Sample apertures dimensions

R is approximately between 4 and 30/mi in the mid-IR range (2.5-20//m) when NA = 0.70. Sometimes it is possible to obtain IR spectra of good quality for samples with dimensions below the theoretical limits. The high diffraction effects of Cassegrain objectives may require the use of two apertures, one below and the other above the sample, to minimize the diffraction in the transmission mode. 

Size and Shape. The dimensions of the standard are more critical In the microenvironment than In the macroenvironment, since microscopic measurements commonly require changes In field apertures and magnification. If a microscopic standard has a small (/im-slzed), well-defined shape, such as a sphere or cylinder, an accurate Intensity/ volume relationship can be established, which should be Independent of the microscope optics. Standardization Is thus valid no matter what microscope parameters are employed, as long as the spectral characteristics of the standard and the sample are quite similar or Identical. 

One additional point of concern in the extension of the PHPMS technique to elevated pressures is that the criteria normally considered to be necessary for accurate aperture sampling of a high-pressure ion source will probably be violated. A condition of molecular flow through the aperture is generally required for accurate sampling. Molecular flow occurs when the critical dimension (such as the diameter of an orifice or the width of an exit slit) is equal to or smaller than the... 

The electron source of the PIMMS is an argon plasma. Inside the plasma chamber the gas is ionized by a 2.45 GHz microwave field, ignited by an electric spark. In the plasma chamber free electrons are created, that are accelerated by a static electric field for impact ionization of the sample gas atoms. The layout of the plasma chamber has to incorporate both the fluidic and the electrostatic requirements. On the one hand the gas apertures of the chamber must have the appropriate dimensions to assure that the gas flow out of the chamber is low. On the other hand the geometry must be such that most of the electrons are generated close to the outlet of the chamber and can be extracted through this small aperture. Electrons should be generated close to the acceleration field, which intrudes the chamber only to a small depth. 

Notwithstanding the capability for measurement of photoemission spectra at pressures up to 1 mbar, the original construction of the gas cell was not suitable for carrying out experiments under catalytic reaction conditions. To provide the short distance between the sample surface and aperture for the exit of electrons (or a short path of the photoelectrons in the zone at the higher pressure), the equipment supplier decreased the dimensions of the gas cell. A section of the cell, together with the standard sample holder and a typical sample, is shown in Figure 4A this design limits the size of the sample so that only thin polycrystalline foils can be... 

Figure 1, The geometry of source and observation planes P - observation (probe) plane 5 - source (sample) plane r- distance between source and observation planes xo- dimension of the source area yn - dimension of the receiving aperture.

At this point, it may be asked if certain parameters can be changed to decrease the measurement time to allow maps to be acquired in reasonable times. As the size of the remote aperture for most applications is smaller than 250 pm, it is valid to suggest that even smaller detectors should be installed in FT-IR microscopes, so that the SNR is optimized for samples that are 50 pm or smaller in dimension. 

Figure 21. Six sequential images of the same sample area of individual carbocyanine dye molecules spread over a polymethylmethacrylate (PMMA) film as recorded by scanning near field optical microscopy. The image dimensions are 2.3 x 2.7-/nn cut out from 4 x 4-fim records of 256 x 256 data points. The excitation polarization was random in (A-D) and linear along y and x, respectively, in ( ) and (F). The emission polarization was measured along y and x in (B) and (C) and not otherwise. Some fluorescence spots are labeled for discussion in the text. The various shapes of the fluorescence peaks (circular spots, rings, arcs, and double arcs) are striking. These shapes can be explained by molecular dipoles being excited by the inhomogeneous electric field at the aperture. (Adopted from [83].)...

As discussed in Chap. 1, digital images are acquired by sampling the pattern of X-rays transmitted by the breast. In practice, this is often accomplished using a detector that is constructed as an array of discrete detector elements or dels, each of which more or less independently measures the X-rays incident on it The pitch or spacing between dels and the dimensions of the active portion of each del (aperture) in part determine the spatial resolution properties of the imaging system. The concept of spatial resolution and its quantification in terms of the modulation transfer function (MTF) were introduced in Chap. 1. 

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