Spectrum wavelength calibration

Standard data reduction, i.e. bias and flat field correction, has been performed with Iraf. The Iraf task APEXTRACT/APALL was used to extract the spectra, with interactively selected background sampling, in order to avoid contamination for the star spectrum. The wavelength calibration has been done using daily He, Ne, HgCd arcs, and, in order to improve the calibration, wavelengths values for the transitions used were taken from http //physics.nist.gov/. 

Wavelength accuracy. In order to evaluate the ability of each system to locate spectral lines, a preliminary wavelength calibration was carred out with the emission spectrum of a mercury pen lamp and then the peak maxima of several atomic lines from an iron hollow cathode lamp were located. The root mean square (RMS) prediction error, which is the difference between the predicted and the observed location of a line, for the vidicon detector system was 1.4 DAC steps. Because it is known from system calibration data that one DAC increment corresponds to 0.0125 mm, the absolute error in position prediction is 0.018 mm. For the image dissector, the RMS prediction error was 7.6 DAC steps, and because one DAC step for this system corresponds to 0.0055 mm, the absolute error in the predicted coordinate is 0.042 mm. The data in Table II represent a comparison of the wavelength position prediction errors for the two detectors. 

If in doubt, check that the wavelength thought to be the one giving optimal sensitivity does indeed do so. Alternatively check the wavelength calibration in that vicinity using a hollow cathode lamp of an element with a much simpler, and thus unambiguous, spectrum in the region of interest. 

The first and/or second dye lasers were tuned to the specific wavelength(s) to populate the desired level(s). The final laser in the excitation sequence (either the second or third laser) was then continuously scanned to obtain the Rydberg or autoionization spectrum. The spectrum and wavelength calibrations were recorded simultaneously on a two pen recorder. Wavelength calibration was obtained by directing a portion of the scan laser radiation to a monochromator that was preset at known U or Th emission lines from an electrodless lamp. 

Figure 3. Photoionization threshold spectra for neodymium. The excitation scheme used in each case is shown on the figure. The scanned laser wavelength calibration is shown at the top of each spectrum. In (a) the 20 300.8 cm 1 level is populated and in (b) the 21 572.6 cm 1 level is populated. The threshold wavelengths indicated yield the same ionization limit value of 5.523 eV. The arrows labeled R. L. indicate the position of the Rydberg convergence limit (3).

The second harmonic output of an excimer (Lambda Physik EMG 201 MSC) pumped-dye laser (Lambda Physik FL 3002) is focused into the photoionization region by a 2(X)-mm, focal-length, fused-silica lens. The wavelength calibration is made using the known ZEKE spectrum of atomic sulfur [194], which is produced by the multiphoton laser photodissociation of HjS. 

Figure 12, Infrared differ-ence spectrum of carbonyl cytochrome c oxidase vs. oxidized oxidase. Water vapor hand at right used for wavelength calibration. Insert at bottom Soret and visible spectra of the CO derivative in the Cap2 cell used to obtain the infrared spectrum.

IR spectrometers must be calibrated for wavelength accuracy. FTIRs are usually calibrated by the manufacturer and checked on installation. Wavelength calibration can be checked by the analyst by taking a spectrum of a thin film of polystyrene, which has well-defined absorption bands across the entire mid-IR region, as seen in Fig. 4.1. Polystyrene calibration standard films are generally supplied with an IR instrument or can be purchased from any instmment manufacturer. Recalibration of the spectrometer should be left to the instmment service engineer if required. 

However, the physical transfer of spectra between instruments is only one step in the complex chain of the standardization in spectra. The ideal is that a given sample provides a constant spectrum for a given physical state and a defined set of recording and sampling conditions. In the past, it was considered adequate to run a simple calibration standard, such as polystyrene. This is often sufficient as a simple validation of an instrument s performance relative to a prerecorded norm. However, it is not adequate for, and does not constitute, instrument standardization. Standardization implies a unified control of parameters, such as spectral resolution and band shape, actual spectral line position (wavelength calibration), and photometric recording accuracy, and all things that can impact these parameters in a practical measurement. 

Continuous xenon (Xe) arc lamps are very high intensity visible light emitters with good yield of UV which decreases in intensity from about 400 nm downwards emission also extends out into the near IR (Fig. 14.3c). Emission is a continuum with some high intensity emission lines from xenon, particularly around 420 80 nm and in the near IR, superimposed. Most fluorimeters use Xe-arc lamps as excitation source and these emission lines can be used for internal wavelength calibration e.g., the line at 467 nm can be used to calibrate the position of the excitation monochromator in a fluorimeter. Xe-arc lamps also make excellent high intensity irradiation sources for photochemical reactors. When fitted with a suitable Air Mass filter they give a reasonable approximation to the solar spectrum and are widely used in solar simulators. 

Figure 12 Calibration spectrum used for the CCD echelle spectrograph. The lower curves represent the white-light response for the different echelle orders used. The lines represent atomic emission lines from a neon lamp used for wavelength calibration. (Adapted with permission from Ref. 133.)...

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