Page 190 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GRB/MBQ P2: GLM Final Pages
Encyclopedia of Physical Science and Technology EN007C-340 July 10, 2001 14:45

Infrared Spectroscopy 801

gas-phase contours of A and C bands have broad, more B. Grating Spectrophotometers
or less symmetric wings and a sharp central peak. The B
Most spectra seen in the literature are of the ratioed or
band has more or less symmetric wings, but is unique in
double-beam type. A double-beam grating instrument is
that it has no central peak. The contours of all these bands
called a spectrophotometer. In this type of instrument the
are dependent on the relative values of the moments of
beam from the source is divided into two beams: a sample
inertia. If the moment of inertia for rotation about the c
beam and a reference beam. The sample is placed in the
axis is relatively large, as in planar molecules, the central
sample beam, and the two beams are alternately passed
peak of the C band is relatively strong compared with the
into the monochromator through the entrance slit, usually
central peak of the A band, as seen in Fig. 8. If the dipole
at 13 Hz.
moment change is not exactly parallel to any of the a, b,
In a monochromator, radiation from the entrance slit
or c axes, a mixed contour results.
goes to a paraboloidal mirror, which makes the radiation
parallel. The parallel radiation goes to a diffraction grat-
ing, which consists of a reflecting surface with straight
II. INSTRUMENTATION a parallel grooves very closely spaced. Each of these
grooves acts as an independent slitlike source of radia-
A. Infrared Spectrometers tion, diffracting it in different directions. The radiation
from the grating is focused onto the exit slit, and only ra-
Infrared spectrometers come in a variety of types but have
dition leaving the grating at the specific angle goes in a
many common features. All have a source that emits all
direction that can pass through the exit slit (Fig. 9). When
the IR radiation of interest. These are usually various solid
radiation leaves the grating at that angle, parallel beams
materials heated to incandescence by an electric current.
coming from any two adjacent grooves have traveled dif-
The radiation energy distribution as a function of wave-
ferent distances and, for one particular wavelength of ra-
length approaches that of a theoretical black body where
diation, will be exactly one wavelength ahead or behind
the energy reaches amaximum at a wavelength (µm) equal
one another. This means that beams of this wavelength
to 2897/T , where T is the absolute temperature (K). The
leaving at this angle from all the grooves will be in phase
operational temperature is such that the radiation energy is
and show constructive interference when they converge at
usually at a maximum near the short-wavelength limit of
the exit slit. Other wavelengths will not be in phase and
the spectrum (usually ∼2 µm) and decreases as the wave-
will show destructive interference at this angle. This is
length gets longer. In the far-IR region, source energy is
called the first order. When parallel beams coming from
very low.
any two adjacent grooves are two, three, or more wave-
All spectrometers must have some kind of detector.
lengths ahead or behind one another, the parallel beams
These are devices that, in one way or another, change
from all the grooves will also be in phase. These are called
radiation energy into an electrical signal that can be ampli-
the second, third, or higher orders. Unwanted grating or-
fied and processed to yield a spectrum. Thermal detectors
ders are removed with filters. This means that, for one
measure the heating effect of the radiation and respond
grating angle, essentially monochromatic radiation leaves
equally to all wavelengths. Examples include thermocou-
ples,bolometers,andpyroelectricdetectors.Detectorsthat
utilize photon energy to free bound electrons in the detec-
tor material are called photodetectors. Photodetectors, un-
like thermal detectors, do not respond to all wavelengths
but have a long-wavelength limit where the photon has
insufficient energy to excite the electrons. One example
is the photoconductive detector, in which the absorption
of photon energy promotes bound electrons to free states.
This results in increased electrical conduction.
In between the source and detector, the spectrometer
must have some means of analyzing the radiation so that
an intensity can be deduced for each wavelength res-
olution element. Two completely different types of de- FIGURE 9 The grating surface is shown enlarged with incoming
vices are used, namely, monochromators and interferom- radiation from the entrance slit and reflected radiation going to
the exit slit. For adjacent grooves, the beams at these angles
eters. Monochromators with gratings or prisms are used
have a path length difference. When this equals one wavelength
in dispersive instruments, and interferometers are used in of radiation as shown, all the grooves will emit radiation of that
Fourier transform instruments. wavelength in phase toward the exit slit.