M. Liess et al., ELECTROABSORPTION SPECTROSCOPY OF LUMINESCENT AND NONLUMINESCENT PI-CONJUGATED POLYMERS, Physical review. B, Condensed matter, 56(24), 1997, pp. 15712-15724
We have measured the quadratic electroabsorption (EA) spectrum of a va
riety of soluble luminescent and nonluminescent pi-conjugated polymer
films in the spectral range of 1.5-4.5 eV. The luminescent polymers in
clude MEH and DOO derivatives of poly(phenylene-vinylene), poly(phenyl
ene ethylene), and polythiophene; the nonluminescent polymers include;
poly(diethynyl silane) and monosubstituted polyacetylene. All EA spec
tra show a Stark shift of the low-lying odd-parity exciton (1B(u)) and
imply the presence of phonon sidebands. There ate also higher-energy
bands due to transfer of oscillator strength to even-parity exciton st
ates (A(g)), the strongest of which (mA(g)) is located at an energy ab
out 1.3 times that of the 1B(u) exciton in both luminescent and nonlum
inescent polymers; in the luminescent polymers the EA spectra also sho
w a second prominent A(g) state (kA(g)) at an energy of about 1.6 time
s that of the 1B(u). We have successfully fitted the EA spectra by cal
culating the imaginary part of the third order optical susceptibility,
Im[chi(3)(-omega;omega,0,0)], using a summation over states model dom
inated by the ground state, the 1B(u) exciton, two strongly coupled A(
g) states (mA(g) and kA(g)), and their most strongly coupled vibration
s, using Frank-Condon overlap integrals. A distribution of conjugation
lengths, which results,in a distribution of excited state energies, w
as also incorporated into the model. The decomposition of the EA spect
ra due to the conjugation length distribution was then used to calcula
te the 1B(u) exciton polarizability (Delta p) using first derivative a
nalysis. For the longest conjugation lengths in our films, we found De
lta p to be of order 10(4) (Angstrom)(3) in luminescent polymers and 1
0(3) Angstrom(3) in nonluminescent polymers, respectively, in good agr
eement with recent subnanosecond transient photoconductivity measureme
nts. We also found that the Huang-Rhys parameter of the 1B(u) exciton
varies between 0.25 and 0.9, being in general smaller for the luminesc
ent polymers, The consequent exciton relaxation energies were calculat
ed to be of order 100 meV. [S0163-1829(97)03948-9].