Normal-incidence laser

Figure 13.3. Two sampling geometries for surface Raman spectroscopy. (A) is 180° backscat-tering, and (B) uses a non-normally incident laser and normal collection through a window in a UHV chamber.

In this paper we present several new aspects of this problem. They include an experimental confirmation of the theoretical results for the oblique-incidence case as well as the theory for the reorientation of normally incident lasers (i.e., when the laser polarization is orthogonal to the director axis of the nematic film) and experimental results for the observed Freedericksz transition field and broadened (or narrowed) radial dependence. 

The optical-field-induced Freedericksz transition for a twist deformation by a normally incident laser beam in a planar-aligned nematic liquid crystal is studied. The Euler equation for the molecular director and the equations describing the evolution of the beam polarization in the birefringent medium are solved simultaneously in the small-perturbation limit. The stability of the undistorted state is investigated. An alternate series of stable and unstable bifurcations is found. This phenomenon has no analog in the Freedericksz transition induced by dc electric and magnetic external fields. 

Figure 17. Reflectivity (O) and Raman scattering intensities (9) for an Al-AlOx-4-pyridine-COOH-Ag tunneling junction prepared on a diffraction grating substrate, as a function of the angle between the incident laser beam and the normal to the grating surface (45). At the same angles that absorption by surface plasmons causes reflectivity dips, the Raman signal shows peaks.

Figure 10.1. Comparison of normal (top) and surface-enhanced (bottom) Raman scattering. The top panel shows the conversion of incident laser light of intensity /(vl) into Stokes scattered light /NRS, which is proportional to the Raman cross section and the number of target molecules N in the probed volume. In the bottom panel

The pump laser is a Quanta-Ray Nd YAG Model DCR-1A with an 8 nsec, 700 mj (max), 1.06 micron output. The multipass cell cavity is bounded by the normal incidence harmonic beamsplitter (>99.5%... 

The first four mirrors (>99.7% reflectance at 1.06 micron) act as a far field isolator which locates the multipass cavity 15 meters away from the laser and effectively isolates the laser from the potentially damaging retroreflected 1.06 micron radiation from the normal incidence beam splitter. The multipass cavity is aligned by monitoring the retroreflected 1.06 micron pulse which is found to emerge from the Nd YAG laser cavity, 120 nsec after the original pulse, when optimum alignment is achieved. 

The intensity of a Raman signal is governed by a number of factors, including incident laser power, frequency of the scattered radiation, efficiency of the grating (in the case of dispersive instruments) and detector, absorptivity of the materials involved in the scattering, molar scattering power of the normal mode, and the concentration of the sample. This situation is further complicated by the fact that many of these parameters are frequency-dependent, as indicated in the following equation ... 

The simple dissolution rate results were obtained using a laser interferometer with a 15 mw/cm He-Ne laser at normal incidence to the wafer surface in the agitated developer bath. The reflected beam was directed by a beam splitter onto a photocell. The photocell output was fed through a Keithly series 500 interface into an IBM-PC. The more complex dissolution data were collected on a Perkin-Elmer dissolution rate monitor using 934 developer at a 1 1 dilution with deionized water at a temperature of 21 C. The stepped exposures were obtained using a calibrated multidensity chrome stepwedge. 

The most complete study of CID at hyperthermal energies involved NO+ scattering on either GaAs(llO) or Ag(lll) at normal incidence conditions for Ts = 298 A laser-based technique, resonance enhanced multi-... 

Raman peaks in the spectrum are displayed as frequency shifts from the incident laser-line, or Av = vq v. Each peak corresponds to the energy of a vibrational normal, which depends on molecular strucmre as well as the characteristics of chemical bonds comprising each normal mode. Hence, Raman spectrum is called the molecular fingerprint of the molecules and materials. Raman spectra of DNA and proteins, for example, contain rich information on their chemical bonds and stmctures. The Raman spectmm not only provides information about the stmcture, conformation, and identity of the sample but also the dynamics and interactions between biomolecules such as protein folding and DNA-protein interactions. 

The method utiHzes the angular dependence of the dielectric filter on impacting photon direction, with its transmission spectral profile shifting to the blue, in fine with the increase in the deviation of photons away from normal incidence. This feature enables the filter to serve as a unidirectional mirror, passing a semi-colhmated laser beam through unhindered from one side while at the other side reflecting any photons emerging from the sample predominantly at random directions, back into the sample. 

Si02(Cab-0-Sil HS5) substrates were prepared by spraying a suspension of SIO2 in ethanol onto a single crystal of NaCl plate, the size of which was 2 cm in diameter and 3 mm thick, followed by drying at 100°C in air. They were then mounted in a glass IR reaction cell. The sample was rotatable in the cell, so that the angle of incident laser beam to the surface normal could... 

Fig. 30 Typical changes in FTIR transmission spectra of the PTFE films deposited on Si(100) substrates at Ts=RT by SR etching (a) and laser ablation (b). The bottom trace is a spectrum for the normal incidence (incident angle=0°) and the top trace is for the oblique incidence (incident angle=80°). Reproduced with permission from J Phys Chem 2000, B26, 6212-6217. Copyright 1998 Am Chem Soc...

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