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At dissociation, the energy difference between the bound molecular state and the dissociation limit along which the molecule fragments is released as translational energy of the atomic fragments. With the laser fixed on a rovibronic absorption line, the second electromagnet can be scanned and a spectrum of the fragment ion's momentum is built up. A centre-of-mass energy release can then be deduced from the momentum profile of the fragment ion neglecting any internal atomic excitation. Measurement of the excess energy of a fragment ion measures the energy of the molecule above the dissociation limit at which it fragments. The profile of the daughter ion's momenta in the lab frame gives an indication of the trajectory of the ion, determining whether the transition moment is perpendicular or parallel to the laser polarisation.
In these types of experiments, the tunable dye laser is tuned to an assigned spectroscopic transition and the first electromagnet set to transmit the parent ion. The second electromagnet is scanned over the region of the daughter ion's mass and the ion current arriving at the multiplier is recorded as a function of the second magnet current. The scan voltage controlling the second electromagnet's current is recorded simultaneously with the daughter ion current to ensure accurate calibration of magnet current and simple conversion to energy releases.
Next: Conversion of momentum to
Up: Experimental
Previous: Laser Dyes
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Tim Gibbon
1999-09-06