We present an analytical treatment of coherent excitation of a Two-Level Atom
driven by a far-off resonant classical field. A class of pulse envelope is
obtained for which this problem is exactly solvable. The solutions are given in
terms of Heun function which is a generalization of the Hypergeometric
function. The degeneracy of Heun to Hypergeometric equation can give all the
exactly solvable pulse shapes of Gauss Hypergeometric form, from the
generalized pulse shape obtained here. We discuss the application of the
results obtained to the generation of XUV.Comment: 9 Pages, 8 Figures. Accepted for Physical Review A as a regular
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