Computational analysis of the quadrupole contribution in the second-harmonic generation spectroscopy for the water/vapor interface.

Second-order susceptibility for water/vapor interface is calculated theoretically using molecular dynamics simulation, which considers both the dipole and quadrupole contributions. We find that the nonresonant second harmonic generation (SHG) signal is dominated by the quadrupole contribution from the bulk. We also elucidate the fact that the nonresonant susceptibility tends to be negative in general, irrespective of the molecular orientation.

Molecular theory on dielectric constant at interfaces: a molecular dynamics study of the water/vapor interface.

Though the local dielectric constant at interfaces is an important phenomenological parameter in the analysis of surface spectroscopy, its microscopic definition has been uncertain. Here, we present a full molecular theory on the local field at interfaces with the help of molecular dynamics simulation, and thereby provide microscopic basis for the local dielectric constant so as to be consistent to the phenomenological three-layer model of interface systems. To demonstrate its performance, we applied the theory to the water/vapor interface, and obtained the local field properties near the interface where the simple dielectric model breaks down.

Electric quadrupole contribution to the nonresonant background of sum frequency generation at air/liquid interfaces.

We study an electric quadrupole contribution to sum frequency generation (SFG) at air∕liquid interfaces in an electronically and vibrationally nonresonant condition. Heterodyne-detected electronic sum frequency generation spectroscopy of air∕liquid interfaces reveals that nonresonant χ((2)) (second-order nonlinear susceptibility) has a negative sign and nearly the same value for all eight liquids studied. This result is rationalized on the basis of the theoretical expressions of χ((2)) with an electric quadrupole contribution taken into account.

Development of a finite-temperature density functional approach to electrochemical reactions.

We present a computational method to calculate the electronic states of a molecule in an electrochemical environment. The method is based on our recently developed finite-temperature density functional theory approach to calculate the electronic structures at a constant chemical potential. A solvent effect is treated at the level of the extended self-consistent reaction field model, which allows considering a nonequilibrium solvation effect.