For a uniaxial sample with sample axis Z is defined as: \[\Delta A_{\text{l}} = A_{Z} - A_{Y}\] where AZ and AY are the absorption spectra measured with the electric vector of linearly polarized light along and perpendicular to the sample axis, respectively. For an isotropic sample ΔAl = 0.
Notes: - Sometimes the reduced @DT07357@ 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 Aiso the @I03353@ @A00028@. Thus, ΔAr is analogous to @ET07370@ and the denominator in the equation corresponds to three times the @A00028@ measured in a similar but @I03353@ sample.
- The dichroic ratio d(λ) is also a frequently used function of the @W06659@. It is defined as \[d(\lambda) = \frac{A_{Z}}{A_{Y}}\]
- Most naturally-occurring solid samples exhibit linear @DT07357@. 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.
- @PT07461@ is a commonly used technique for the production of aligned samples; both the photoselected subset and the set of remaining molecules may be studied.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 365 (https://doi.org/10.1351/pac200779030293)