Electron Correlation Theory and Experimental Measurements of the ThirdOrder Nonlinear Optical Properties of Conjugated Linear Chains.
Abstract
Comprehensive theoretical and experimental studies of the magnitude, sign, dispersion, and length dependence of the third order molecular susceptibility gamma _{ijkl}(omega_4 ;omega_1, omega_2, omega_3 ) demonstrate that the microscopic origin of the nonresonant third order nonlinear optical properties of conjugated linear chains is determined by the effects of electron correlation due to electronelectron repulsion. Multiple excited configuration interaction calculations of gamma_{ijkl}(omega _4;omega_1, omega_2, omega _3) for the archetypal class of quasione dimensional conjugated structures known as polyenes reveal for the first time the principal role of strongly correlated, energetically highlying, two photon ^1 A_{rm g} virtual states in the largest of the two dominant, competing virtual excitation processes that determine gamma _{ijkl}(omega_4 ;omega_1,omega _2,omega_3). It is also found in studies of the effects of conformation on gamma_{ijkl}( omega_4;omega _1,omega_2, omega_3) that the origin of the third order optical properties remains basically the same for the alltrans and cistransoid polyenes, and the results for the two conformations are unified by a common power law dependence of the dominant tensor component gamma_{xxxx}( omega_4;omega_1 ,omega_2,omega _3) on the physical endtoend length L of the chain with an exponent of 3.5. Calculations for a noncentrosymmetric conjugated chain demonstrate that virtual excitation processes involving diagonal transition moments that are forbidden in centrosymmetric structures lead to a more than an order of magnitude enhancement in gamma_{xxxx}(omega _4;omega_1, omega_2,omega _3) compared to the analog centrosymmetric structure. Experimental measurements of the dispersion in the isotropically averaged dcinduced second harmonic susceptibility <gamma( 2omega;omega,omega,0) > and third harmonic susceptibility <gamma(3omega ;omega,omega ,omega)> in two important polyene structures confirm the electron correlation description of gamma _{ijkl}(omega_4 ;omega_1,omega _2,omega_3). The measured values of <gamma( 2omega;omega, omega,0)> and <gamma(3 omega;omega, omega,omega) > at several near infrared fundamental wavelengths for hexatriene, the N = 6 carbon site polyene chain, are in excellent agreement with the calculated magnitude, sign, and dispersion. Corresponding measurements for betacarotene, a substituted N = 22 polyene chain, together with the results for hexatriene, also verify the calculated power law dependence of <gamma(omega_4 ;omega_1,omega _2,omega_3) > on L. It was further found in the course of the experimental measurements that the common reference standard for the macroscopic susceptibility chi^{(3)}(3 omega;omega, omega,omega) is too large by a factor of 2.0, and an improved standard value is proposed.
 Publication:

Ph.D. Thesis
 Pub Date:
 January 1990
 Bibcode:
 1990PhDT.......177H
 Keywords:

 Physics: Condensed Matter; Physics: Optics; Physics: Molecular