Given, ionisation energy = 13.6 eV
Energy absorbed by the electron = 13.6×1.5=20.4 eV
So, the maximum kinetic energy of the electron
=20.4−13.6 eV=6.8 eV
Again, from de Broglie wavelength,
λ=h√2meV
⇒λ=6.626×10−34√2×6.8×1.6×10−19×9.1×10−31
⇒λ=6.626×10−3414×10−25
⇒λ=4.73 Ao≈5 oA