The beam of light has three wavelengths $4144$ \AA, $4972$ \AA and $6216$ \AA with a total intensity of $3.6 \times 10^{-3}$ W/m$^2$ equally distributed amongst the three wavelengths. The beam falls normally on an area $1$ cm$^2$ of a clean metallic surface of work function $2.3$ eV. Assume that there is no loss of light by reflection and that each energetically capable photon ejects one electron. Calculate the number of photoelectrons liberated in $2$ s.