LATTICE-RELAXATION IN LINEAR POLYENES

We present a multi-phonon approach to analyze the lattice relaxation i n the first B(u) excited state of linear polyenes, from trans-butadien e to polyacetylene. Photo-absorption experiments carried out on a seri es of linear polyenes, beta-carotene, polyenals/polyenones, and polyac etylene are discussed. The Huang-Rhys factor S and the relaxation ener gy are calculated for polyenes with various lengths. A major result is that we find that the S value first tends to decrease with increasing chain length but after some critical length the evolution completely modifies and S starts increasing. We therefore caution that extrapolat ions based on observations on short polyenes might be invalid for the polymer.