
TY  - JOUR
A1  - Unnikrishnan, K 
T1  - Renormalised Perturbation Theory of Resonant Multiphoton Ionisation
JF  - Australian Journal of Physics
JO  - Aust. J.  Phys.
Y1  - 1995
VL  - 48
IS  - 5
SP  - 835
EP  - 848
KW  - 
UR  - https://doi.org/10.1071/PH950835 
N2  - Perturbation theory of multiphoton ionisation due to a classical electromagnetic field is modified to allow for intermediate resonances with bound states. Complex energies, generally associated with resonances, do not enter into this formalism. For a monochromatic field of frequency u, a constant ionisation rate can then be defined unambiguously and only such continuum states are excited which correspond to the absorption of energy in integral multiples of hu. As an application, differential and total cross sections for the two-photon ionisation of hydrogen, for frequencies below the n = 3 resonance region, are obtained in closed form. Existing data for generalised cross sections, calculated numerically using the complex coordinate method, are in good agreement with the present results. Finally, the mean fractional ionisation resulting from a pulse of finite duration is estimated on the basis of the associated power spectrum. For short pulses, the time dependence of ionisation exhibits a departure from that expected of a time-independent rate. 
ER  -

