Excess-photon detachment in the negative gold ion

Multiphoton detachment of ${\mathrm{Au}}^{\mathrm{-}}$ is studied at an irradiation wavelength of 1064 nm and for optical pulses of ∼40 ps duration and maximum intensity ∼3×${10}^{12}$ W/${\mathrm{cm}}^2$. The negative gold ion can be detached by absorption of a minimum of two 1.165-eV photons which leads to ejection of ∼20-meV electrons. Utilizing a time-of-flight electron spectrometer, the absorption of up to two excess photons is observed with a probability, normalized to the lowest-order peak, of ∼5% and ∼0.6%, respectively, for first- and second-order excess-photon detachment.