Incident wavelength, \(\lambda=200\, \mathrm{nm}\)

Work function, \(\phi_{0}=5.01 \,\mathrm{eV}\)

According to Einstein's photoelectric equation

\({e V_{s}=h v-\phi_{0}}\)

\({e V_{s}=\frac{h c}{\lambda}-\phi_{0}}\)

where \(V_{s}\) is the stopping potential

\(e V_{s}=\frac{(1240 \,\mathrm{eV} \mathrm{nm})}{(200\, \mathrm{nm})}-5.01 \,\mathrm{eV}\)

\(=6.2\, \mathrm{eV}-5.01\, \mathrm{eV}=1.2\, \mathrm{eV}\)

Stopping potential, \(V_{s}=1.2 \,\mathrm{V}\)

The potential difference that must be applied to stop photoelectrons \(=-\,V_{s}=-\,1.2\, \mathrm{V}\)