NONRESONANT FREQUENCY DISPERSION OF THE ELECTRONIC 2ND HYPERPOLARIZABILITY OF ALL-TRANS POLYSILANE CHAINS - AN AB-INITIO TDHF OLIGOMERIC APPROACH

The frequency-dependent electronic second hyperpolarizability of incre asingly large polysilane chains is computed for the most common nonlin ear optical (NLO) processes at the time-dependent Hartree-Fock level w ith the 6-31G atomic basis set. Due to a-conjugation, the longitudinal component (gamma(L)(e)) turns out to be dominant. Its nonresonant dis persion relations are described by the coefficients of the power expan sion formula, gamma(L)(e)(- omega(sigma); omega(1), omega(2), omega(3) ) = gamma(L)(e)(0; 0, 0, 0)[1 + A omega(L)(2) + B omega(L)(4) + C omeg a(L)(6) + ...], where omega(L)(2) = omega(sigma)(2) + omega(1)(2) + om ega(2)(2) + omega(3)(2) and gamma(L)(e) (0; 0, 0, 0) is the static lim it value. In the infinite chain length limit, the CHF/6-31G static lon gitudinal electronic second hyperpolarizability per Si,H, unit cell is estimated to attain 463 +/- 10 X 10(3) a.u. whereas the A coefficient reaches 27.8 +/- 0.9 a.u. The accuracy that could be reached from usi ng this power expansion expression for estimating the second hyperpola rizability for other optical frequencies is discussed. (C) 1998 John W iley & Sons, Inc.