6.2.2 Laboratory Measurements of Rate Constants  6.2 Laboratory Measurements of Chemical Parameters  6.2 Laboratory Measurements of Chemical Parameters

6.2.1 Absorption Cross Sections

Absorption of a photon of radiation corresponds to a change in the internal state of a molecule between two energy levels. The energy of the transition is precisely determined by the separation of the energy levels. A good overview of atomic and molecular spectroscopy is to be found in the textbook on Physical Chemistry by Atkins. The absorption cross section is proportional to the intensity of the absorption (or emission) between the two levels involved. The absorption of light is in general governed by the Beer-Lambert Law, for optically thin samples:
 

$$ln\left({I_o(\lambda )\over I(\lambda )}\right) = \sigma (\lambda) \times l\times c$$ (6.6)

 where $I(\lambda)$ and $I_o(\lambda)$ are the transmitted light intensities at a wavelength $\lambda$ with and without sample present, l  is the path length, and c is the concentration. The constant $\sigma$ is the absorption cross section. If l is in centimeters, and c in molecules cm^-3, then $\sigma$ has units of cm² molecule^-1. In the older literature, the absorption coefficient is sometimes given in base 10 and molar units, in which case it is given the symbol $\epsilon$. The Beer-Lambert Law forms the basis of all laboratory determinations of absorption cross sections. In the literature on atmospheric sensing, one also encounters the symbol $\tau$. This is simply the product $\sigma cl$ summed over all the absorbing species, i.e., the total optical depth along the line of sight. Following the absorption of a photon, the atom or molecule can re-emit radiation at a characteristic frequency. This forms the basis of the fluorescence techniques described later.

A typical laboratory experiment to measure absorption cross sections requires a light source, an evacuable absorption cell, some device for selecting the wavelength, and a detector. The wavelength selector can be dispersive (prism, grating) or interferometric (see Figure 6.1). If a laser is used as the light source, the inherent high monochromaticity of the laser normally circumvents the requirement for a wavelength selector (although an independent measurement of the wavelength is sometimes necessary). The principles described here are common to measurements throughout the infrared, visible and ultraviolet spectral regions. A major factor influencing the complexity of a spectrum is the existence of individual rotational lines, whose separation and width are both often comparable to the resolution of the instruments. An atmospheric absorption or emission spectrum will consist of the superposition of the individual lines of many species. As described in the section on satellite measurements, large databases of spectral parameters exist to assist in the interpretation of atmospheric spectra.

Figure 6.1:   Schematic of apparatus used to measure absorption cross sections in the ultraviolet and visible spectral regions.

  
 6.2.2 Laboratory Measurements of Rate Constants  6.2 Laboratory Measurements of Chemical Parameters  6.2 Laboratory Measurements of Chemical Parameters 
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