Wavenumber precision

Wavenumber precision and low noise levels allow spectra with slight differences to be subtracted from each other to expose those differences. Fourier transform instruments are not as accurate as dispersive spectrometers for measuring transmittance. 

A third advantage is proffered by the laser referencing system that affords high wavenumber precision (the Connes advantage [23]), aiding the attain-... 

A third advantage FTlRs enjoy over other infrared spectrometers is wavenumber precision (remember that precision is a measure of reproducibility and is not the same thing as accuracy [2]). It is vital that the wavenumbers, and hence peak positions, in an infrared spectrum be measured reproducibly. FTlRs contain a laser that acts as an internal wavenumber standard. This allows the wavenumbers in a measured FTIR spectrum to be determined with a precision of 0.01 cm. The reasons for this will be discussed in detail in Chapter 2. 

An analysis of the Fourier Transform i.r. spectrum of PF around 946 cm-1 gave rise to nine spectroscopic constants for the Vg band and five for the Vg + v -v band which allowed calculation of the wavenumbers of the Vg band with a precision of 1 x 10-3 cm-1.11... 

Included in Table III is the comparison of the transition frequencies calculated from the energies obtained in our calculations with the experimental transition frequencies of Dabrowski [125]. To convert theoretical frequencies into wavenumbers, we used the factor of 1 hartree = 219474.63137 cm . For all the frequencies our results are either within or very close to the experimental error bracket of 0.1 cm . We hope that the advances in high-resolution spectroscopy will inspire remeasurements of the vibrational spectrum of H2 with the accuracy lower than 0.1 cm. With such high-precision results, it would be possible to verify whether the larger differences between the calculated and the experimental frequencies for higher excitation levels, which now appear, are due to the relativistic and radiative effects. 

For a precise determination of the structures, ab initio calculations on the energies of formation and characteristic harmonic wavenumbers have been performed at the HF and MP2/6-31G(d,p) level of theory with the Gaussian 92 program for the species of interest . The ab initio calculated energies and harmonic vibrational wavenumbers are given in Table 5. 

In Table V we summarize the spectra of the principal adsorbed species on Pt surface planes and include more precise band positions for the di-ct (158) and ethylidyne species (159) from RAIR spectra. The spectra are listed in the order of increasing temperatures for the adsorbed species. As we shall see, the patterns of spectra for the n- and 7i -species and for ethylidyne vary little from metal to metal, but that of the di-cr species is more variable, with other metals giving an increased wavenumber of ca. 1140 cm-1 for the strongest feature replacing 1040/980 cm 1 for Pt (17). The other surface species, except for CCH are rarely found on other metals. 

The precise definition of resolution depends on the lineshape, but usually resolution is taken as the full line width at half maximum intensity (FWHM) on a wavenumber, 8v, or frequency, 8v, scale. 

Table 7.1 are continuous wave (CW), not pulsed. Second, frequency stability to < 1 cm" is important to assure Raman shift precision and avoid line broadening. Although the Raman shift axis is usually calibrated periodically, the laser frequency must remain stable between calibrations. Third, lasers vary significantly in output linewidth, from hundreds of reciprocal centimeters to much less than 1 cm". For the majority of samples of analytical interest, a laser linewidth below 1 cm" is sufficient. Laser linewidths are often quoted in terms of frequency rather than wavenumber, in which case 1 cm" equals 30 GHz. Lasers are available with < 1 MHz linewidths (< 10 em ), but such lasers would be unnecessarily narrow for most analytical Raman applications. Fourth, lasers differ in their output of light at wavelengths other than the laser line itself. Gas lasers (Ar+, Kr+, He-Ne) emit atomic lines (plasma lines), and solid-state lasers luminesce, both of which can interfere with Raman scattering. Essentially all lasers require a bandpass filter or monochromator to reduce these extraneous emissions. 

Nicolet and Laurence11 studied the temperature dependence of the wavenumber of the lowest energy absorption band of the relevant probes. In the gas phase the effect was negligible, but not so in both non-EPD solvents (perfluorodecalin, n-heptane and 1,2-dichloroethane) and in EPD/HBA solvents (dibutyl ether, HMPA and triethylamine). The hypsochromic effect noted was 5 to 17 cm-1 K-1 for 8 and 3 to 8 cm-1 K-1 for 7. This magnitude (compared with /iS 7 = 2759 cm-1 at 25 °C for HMPA) is of little consequence for measurements near ambience except for the most precise requirements and is commonly ignored in ordinary work. 

Conners Advantage (Frequency Precision). The dispersive instrument depends on calibration (polystyrene at 1601 cm 0, and the ability of gears and levers to move slits and gratings in a reproducable fashion. The FTIR carries its own internal frequency standard, usually a He-Ne gas laser, that serves as the master timing clock, tracking mirror movement and frequency calibration to a precision and accuracy of better than 0.01 wavenumbers (cm ). 

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