Saturation Spectroscopy in Fast Beams

For the elimination of the residual Doppler width the FIBLAS technique allows an elegant realization of saturation spectroscopy with a single fixed frequency laser (Fig.9.27). The ions are accelerated by the voltage U which is tuned to a value where the laser radiation is absorbed on a transition i) k) in the first part of the interaction zone at the fixed laser frequency 

In the second part of the interaction zone an additional voltage AU is applied which changes the velocity of the ions. If the laser-induced fluorescence is monitored by PM2 as a function of AU a Lamb dip will be observed at AU - 0 because the absorbing level i) has already been partly depleted in the first zone. 

If several transitions are possible from level i with frequencies w within the Doppler tuning range 

Fast ion and neutral beams are particularly useful for very accurate measurements of lifetimes of highly excited ionic and neutral molecular levels (Sect. 11.3). 

4 Laser Spectroscopy in Molecular Beams turbomolecular pump laser beam 

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The laser wavelength was kept at X = 5815 % and the velocity of the 0 ions was continuously tuned by controlling the acceleration voltage. More examples, including saturation spectroscopy in fast ion beams, can be found in the review by DUFAY and GAILLARD [10.22]. 

An electronic or vibrational excited state has a finite global lifetime and its de-excitation, when it is not metastable, is very fast compared to the standard measurement time conditions. Dedicated lifetime measurements are a part of spectroscopy known as time domain spectroscopy. One of the methods is based on the existence of pulsed lasers that can deliver radiation beams of very short duration and adjustable repetition rates. The frequency of the radiation pulse of these lasers, tuned to the frequency of a discrete transition, as in a free-electron laser (FEL), can be used to determine the lifetime of the excited state of the transition in a pump-probe experiment. In this method, a pump energy pulse produces a transient transmission dip of the sample at the transition frequency due to saturation. The evolution of this dip with time is probed by a low-intensity pulse at the same frequency, as a function of the delay between the pump and probe pulses.1 When the decay is exponential, the slope of the decay of the transmission dip as a function of the delay, plotted in a log-linear scale, provides a value of the lifetime of the excited state. 

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