Electronic states of Ar2F and Kr2F

Abstract

Ab initio POL CI calculations have been performed on the low‐lying states of Ar₂F and Kr₂F. The results support a simple diatomics‐in‐molecules description of Rg₂F (Rg=rare gas). In each case only one ionic state (2 ²B₂) is found to be stable (by 0.6 eV) relative to Rg⁺F⁻+Rg. The equilibrium geometry is found to be an isosceles triangle where the Rg–Rg distance is 0.02 Å shorter than that in the ²Σ⁺_u state of Rg⁺₂ and the Rg–F distance is 0.14 Å greater than that in the 2 ²Σ⁺ state of RgF. There are two dipole‐allowed emissions (2 ²B₂→1 ²A₁, 2 ²B₂→1 ²B₂) of comparable strength and wavelength (267 and 274 nm, respectively, in Ar₂F and 357 and 368 nm, respectively, in Kr₂F). The calculated wavelengths are in good agreement with the broad bands at 290 and 400 nm observed in e‐beam pumped Ar or Kr/F₂ mixtures, confirming the earlier assignment of these bands to Ar₂F and Kr₂F, respectively. The radiative lifetime of the 2 ²B₂ is calculated to be 132 ns for both Ar₂F and Kr₂F. There are four dipole‐allowed absorptions from the 2 ²B₂ state, one of which (2 ²B₂→3 ²A₁) is very strong and corresponds to the ²Σ⁺_u→²Σ⁺_g transition in Ar⁺₂ and Kr⁺₂. The effect of the F⁻ on this transition is negligible, so that the calculated wavelengths (320 nm in Ar₂F and 344 nm in Kr₂F) and oscillator strengths are basically unchanged from the rare gas dimer ions. Therefore, Ar₂F and Kr₂F may be at least as important as the Ar⁺₂ and Kr⁺₂ absorbers in the KrF laser. Spin–orbit coupling has been included at the equilibrium geometry of the 2 ²B₂ state using a simple atoms‐in‐molecules approach. The stability of the 2 ²B₂ (now 4 ²Γ) state is decreased slightly so that D_e(Rg⁺F⁻+Rg) is 0.57 eV for Ar₂F and 0.49 eV for Kr₂F. The emission wavelengths and radiative lifetime for the 2 ²B₂ state in Ar₂F are unchanged, as are the wavelengths and strength of the 2 ²B₂→3 ²A₁ absorption. In Kr₂F the emission wavelengths are shifted slightly to 361 and 371 nm, while the radiative lifetime becomes 133 ns. The 2 ²B₂→3 ²A₁ absorption in Kr₂F shifts to 336 nm and the strength decreases by 5%. Consideration of the geometry dependence of the transition moment and emission wavelength indicates that vibrational averaging should not change the radiative lifetimes for Ar₂F and Kr₂F significantly. Recent experimental measurements of the Ar₂F (185±45 ns) and the Kr₂F (176±20 ns) lifetimes are in good agreement with the calculated values. Finally, the calculations indicate that Rg⁺₂F⁻ should be formed by three‐body quenching of Rg⁺F⁻ rather than reaction of rare gas excimers with molecular fluorine. The latter exothermic reaction produces highly excited Rg⁺₂F⁻, which falls apart into Rg⁺F⁻+Rg. These conclusions are supported by recent experimental results.