Observation of a step change in the optical absorption of gold in a vacuum

In this paper, we study the ambient pressure dependent optical absorption of gold following femtosecond laser pulse excitation. We find that gold exhibits a clear pressure dependent step function in its optical absorption. For the $p$-polarized $60\text{\ensuremath{-}}\text{femtosecond}$ laser pulses at $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the pressure threshold for the step change is found to be $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\text{torr}$ and is nearly independent on the pump laser intensity. The enhanced absorption is attributed to the contribution of a layer of emitted electrons from the metal surface due to intense pulse excitation.