  We report a quantum mechanical method for calculating the momentum
distributions of constituent atoms of polyatomic molecules in
rotational-vibrational eigenstates. Application of the present theory to
triatomic molecules in the rovibrational ground state revealed that oscillatory
changes appear on the proton momentum distribution in the nonlinear
$\mathrm{H_2O}$ molecule, whilst no such modulation is present in the case of
an oxygen atom in the linear $\mathrm{CO_2}$ molecule. The atomic momentum
distributions were analyzed in detail by means of a rigid rotor model, and it
was found that the oscillation originates from quantum-mechanical
delocalization of the target atom with respect to the other atoms.
