Photoemission yield under two-quantum excitation in Si

It is shown for the first time that under pulsed laser excitation, photoemission from cleaned (7×7) Si (111) surfaces occurs at photon energies (3.7 to 2.3 eV) below the work function ($\phi =4.6\mathrm{}$ eV). We demonstrate its two-quantum origin by: (i) establishing the characteristic flux law, i.e., electron flux proportional to the square of photon flux, over six orders of magnitude; (ii) showing that the two-quantum yield falls very rapidly when photon energy decreases, no photoemission being observed below $\frac{1}{2}\phi$.