16. Polarization Spectroscopy of Ordered Samples
16.2 OCCURRENCE, PRODUCTION, AND OPTICAL
PROPERTIES OF ALIGNED, SOLID SAMPLES
16.2.1 Perfectly and Partially Aligned Samples
While the molecular orientation distribution in liquid solutions is isotropic, many
naturally occurring samples have some kind of order (are aligned). This is not
only the case for crystals; many biological samples, for example, from plants or
animals, have alignment, although this alignment tends to be only partial, not
perfect. Also, many industrial products are aligned samples: the strength of
common plastic bags, for example, is due to the fact that they consist of two
crossed layers of stretched or extruded, in other words, aligned polyethylene.
Such polyethylene material cannot be stretched further in the direction of
stretching; it is therefore very strong in this direction, and by crossing two layers
a very sturdy material is produced. The alignment in the polyethylene bags and
in many naturally occurring or industrial samples is not perfect, but only partial.
However, it turns out that such samples, especially those produced by means of
aligning (solid) solvents, are often the most useful. Therefore, after a brief
discussion of other alignment techniques, we shall concentrate on solutes that are
partially aligned in solid solvents.
Intuitively, one might expect that perfectly aligned crystals would provide
ideal samples for LD spectroscopy. It is also true that both single crystals and
mixed crystals have been used for polarization spectroscopic purposes [2].
However, spectroscopic experiments with polarized light on crystalline samples
have often been difficult, for example, because very thin slices of single crystals
are required, especially for studies of electronic transitions. An alternative
possibility, to measure reflection spectra, leads to new complications. In
addition, interpretation of the spectra in terms of single molecular transitions
requires knowledge of the crystal structure, as well as of the often very strong
intermolecular interactions (Davydov splitting).
Instead, mixed crystal methods have sometimes been used. This requires that
a host crystal be found that does not absorb in the region of interest and in which
the molecule to be studied fits perfectly, without changing the crystal structure
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In all two-photon experiments, new information may be obtained, both on
the sample structure and on the optical properties of individual molecules. While
the only information on the sample structure that may be provided by absorption
spectroscopy are average values over the sample molecules of cosine squares of
angles between transition moments and the sample axis, two-photon
spectroscopy may in addition provide averages of the fourth powers of such
angles. This is important information; one may say that while LD absorption
spectroscopy provides information on the average molecular alignment, the
addition knowledge from two-photon spectroscopy may provide information
about the spread of the molecular orientation distribution function around these
average alignments.