Electronic transition dipole moment function of the f' 0⁺(¹D₂) - X^{1 +} transition of ICl

The transition dipole moment (TDM) function of the f' 0+ (D-1(2)) - X-1 Sigma(+) transition of ICl is determined in the range 2.76 - 3.49 angstrom from analyses of fluorescence spectra and data on the f' 0(+) (D-1(2)) state lifetime. The TDM function in the specified range shows non-monotonic dependence with a maximum at R approximate to 2.9 angstrom. Ab initio calculations carried out using the complete active space self-consistent field method followed by calculations that take into account the dynamic electron correlations and spin-orbit interaction reproduce reasonably well the shape of the experimental TDM function. It is shown that the f' 0(+) (D-1(2)) state near its equilibrium internuclear distance (R-e = 3.226 angstrom) consists mainly of the ls-state 3(1) Sigma(+) which is optically coupled with the ls-state 1 (1) Sigma(+), the dominant component of the X-1 Sigma(+) state in this R-region. The non-monotonic behavior of the f' 0(+) (D-1(2)) - X-1 Sigma(+) TDM function at shorter distances is mainly due to spin-orbit interaction between the ls-states 3 (1) Sigma(+) and 3 (3) Pi. The potentials of these ls-states cross at R approximate to 2.6 angstrom that leads to decrease in the weight of 3 (1) Sigma(+) in f' 0(+) (D-1(2)) and, as a consequence, to a non-monotonic TDM function of the f' 0(+) (D-1(2)) - X-1 Sigma(+) transition. The TDM functions of the transitions from f' 0(+) (D-1(2)) to other 0(+) valence states were also calculated. The behavior of these TDM functions is qualitatively interpreted in terms of the R-dependence of the weights of the optically coupled ls-states. (C) 2021 Elsevier Ltd. All rights reserved.