Spectrometer bandwidth

The photoabsorption cross section Is measured by the split-beam technique, l.e., by simultaneously monitoring the light Intensities both before and after the photon beam passes through the heat-pipe oven. The spectrometer bandwidth was set at 0.08 nm In the present measurement. 

Although the relative intensities of spectral lines in the ICP differ from those observed in the DC arc and AC spark, the published tables are invaluable for the selection of analyte lines in ICP sources, and the identification of spectral interferences in the spectrometer bandwidths. However, spectral lines are emitted by ICP sources that are not emitted by DC arcs and sparks. In order to facilitate spectral line selection in ICP-AES, numerous spectral line atlases are now available which list the best analytical lines and the potential interferences due to coincidences from major and minor constituents. Simulated... 

For a lamp-monochromator combination as an irradiation source for photochemical reactions the compromise is between maximum photon flux and spectral selectivity. Due to the need for a high photon flux, irradiations for photochemical reactions are usually carried out using wider bandwidths than those used in absorption or emission spectrometers. Bandwidth is a particularly important consideration when using sources that contain some line emission such as Hg or Xe lamps, since line emission within the bandwidth can dominate even though it is not the wavelength shown on the monochromator setting or readout. 

In practice, the NEP of a room-temperature THz spectrometer is usually limited by fluctuations (shot-noise) in the ambient blackbody radiation. Usmg an optical bandwidth Av = 3 THz (limited by, for example, a polyethylene/diamond dust window), a field of view (at nomial incidence) 0 = 9 and a detecting diameter (using a so-called Winston cone, which condenses the incident radiation onto the detecting element) laboratory applications, the background-limited NEP of a bolometer is given by... 

A microwave pulse from a tunable oscillator is injected into the cavity by an anteima, and creates a coherent superposition of rotational states. In the absence of collisions, this superposition emits a free-mduction decay signal, which is detected with an anteima-coupled microwave mixer similar to those used in molecular astrophysics. The data are collected in the time domain and Fourier transfomied to yield the spectrum whose bandwidth is detemimed by the quality factor of the cavity. Hence, such instruments are called Fourier transfomi microwave (FTMW) spectrometers (or Flygare-Balle spectrometers, after the inventors). FTMW instruments are extraordinarily sensitive, and can be used to examine a wide range of stable molecules as well as highly transient or reactive species such as hydrogen-bonded or refractory clusters [29, 30]. 

Isotope shifts for most elements are small in comparison with the bandwidth of the pulsed lasers used in resonance ionization experiments, and thus all the isotopes of the analyte will be essentially resonant with the laser. In this case, isotopic analysis is achieved with a mass spectrometer. Time-of flight mass spectrometers are especially well-suited for isotopic analysis of ions produced by pulsed resonance ionization lasers, because all the ions are detected on each pulse. 

The narrowest PFI bands in the present study are 3 cm-1 FWHM, using a 0.5 V/cm extraction field with the lasers attenuated to minimize effects of space charge. We measure band positions at the intensity maxima. These are reproducible to better than 1 cm-1. The bandwidth is limited by the rotational contour and also by the ionization process. A major advantage of ZEKE-PFI over more traditional photoelectron techniques is that the energy calibration is that of the tunable dye lasers, which are quite stable from day to day. In contrast, both electrostatic analyzers and time-of-flight photoelectron spectrometers require frequent calibration. 

At first sight, this may appear to be a lousy function to excite evenly all the frequencies in a spectrum but because we use such a short pulse, we only use the bit of the function around x = 0. The first zero-crossing point is at 1/(2 x pulse width) - this would be at about 150 kHz for a 3 qs pulse. For a 400 MHz spectrometer, we need to cover a bandwidth of about 8 kHz for a proton spectrum. As Figure 3.4 shows, there is minimal power fall off for such a small pulse. 

Artifacts may be roughly categorized into those due to inherent limitations (e.g. pulses cannot excite unlimited bandwidths even if all hardware components work perfectly) and those that result from improper set-up of the experiment or nonideal functioning of the NMR spectrometer system. In this chapter we will mainly focus on the latter two. These artifacts are more likely to appear in multiple-pulse experiments. Quite often, they are avoided by clever programming of the experiments (e.g. interleaved acquisition of data for NOE spectra, use of pulsed-field gradients instead of phase-cycling). 

X-Ray Absorption Spectroscopy (XAS). The XAS measurements were similar to those described elsewhere.Grazing incidence (GI)-XAS measurements were performed at beamline 11-2 at Stanford Synchrotron Radiation Laboratory (SSRL). A double Si(220) crystal spectrometer was used to select the energy of the synchrotron X-rays, and the beam size was set to 400 pm x 2 mm. The bandwidth of the spectrometer was about 1 eV. Routine procedures were used to optimize the positions of the samples so that the angle of incidence was about 0.17°, with the X-ray... 

The Fourier frequency bandpass of the spectrometer is determined by the diffraction limit. In view of this fact and the Nyquist criterion, the data in the aforementioned application were oversampled. Although the Nyquist sampling rate is sufficient to represent all information in the data, it is not sufficient to represent the estimates o(k) because of the bandwidth extension that results from information implicit in the physical-realizability constraints. Although it was not shown in the original publication, it is clear from the quality of the restoration, and by analogy with other similarly bounded methods, that Fourier bandwidth extrapolation does indeed occur. This is sometimes called superresolution. The source of the extrapolation should be apparent from the Fourier transform of Eq. (13) with r(x) specified by Eq. (14). 

Atomic emission instruments have resolving powers in the order of 30000 to 100 000. The use of a wide exit slit (bandwidth perceived by the photomultiplier tube) degrades the resolving power of the spectrometer. 

The CD spectrometer is usually required to work near the limits of sensitivity—e.g., reading AA values of <10 4 at a total absorbance of 1. Thus, particular care needs to be taken with cleanliness and orientation of cells and with settings of scan rate, time constant, and bandwidth. It is also important, especially when recording far-UV spectra, that the lamp is not old and that the mirrors are not clouded from radiation and traces of ozone. Because the spectrometer is a single-beam instrument, it is essential always to watch carefully for evidence of instrumental drift during measurements of sample and baseline. 

Figure B3.6.1 Rayleigh and Raman bands in fluorescent spectra, as seen in scans for solvent baseline and hen egg white lysozyme (EWL) solutions (solid lines). Circles represent the spectrum of EWL with baseline subtracted. Parameters EWL A2ao = 0.05 Xex = 280 nm excitation and emission bandwidths, 2.5 nm scan rate, 100 nm/min five scans accumulated. Spectra were measured using a Perkin Elmer LS50B fluorescence spectrometer.

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