The production and spectroscopy of molecular ions isolated in solid neon

Abstract

Studies of the molecular spectra of small polyatomic molecular ions are stlll in therr infancy. The availability of survey spectra would facilitate the search for individual vibrational and vibronic bands of gas-phase ion species using sophisticated, highly sensitive laser techniques. Deeectable concentrations not only of simpee ions such as CO+2 and H+2 but also of dimer cations and anions such as O+4 and O-4 have been stabilized in solid neon when the parett molecule is codeposited with a beam of excited neon atoms. The vibrational fundamentals heretofore observed for the simpee ions isolated in solid neon lie very close to the positions of the corresponding gas-phase band centers. Molecular spectroscopic data are not yet available for the gas-phase dimer ions. Therefore, it is difficutt to estimate how closely the neon-matrix vibrational frequencies here reported for a numbrr of dimer ions correspond to the gas-phase band centers. Some guidance is available from comparison of the argon-matrix spectra of small molecules hydrogen-bonded to HF with the gasphaee spectra of theee hydrogen-bonded species [55]. In theee studies, the effect of the argon matrix is to enhance the apparent hydrogen-bond strength; the HF stretching frequency is lowered by 1 to 5% from the gas-phase value, and the absorptions contributed by the flexing of the HF with respect to the other molecule are raised by 5% or more. Since neon matrices are generally less perturbing than argon matrices, the deviation from the gas-phase frequencies shoudd be somewhat less in neon-matrix studies. In the present experiments, only the high frequency stretching fundamentals have been observed, suggesting that matrix shifss shoudd amoutt to less than 3 or 4%. Therefore, matrix isolation studies such as theee promise to provide a valuable new tool for the detection and spectroscopic characterization of small molecular ions and cluster ions.