Cavity ring down spectroscopy of molecular ions in the 3 micron region

Computer Science – Performance

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Many ions, both + and -, have been found in space. They play a role in gas-phase reaction schemes through reactions with molecules. Ionic complexes may then be thought of as intermediates whose potential energy surfaces will provide information on the reaction dynamics. Because ionic complexes possess significantly higher binding energies than their van der Waals analogues, they may survive the intense radiation conditions in space. Candidate complexes include (CO-CO)+, H_2-HCO+, H_2-HN_2+, H_2O-H_2O+, N_2-N_2+, N_2-H+-N_2. An added advantage in the cases of H_2-HCO+ and H_2-HN_2+ is that a dipole moment is given to a molecule that is intrinsically invisible in the IR. The ionic complexes are created in a continuous plasma jet emanating from a slit-nozzle. The gas mixture and pressure behind the nozzle are varied to favour desired products, which are identified with a quadrupole mass spectrometer. Cavity ringdown spectroscopy (CRDS) is used to map the potential energy surfaces and to determine molecular parameters. A cw optical parametric oscillator (OPO) is used as a light source. It combines a broad spectral coverage and narrow bandwidth with a high output power and excellent mode characteristics. The performance of the technique is demonstrated by recording a rotationally cold spectrum of the formyl cation, HCO+, in the CH stretching region. Absolute ion densities are directly derived. Presently, experiments are prepared to record fully resolved spectra of ionic complexes of astrophysical relevance, with a focus on species that may be considered reactive intermediates in ion-molecule reactions in space [2]. Some experience measuring such species in direct absorption using plasma methods exists [5]. An extension to species such as (CO-CO)+ or H_2-HCO+ is planned.

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