Measurement Techniques for Optical Transients

In this subsection we will discuss various techniques for recording a decaying light intensity. A short laser pulse is used to excite the atoms at time t=0. Fluorescence photons are detected by a fast photomultiplier tube. The corresponding electrical signal can be handled in different ways which can conveniently be discussed with reference to Fig.9.22. 

Transient-Digitizer Technique. The most direct way of capturing the transient signal from the photomultiplier tube is to use a normal fast oscilloscope and take a photograph of the screen with open camera shutter while the electron beam, whicli is trigged by the laser, sweej j the screen drawing the decay curve. Clearly, it is inconvenient to further process data 

Examples of atomic excited-state decay curves recorded with the transient digitizer technique for a sequence of states in sulphur are shown in Fig. 9.20. The lifetimes increase with a r oc trend as expected for a Rydberg sequence 

Delayed-Coincidence Techniques. This method operates in the extremely low intensity regime, in which single-photon counts axe recorded. The principle is illustrated in Fig. 9.22. 

Optical detection methods (streak-camera and pump-probe techniques) exist that are even faster then those discussed here. Ultrafast spectroscopy is discussed in Sect. 9.5. 

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