To study fast processes such as collision induced decays of excited molecules, isomerization of excited molecules, photoexcitation of rhodopsim molecules of the vetina in the eye and several other ultrafast processes, we need good time resolution which can be obtained by generating ultrashort laser pulses. Time resolution in the femtosecond (1fs = 10 -15 s) has been achieved now. In this section we outline some features involved in getting these ultrashort pulses. If t is the duration of the light pulse, the spectral resolution is given by 1 /t. The duration of the pump pulse is dependent on the electric discharge which could be of the order of nanoseconds.
The intensity vs time of the laser pulse (referred to as the time profile) is determined by the relaxation times ( ) of the upper and lower levels ( u and L). If these times are short compared to the rise time of the pump pulse, then a smooth time profile of the laser pulse PL(t) is obtained. The intensity of PL (t) depends on the inverted population difference N (t) of the upper and lower states. The pump and laser time profiles of gas lasers and solid state lasers are shown below
Figure 31.11 Figure showing the growth of the pump pulse, the laser pulse and the inverted population difference N(t). Laser time profile" is large when N(t) is large. a) gas lasers, b) N2 laser and c) solid state lasers.
If the values of u and L are large, the induced emission does not deplete the upper level. These can be combined with amplification of induced light emission to get the spikes shown in Fig 31.11 (c)
t is the duration of the light pulse, the spectral resolution 
is given by 1 /
) of the upper and lower levels ( 
