High Sensitivity Picosecond Laser Spectroscopy
Studies of fast spectroscopic events in atomic and molecular systems occurring on the picosecond or sub-picosecond time scales generally require short pulse excitation sources to create a population perturbation among discrete energy states and a means of monitoring the perturbation process or the recovery of the system towards its equilibrium conditions. In recent years excitation methods have benefited dramatically from the development of picosecond and femtosecond lasers operating in the visible regions of the spectrum, and derivatives of these excitation sources operating in the ultraviolet region (by frequency doubling and trebling) and in the infrared region (by difference frequency generation). Following hand-in-hand with these developments in short pulse sources of electromagnetic radiation have been improved fast detection methods, such as streak cameras, fast response photomultipliers and photodiodes, and pump-probe spectroscopic methods, which permit these perturbations to be followed to the picosecond and in some cases to the femtosecond time domains The extent to which such experimental methods may be employed to monitor transient spectroscopic events, however, depends not only on the time response of the overall system but also on the sensitivity of the detection methods to the perturbation created. In seeking to investigate the applicability of these methods to chemical systems there is, not surprisingly, a limitation set by the Heisenberg uncertainty principle between the temporal characteristics of the radiation source employed and the quantised energy states which may be interrogated. This limitation is most readily seen by an appreciation of the transform relationship which exists between the time duration of a pulse of laser radiation and the spectral width of the band of radiation of which it is comprised.
KeywordsProbe Beam Pump Pulse Pump Laser Probe Pulse Probe Laser
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