J. M. Turlet

Polarization Modulation FT-IR Spectroscopy of Surfaces and Ultra-Thin Films: Experimental Procedure and Quantitative Analysis:

Polarization modulation of the incident electromagnetic field is used to increase the sensitivity and the in situ experiment ability of the well-known method of reflexion absorption FT-IR spectroscopy, for the characterization of surfaces and ultra-thin films. The experimental procedure and signal processing of the detected intensity are described and illustrated with the use of results obtained with Langmuir-Blodgett monolayers. The quantitative analysis of the spectra is then developed, and a linear behavior of the band intensities is found for ultra-thin films exhibiting no strong absorptions. This result is checked with the use of organic and inorganic ultra-thin films of increasing thicknesses.

Polarization-modulated FT-IR spectroscopy of a spread monolayer at the air/water interface

This study devoted to the FT-IR spectroscopy of monolayers spread at the air/water interface is, to our knowledge, the first report presenting complete mid-infrared monolayer spectra perfectly extracted from the strong water vapor bands. This has been possible with the use of the polarization-modulated IRRAS method, which is not sensitive to the isotropic absorptions of the sample environment. On the basis of theoretical modeling and experiments, the best angle of incidence has been found near 76° for detection of intraplane as well as out-of-plane oriented monolayer absorptions. With the use of such experimental conditions, on the normalized difference (covered vs. uncovered water) PM-IRRAS spectra, monolayer vibrational bands come out upward or downward, depending on the orientation of their transition moment with respect to the interface. Application to the study of deuterated arachidic acid and arachidate monolayers allows observation of the vibrational modes of the polar head groups interacting with the liquid water molecules and provides some evidence of their symmetrical anchoring. The vibrational modes of the liquid water subphase contribute to these difference spectra as broad dips that certainly contain information on a possible restructuring of the water molecules at the interface.

Investigations at the air/water interface using polarization modulation IR spectroscopy

The ability of polarization modulation IR reflection absorption spectroscopy (PM–IRRAS) to study the air/water interface is presented. A brief description of the set-up and of the experimental procedure is given. Theoretical simulations accounting for the uniaxial nature of the spread monolayer lead to optimum experimental conditions (71 ° for the angle of incidence) and to a specific surface selection rule. Application to the study of cadmium arachidate, dimyristolyl phosphatidylcholine (DMPC) and polypeptidic Langmuir films illustrates the potential uses of this method.

Polarization modulation FTIR spectroscopy at the air-water interface

Abstract Mid-infrared spectra of monolayers spread at the air-water interface have been obtained, completely devoid of strong water vapor absorptions, using polarization modulation infrared reflexion absorption spectroscopy (PM-IRRAS). On normalized difference (covered vs. uncovered water) PM-IRRAS spectra, the monolayer absorption bands appear upwards or downwards depending on the orientation of their transition moment with respect to the water surface. Vibrational modes of the water subphase contribute to these difference spectra as broad dips. Study of a monolayer of cadmium arachidate has allowed observation of the vibrational modes of the polar heads and provides some evidence of their symmetrical anchoring at the water surface. Under surface compression, a monolayer of deuterated arachidic acid undergoes a molecular reorganization leading to a better ordering of the deuterated chains.

Calibration Procedure to Derive IRRAS Spectra from PM-IRRAS Spectra

A calibration procedure is presented to relate Fourier transform infrared reflection-absorption spectroscopy (IRRAS) spectra obtained by polarization modulation (PM) and conventional techniques. This procedure concerns only PM-IRRAS measurements obtained with a lock-in amplifier. At the first stage, it consists in measuring two calibrated spectra, by adding a linear polarizer after the sample. The corresponding corrected PM-IRRAS spectra display band intensities close to those observed on IRRAS spectra (at about 6%), but the value of the baseline is lower. The calibration procedure can be performed at the second stage, by normalizing the corrected PM-IRRAS spectra of the sample and the bare substrate. In this case, the band intensities and the baseline of the normalized corrected PM-IRRAS and IRRAS spectra are similar.

Study of boundary film formation with overbased calcium sulfonate by PM-IRRAS spectroscopy

Abstract Overbased calcium sulfonates can be used to prevent wear occurring during friction phenomenon by forming a boundary film on friction surfaces. The formation mechanism of the boundary film was investigated using polarization modulated infrared reflection absorption spectroscopy. It appears that the film mainly consists of CaCO 3 crystallites and that the calcium sulfonate chains are expelled from the friction zone. The expulsion of the sulfonate chains occurs in a relatively fast step and is followed by the adsorption and the packing of calcium carbonate particles on the surface. This adsorbed mineral phase rapidly crystallises into a calcite lattice with the c axis preferentially oriented perpendicular to the surface.

Characterization of Langmuir-Blodgett monolayers using polarization modulated FTIR spectroscopy

Abstract Structural and conformational analysis of cadmium arachidate monolayers deposited on a gold substrate has been performed on the basis of the well-defined spectra obtained by polarization modulated infrared reflection spectroscopy. This quantitative analysis gives accurate values for the tilt and twist orientations of the arachidate chains relative to the metallic substrate in agreement with a high organization of the LB molecules that decreases as the film goes to a single layer and gauche conformations appear.

Organization in pure and alternate deuterated cadmium arachidate monolayers on solid substrates and at the air/water interface studied by conventional and differential Fourier transform infrared spectroscopies

The molecular organization in monolayers of deuterated cadmium arachidate deposited on solid metallic and dielectric substrates or spread at the air/water interface is investigated using different techniques of Fourier transform infrared spectroscopy. For metals, the polarization modulated reflectivity shows that the molecules have a high molecular order and are oriented quasi perpendicularly with respect to the surface. The in‐plane organization is determined by complementary transmission experiments on CaF2 plates. The CD2 bending vibration in mono‐ and multilayer Langmuir–Blodgett stacks reveals the hexagonal packing of a single monolayer irrespective of the hydrophilic or hydrophobic character of the substrate. Probing the stack organization using selective monolayer deuteration in hydrogenated Langmuir–Blodgett films shows that the first deposited monolayer of an odd‐numbered stack keeps its hexagonal structure while additional bilayers adopt an orthorhombic packing induced by the strong polar head/h...

Pure electronic, vibronic, and two‐particle surface excitons on the anthracene crystal. I. General theoretical basis, experimental study, and analysis of the (0,0) region

These two papers (papers I and II) present a complete study—theoretical and experimental—of the first singlet–singlet electronic and vibronic transitions of the anthracene crystal. We interpret the new observed structures in terms of pure electronic, vibronic, and two‐particle site shift surface exciton states. The first paper (I) begins by a model description of the bulk and surface excitons in a rigid lattice. We then describe the experimental study of the anthracene crystal reflectivity and surface emission excitation at low temperatures with and without nitrogen coating of the crystal surface. For the first time, we clearly established the following results: (1) The existence and the accurate position of the upper a polarized Davydov component for the pure electronic surface exciton; (2) The equality within our experimental accuracy (≊2 cm−1), of the surface and bulk excitonic Davydov splitting (Δ=223 cm−1) and the translational equivalence (δ=206 cm−1) of the surface structures with their bulk coun...

Pure electronic, vibronic, and two‐particle surface excitons on the anthracene crystal. II. Experimental study and analysis of the (0,1) region

In this second part, we present the experimental and theoretical study concerning vibronic surface transitions for the vibration modes 390 and 1400 cm−1. We begin by a theoretical discussion of the different limiting cases of exciton‐vibration coupling. In the experimental part, evidence of the surface character for the observed structures in surface emission excitation spectra is provided by their shifts upon surface coating. A general smilarity of the surface states with the previously described bulk states, including a translational equivalence (δ=206 cm−1), is established. The observed structures are assigned to interacting single particle (vibron) resonances and two‐particle bands. However, some slight discrepancies between surface and bulk band structures lead us to question the complete validity of their translational equivalence. Finally, our observations show that the surface two‐particle excitation profile (peak shaped) is quite different from the surface two‐particle absorption profile (stepwise threshold profile). We propose a model mechanism of the surface excitations relaxation which accounts qualitatively for this specific surface excitation profile.