Time-Resolved Measurement of Excited-State Populations

Introduction The absorption of light is the most common method for the investigation of the energy levels and transfer processes of atomic and molecular systems. The measurement of the ground-state absorption (GSA) provides information on the energy level scheme. If the density of the atoms or molecules in the stable ground state is known then the cross sections of the GSA transitions can be calculated from a Lambert-Beer law (see (2 ) of the next Section). Once the cross section of a certain transition has been determined an unknown density of atoms can be ascertained by measuring the absorption of light on this transition. The case is much more difficult if the populations of excited states and the cross sections of ESA are to be investigated because these states are unstable and decay, e.g. by the emission of light. In most cases it is impossible to obtain a defined distribution of the energy over the excited states and, therefore, the calculation of the ESA cross sections seems to fail. In this contribution we show that time-resolved ESA measurements, without the knowledge of the corresponding cross sections, provide enough information for the calculation of the excited-state populations. The ESA cross sections can then easily be calculated, and with the knowledge of the cross sections the distribution of the energy after any excitation can be derived.