One- and Two-Photon-Pumped Fluorescence from Rhodamine B in Solid Poly(acrylic acid) under High Pressure

We investigate the effect of pressure on one- and two-photon-excited fluorescence of Rhodamine B dissolved in solid poly(acrylic acid). We present the pressure dependence of the fluorescence intensity as well as the energy and lifetime of the emitting state for one- and two-photon excitation for three different excitation energies. All these parameters decrease with increasing pressure regardless of the mode and energy of excitation. The similar change with pressure of the lifetime and emission energy for one- and two-photon excitation reveals that the fluorescence originates in the same state for both excitations. The decrease of fluorescence intensity with pressure is attributed (i) to an increase of the nonradiative rate from the emitting state and (ii) to a decrease of the absorption cross section. The first process is correlated with a pressure-induced shift of the emission energy that coincides with the energy gap law. The observed larger pressure change in the absorption cross section for two-photon than for one-photon excitation indicates an additional pressure dependence of the transitions involving the intermediate states.