Heavy-atom effect on second hyperpolarizabilities of thiophene homologues investigated by a femtosecond optical-Kerr-effect experiment and ab initio molecular orbital calculation

The second hyperpolarizabilities of three heterocyclic molecules of ${\mathrm{C}}_4{\mathrm{H}}_{4}X,$ where $X=\mathrm{O}$ (furan), S (thiophene), or Se (selenophene), were investigated by an optical-heterodyne-detected optical-Kerr-effect experiment and an ab initio Hartree–Fock molecular orbital calculation with largely augmented basis sets. From the observed third-order responses the electronic hyperpolarizabilities were determined after the nuclear nonlinear optical responses were removed by Fourier-transform analysis. Results of both experiment and calculation exhibited $\sim 2$ times enhancement when the heteroatom of O was replaced by Se. The origin of the enhancement and the contribution of the heteroatom to the hyperpolarizability are discussed. Also, a direct comparison of the experimental values with calculated ones is made.