectrum (InCl+) these band systems were identified as B 2Σ+–X 2Σ+ transitions. It is concluded that the excited state potentials are considerably displaced against the ground state, and their energetics are given. (2) For six species AX+, narrow band systems were observed in the 2500 Å region. They could be clearly identified as being due to C 2Π–X 2Σ+ transitions by means of comparison with the systematics of the analogous A 2Π–X 2Σ+ transitions of the isoelectronic alkaline earth halides, by the resolved fine structure, and, in the case of AlF+, by an ab initio calculation. (3) In the GaCl+, GaBr+, and InBr+ spectra, narrow features accompany the C–X transitions. They are attributed to D 2Σ+–X 2Σ+ transitions, analogous to the alkaline earth halide B 2Σ+–X 2Σ+ band systems. Qualitative electronic state correlations are discussed, and the expected dominant configurations in different regions of the AX+ ground and excited states are given. These are in accord with recent ab initio results on AlF+.

A laser-induced fluorescence technique, in combination with optical emission spectroscopy, was applied to investigate the spatial distribution of the number densities of neutral species in an asymmetric low-pressure 27.12 MHz discharge in pure nitrogen. The primary targets of this investigation were the three nitrogen triplet states C 3 u , B 3 g and the metastable A 3 + u each in their lowest vibrational levels v = 0, 1, 2 and additionally v = 8 of A 3 + u which can be populated very efficiently in the plasma sheath. The absolute number densities of all these states were determined in a Rayleigh scattering experiment, the results of which have been published elsewhere (Krames B, Glenewinkel-Meyer T and Meichsner J 2001 J. Appl. Phys. 89 3115). The spatial profiles as a function of the discharge parameters were studied by varying the gas pressure between 10 and 100 Pa and the effective rf voltage up to 260 V. These profiles of the different molecular states vary markedly in the plasma bulk as well as in the sheath near the powered electrode. A model, which includes the production, diffusion and quenching rates of contributing species and also fluorescence cascade processes, explains these varieties. A comparison with the experimental data at 50 Pa confirms the results of the model calculations.

No abstract

We observe the photodissociation of SiH4 in a pulsed molecular beam after excitation with 125.1 nm vacuum ultraviolet light generated via resonant four-wave mixing in mercury vapor. Ultraviolet radiation from a Nd:YAG/dye laser combination ionizes the neutral photodissociation fragments and a time-of-flight mass spectrometer detects the ions. The photodissociation signal consists entirely of silicon atoms and silylidyne (SiH) in its first electronically excited state. We see no silylene (SiH2) or silyl radicals (SiH3). Thus, the photodissociation cleaves almost all of the silicon–hydrogen bonds, but the energetics of the dissociation require the production of at least one hydrogen molecule per dissociation event. These results imply that the high energy content of the initially excited Rydberg state prevents formation of the silylene and silyl radicals in stable vibronic states and that dissociation pathways exist that connect the Rydberg state directly to the corresponding silicon atom and silylidyne asymptotes. These pathways are likely to exist because of Jahn–Teller distortion from the initial Td symmetry. Very little of the available energy appears as kinetic energy of the fragments but rather as electronic excitation of the products. Our results differ from those of earlier studies that concluded silylene and silyl are the principle products.