THE EFFECT OF HIGH-PRESSURE ON THE DYNAMICS OF DOPED ORGANIC GLASSES - A STUDY BY SPECTRAL HOLE-BURNING

We report time-resolved spectral hole-burning experiments on bacterioc hlorophyll-a (BChl-a) doped into the glass triethylamine (TEA) at ambi ent pressure (Delta p = 0) and at a pressure of Delta p = 3.4 GPa. We observe a number of remarkable effects: (a) from the change in the tem perature dependence of the ''effective'' optical homogeneous linewidth Gamma(hom)', we conclude that local order is introduced in TEA under high pressure; (b) from the change in the time dependence of Gamma(hom )' we conclude that spectral diffusion is induced by ''downhill'' ener gy transfer among BChl-a molecules (at a concentration of c similar to 5 x 10(-4) M) within the S-1 <-> S-0 0-0 band, both at ambient and hi gh pressure; and (c) from the wavelength dependence of Gamma(hom)', we conclude that an energy barrier (Delta E similar to 135 cm(-1) at Del ta p = 0, and Delta E similar to 70 cm(-1) at Delta p = 3.4 GPa) has t o be crossed in order to excite two-level systems (TLSs) of the TEA ho st which, subsequently, leads to spectral diffusion. We present a qual itative molecular model for the interpretation of these effects. (C) 1 997 American Institute of Physics.