EXCESS-PHOTON ABSORPTION VIA AUTODETACHING STATES - POTENTIAL FOR SPECTROSCOPY BEYOND THE ISOLATED CORE EXCITATION METHOD

We report the observation and study of the excess-photon absorption (E PA) phenomenon in the region of short-lived autodetaching states of ne gative ions at low intensities (10(4)-10(7) W cm(-2)). A sensitive res onant-ionization-based technique is utilized here for the first time t o investigate nonlinear processes in the continuum, including detailed studies of signal response to laser power. We demonstrate that the se nsitivity of the new method is sufficient to obtain actual positions a nd widths of the states at the limit of low intensities. Higher intens ities (i.e. above similar to 10(5) W cm(-2)) can lead to significant d isplacement of the EPA signals from the autodetaching state positions and, depending on the shape of the one-photon detachment cross section , to either broadening or narrowing of the signal peaks. The amplitude of the signal at these higher intensities typically displays a sublin ear dependence on laser power.