A Silver surface $($work function $=4\mathrm{.}26$eV$)$ is illuminated with $240$nm light from a $5$mW laser.

How many photons per second are emitted by the laser?

$6\mathrm{.}041015$ photons per second

Only a small fraction of these photons can interact with electrons at the surface. Suppose this fractional efficiency is $2\mathrm{.}1\%.$

What is the current in the detector assuming all the emitted electrons are captured?

nA

Note that current is charge $($in Coulombs$)$ per second. The charge of one electron is $1\mathrm{.}61019$C