Host–guest charge transfer states: CN doped Kr and Xe

Abstract

The host–guest charge transfer absorption of CN doped krypton and xenon matrices are identified through direct analogy with the previously assigned transitions of Cl/Kr and Cl/Xe. These intense, structured absorption bands appear with the onset at 245 nm in Kr and 360 nm in Xe. Excitation of the CN/Kr charge transfer band at 193 nm leads to emission over CN(A(2Π)→X(2Σ)) transition, indicating that an efficient curve crossing precludes the ionic state from radiating. No emissions were seen in CN/Xe when excited at 193 nm. The charge transfer absorption spectrum of CN/Kr is reproduced through an extended diatomics-in-ionic-systems treatment, using accurate ab initio pair potentials and transition dipoles as input, without further adjustment. The delocalized hole states are then analyzed in real-space, using atomic bases distributed over as many as eleven shells surrounding the CN− center. The ionic states are well described as J=1/2, 3/2 valence bands bound to CN−, with a substructure that cannot be exclusively assigned to a single quantum number. The strong absorptions terminate on states in which 70%–95% of the hole density remains on the first nearest neighbor shell, with hole densities of 1%–5% extended out to R=8 Å. In higher ionic states, with weaker transition dipoles, the hole density maximizes on shells removed by 10 Å from the ionic center. Although these delocalized states provide channels for charge separation via self-trapping of holes, save for a weak signal from the impurity trapped hole at H+ centers, the experiments do not provide evidence for significant charge separation.