Technique used in
femtochemistry spectroscopy to study processes that occur during the evolution of the
excited state. The concept is to use two pulses, one to excite the system to a high electronic state (the pump pulse) and the next to monitor the system in the excited state (the probe pulse).
The advantage to this is that the pump pulse sets time zero of the experiment. Since all the molecules in the sample beam are excited at the same time, their vibrational modes are synchronized and the evolution of bond vibration can be monitored as a fucntion of time using the probe pulse. This was how Ahmed Zewail and his laboratory where able to study the dynamics and breaking of the ionic bond in sodium chloride. Similar techniques have been applied to study roational motion in molecules such as stilbene.