Longer wavelengths require lower intensity in multiphoton detachment of negative ions.

Abstract

Negative chlorine ions, held in a Penning ion trap, are irradiated by intense (up to 2\ifmmode\times\else\texttimes\fi{}${10}^{16}$ W/${\mathrm{m}}^{2}$) Raman-shifted light of a Nd:YAlG laser with a wavelength of 1098 nm and a pulse duration of 30 ps. Electron-energy spectra are obtained showing up to 11-photon absorption. In the case of circularly polarized light suppression of the low-energy channels is observed. A saturation intensity of (8.5\ifmmode\pm\else\textpm\fi{}2.5)\ifmmode\times\else\texttimes\fi{}${10}^{15}$ W/${\mathrm{m}}^{2}$ is found for multiphoton detachment of ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ by 1908-nm light. This result is counterintuitive because it is lower than the saturation intensity for multiphoton detachment at 1064 nm.