Keldysh parameter

Fig. 4.2. Complex saddle points t s (left panel), ts (middle panel), and kxs (s = i,j) (right panel) for the pair of solutions having the shortest travel times as discussed in the text. The figure is for ATI, for a Keldysh parameter of 7 = 0.975, and emission parallel to the laser held. The panels present the paths in the complex plane that are followed by the saddle points as a function of the final energy of the electron at the detector, which is indicated by the numbers (in multiples of Up). The figure shows how the saddle points of a pair approach each other very closely near the classical cutoff at 10 Up, which is the classical cutoff of the ATI energy spectrum. The contribution of the orbit that is drawn dashed has to be dropped after the cutoff. From [30]...

Strong field excitation may be divided into three different regimes, according to the value of a parameter 7k introduced by Keldysh [494], defined... 

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