ITERATIVE FOURIER RECONVOLUTION SPECTROSCOPY - VANDERWAALS BROADENING OF RYDBERG TRANSITIONS - THE B[-X (5P, 6S) TRANSITION OF METHYL-IODIDE

The pressure-induced shifting and asymmetric broadening of the B approximately <-- X approximately (5p, 6s) Rydberg transition of CH3I by the perturbers He, Ar, H2, CH4, and SF6 is analyzed using an iterative Fourier reconvolution (IFR) method. Perturbed spectra are analyzed globally, and the full details of the pressure-induced perturbation are accounted for in terms of ground- and excited-state van der Waals complexes between the absorber and perturber. The variance of the observed and calculated perturbed spectra is 0.0123 for an Ar pressure of 81.6 atm. Centrosymmetric potentials are used: 6-12 for the ground state, and exp-6 for the excited state. 6-12 parameters obtained from the IFR analysis are compared with those approximated from simple combining rules. In all cases, the position of the well minimum increases in the excited state whereas the well depth of the van der Waals complex is smaller in the excited state. For the CH3I-He system, repulsive exponential potentials were used for both ground and excited states. The oscillator strength of the CH3I transition appears to be conserved between the gas phase and the condensed phase, n-hexane solution (after n correction); at 295 K f(g)obs = 0.0580 and f(c)obs = 0.072(4). The role of pure homogeneous collisional broadening as an independent, competitive process is discussed.