Answer : The energy of a photon absorbed by
ions is 
Explanation :
The energy of an electron in the nth level is,

where,
= energy of an electron in the nth level
B = constant = 
n = number of energy level
Z = charge on nucleus or number of protons
First we have to calculate the energy of an electron for n = 1 level.

Z = charge on nucleus or number of protons for helium atom = 2


Now we have to calculate the energy of an electron for n = 4 level.

Z = charge on nucleus or number of protons for helium atom = 2


Now we have to calculate the energy of a photon that is absorbed by
ions.



Therefore, the energy of a photon absorbed by
ions is 