Advanced methods of spectroscopy

Spectroscopic methods continue to advance, and new techniques are emerging as the need for greater resolution and shorter pulse duration become necessary. With current techniques, it is possible to track very fast chemical events occuring on the femtosecond (10$^{-15}$ s) time scale and even now attosecond (10$^{-18}$ s) time scales. On the femtosecond time scale, it is possible to follow the motion of a single proton in a proton-transfer reaction in, for example, an acidic solution. On the attosecond time scale, the nuclei in a molecule hardly move at all, however, electron transfer processes occur on this time scale, so it is possible to track changes in the electron wave functions.

One of the method of obtaining time-resolved phenomena is known as time-resolved spectroscopy or pump-probe spectroscopy. Basically, a ``pump'' pulse is used to excite a molecule, and a second pulse is used to track the changes that occur as a result of the excitation. Thus, one is interested in the changes as a function of the delay time between the two pulses. Typically, one plots the so-called difference spectrum between the system at a time $t$ after the pump pulse (based on the probe pulse) and the spectrum of the system in its ground state so that one can clearly see where the changes occur at different frequencies.