Spectroscopy films

Figure 8. IR spectra (at 8 to 10 pm) of DPL films either deposited directly (Curve 1) or collected from the interface onto germanium plate (other curves). Curve 2, DPL on 0.15M NaCl Curve 3, DPL on 0.001M CaCU Curve 4, DPL on O.OiOM CaCU Curve 5, DPL on 0.075M CaCU and Curve 6, interface of 0.075M without DPL film. Spectroscopy technique as in Figure 7.

Ultraviolet, visible, infrared and EPR s[ tra are u ful in studying pyridinyl radicals. The major recent developments have come from experiments using new apparatus for known techniques. The developments, especially that for thin film spectroscopy, will be useful for other chemical problems. Therefore, the techniques are described in detail. 

The blue film, formed by distillation of l-alkyl-2-carbomethoxypyridinyl radical, 2, onto a 77 K surface, disappears on warming. The thin film spectroscopy apparatus depicted in Fig. 3 allowed a) direct cooling of the condensing surface so that tern-... 

Fig. 3. Exploded view of the thin film spectroscopy apparatus, showing the relative positions of the optical cooling surface, the interferometer, the optical pathway for measurement and the material source for the film. W, window (polished quartz or sapphire), O, ground surface onto which windows are glued, LED, light emitting diode, PD, photodiode...

The infrared spectra of pyridinyls are measured conveniently with the thin film spectroscopy apparatus and the changes which ensue on dimerization have been examined. A complete inO red spectrum for 4 at 77 K has been reported External sapphire windows on the thin film apparatus allow spectra to be measured... 

Irradiation of the matrix (420-500 nm, a range not corresponding to the pimer charge-transfer transition) produced an additional triplet (ti j 10 hours) with D = 0.0175 cm and E = O, r = 5.4 A. Careful examination of the spectra of concentrated solutions of 4 revealed absorption in the range of 420-500 nm Thin film spectroscopy of 4 revealed that covalent dimers (rather than singlet pairs) gave rise to the new triplet via a no ncr transition Formation of a triplet [D = 0.0146 cm" and E = O, r = 5.5 A] from the dimer of 2 , 2-2, has been accomplished by irradiation in an MTHF glass. The equilibria describe the relationship of the triplets and radicals in Eq. 13. The two triplet pairs (A and B) are not readily interconvertible. 

Pyridinyl radical studies have stimultated the development of useful equipment, such as those for thin film spectroscopy, for variable i th measurements under oxygen-free conditions (the VV-cell ) and a spectr< lectrochemical cell. In addition, a new technique for measuring the photodissodation spectra of unstable species which can be generated electrochemically was introduced. 

Film Spectroscopy. Each polymer (IX, X, XI) was dissolved in cyclohexanone at 3% by weight, and the sensitizer, 3,3 -carbonylbis(5,7-dipropyloxy coumarin) concentration was 3% by weight of the polymer. The three polymer solutions were each cast onto a quartz substrate via spin coating. The thickness of the films were adjusted until an optical density (O.D.) of between 2.0 and 3.0 was obtained at a wavelength of 300 nm. At this film thickness the concentration of the sensitizer was approximately 3xlO 3 M. Figure 3 shows the absorption characteristics of the non-... 

Absorption spectroscopy provides a means to study particular details about a monolayer. Transmission spectroscopy is difficult because the film, which is thin, absorbs little. Gaines [1] describes multiple-pass procedures for overcoming this problem. Reflection spectroscopy in the UV-visible range has been reported for lipid monolayers [150,151] and in the IR range for oleic acid [152]. 

Madey and co-workers followed the reduction of titanium with XPS during the deposition of metal overlayers on TiOi [87]. This shows the reduction of surface TiOj molecules on adsorption of reactive metals. Film growth is readily monitored by the disappearance of the XPS signal from the underlying surface [88, 89]. This approach can be applied to polymer surfaces [90] and to determine the thickness of polymer layers on metals [91]. Because it is often used for chemical analysis, the method is sometimes referred to as electron spectroscopy for chemical analysis (ESCA). Since x-rays are very penetrating, a grazing incidence angle is often used to emphasize the contribution from the surface atoms. 

Vibrational Spectroscopy. Infrared absorption spectra may be obtained using convention IR or FTIR instrumentation the catalyst may be present as a compressed disk, allowing transmission spectroscopy. If the surface area is high, there can be enough chemisorbed species for their spectra to be recorded. This approach is widely used to follow actual catalyzed reactions see, for example. Refs. 26 (metal oxide catalysts) and 27 (zeolitic catalysts). Diffuse reflectance infrared reflection spectroscopy (DRIFT S) may be used on films [e.g.. Ref. 28—Si02 films on Mo(llO)]. Laser Raman spectroscopy (e.g.. Refs. 29, 30) and infrared emission spectroscopy may give greater detail [31]. 

Shen Y R 1998 Sum frequency generation for vibrational spectroscopy applications to water interfaces and films of water and ice Solid State Commun. 108 399... 

Freunscht P, Van Duyne R P and Schneider S 1997 Surface-enhanced Raman spectroscopy of trans-stilbene adsorbed on platinum- or self-assembled monolayer-modified silver film over nanosphere surfaces Chem. Phys. Lett. 281 372-8... 

Al-Obaidi A H R, Rigby S J, Hegarty J N M, Bell S E J and McGarvey J J 1996 Direct formation of silver and gold metal liquid-like films (MELLFS) from thiols and sols without organic solvents SERS and AFM studies ICORS 96 XVth Int. Conf on Raman Spectroscopy ed S A Asher and P B Stein (New York Wiley) pp 590-1... 

Zeisel D, Deckert V, Zenobi R and Vo-Dinh T 1998 Near-field surface-enhanced Raman spectroscopy of dye molecules adsorbed on silver island films Chem. Phys. Lett. 283 381... 

For bulk structural detemiination (see chapter B 1.9). the main teclmique used has been x-ray diffraction (XRD). Several other teclmiques are also available for more specialized applications, including electron diffraction (ED) for thin film structures and gas-phase molecules neutron diffraction (ND) and nuclear magnetic resonance (NMR) for magnetic studies (see chapter B1.12 and chapter B1.13) x-ray absorption fine structure (XAFS) for local structures in small or unstable samples and other spectroscopies to examine local structures in molecules. Electron microscopy also plays an important role, primarily tlirough unaging (see chapter B1.17). 

One interesting new field in the area of optical spectroscopy is near-field scaiming optical microscopy, a teclmique that allows for the imaging of surfaces down to sub-micron resolution and for the detection and characterization of single molecules [, M]- Wlien applied to the study of surfaces, this approach is capable of identifying individual adsorbates, as in the case of oxazine molecules dispersed on a polymer film, illustrated in figure Bl.22,11 [82], Absorption and emission spectra of individual molecules can be obtamed with this teclmique as well, and time-dependent measurements can be used to follow the dynamics of surface processes. 

Lesieur P, Vandevyver M, Ruaudel-Teixier A and Barraud A Orientational studies of Langmuir-Blodgett films of porphyrins with polarized resonant Raman spectroscopy Thin Soiid Fiims 159 315-22... 

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