EFFECTS OF A PULSE-WIDTH ON THE QUANTUM SPECTRA

A theoretical study is made of the absorption cross section by a finit e width laser pulse for atoms or negative ions in external fields. A m odified closed-orbit formula for the cross section is derived. It is s hown that the finite width of the laser pulse reduces the contribution s from the short closed orbits and completely eliminates the contribut ions from the long closed orbits whose transit times are much greater than the pulse width. The theory is applied to study the photodetachme nt of H- in the presence of a static electric field by a laser pulse.