Question

Radiation coming from transitions $n=2$ to $n=1$ of hydrogen atoms falls on helium ions in $n=1$ and $n=2$ states. What are the possible transition of helium ions as they absorb energy from the radiation ?

Answer 1

By Bohr's model, energy in nth orbit is given by $\frac{-13.6Z^2}{n^2}$ where Z is the atomic number of the atom. Using the given formula, energies of different states are obtained as shown in the image. Now when transition is made from n=2 to n=1, a photon of 13.6-3.4=10.2 eV energy is released.

When this falls on helium atom:

When it falls on n=1 state, the minimum energy required to make a transition is 54.5-13.6=40.2 eV. Hence non transition is possible with this photon,

When it falls on n=2 state, a transition can directly be made from n=2 to n=4 as the energy required for this is 10.2 eV only. Or it can first make a transition to n=3 state and then n=4.