SPECTRAL HOLE-BURNING STUDY OF ELECTRON-PHONON COUPLING IN POLYMERS

Persistent hole burning in the S-1<--S-0 transition of tetra-tert-buty l-tetraazaporphine was used to investigate the electron-phonon couplin g in a broad range of polymeric solids between 5 and 30-50 K. The maxi ma of pseudophonon sidebands (v(ph)) are displaced by 5-20 cm(-1) from the 0-0 hole, The relationships between the v(ph) values and the velo cities of transversal and longitudinal ultrasound waves as well as the Young's modulus of polymers were established, At the same time v(ph) compare well to inelastic neutron scattering maxima, the first boson p eaks in the Raman scattering spectra, and the characteristic modes res ponsible for extra specific heat and heat conductivity plateau, Mutual correlations of the molecular structure, nanoscopic, and bulk propert ies in glassy and partially crystalline polymers are pointed out, The quasihomogeneous hole width (Gamma(qh)) at fixed temperature (T) incre ases when v(ph) becomes smaller and the polarity of the host increases , Hole widths measured at 15 and 25 K also display a common linear rel ationship with total heat content (J/cm(3)) of the matrix in less pola r hosts, Irreversible broadening of holes as a function of excursion t emperature was investigated by means of T cycling, The contribution of slow irreversible broadening processes (spectral diffusion) to Gamma( qh) does not exceed 20%. The shift of holes burned at 4-8 K upon the r ise of T was measured. The pure thermal hole shift was calculated by t aking into account the solvent shift contribution due to the density c hange of the matrix. This pure phonon-induced shift is always bathochr omic with increasing T. The T dependence of both the hole width and sh ift can be equally well fitted with the power law and a coth function, In most systems both the width and shift obey the power law with simi lar T coefficients of 2.8+/-0.5 and 2.4+/-0.5, respectively. A consist ent description of the T dependence of the Debye-Waller factor, the ho le shift, as well as the width in terms of an anharmonic single-mode m odel can be achieved for most of the polymers with the same characteri stic energy (entering the coth function) which is approximately by a f actor of 4 larger than v(ph). The influence of crystallinity, tacticit y, molecular weight, polarity, and chemical structure of the macromole cular host on the strength of electron-phonon coupling is analyzed in detail. The relative importance of the Stark effect and intermolecular dispersive forces in the dynamic modulation of electronic energy leve ls causing the optical dephasing is discussed. (C) 1997 American Insti tute of Physics.