The rotationally resolved infrared spectrum of the Na+H2 cation complex is recorded in the H–H stretch region (40674118cm1) by monitoring the production of Na+ photofragments. Altogether 42 lines are identified, 40 of which are assigned to Ka=1-1 transitions (associated with complexes containing ortho-H2) and two tentatively assigned to Ka=0-0 transitions (associated with complexes containing para-H2). The Ka=1-1 subband lines were fitted using a Watson A-reduced Hamiltonian, yielding effective spectroscopic constants. The band origin is estimated as 4094.6cm1, a shift of 66.6cm1 with respect to the Q1(0) transition of the free H2 molecule. The results demonstrate that Na+H2 has a T-shaped equilibrium configuration with the Na+ ion attached to a slightly perturbed H2 molecule but that large-amplitude vibrational motions significantly influence the rotational constants derived from the asymmetric rigid rotor analysis. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 2.493Å, increasing slightly (by 0.002Å) when the H2 subunit is vibrationally excited. A new three-dimensional potential energy surface is developed to describe the Na+H2 complex. Ab initio points calculated using the CCSD(T) method and aug-cc-pVQZ basis set augmented by bond functions are fitted using a reproducing kernel Hilbert space method [Ho et al, J. Chem. Phys.104, 2584 (1996)] to give an analytical representation of the potential energy surface. Ensuing variational calculations of the rovibrational energy levels demonstrate that the potential energy surface correctly predicts the frequency of the νHH transition (to within 2.9cm1) and the dissociation energies [842cm1 for Na+H2(para) and 888cm1 for Na+H2(ortho)]. The B and C rotational constants are slightly underestimated (by 1.7%), while the vibrationally averaged intermolecular separation is overestimated by 0.02Å.

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