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Detection Electronics

The daughter ions are directed onto the first dynode of an electron multiplier (gain $\approx 10^{6}$) by the second electromagnet. The current from the multiplier is fed into a current preamplifier (Brookdeal 5002 or in-house built devices) and the DC voltage output monitored with an oscilloscope. The AC voltage component is fed into a lock-in amplifier (incorporating a 3 kHz filter) with a reference signal from the chopper. This discriminates against daughter ions arising from collisional or unimolecular dissociations. The 0-10 V output from the lock-in amplifier is fed into the input of the Coherent Autoscan interface system. Lock-in time constants of between 300 ms and 10 s are typical, depending on laser power, ion beam strength and the intensity of transitions.



Tim Gibbon
1999-09-06