Br~--H_2 and I~--H_2 anion complexes: Infrared spectra and radial intermolecular potential energy curves

摘要

Midinfrared spectra of the ~(81)Br~--H_2 and I~--H_2 anion complexes are measured in the H-H stretch region by monitoring the production of halide anion photofragments. The spectra, which are assigned to complexes containing ortho H_2, exhibit rotationally resolved sum-sum bands whose origins are redshifted from the molecular hydrogen Q_1(1) transition by 110.8 cm~(-1) (Br~--H_2) and 74.1 cm~(-1) (I~--H_2). The complexes are deduced to possess linear equilibrium structure, with vibrationally averaged intermolecular separations between the halide anion and H_2 center of mass of 3.461 A (Br~--H_2) and 3.851 A(I~--H_2). Vibrational excitation of the H_2 subunit cases the intermolecular bond to stiffen and contract by 0.115 A (Br~--H_2) and 0.112 A (I~--H_2). Rydberg-Klein-Rees inversion of the spectroscopic data is used to generate effective radial potential energy curves near the potential minimum that are joined to long-range potential energy curves describing the interaction between an H_2 molecule and a point negative charge. From these curves the dissociation energies of Br~--H_2 and I~--H_2 with respect to isolated H_2 (j = 1) and halide fragments are estimated as 365 and 253 cm~(-1), respectively.