Specular beam

Fig. 3. Temporal evolution of RHEED specular beam intensity (along [1101 azimuth) for GaAs at 600°C, GaAs at 250°C, and (Ga,Mn)As at 230°C from top to bottom (Shen et al. 1997a).

The RHEED technique has recently been used successfully in the in-situ compositional control of the MBE growth of InxGai.xN, 0 < x < 0.4 [12], This was achieved by monitoring the oscillations of the specular beam intensity during deposition of the alloy. 

Diffraction intensities are shown in Fig. 34 with the notation [00], [10], and [20] indicating the specular, first- and second-order diffracted beams, respectively. It is immediately apparent from the contrast between the three figures that the availability of the PES sink has a marked effect on all the beams. The specular beam does not go through a minimum at 350 meV because as the energy is raised, dissociation becomes more probable, and thus more of the flux is channeled into this dissociation channel instead of into the reflected specular channel. Similarly, the peak values of the [10] and [20] beams are significantly reduced. In a nonreactive system, it is... 

Figure 34. (a) Hj intensities for the first three diffraction states as a function of initial energy for a primary beam at normal incidence to a potential having no reactive channel. The sharp minimum occurring in the specular beam at 370 meV arises as a consequence of destructive interference between molecules scattered at top and center sites (b) same as (a) but here the minimum value of the activation barrier is above an atop site (c) same as (a) but here the minimum barrier value is located above a center site. The figure is reprinted with permission from Halstead and Holloway (1986). 

Fig. 9. Intensity-voltage plot (intensity versus A) of 00 beam from a clean W(112) surface (see Fig. 6d). Crystal rotated 5° around [01] axis so that specular beam could be measured (incidence brought 5" closer to [llT] surface direction). The experiment was performed in a post-acceleration tuhe and intensity on screen monitored hy telescopic photometer [data of Chang, (203, pp. 24—25)]. Heavy arrows show theoretical positions Fmax of Bragg maxima of order n calculated according to nX = n( 150.4/ Umax) = 2Uo(6) cos 5 . Identifiable Bragg maxima stand out clearly in the higher voltage range and are displaced by an inner potential of about 19 V. At lower voltage, the curve is complicated and is not well understood.

Figure 29. Time-of-flight spectra for a series of temperatures at the specular angle (45°) for NaCl(001), transformed to energy transfer distributions. The laige peak at zero energy transfer is the coherent elastic scattering (specular beam) which diminishes in intensity with temperature according to the Debye-Waller factor. The solid curves fitting the multiphonon foot of the elastic peaks are calculated from the Manson theory as described in the text. The incident helium wavevector for these experiments was 9.25 A , corresponding to an incident energy of 44 meV. (Reproduced from Fig. 1 of Ref. 96, with permission.)...

This follows from the fact that the parallel component of the electron momentum is conserved to within a reciprocal lattice vector and for the first order non-specular beams, the reciprocal lattice vectors are the unit vectors of the two-dimensional reciprocal lattice of the surface planeThe relationship between the diffraction angle (where h,k label the two-dimensional order of the diffracted beam) and the primary electron energy is... 

LEED intensity curves involving data and computations at off-normal incidence for all the principal non-specular beams leads to the same result. Similar analyses have also been completed for the GaAs (110) and ZnO (1120) surfaces with similar results best agreement between measured and computed LEED intensity curves is achieved when the computations are performed with the surface ideally terminated,... 

For the 180° geometry, a problem exists with the inclusion of the specular component of reflectance, which can be a major source of error in the infrared. The problem can be alleviated by the inclusion of a small baffle that blocks the specular beam but allows the diffuse component to pass through unhindered (Figs. 14 and 15). 

It is common practice to plot BRDF with respect to the angle from the specular beam, AO. If scatter is measured only in the PLIN, A0 = 0 —. ... 

TIS can be calculated from BRDF by integrating BRDF over the hemisphere (12). Typically, a 5° total angle hole is left around the specular beam because specular light is not included in total integrated scatter (13). 

Figure 3 shows how light may be scattered in both reflection and transmission. In both cases, the hght may be scattered throughout a hemisphere, whereas the specular beam is confined to a single angle. 

Figure 5 shows the BRDF plot for two randomly polished mirrors. As explained previously, this is a plot of scattered intensity versus angle. The 0° point on the horizontal axis indicates the direction of the specular reflection. All other directions on the plot are relative to this direction (0, — 0j in Fig. 4). The vertical axis is the BRDF scatter (on a logarithmic scale). BRDF is typically plotted on a logarithmic scale because it changes over several orders of magnitude within a few degrees of the specular beam.

Figure 3.2.1.11 Kinematic intensity spectra of the specular beam for normal incidence and normalized to atomic scattering, ho/ fo, for (s) gradual electron attenuation according to Vo = —5 eV and (b) for...

The reader should keep in mind that Eq. (3.2.1.25) holds only when surface and subsurface layer contributions are not additionally weakened by different atomic vibrations. Also, it applies quantitatively only for the specular beam at normal incidence, although qualitatively similar dependences result for gw 0. 

Figure 3.2.1.25 displays FD spectra and their dependence on structural and nonstructural parameters. As an example, in panel (a), the specular beam spectrum for the bulk-terminated Pt(lOO) surface is compared to that for the surface with... 

Figure 3.4.2.27 shows the relation between Ijdn and Idy for the specular beam, where the effects are largest. Below the critical angle (at Inn = 0.07 in the example), there is no perpendicular momentum transfer, that is, ldy = 0. For small incoming and/or exit angles, the I index of measured reflections needs to be corrected for this shift. 

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