A Hydrogen-like atom has atomic number 𝑍. When it collides with stationary hydrogen  atom in ground state, it needs a minimum KE of 51 eV to excite the hydrogen atom.  Photons emitted in the  electronic transitions from level 𝑛 = 4 to level 𝑛 = 2 in these  atoms are used to perform photoelectric effect experiment on a target metal.  The  maximum kinetic energy of the photoelectrons generated is 4 eV. The photoelectric  threshold wavelengths for the target metal is close to 100X nm where X is