Your operator is a rank one perturbation of the multiplication operator $(Mf)(x) = (x^3/2)f(x)$$(Mf)(x) = (x/2)f(x)$, which has (purely) absolutely continuous spectrum equal to $[0,1/2]$. Since the ac spectrum is invariant under trace class perturbations (so certainly under rank one perturbations), your operator $L$ still has the same ac spectrum, so doesn't even come close to having pure point spectrum (and thus it isn't "diagonalizable," if you want to put it this way, though I personally don't think it's very good terminology).