Second-harmonic probe of pressure-induced structure ordering within long-chain fluorescein monolayer at the air/water interface

Pressure-induced structure ordering of long-chain Fluorescein O322 molecules within the monolayer at the air/water interface was studied by rotating a sample trough using the second-harmonic generation technique. Azimuthal angle dependences of the polarized second-harmonic intensity (I_pp, I_ps, I_ss, and I_sp) were measured for different degrees of monolayer compression. I_pp and I_ss showed a 2-fold pattern, whereas I_ps and I_sp showed a 4-fold pattern. A set of theoretical equations was derived to interpret the observed pattern. The observed patterns agreed with the equations based on C_2v symmetry, and the monolayer showed a definite crystalline structure in the 0.21 nm²/molecule region. These results showed that the transition of an initially disordered and heterogeneous dye monolayer into a fairly regular crystalline-like structure was characterized by continuous changes in the structure ordering of constituent molecules maintaining C_2v symmetry.