In-plane polar structure in 2D liquid crystalline monolayers probed by optical second harmonic generation spectroscopy

Tabe, Yuka; Yokoyama, Hiroshi
June 2000
AIP Conference Proceedings;2000, Vol. 519 Issue 1, p133
Academic Journal
The alleged in-plane polar structure in two-dimensional (2D) smectic-C liquid crystalline phases of Langmuir monolayers was directly confirmed by the optical second harmonic generation (SHG) spectroscopy for the first time. The SH signal associated with the in-plane polar structure was found to be comparable with that coming from the out-of-plane polar distribution of molecular orientation inherent at the air-water interface, This result indicates that there exits a strong preference for the heads and tails of the molecules to spontaneously associate one another over the otherwise isotropic water surface as a resuk of the 2D liquid crystalline order. When the monolayer underwent a first order phase transition from SmC to a solid-like phase under compression, both of the in-plane and out-of-plane components of SH signals were considerably reduced despite an increase in density, implying the rather subtle nature of the centro-symmetry breaking in the SmC phase.


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