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Infrared and Microwave studies

Oka and co-workers were the first group to report a direct infrared absorption spectrum of a molecular ion. They passed coherent infrared radiation through a multiple reflection, liquid nitrogen cooled, discharge cell and recorded a vibration-rotation spectrum of H3+; the simplest triatomic molecular ion. The laser radiation source was constructed from the output of a dye and argon laser mixed in a LiNbO3 crystal. The ratio of ions to neutral species in such a cell is approximately 1:106, and so an inherent disadvantage of such a technique is that the molecular ion absorption spectra can become obscured beneath (even weak) neutral species absorptions. Three modulation techniques have been developed to overcome these disadvantages and to enable the direct infrared absorption spectra of molecular ions to be recorded:

Microwave spectroscopy generally involves transitions between rotational states in the ground vibrational state. It is an extremely high resolution technique which allows a very precise determination of many terms in the Hamiltonian. Laboratory studies of microwave spectra for molecular ions were first performed in a glow discharge spectrometer on CO+ by Dixon and Woods [26].

Microwave spectroscopy has also recently been used in conjunction with Fourier Transform techniques to study the rotational spectra of a number of polyatomic ions using a Pulsed Discharge Nozzle (PDN) coupled with a Fourier-transform microwave spectrometer. Shortly after leaving the nozzle the ions are subject to a Lorentz force due to a static magnetic field passing through a coil. This focusses ions into a region in the centre of the microwave cavity, where spectra can be recorded. Spectra of HOCO+, HOCS+, D3+-Ar and D ${\rm _{2}H^{+}}$-Ar have been recorded in this manner [27].


next up previous contents
Next: Laser Magnetic Resonance Up: Molecular ions Previous: Emission Spectroscopy   Contents
Tim Gibbon
1999-09-06