Kenneth B. Eisenthal

Polarization of water molecules at a charged interface: second harmonic studies of the silica/water interface

Abstract Surface second harmonic generation was used to probe the silica/water interface. At pH values where the silica surface is charged, we found that the water molecules near the interface are polarized by the interfacial electric field and are responsible for the observed second harmonic light. Two types of silanol sites having pKa values of 4.5 and 8.5 were inferred from the data. A promising result of the studies was the development of a new method for obtaining electric potentials at charged liquid interfaces.

Theory of Fluorescence Depolarization by Anisotropic Rotational Diffusion

The general expressions for the time‐dependent fluorescence depolarization caused by anisotropic rotation diffusion have been obtained. It is shown that after an instantaneous exciting light pulse, the parallel and perpendicular components of fluorescence can have a maximum of six exponential decays and the difference of these two components a maximum of five decays. The present results differ from those of previous studies and the differences are discussed.

The dynamics of barrier crossings in solution: The effect of a solvent polarity-dependent barrier

Abstract Experiments on the role of the solvent in the dynamics of photoisomerization involving large dipole moment changes are discussed. Variation of the photoisomerization dynamics of dimethylaminobenzonitrile in a series of solvents is attributed chiefly to polarity-induced barrier height changes rather than viscosity changes. Implications of these findings regarding the use of barrier crossing theories are discussed.

Polarity-dependent barriers and the photoisomerization dynamics of molecules in solution

Abstract The dynamics of molecular isomerizations that involve major charge redistributions are studied using picosecond lasers. The usual assumptions that the isomerization barrier is independent of temperature and constant within a solvent series are found to be incorrect due to solvent polarity effects. Polarity and hydrogen bonding effects on isomerizations involving large dipole moment changes ( p -dimethylaminobenzonitrile) and those involving a polar intermediate ( t -stilbene) are discussed.

Studies of effects of hydrogen bonding on orientational relaxation using picosecond light pulses

Abstract Orientational relaxation of rhodamine 6G in a series of normal alcohols, ethylene glycol, chloroform and formamide has been studied using picosecond light pulses. The effects of hydrogen bonding and the structure of the liquids on the molecular rotational motion are examined.

The spine neck filters membrane potentials.

Dendritic spines receive most synaptic inputs in the forebrain. Their morphology, with a spine head isolated from the dendrite by a slender neck, indicates a potential role in isolating inputs. Indeed, biochemical compartmentalization occurs at spine heads because of the diffusional bottleneck created by the spine neck. Here we investigate whether the spine neck also isolates inputs electrically. Using two-photon uncaging of glutamate on spine heads from mouse layer-5 neocortical pyramidal cells, we find that the amplitude of uncaging potentials at the soma is inversely proportional to neck length. This effect is strong and independent of the position of the spine in the dendritic tree and size of the spine head. Moreover, spines with long necks are electrically silent at the soma, although their heads are activated by the uncaging event, as determined with calcium imaging. Finally, second harmonic measurements of membrane potential reveal an attenuation of somatic voltages into the spine head, an attenuation directly proportional to neck length. We conclude that the spine neck plays an electrical role in the transmission of membrane potentials, isolating synapses electrically.

Picosecond studies of the cage effect and collision induced predissociation of iodine in liquids

Abstract In this work we report the first time dependent study of the role of the cage effect in the geminate recombination of iodine atoms produced by photodissociation of I 2 . We have also directly observed the rate of the collision induced predissociation of the excited iodine molecule, I 2 (B 3 Π o+u ).

Second harmonic generation from the surface of centrosymmetric particles in bulk solution

Second harmonic generation (SHG) is reported for the first time from the surfaces of centrosymmetric particles in bulk isotropic solution. Although SHG is generally described as electric dipole forbidden in centrosymmetric systems, we show that this requires the system to be centrosymmetric on length scales much less than the coherence length of the process. This condition is not satisfied for micron-size particles, and accordingly we have observed a strong SH signal from various particles of this length scale. This promising discovery provides a powerful spectroscopic method for the investigation of physical and chemical processes on the surfaces of microscopic centrosymmetric particles.

Influence of Resonance Transfer on Luminescence Decay

The effects of dipole—dipole resonance transfer on the population and decay of excited donor molecules are discussed. The limitation of the previous treatments of this problem to flash excitation is demonstrated. The present treatment is applicable to systems which have achieved a steady state or have been flashed. Expressions are given for the donor quantum yields, the decay of excited donor molecules for flashed and steady‐state systems, and the steady‐state population of excited donor molecules.

Picosecond dynamics of twisted internal charge transfer phenomena. The role of the solvent

To probe the role of the solvent in intramolecular charge transfer processes, and in particular, the origin of the well‐known dual fluorescence phenomena of p‐dimethylamino benzonitrile (DMABM), picosecond laser studies in mixed polar/nonpolar solutions were undertaken. The anomalous long wavelength emission is attributed to a complex formed between excited DMABN and butanol with a rate constant of (9.7±1.5)×108 M−1 s−1. The dominant stabilization of the twisted intramolecular charge transfer state is therefore concluded to be due to a short range specific interaction with a polar solvent molecule. A secondary solvent effect arises from a further stabilization of the complex by long range polarization interactions with solvent molecules. Evidence on the existence of ground state complexes between DMABN and butanol are also presented. Excitation of these ground state complexes leads to the rapid formation of the excited state complexes in 30 ps, which we have interpreted to be the time required for the com...