Ab initio configuration interaction study of the predissociation of the (4s), (4pσ) 1,3Π, and (4pπ) 3Σ+ Rydberg states of HCl and DCl

Ab initio configuration interaction (Cl) calculations are carried out for the lowest-lying six ${}^{\text{1,3}}\Pi$ and three ${}^3{\Sigma }^{+}$ states. In addition, the first and second radial couplings ${\mathrm{〈\phi }}_i{\mathrm{(R,r)|\partial /\partial R|\phi }}_j\mathrm{(R,r)〉}$ and ${\mathrm{〈\phi }}_i{\mathrm{(R,r)|\partial }}^2{\mathrm{/\partial R}}^2{\mathrm{|\phi }}_j\mathrm{(R,r)〉,}$ spin-orbit, and rotational couplings are also evaluated based on the resulting highly correlated wave functions. Energy positions and predissociation linewidths of rovibrational levels associated with the above electronic states are determined in the adiabatic representation by means of the complex scaling method employing a basis of complex scaled Hermite polynomials and Gauss–Hermite quadrature. The present treatment correctly reproduces a number of observed trends in energy and line broadening for individual rovibrational levels of the ${\text{(4s)b\hspace{0.05em}}}^3\Pi$ and ${\mathrm{C }}^1\Pi$ states of HCl and DCl. The calculated linewidths for the $\text{v=0,}$ $\text{J=2,}$ and $\text{J=8}$ rovibrational levels of the ${\text{(4s)C\hspace{0.05em}}}^1\Pi$ state of HCl are 1.2 and 2.1 cm⁻¹, respectively, which is in good agreement with the recently observed rotational dependence of line broadening for the $\text{v=0}$ level of this state [M. H. Alexander et al., Chem. Phys. 231, 331 (1998)]. The present calculations find that the linewidths of the ${\text{(4pσ)D\hspace{0.05em}}}^1\Pi$ state are greater than for the ${\text{(4pσ)d\hspace{0.05em}}}^3\Pi$ state. The distinctive predissociation mechanisms of the ${\mathrm{D }}^1\Pi$ and the ${\mathrm{d }}^3\Pi$ states are discussed on the basis of the calculated radial couplings. The calculations predict very broad lines for the $\text{v=0}$ and 1 vibrational levels of the lowest adiabatic bound ${}^3{\Sigma }^{+}$ state, consistent with the fact that the corresponding state has not yet been identified by spectroscopic means.