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Direct absorption techniques

A recent absorption study has recorded the first spectrum for TiF+ using a sensitive high resolution absorption technique [31]. Solid phase parent TiF4 was heated with a thermal resistor to create TiF4 vapour in a cell. Helium gas was flowed through a central inlet, carrying the precursor gas, which was ionised using a AC glow discharge source. The oscillatory discharge allows accurate detection using the velocity modulation technique [18] which eliminates signals from neutral molecules. A ring dye laser operating in the 16,800 - 18,600 cm-1 region was used in conjunction with a two photodiodes to record the absorption spectrum through the cell.

Cavity ring down spectroscopy is a very sensitive technique [32], used to measure absorption spectra. It has recently [33] been combined with a molecular ion source to study the (6,0) band of the A ${\rm ^{2}\Pi_{u}-X^{2}\Sigma_{g}^{+}}$ transition of N2+. The technique involves a hollow cathode discharge cell consisting of two highly silvered mirrors (99.99 % reflectivity) at either end. A pulsed laser is focussed into the chamber and the ring-down time (which is the lifetime of the decay curve for residual laser power leaving the cavity) is shorter with the laser on resonance, and longer when it is off resonance. A photomultiplier is used to detect the residual laser power emanating from the cell and electronic timing gates convert this to an absorption signal. Due to the high reflectivity of the mirrors and long path length, the laser pulse passes through the chamber many times thus allowing very weak absorptions to be recorded.


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