The spectroscopy of AgF: CASSCF+CASPT2 calculations on the lowest Σ+3, Σ+1, Π1, Π1, Δ3, and Δ1 excited states

Abstract

The spectroscopic properties of the three lowest-lying (X,2 and 3)1Σ+, the first Σ+3, the two lowest-lying (1 and 2)3Π, the first Π1, and the Δ3,1 states of the AgF molecule have been studied through extensive CASSCF (complete active space self-consistent field)+CASPT2 (complete active space second-order perturbational) calculations, using a 19-active-electron relativistic effective core potential for Ag and large Gaussian basis sets for both atoms. Strong mixtures of the Ag+(4d95s1)F−(2s22p6) ionic and Ag(4d95s2)F(2s22p5) or Ag(4d105s1)F(2s22p5) neutral configurations were found for the Σ+3, 2 1Σ+, and 1 3Π states between 4.0 and 4.4 a.u., while for the higher lying states no evident neutral-ionic crossings were found. This leads to curves that present local maxima at 4.3 a.u. for the 2 1Σ+ and Σ+3 states as well as for the 1 3Π state at 4.0 a.u. The 2 3Π excited state shows the lowest ionic character of all the states. The calculated spectroscopic constants for all the studied states are reported and found in good accordance with available experimental data. The question of the nature of the electronic parent state of the observed B0+ state, responsible for the most intense transition and which is the shortest lived excited state of AgF, is thoroughly addressed in the light of the present results. They clearly indicate that the B0+ state is not correlated with the Rydberg Ag+(4d95p1)+F−(2s22p6) ionic structure, as previously proposed [J. Chem. Phys. 102, 4482 (1995)]. Since the 2 1Σ+ state has been shown to be the ΛSΣ electronic parent state of the fine-structure A0+ state (these results confirm this idea), and given the difference between the calculated Te (1513 cm−1) of the 2 1Σ+ and 1 3Π states, these calculations point to this latter state as the ΛSΣ parent of the experimental B0+ state. At this level of calculation, the next higher lying state that could contribute (3 1Σ+) through spin–orbit couplings to this B0+ state lies more than 8000 cm−1 away. This, however, is not consistent with the accurately measured radiative lifetimes of 7.1 μs (A′Ω1), 9.1 μs (aΩ1), 240 ns (A0+), 21 ns (B0+) for the four observed excited states, which seem to indicate that the two Ω=0+ excited states are of singlet character. Therefore, only a theoretical study including a substantially more accurate and complete account of the electronic+spin–orbit interactions will yield a reliable answer to this complex problem in the spectroscopy of AgF.