Laser setup that allows for femtosecond time-scale measurements of excited state molecular processes.

Current lasers can, through mode-locking and other techniques, generate a train of very short light pulses. There are many technical advantages to having a short light pulse when doing spectroscopic measurements. It requires less post-experimental data processing if you dont have to deconvolute a wide excitation pulse from your signal. It also ensures a better idea of time-zero in the experiment and better synchronization of post-excitation events in the sample. Unfortunately, the shortest current pulse widths are on the order of a few picoseconds. To get better time resolution, the technique of up conversion becomes useful.

Up conversion requires splitting the exciting laser train into two beams. One beam is used to excite the sample. The other is directed through an alternate path with a variable time delay in it. The signal and the alternate beam are then mixed in a birefringent crystal and only those beams which are temporally synchronous are detected. That is to say, the second beam is acting as a shutter on the signal, only allowing detection of light at a time after excitation. This time is dependent on the amount of delay inserted in the second beam path. The delay is introduced by a mirror which is on a mobile stage. Moving this mirror increases or decreases the path length of the beam, therefore increasing its transit time before it reaches the crystal.

Nearly all current spectroscopic work in the femtosecond time domain requires some sort of up conversion setup. As a result, the Nobel Prize winning work of Ahmed Zewail and many others has since been possible. Ultrafast events such as modes of molecular vibration, formation and breaking of atomic bonds, and light induced conformational changes (e.g. in stilbene) have been studied using this method.