EFFECTS OF A PULSE-WIDTH ON THE QUANTUM SPECTRA

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