Spectrometer surface scatter

The contribution of spectrometer surface scatter depends upon the design, quality, and alignment of the spectometer components. Surface scatter may originate from the grating, slits, mirrors, refractor plates, photomultiplier(s), light baffles, interference filters, dust, and the spectrometer interior surfaces. Essentially all internal surfaces may contribute to scatter. 

Figure 2 Schematic view of the apparatus used in studies of the steric effects in gas-surface scattering. A detail of the crystal mount with die orientation rod at 1 cm in front of the surface is shown in die right hand corner. A detailed drawing of the hexapole state selector is given below the main figure. The voltage is applied to die six small rods indicated by an arrow. Key Q quadrupole mass spectrometer R Rempi detector M, crystal manipulator SI, beam source for state selected molecules H electric hexapole state selector C mechanical beam chopper V pulsed gas source S2, continuous molecular beam source. From Tenner et al. [34].

Instrumentation. In both cases, a near field probe is employed—either a metal-coated fiber (aperture-based) or a metal tip (apertureless). Distance regulation, as used with scanning probe methods (see Sect. 7.2), controls the probe-surface gap it may also be used to obtain a topographic mapping of the studied surface. Scattered light is collected and guided to a Raman spectrometer. In a (non-electrochemical) study, dye-labeled DNA that had adsorbed onto evaporated silver layers on FIFE nanospheres was observed [531]. Special surface sites with particularly high enhancement could be identified. 

Elastic and inelastic colUsions of atoms and molecules at surfaces are also of importance. The scattering of hydrogen and helium from surfaces leads to diffraction patterns in the same manner as with LEED, but since the atoms penetrate the surface far less deeply than even low-energy electrons, the structures obtained reflect the very surface of the sample. The inelastic surface scattering of molecules can be examined in detail using laser and mass spectromet-ric detection for the scattered molecules. Such measurements can be used to model the form of the gas-surface interaction potential, knowledge of which is a prerequisite for any detailed picture of gas-surface reaction dynamics. 

A SERS apparatus consists basically of a laser excitation source, the potential-controlled electrochemical cell, the optics for collecting the surface scattering, the com-putercontrolled spectrometer (double or triple monochromator), the photon-counting electronic or multichannel detection system and the display unit. 

See also Bloflulds Studied By NMR Chiroptical Spectroscopy, General Theory Forensic Science, Applications of IR Spectroscopy Industrial Applications of IR and Raman Spectroscopy IR Spectrometers Medical Science Applications of IR Membranes Studied By NMR Spectroscopy Nucleic Acids and Nucleotides Studied Using Mass Spectrometry Nucleic Acids Studied By NMR Peptides and Proteins Studied Using Mass Spectrometry Raman Spectrometers Surface-Enhanced Raman Scattering (SERS), Applications. 

Beck, R. D. Weis, P. Brauchle, G. Rockenberger, J. Tandem time-of-flight mass spectrometer for cluster-surface scattering experiments. Rev. Sci. Instr. 1995, 66, 4188-4197. 

Grizzi O, Shi M, Bu H, and Rabalais J W 1990 Time-of-flight scattering and recoiling spectrometer (TOF-SARS) for surface analysis Rev. Sc/. Instrum. 61 740-52... 

If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

It is difficult to observe tliese surface processes directly in CVD and MOCVD apparatus because tliey operate at pressures incompatible witli most teclmiques for surface analysis. Consequently, most fundamental studies have selected one or more of tliese steps for examination by molecular beam scattering, or in simplified model reactors from which samples can be transferred into UHV surface spectrometers witliout air exposure. Reference [4] describes many such studies. Additional tliemes and examples, illustrating botli progress achieved and remaining questions, are presented in section C2.18.4. 

When the spectral characteristics of the source itself are of primary interest, dispersive or ftir spectrometers are readily adapted to emission spectroscopy. Commercial instmments usually have a port that can accept an input beam without disturbing the usual source optics. Infrared emission spectroscopy at ambient or only moderately elevated temperatures has the advantage that no sample preparation is necessary. It is particularly appHcable to opaque and highly scattering samples, anodized and painted surfaces, polymer films, and atmospheric species (135). The interferometric... 

REELS will continue to be an important surface analytical tool having special features, such as very high surface sensitivity over lateral distances of the order of a few pm and a lateral resolution that is uniquely immune from back scattered electron effects that degrade the lateral resolution of SAM, SEM and EDS. Its universal availability on all types of electron-excited Auger spectrometers is appealing. However in its high-intensity VEELS-form spectral overlap problems prevent widespread application of REELS. 

In Raman measurements [57], the 514-nm line of an Ar+ laser, the 325-nm line of a He-Cd laser, and the 244-nm line of an intracavity frequency-doubled Ar+ laser were employed. The incident laser beam was directed onto the sample surface under the back-scattering geometry, and the samples were kept at room temperature. In the 514-nm excitation, the scattered light was collected and dispersed in a SPEX 1403 double monochromator and detected with a photomultiplier. The laser output power was 300 mW. In the 325- and 244-nm excitations, the scattered light was collected with fused silica optics and was analyzed with a UV-enhanced CCD camera, using a Renishaw micro-Raman system 1000 spectrometer modified for use at 325 and 244 nm, respectively. A laser output of 10 mW was used, which resulted in an incident power at the sample of approximately 1.5 mW. The spectral resolution was approximately 2 cm k That no photoalteration of the samples occurred during the UV laser irradiation was ensured by confirming that the visible Raman spectra were unaltered after the UV Raman measurements. 

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