Photoelectric Sphere!

A monochromatic light of wavelength $\lambda$ is incident on an isolated metallic sphere of radius $R$ . The threshold wavelength is $\lambda _{0}$ which is larger than $\lambda$ . Find the number of photoelectrons emitted before the emission of photoelectrons will stop.If answer is of the form $\frac{a\pi \epsilon _{0}Rhc}{be^{d}}(\frac{1}{\lambda }-\frac{1}{\lambda _{0}})$ Find the value of $a+b+d$ . Here $a,b,d$ are positive integer constants. $h=$ Plancks constant, $c=$ Speed of light, $e=$ Charge of an electron. All units are in SI system.