Wavelength and Wavenumber

The effect of this wavekngth-to-wavenumber conversion is iUusIrated in Appendix I. Mullqdicadon by X results in a selective enhancement of the long-wavelength side of the emission, and there is a shift in the apparent emission maximum. It should be noted that even afierthis correction is petfotmed, the resolution of the spectrum sdll varies with wavenumber. 

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All infrared spectrophotometers are provided with chart recorders which will present the complete infrared spectrum on a single continuous sheet, usually with wavelength and wavenumber scales shown for the abscissa and with absorbance and percentage transmittance as the ordinates. More advanced instruments also possess visual display units on which the spectra can be displayed as they are recorded and on which they can be compared with earlier spectra previously obtained or with spectra drawn from an extensive library held in a computer memory. These modern developments have all led to quantitative infrared spectrophotometry being a much more viable and useful analytical procedure than it was just a few years ago. 

Understand the relationship between the energy of a transition and its frequency, wavelength and wavenumber... 

The following table provides a conversion between wavelength and wavenumber units, for use in infrared spectrophotometry. 

Note The values in Tables 2-1 and 2-2 are expressed as wavelengths and wavenumbers in air. The difference between Av (air) and Av (vacuum) is usually less than 1cm-1 and can be ignored in Raman spectroscopy. When molecular constants are calculated from absolute Raman frequencies, Av (air) must be converted to Av (vacuum). 

Figure 9.1 Energy, frequency, wavelength and wavenumber ranges of electromagnetic waves. Frequency range of molecular vibrations is in infrared region close to visible light. (Reproduced from A. Fadini and F-M. Schnepel, Vibrational Spectroscopy Methods and Applications, Elbs Horwood, Chichester, 1989.)...

Spectroscopy is the study of the interactions between radiant energy and matter. The frequencies, wavelengths and wavenumbers associated with different regions of the electromagnetic spectrum are shown in Fig. 10.4. 

It is common to use wavenumber, symbolized by either v or v, with units of cm, in describing IR spectra. The first symbol is called nu tilde the second symbol is called nu baf both symbols are used in the literature. The unit cm is called a reciprocal centimeter. The wavenumber is the reciprocal of the wavelength. Wavenumber is the number of waves of radiation per centimeter, 1 /A frequency is the number of waves per second, c/A. Wavelength and wavenumber are related by ... 

The frequency of modulation is therefore proportional to the velocity of the mirror and inversely proportional to wavelength of the incident radiation. The frequency is therefore also proportional to the wavenumber of the incident radiation, as we know from the relationship between wavelength and wavenumber. 

The units of wavenumber are almost always chosen as reciprocal centimeters (cm ), so we can picture the wavenumber of radiation as the number of complete wavelengths per centimeter. The frequencies, wavelengths, and wavenumbers of the various regions of the electromagnetic spectrum were summarized in Fig. F.7. In this chapter we concentrate on vibrational and electronic transitions, which can be excited by the absorption of infrared and ultraviolet-visible radiation, respectively. 

The following table provides a conversion between wavelength and wavenumber units, for use in infrared spectrophotometry (Refs. 1 and 2). Because spectra are presented in different formats, this table is an aid in interpretation. 

Interconvert given values of energy, frequency, wavelength and wavenumber units. (Question 5.1)... 

The Speed, Frequency, Wavelength, and Wavenumber of an Electromagnetic Wave... 

If the speed, frequency, wavelength, and wavenumber of an electromagnetic wave, including a light wave, in a medium is denoted, respectively, by c, v, A, and v, the following relations exist between them. 

It is clear from Equation (A3) that the speed c of an electromagnetic wave in a medium changes from that in vacuum Cq, but the dependence of the frequency v, wavelength A, and wavenumber v on the medium is not self-explanatory in Equations (Al) and (A2). It should be remembered that the wavelength and wavenumber depend on the medium but... 

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