Infrared photodissociation spectroscop

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

Since Ar has an I value lower than that of F, it is not unreasonable to expect that argon fluorides will be synthesized soon. Recently HArF was detected in a photodissociation study of HF in an Ar environment (see Section 5.8.1). With infrared spectroscopic studies of species such as H40ArF, D40ArF, and H36ArF, it was determined that the bond lengths of H-Ar and F-Ar are 133 and 197 pm, respectively. As of now, He and Ne are the only two elements that do not form any stable compounds. 

The infrared and UV spectra of van der Waals molecules do, however display many sharp lines.This indicates that the excited states often have sufficiently long lifetimes to display sharp spectral features, despite the fact that they have more than enough energy to dissociate. In principle every observed spectral line corresponds to a photodissociation process. If the line is sharp the dissociation proceeds through a long lived intermediate resonance state and, in spectroscopic parlance, is termed a predissociation process. In the present brief overview I will discuss the spectra of van der Waals molecules from this view point. The main objective of the chapter will be to outline the different possible treatments of the process and their relationship to each other as well as to collect together a few key references on the theory of these processes. 

Cold, trapped HD+-ions are ideal objects for direct spectroscopic tests of quantum-electrodynamics, relativistic corrections in molecules, or for determining fundamental constants such as the electron-proton mass ratio. It is also of interest for many applications since it has a dipole moment. The potential of localizing trapped ions in Coulomb crystals has been demonstrated recently with spectroscopic studies on HD+ ions with sub-MHz accuracy. The experiment has been performed with 150 HD+ ions which have been stored in a linear rf quadrupole trap and sympathetically cooled by 2000 laser-cooled Be+ ions. IR excitation of several rovibrational infrared transitions has been detected via selective photodissociation of the vibra-tionally excited ions. The resonant absorption of a 1.4/itm photon induces an overtone transition into the vibrational state v = A. The population of the V = A state so formed is probed via dissociation of the ion with a 266 nm photon leading to a loss of the ions from the trap. Due to different Franck-Condon factors, the absorption of the UV photon from the v = A level is orders of magnitude larger than that from v = 0. 

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