https://doi.org/10.1351/goldbook.LT07415
For a @[email protected] with sample axis \(Z\) is defined as: \[\Delta A_{\text{l}} = A_{Z} - A_{Y}\] where \(A_{Z}\) and \(A_{Y}\) are the absorption spectra measured with the electric vector of linearly polarized light along and perpendicular to the sample axis, respectively. For an @[email protected] sample \(\Delta A_{\text{l}} = 0\).
Notes:
- Sometimes the reduced @[email protected] is used instead. It is defined as \[\Delta A_{\text{r}} = \frac{A_{Z}\,-\,A_{Y}}{A_{Z}\,+\,2\,A_{Y}} = \frac{A_{Z}\,-\,A_{Y}}{3\,A_{\text{iso}}}\] with \(A_{\text{iso}}\) the @[email protected] @[email protected] Thus, \(\Delta A_{\text{r}}\) is analogous to @[email protected] and the denominator in the equation corresponds to three times the @[email protected] measured in a similar but @[email protected] sample.
- The dichroic ratio \(d(\lambda)\) is also a frequently used function of the @[email protected] It is defined as \[d(\lambda) = \frac{A_{Z}}{A_{Y}}\]
- Most naturally-occurring solid samples exhibit linear @[email protected] It may also be produced in the laboratory by dissolving the sample molecules in anisotropic solvents such as nematic liquid crystals or stretched polymers. This procedure tends to produce uniaxial samples. Also crystals may be used as aligned solvents and if the sample forms suitable crystals by itself these may be used directly. Other molecular alignment techniques include application of electric or magnetic fields.
- @[email protected] is a commonly used technique for the production of aligned samples; both the photoselected subset and the set of remaining molecules may be studied.