Nonlinear susceptibility of a cyanobiphenyl derivative at the air/water interface: improved measurement of molecular orientation by optical second harmonic generation.

The nonlinear susceptibility tensor, chi(2), for second harmonic generation (SHG) by a monolayer of the cyanobiphenyl derivative 4-n-octyl-4'-cyanobiphenyl (8CB) at the air/water interface has been measured with the fundamental frequency variant Planck's over 2piomega=1.55 eV (lambda=800 nm). The contribution of the water subphase was estimated by measuring the magnitudes and relative phases of the nonlinear susceptibilities of the bare and monolayer-covered water surfaces. All nonzero elements of chi(2) are placed on an absolute scale by comparison with SHG in reflection from a z-cut quartz crystal. The experimental measurements for the 8CB monolayer are compared with computed susceptibilities derived by using standard time-dependent perturbation theory in conjunction with a semiempirical electronic structure model. Good agreement has been found between experimental and computational results when the average tilt angle of the cyanobiphenyl chromophore of 8CB is in the range 60-70 degrees relative to the surface normal. A critique is given of an alternative, simplified measurement procedure of the tilt angle in which only the dominant element of the molecular hyperpolarizability tensor is considered. It is shown that the simplified procedure is invalid for 8CB monolayers when the tilt angle is greater than approximately 70 degrees.