Page 12 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

Analytical Chemistry 551

components, the components can pass through the sample thickness of ultrathin films. Circularly or elliptically polar-
and be unequally retarded so that the recombination after ized light is reflected from a surface on which the sample
exiting the sample results in constructive interference with is coated. The reflection through the sample alters the po-
a phase difference from that originally observed. This re- larization, and the degree of alteration provides thickness
sults in a rotation of the plane of polarization and propaga- data to 0.1-nm precision. These instruments have wide ap-
tion of circularly or elliptically polarized light dependent, plicability in the semiconductor industry and as tools for
1
1
respectively, on phase separations of multiplies of 0, , , the measurement of organic films of monolayer dimen-
4 2
or combinations of these numbers. sions.
Polarimetry is a technique suitable for structural de-
termination and quantitative analysis and simply studies Ultraviolet–visible. This form of electromagnetic
the direction and degree of the rotation of plane-polarized radiation is of sufficient energy to cause electronic ab-
monochromatic radiation of transparent substances. In- sorption transitions from the ground state to an excited
struments generally make use of vapor lamps, which iso- state within molecules. In contrast to sharp atomic elec-
late the sodium D line or mercury line at 546 nm. The light tronic absorptions, molecular spectra show broad bands
is then processed by a Nicol polarizer prism assembly, covering tens of nanometers due to the availability of a
passed through a half-shadow Lippich prism (to assist de- multitude of distinct energy states caused by the overlap
tection of the optical minimum intensity), passed through of the vibrational and rotational energy sublevels with the
a thermostated transparent sample holder, passed through primary electronic level. Only high-resolution spectrom-
a second Nicol prism known as the analyzer, and directed eters can clearly resolve these energy levels.
to an eyepiece or photoelectric detector. Rotation of the The design of a typical double-beam instrument that al-
analyzer allows matching of the split-field bright image. lows for relative background correction is schematically
The instrument is calibrated by analyzing the rotation of shown in Fig. 6 and consists of a radiant source, such as
a standard solution of known concentration and rotatory a hydrogen–deuterium lamp for ultraviolet and a tung-
power and provides precision to 0.001 when coupled to sten lamp for visible radiations; a filter, prism, or grating
◦
photoelectric detectors. monochromator for wavelength selection; sample and ref-
Other techniques that find widespread application are erence cells; and a photocathode, usually associated with
optical rotatory dispersion and circular dichroism. These a photomultiplier for detection.
are closely related methods that rely on the wavelength The most important types of electronic transition in-
dependence of refractive index and molar absorptivity, re- volve the following:
spectively, toward the direction (d or l ) of circularly po-
larized radiation. Optical rotatory dispersion experiments 1. n, σ, and π electrons, covalent compounds
provide a measurement of molecular rotations as a func- 2. Charge-transfer electrons
tion of wavelength. Circular dichroism results are pre- 3. Electrons, ligand field strength
sented in a plot of wavelength vs a value [θ], and [θ]is 4. f electrons, lanthanide and actinide ions
a corrected value of θ, the molecular ellipticity measured
as the singular difference between the emergent ellipti- Of particular importance are those transitions involving
cal beam and the original incident plane of polarization. bonding electrons, since particular absorption energies are
These techniques have been important tools in the elucida- characteristic of the bonding of certain functional groups,
tion of structural information about complex organic and known as chromophores, providing chemical identifica-
biological systems and metal–ligand complexes. tionandconcentrationinformation.Asummaryofapprox-
The use of elliptically polarized light has also found imate electronic energy transitions for bonding and non-
application in the determination of refractive index and bonding electrons is given in Fig. 7. This can be correlated

FIGURE 6 Conventional double-beam ultraviolet–visible absorption spectrophotometer.