Detected Scattered Light

The detected, integrated SERS intensity, //, in photons per second is given 

The total number of molecules involved in enhanced scattering will be NA if only adsorbed molecules contribute however, it is possible that some molecules in a volume element, 8V, contiguous to the surface will also produce enhanced scattering and then NA in Eq. (1) would have to be expanded to (iVA+ C5V) to include those solutions-soluble molecules which are within the enhancement range. 

In practice it is not necessary to include the area of the laser beam on the surface in a calculation of 7/ since AJ can be replaced by 1, the total number of photons striking the surface per second. Il can be calculated by measuring the total power, P, in the laser beam which impinges on the metal surface. If which is in watts, i.e., joules per second, is divided by the energy per photon, the result is II- Thus, 

This type of calculation was not encouraging to early investigators who envisioned using the Raman technique for surface studies. However, when a surface pretreatment is used to roughen a Ag surface, a peak intensity of ca. 20,000 Hz can be observed for pyridine with conditions similar to those in the above calculation except at one-tenth the laser power. This means that a 10 -fold enhancement has occurred for the SERS peak intensity. This enhancement can be accounted for by large increases in both the differential Raman cross section, dor/dft, and the electromagnetic factor L (Oi)L (Os)- We will treat the origin of these enhancements in the theoretical sections. 

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The specific intensity is an important quantity because it depends mainly on sample D) and laser (Pd) variables and not on spectrometer parameters such as collection angle, quantum efficiently, and the like L indicates what the spectrometer has to work with while collecting and detecting scattered light. If we consider the example of a clear sample and 180° backscattered geometry with = 0.1 cm (as in Fig. 2.5), then L can be calculated for a variety of samples. Table 2.4 lists several specific intensities for samples of... 

Fig. 4.56. Schematic diagram of a SERS-active substrate and the measurement arrangement. Alumina nanoparticles are deposited on a glass surface and produce the required roughness. A thin silver layer is evaporated on to the nanoparticles and serves for the enhancement. Organic molecules adsorbed on the silver surface can be detected by irradiation with a laser and collecting the Raman scattered light.

The observation may be by a lamp illuminating the surface and a photocell to detect the scattered light due to the water droplets on the surface. The accurate measurement of the surface temperature, which is the dewpoint temperature, is critical. If a coolant is used, a close approximation for the surface temperature is the fluid temperature otherwise a small thermocouple or resistance sensor can be attached to or embedded into the surface. 

A modern laser Raman spectrometer consists of four fundamental components a laser source, an optical system for focusing the laser beam on to the sample and for directing the Raman scattered light to the monochromator entrance slit, a double or triple monochromator to disperse the scattered light, and a photoelectric detection system to measure the intensity of the light passing through the monochromator exit slit (Fig. 7). 

Fiq. 20a. The pulsed Raman spectrum of Mn-doped ZnSe single crystal using a detection interval of 200 nsec. Broad band fluorescence superimposed on a large instrumental scattered light component was observed. Recordings taken with ratemeter time constants (TC) of 1 sec and 10 sec are shown (37). 

Fig. 3—Measurement of surface by HDI surface reflectance analyzer. In electromagnetic radiation (light), the polarization direction is defined as the direction of the electric field vector. The incident polarization of the light can be controlled. The instrument uses a variety of detectors to analyze the reflected polarization state of the light. (U.S. Patent 6,134,011). (a) Plane of the disk The SRA uses a fixed 60 degree (from the surface normal) angle of incidence. The plane of incidence is the same as the paper plane (b) Pit on a surface detected by reflected light channels of HDI instrument (c) Scratches on disk surface measured by HDI surface reflectance analyzer (d) Particles on the surface of disk detected by reflected light (black spot) and by scattered light (white spot) [8].

Raman spectroscopy detects the scattering of light, not its absorption. Superposed on the frequency of the scattered light are the frequencies of the molecular vibrations. The detection occurs in the IR spectral region while the excitation happens in the visible region. Since laser light sources have become well developed, Raman spectroscopy has become an important tool for the analysis of biomolecules. 

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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