Daniel Ducharme

Design and characteristics of a cell for photoacoustic spectroscopy of condensed matter.

We describe a photoacoustic cell which is simple to use, easy to construct, and which gives a great sensitivity, expressed as a signal-to-noise ratio of 3000. Calibration of the cell indicates that it can be used with confidence as a general purpose photoacoustic cell.

Ellipsometric studies of rod outer segment phospholipids at the nitrogen-water interface☆

Abstract Ellipsometry is an optical method which uses the principle that the state of polarization of polarized light changes on reflection at an interface. We have constructed a null ellipsometer for the study of the optical properties of monomolecular films of biological interest at the nitrogen-water interface. With this apparatus we have reproduced the known ellipsometric properties of arachidic acid spread on acidic aqueous solution. Then, we have performed surface potential, surface pressure and ellipsometric measurements of PC (18:1) and rod outer segment phospholipids, i.e. PE ros , PC ros and PS ros , at the nitrogen-water interface. Based on literature refractive indices data, results provide information about the thickness of a lipid film in the close-packed region. Pertinent information about molecular organization of the film is deduced from the close relationship observed between surface potential and ellipsometric isotherms. The importance of these results on the discal membrane structure and function is discussed.

Null ellipsometer for the studies of thin films at gas–water interface

Ellipsometry is a sensitive nondestructive optical technique which can be successfully used for determining the thickness of an optically transparent thin film deposited on a dielectric substrate. In this laboratory we have constructed a vertical null‐type ellipsometer which has been used with a homemade aluminum Teflon‐coated Langmuir trough, to study thin films at gas–water interface. Surface pressure, surface potential, and ellipsometric measurements have been performed simultaneously. Ellipsometric measurements have been done directly on the aqueous substrate contained in the trough, with and without the film, with reproducibility (±0.02°) and good signal stability (±0.1 mV) on the 10‐mV scale. The performance of the homemade instrument had been checked against the known ellipsometric properties of spread arachidic acid on acidic water solution. In this paper, details of construction and performance of the ellipsometer as well as characteristics of the trough and ellipsometric study of β‐palmitoyl‐α‐o...

Direct evidence for the formation of a monolayer from a bilayer. An ellipsometric study at the nitrogen-water interface

Direct evidence for the formation of a monolayer from a bilayer was measured by ellipsometry after spreading unilamellar vesicles of dioleoyl phosphatidylcholine (DOPC) at the nitrogen-water interface. The ellipsometric isotherms of DOPC vesicles and DOPC spread from an organic solvent were compared and found similar. From the observed ellipsometric angle (delta delta) in the plateau region (-1.04 degrees) and literature data for refractive indices of an anisotropic film similar to DOPC, we have calculated a thickness of 20 +/- 1 A. These results strongly suggest that, similarly to DOPC spread from an organic solvent, DOPC vesicles form a monolayer when spread at the nitrogen-water interface.

Multiple-wavelength ellipsometry in thin uniaxial nonabsorbing films

We develop and solve the equations of multiple-wavelength ellipsometry for thin uniaxial nonabsorbing films. We show that the values of the thickness and the real refractive indices (parallel and perpendicular to the optical axis) of the thin film can be obtained by three measurements, at three different wavelengths, of the phase difference (between the filmed and bare substrate phase changes caused by reflection), provided that the refractive indices of both the incident medium and the substrate are significantly dispersive. We also show, in the case of a dispersive thin film, how to determine the indices of refraction in terms of wavelength up to any desired accuracy by making the appropriate number of measurements of the phase difference at different wavelengths.

Surface properties of valine-gramicidin A at the air-water interface

Abstract Fluorescence microscopy, X-ray reflectivity and ellipsometry have been applied to the study of a monolayer of valine-gramicidin A (VGA) at the air-water interface to obtain insight into the conformation and orientation of VGA, and to clarify the shoulder observed in its π - A curve (12.5–16 mN m −1 , 240 – 175 A 2 molecule −1 ). At low molecular areas, the X-ray reflectivities are consistent with formation of a single layer of tubular-shaped dimers. Fluorescence microscopy yielded homogeneous pictures suggesting that on the macroscopic scale the shoulder cannot be attributed to domain formation. Thicknesses calculated from the ellipsometric isotherm are consistent with the X-ray data.

Standards for Which the Ellipsometric Parameter psi Remains Insensitive to Variations in the Angle of Incidence.

We examine the possibility of using two- and three-phase systems suitable as standards for which the ellipsometric parameter ? remains insensitive to variations in the angle of incidence. These standards avoid propagation of errors in the angle of incidence with respect to the measured standard ? value. Different materials (dielectrics, metals, and semiconductors), adequate for the above purpose, are considered in different structure combinations, and their optical response are analyzed.

Properties of mixed chlorophyll a-glycolipid monolayers

Abstract Surface pressure, ellipsometric and surface potential-area isotherms have been measured to investigate the properties of chlorophyll a-digalactosyldiacyglycerol (Chl a-DGDG) and Chl a-sulfoquinovosyldiacylglycerol (SQDG) monolayers at the nitrogen-water interface. The surface pressure-area isotherms of Chl a-DGDG exhibit a pronounced negative deviation with respect to ideality, whereas ellipsometric and surface potential isotherms of the mixed monolayers fall between those of the pure components. Hydrophilic interactions have been ruled out on the basis of the additivity law followed by the surface dipole moment of the mixed monolayers. The observed deviation exhibited by the π-A isotherms is interpreted in terms of an intermolecular cavity effect.

Polynomial solution for two thicknesses of a multilayer system from a single ellipsometric measurement

It is shown that the unknown thicknesses of any two transparent layers in an arbitrary multilayer system from a single ellipsometric measurement can be found by the solution of an eighth-degree real polynomial. The method gives directly all the possible physical solutions, which are computed from the real roots of the polynomial. The coefficients of the polynomial are determined by the angle of incidence, the refractive indices of all the phases, and the thicknesses of the other layers. The method is used on a simulated system of air/silicon nitride/silicon oxide/silicon.

Langmuir-Blodgett films based on substituted 1,2-dithiolium-tetracyanoquinodimethane radical anion salt

Abstract A new semi-amphiphilic radical-anion salt, 1,2-dithiolium-tetracyanoquinodimethane has been synthesized for developing electroactive LB films. From in situ surface potential and ellipsometric measurements, it has been shown that the monolayer thermodynamic state is crucial for the building up of controlled multilayers.