EFFECTS OF HIGH PRESSURE ON THE SPECTRAL SENSITIZATION OF ZINC OXIDE BY RHODAMINE B (ADSORPTION, DYES, PHOTOELECTROCHEMISTRY, SEMICONDUCTORS)

High pressures up to 10 kilobars were used to investigate the effects of adsorption on the properties of rhodamine B, an organic dye. The adsorbed dye was studied using a photoelectrochemical cell in which rhodamine B was adsorbed onto the surface of a zinc oxide single crystal electrode. From measurements of the photocurrent as a function of the energy of the incident light, the electronic absorption spectrum of the adsorbed dye was obtained.; Rhodamine B exists as an equilibrium of monomers and dimers both in solution and in the adsorbed state. The effects of pressure on the peak locations and equilibrium characteristics of the monomer and dimer, both in solution and adsorbed to the semiconductor crystal, are established by comparing, at corresponding pressures, the optical absorption spectra of rhodamine B in solution with the electronic spectra of rhodamine B adsorbed on zinc oxide. The peak locations of the monomer and dimer shift red with increasing pressure at approximately the same rate both in solution and in the adsorbed state. A red shift, characteristic of (pi) (--->) (pi)* excitations, indicates that the polarizability and the dipole moment of the species increases after excitation. The change in volume for dimerization in solution and the change in volume on adsorption for both the monomer and the dimer are established. The results are discussed in terms of the mode of attachment on adsorption.