Rydberg states of the ArCO2 and KrCO2 van der Waals molecules

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Abstract
Relative photoionization cross sections were determined for ArCO2 at a wavelength resolution of 0.24 Å in the regions of both the Ar*+CO2 and the Ar+CO*2 dissociation limits (690–920 Å) and for KrCO2 at a wavelength resolution of 0.4 Å in the region of the Kr+CO*2 dissociation limits (690–780 Å). The dimer ionization potentials are 13.59±0.04 eV (912.5±2.5 Å) for ArCO2 and 13.33±0.04 eV (930.0±3.0 Å) for KrCO2. Combining these values with the known value of the CO2 ionization potential and the known (or estimated) values of the ArCO2 and KrCO2 dissociation energies yields values for the dimer ion ground state dissociation energies of 0.20±0.04 eV for ArCO+2 and 0.47±0.05 eV for KrCO+2. Rydberg structure is observed in the photoionization cross section of ArCO2 corresponding to dissociation limits of the type Ar*(nd,ns)+CO2; however, discrete structure is observed only near the Ar*(3d,5s)+CO2 dissociation limits. van der Waals structure corresponding to the higher Ar*+CO2 limits is diffuse and red degraded. Molecular Rydberg structure also is observed in the photoionization cross sections of both ArCO2 and KrCO2 corresponding to dissociation limits of the type Ar+CO*2. Prominent van der Waals structure is observed corresponding to dissociation limits based on members of the CO2 Henning sharp and diffuse Rydberg series, which converge to CO+2B 2Σ+u; however, no van der Waals structure is observed corresponding to dissociation limits based on members of the CO2 Tanaka–Ogawa Rydberg series, which converge to CO+2A 2Πu. Various experimental evidence indicates that neither vibrational predissociation of the excited van der Waals dimer nor vibrational predissociation of the dimer ion can account completely for the absence of van der Waals structure corresponding to the Tanaka–Ogawa series in CO2.