Polarization and Surface Density Dependence of Pyrenehexadecanoic Acid at the Air−Water Interface under Compression Studied by a Laser Two-Photon Ionization Technique

Abstract

Orientation of pyrenehexadecanoic acid (PyHA) on the water surface has been investigated using surface compression, fluorescence spectra, and two-photon ionization measurements. PyHA stays as a monomer at surface densities lower than 0.025 nmol/cm2, while at high surface densities excimer emission was observed. The two-photon ionization signal had a peak at 90° (p-polarized laser beam) upon the rotation of the polarization of the incident laser beam. The difference in the signal intensity generated by p-polarized and s-polarized lights became smaller as surface density increased. It is suggested that the tilt angle of the transition moment of the pyrene ring is ca. 70° at a low surface density, and it increases with the surface density increase. Problems in determining orientation of the molecules on water are discussed.

Characterization of Langmuir Monolayers of the Amphiphilic Ru Complex at the Air/Water Interface by Ultraviolet Photoelectron Yield Spectroscopy

“…The only analytical technique that can be applied to the characterization of chemical species on the air/water interface employing a photoelectron as a probe is the laser two-photon ionization method. This technique has been used to determine the concentration of chemical species on the water surface in ambient air. − The laser two-photon ionization method is very sensitive to the surface and actually has been applied to the characterization of the Langmuir layer of pyrene hexadecanoic acid on water which gives information on the structure of the surface species, because the pulsed laser is an essential light source for two-photon excitation.…”

“…[18][19][20] The laser twophoton ionization method is very sensitive to the surface and actually has been applied to the characterization of the Langmuir layer of pyrene hexadecanoic acid on water. 21 With this technique, however, it is difficult to obtain a continuous photoexcitation spectrum, 22 which gives information on the structure of the surface species, because the pulsed laser is an essential light source for two-photon excitation.…”