When electrons jumps from ground state level(n=1) to another another energy level(n=2) the corresponding energy is called 1st excitation potential energy and corresponding potential is called 1st excitation potential.

ionization potential energy is defined as the minimum energy required in ev to excite a neutral atom to its ionized state i.e it is the minimum energy required in ev to excite an electron from n=1 to infinity, and corresponding potential is called ionization potential.

Given, 

Ionization potential of the atom= 16 v

So, first excitation potential of this atom is-

3.4 - 16= 12.6 v