Autoionization in diatomics: Measured line shape parameters and predicted photoelectron spectra for some autoionizing states of the hydrogen halides

Using a Hopfield continuum source and a computer-controlled data acquisition system, we have measured relative absorption cross sections for HCl, DCl, HBr, DBr, and HI at 0.2 Å resolution from 1000 to 700 Å, a region dominated by autoionizing Rydberg states converging to the A 2Σ+ state of the HX+ ion. In order to analyze the shapes of autoionizing vibrational bands, we applied Fano’s theory of line shapes to homogeneous autoionization in diatomics, and found that the line shape parameters q, Γ, and ρ2 are independent of rotational quantum number but may depend upon the vibrational quantum number of the autoionizing state. Calculations of this vibrational dependence in HCl shows it to be most pronounced at high v. For progression I in HCl and DCl (n*=2.795) and in HBr and DBr (n*=2.748), we fit computer-generated vibronic band contours to experimental absorption cross sections to obtain q, Γ, and ρ2 which we then used to predict the photoelectron spectra of these autoionizing states. The autoionization lifetime h//Γ in HCl and DCl is 1.1×10−14 sec; in HBr and DBr it is 9.5×10−15 sec. The measured autoionization widths of bands in the next member of the Rydberg series obey the scaling rule Γ∝n*−3. We attribute an additional broadening of levels with v′?3 in progression I of HBr as the onset of predissociation, and estimate the relative rates of predissociation and autoionization. A vibrational reassignment of progression II in HCl and DCl gives n*=2.310 for these states. We saw only a single broad peak (with n*=2.78) in the absorption spectrum of HI.