Quantum dynamics of the van der Waals molecule (N2)2: An ab initio treatment

Abstract

Starting with an available ab initio N2–N2 potential, which favors a crossed equilibrium structure for the (N2)2 dimer with well depth De=122 cm−1, Re=3.46 Å, and barriers to internal rotations of 25 and 40 cm−1, we calculate the bound rovibrational states of this dimer for J=0, 1, and 2. This is done by solving a secular problem over the exact (rigid monomer) Hamiltonian including centrifugal distortions and Coriolis interactions, using a product basis of radial (Morse oscillator) functions and angular momentum eigenfunctions. The full permutation-inversion symmetry of the system, in relation to the nuclear spin coupling, is used in order to simplify the calculations and to derive selection rules for IR absorption. We find that the (N2)2 dimer has a large number of bound rovibrational states (92 already for J=0). These are analyzed by correlation with rigid molecule (harmonic oscillator/rigid rotor) results, on the one hand, and with the states of two freely rotating N2 monomers, on the other, and by plotting some characteristic vibrational wave functions. In the ground state, the vibrations are nearly harmonic, with a small tunneling splitting; the dissociation energy D0 ranges from 74.9 cm−1 for oN2–oN2 to 80.5 cm−1 for pN2–pN2, the mean distances 〈R〉 equal 3.79 and 3.76 Å, respectively. In the lower vibrationally excited states, the monomer rotations are still locked in, but strongly anharmonic and coupled, also with the dimer stretch. With increasing energy, the internal rotations become successively delocalized in the different angles, starting with the torsion (φ) about R. The resulting energy level diagram is so complex that it is hard to discover regularities. The results are compared with the experimental IR spectrum.