Detection of Molecular Monolayers by Optical Second-Harmonic Generation

Abstract

LBL-12003Second harmonic generation is shown to be sensitive enough to deteet molecular monolayers adsorbed on a silver surface. Adsorption of AgCt and pyridine on silver during and after an electrolytic cycle can be easily observed,

Nonlinear Optical Techniques for Surface Studies

“…Power-law dependences of the nonlinear signal on silver. The upper and lower solid curves show the quadra-, tic dependence of the diffuse SH signal from the rough bulk sample and of the sorbed molecular monolayers on the rough metal surfaces as long as the nonlinear susceptibility of the adsorbed molecules is larger than that of the metal (10). This is easily demonstrated by monitoring the second 'harmonic signal from the silver electrode submerged in the electrolytic cell (see Fig.…”

“…Because of the big local field enhancement due to local surface plasmon resonance, surface nonlinear optical effects at a very rough metaldielectric interface can be greatly enhanced. Since the nonlinearly induced dipole is now given by p(n)(w) = a(n)E£oC(Wl) ---E~oc(wn) instead of p(n)(w) = a(n)E(Wl) ---E(w n ) which is true in the absence of the local metal structure, the output, being proportional to L 2 (w)!p(n)(w) 1 2 , is enhanced by a factor (10) This has not taken into account possible further enhancement from increase of a(n) due to molecule-metal interaction at the interface. The latter certainly contributes to the recently discovered surface enhancement on Raman scattering [26], but is not generally believed to be the dominant mechanism for the observed enhancement.…”

“…We then realize that such surface enhancement should also happen to other nonlinear optical processes [9]. Study of adsorbed molecules on those rough metal surfaces by surface nonlinear optics then also becomes feasible [10]. In fact, a simple estimate would show that even a monolayer of adsorbed molecules on smooth surfaces can yield a detectable second-harmonic or sum-frequency signal.…”

Optical second-harmonic generation from a monolayer of centrosymmetric molecules adsorbed on silver

“…By way of comparison, the adsorption of pyridine gave 4 LBL-l2735 rise to a -50-fold increase in SHG. 2 The SH radiation was angularly diffuse and unpolarized, as one would expect in view of the rough profile of the silver surface on the scale of an optical wavelength. That the signal was, in fact, SH was verified by checking its monochromaticity and its quadratic variation with the intensity of the fundamental excitation.…”

“…Lacking inversion symmetry, the free pyridine molecule has an electric- (2) dipole contribution to the second-order nonlinear polarizability a Therefore, a simple alignment of molecules upon adsorption could explain the increase in'SilC. For the adsorption of centrosymmetric molecules, however, such an alignment alone cannot account for a change in the SH intensity.…”