The wavelength, frequency, or photon energy can be used to characterize electromagnetic radiation.
As background to specific spectroscopic studies, we first consider the regions of electromagnetic radiation and the ways in which absorptions or emissions are related to molecular level phenomena. Electromagnetic radiation, of which visible light is in example, can be treated in terms of the frequency v of the oscillating electric or magnetic effects that the radiation falls. This frequency and the speed with which the radiation can produce in a receiver on which the radiation falls. This frequency and the speed with speed with which the radiation falls. This frequency and the speed with which the radiation falls. This frequency and the speed with which the radiation travels, c = 2.99792 × 108 m s-1 in vaccum, let us deal also with the wave length λ of the radiation. Since the frequency is equal to the number of waves in the radiation of an approaching beam that can arrive at a receiver in 1 s, and this number is equal to c/λ, we can write
v = c/ λ or λ = c/v
The range of the frequencies and wavelengths that we will deal with, and the names of regions within these ranges. [In that figure some values of the frequently like quantity known as the wave number v? are included. This quantity is defined as v? = 1/( λ , in cm) or v? = v/(3 × 1010 cm s-1). You can see that v?has convenient numerical values in the infrared region.]
When electromagnetic radiation is treated as a stream of energy packets, called photons, it can be characterized by the energies of the photons. These energies are related to the wave model quantities through the Planck relation
ε = hv with h = 6.6262 × 10-34 J s
For chemical purposes we sometimes deal with the energy, in kilojoules, of 1 mol of photons. Some photon energies in these units are included in this aspect.
Example: when we deal with visible and ultraviolet radiation, we often express the wave length in units of nanometers and the photon energy in units of kilojoules per mole. Obtain the relation between these two quantities.
Solution: we begin with the fundamental relations ε = hv and v = c/ λ to obtain ε = hc/ λ .
Then unit changes lead to
E(kJ mol-1) = (6.6262 × 10-34 J s)(2.9979 × 108 m s-1)(6.0220 × 1023)/(1000 J/kJ)( λ × 10-9)
= 119,620/ λ in nanometers
The regions of electromagnetic radiation that are recognized are based primarily on the experimental techniques used in the generation, dispersion, or detection of the radiation. The "visible" region, for example, is recognized because of the range of our detection system. The regions are also helpful in organizing the molecular level phenomena studied by spectroscopic using the radiation in the particular regions.
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