•The 2Π(4p) state of the AlAr van der Waals complex has been observed using two-photon excitation.•2Π(4p) zero-point spin–orbit coupling constant was found to be 18.5 cm−1.•Potential curve fitting yielded an equilibrium distance of 3.33 Å and a well depth of De = 495 cm−1.The 2Π(4p)–X2Π(3p) band system of AlAr has been observed using two-photon excitation. The spectrum consists of a short progression of doublet bands, with spin–orbit intervals that are close to that of Al(4p). Potential energy curve fitting yielded a bond dissociation energy for 2Π(4p) of De = 495(5) cm−1 and an approximate bond length of Re = 3.33(4) Å.Graphical abstract

Low lying electronic states of the beryllium dimer were investigated by laser induced fluorescence (LIF) and resonance enhanced multiphoton ionization (REMPI) techniques. Be2 was formed by pulsed laser ablation of Be metal in the presence of helium carrier gas, followed by a free jet expansion into vacuum. Several previously unobserved states of the dimer were characterized. These included transitions of the triplet manifold (2)3Πg ← (1)3Σ+u and (3)3Πg ← (1)3Σ+u, for which rotationally resolved bands were obtained. In addition, transitions to the v′ = 10–18 vibrational levels of the A1Πu state were recorded. Photoionization efficiency (PIE) measurements were used to determine an accurate ionization energy (IE) for Be2 of 7.418(5) eV and the term energy for (1)3Σ+u. Above the ionization threshold the PIE spectrum was found to be highly structured, consisting of overlapping Rydberg series that converged on excited vibrational levels of Be2+. Analysis of these series yielded a vibration frequency for the X2Σ+u state of 498(20) cm−1. The bond dissociation energy for Be2+, deduced from the IE measurement, was 16072(40) cm−1. Multi-reference configuration interaction (MRCI) calculations were carried out for Be2 and Be2+, yielding results that were in excellent agreement with the experimental observations.

A study of NH/D–Ne was undertaken to investigate the structure of this complex and examine the ability of high-level theoretical methods to predict its properties. The A 3Π–X 3Σ− transition was characterized using laser induced fluorescence measurements. Results from theoretical calculations were used to guide the interpretation of the spectra. Two-dimensional potential energy surfaces were calculated using second-order multireference perturbation theory with large correlation consistent basis sets. The potential energy surfaces were used to predict the ro-vibronic structure of the A–X system. Calculated ro-vibronic energy level patterns could be recognized in the spectra but quantitative discrepancies were found. These discrepancies are attributed to incomplete recovery of the dynamical correlation energy.

The effects of relative orientation on collision and reaction dynamics can be examined by characterizing the unimolecular decay of van der Waals complexes. Most commonly, decomposition is initiated by exciting one of the monomers within a complex, and the relative orientation is defined by the zero-point motions for the intermolecular degrees of freedom. However, if simultaneous excitation of the intermolecular degrees of freedom is achieved, a considerable range of starting configurations may be accessed. We are currently studying I2–Rg and CN–H2/D2 complexes with the goal of using these systems to examine oriented dynamics. For I2(B)–Ne we have observed excited intermolecular vibrations and the effects of these motions on the predissociation dynamics. The correlation between structure and dynamics suggested by these results can be understood using classical mechanics, but the implications of this model are at variance with accepted ideas about the topology of I2(B)–Rg potential surfaces. Spectroscopic and theoretical studies of CN–H2/D2 indicate that this pre-reactive complex may be used to examine steric effects in the H2+CN→H+HCN reaction. The complex was characterized using the A–X and B–X electronic transitions. As is often the case for weakly bound systems, the insights provided by high-level theoretical calculations were essential for interpretation of the spectra. The properties of excited intermolecular vibrations of CN–H2 were predicted as a prelude to studies of the intra-cluster reaction dynamics.

Low lying electronic states of the beryllium dimer were investigated by laser induced fluorescence (LIF) and resonance enhanced multiphoton ionization (REMPI) techniques. Be2 was formed by pulsed laser ablation of Be metal in the presence of helium carrier gas, followed by a free jet expansion into vacuum. Several previously unobserved states of the dimer were characterized. These included transitions of the triplet manifold (2)3Πg ← (1)3Σ+u and (3)3Πg ← (1)3Σ+u, for which rotationally resolved bands were obtained. In addition, transitions to the v′ = 10–18 vibrational levels of the A1Πu state were recorded. Photoionization efficiency (PIE) measurements were used to determine an accurate ionization energy (IE) for Be2 of 7.418(5) eV and the term energy for (1)3Σ+u. Above the ionization threshold the PIE spectrum was found to be highly structured, consisting of overlapping Rydberg series that converged on excited vibrational levels of Be2+. Analysis of these series yielded a vibration frequency for the X2Σ+u state of 498(20) cm−1. The bond dissociation energy for Be2+, deduced from the IE measurement, was 16072(40) cm−1. Multi-reference configuration interaction (MRCI) calculations were carried out for Be2 and Be2+, yielding results that were in excellent agreement with the experimental observations.