Rydberg-Rydberg molecular transitions in the condensed phase: towards the ionisation limit

Abstract

The Rydberg states of NO in rare gas matrices show a strong interaction with the surrounding, expressed by shifts to higher energies with respect to the gas phase of up to 1 eV, large absorption-emission Stokes shifts due to extensive cage relaxation and drastic lengthening of the lifetimes. Up to now absorption measurements from the ground state have shown Rydberg series up to n=4 in Ne and n=3 in the other matrices. It was suspected that the cut-off might be due to a competition between vertical Rydberg excitations and the adiabatic ionization limit. To observe infinite Rydberg series, we performed fluorescence dip experiments on the lowest Rydberg state $(3s\sigma)$ in order to observe its depopulation by Rydberg-Rydberg transitions to upper states. The $3s\sigma$ state was excited by means of an ArF laser (193nm), while a tunable second laser (0.48 - 1.3 um) was used to probe the higher Rydberg states after a delay of about 20 ns, which allows for cage relaxation around the lowest Rydberg state. As all Rydberg states lie at more or less the same internuclear distances only $v^{\prime\prime}=0 \rightarrow v^{\prime}=0$ transitions are observed. The n=4 states in Ar could be identified with this method. A quantum defect model discribes the $s\sigma, p\pi$ and $p\sigma$ series successfully providing matrix-induced quantum defects for each as well as consistent ionisation energies of NO in the rare gas matrices.