The rigid rotor model can be improved by recognizing that in a realistic anharmonic potential, the bond length increases with the vibrational quantum number n. Therefore, the rotational constant depends on n, and it can be shown that B_n = B − (n + 1/2)α , where B is the rigid rotor value. The constant α can be obtained from experimental spectra. For ¹H⁸¹B          r,B = 8.46488 cm⁻¹ and α = 0.23328 cm^–1. Using this more accurate formula for B_n, calculate the bond length for HBr in the ground state and for n = 3.