Infrared Spectroscopy is the analysis of infrared
light interacting with a molecule.
This can be analyzed in three ways by measuring
absorption, emission and reflection.
The main use of this technique is in organic
and inorganic chemistry.
It is used by chemists to determine functional
groups in molecules.
IR Spectroscopy measures the vibrations of
atoms, and based on this it is possible to
determine the functional groups.5 Generally,
stronger bonds and light atoms will vibrate
at a high stretching frequency (wavenumber).
The use of infrared spectroscopy began in
the 1950's by Wilbur Kaye.
He had designed a machine that tested the
near-infrared spectrum and provided the theory
to describe the results.
Karl Norris started using IR Spectroscopy
in the analytical world in the 1960's and
as a result IR Spectroscopy became an accepted
technique.
There have been many advances in the field
of IR Spec, the most notable was the application
of Fourier Transformations to this technique
thus creating an IR method that had higher
resolution and a decrease in noise.
The year this method became accepted in the
field was in the late 1960's.
Infrared spectroscopy (IR spectroscopy or
Vibrational Spectroscopy) is the spectroscopy
that deals with the infrared region of the
electromagnetic spectrum, that is light with
a longer wavelength and lower frequency than
visible light.
It covers a range of techniques, mostly based
on absorption spectroscopy.
As with all spectroscopic techniques, it can
be used to identify and study chemicals.
For a given sample which may be solid, liquid,
or gaseous, the method or technique of infrared
spectroscopy uses an instrument called an
infrared spectrometer (or spectrophotometer)
to produce an infrared spectrum.
A basic IR spectrum is essentially a graph
of infrared light absorbance (or transmittance)
on the vertical axis vs. frequency or wavelength
on the horizontal axis.
Typical units of frequency used in IR spectra
are reciprocal centimeters (sometimes called
wave numbers), with the symbol cm-1.
Units of IR wavelength are commonly given
in micrometers (formerly called "microns"),
symbol �m, which are related to wave numbers
in a reciprocal way.
A common laboratory instrument that uses this
technique is a Fourier transform infrared
(FTIR) spectrometer.
Two-dimensional IR is also possible as discussed
below.
The infrared portion of the electromagnetic
spectrum is usually divided into three regions;
the near-, mid- and far- infrared, named for
their relation to the visible spectrum.
The higher-energy near-IR, approximately 14000�4000
cm-1 (0.8�2.5 �m wavelength) can excite
overtone or harmonic vibrations.
The mid-infrared, approximately 4000�400
cm-1 (2.5�25 �m) may be used to study
the fundamental vibrations and associated
rotational-vibrational structure.
The far-infrared, approximately 400�10 cm-1
(25�1000 �m), lying adjacent to the microwave
region, has low energy and may be used for
rotational spectroscopy.
The names and classifications of these subregions
are conventions, and are only loosely based
on the relative molecular or electromagnetic
properties.
Infrared spectroscopy exploits the fact that
molecules absorb specific frequencies that
are characteristic of their structure.
These absorptions are resonant frequencies,
i.e. the frequency of the absorbed radiation
matches the transition energy of the bond
or group that vibrates.
The energies are determined by the shape of
the molecular potential energy surfaces, the
masses of the atoms, and the associated vibronic
coupling.
In particular, in the Born�Oppenheimer and
harmonic approximations, i.e. when the molecular
Hamiltonian corresponding to the electronic
ground state can be approximated by a harmonic
oscillator in the neighborhood of the equilibrium
molecular geometry, the resonant frequencies
are associated with the normal modes corresponding
to the molecular electronic ground state potential
energy surface.
The resonant frequencies are also related
to the strength of the bond and the mass of
the atoms at either end of it.
Thus, the frequency of the vibrations are
associated with a particular normal mode of
motion and a particular bond type.
Fourier transform infrared (FTIR) spectroscopy
is a measurement technique that allows one
to record infrared spectra.
Infrared light is guided through an interferometer
and then through the sample (or vice versa).
A moving mirror inside the apparatus alters
the distribution of infrared light that passes
through the interferometer.
The signal directly recorded, called an "interferogram",
represents light output as a function of mirror
position.
A data-processing technique called Fourier
transform turns this raw data into the desired
result (the sample's spectrum): Light output
as a function of infrared wavelength (or equivalently,
wavenumber).
As described above, the sample's spectrum
is always compared to a reference.
An alternate method for acquiring spectra
is the "dispersive" or "scanning monochromator"
method.
In this approach, the sample is irradiated
sequentially with various single wavelengths.
The dispersive method is more common in UV-Vis
spectroscopy, but is 
less practical in the infrared than the FTIR
method.
One 
reason that FTIR is favored is 
called "Fellgett's advantage" or the "multiplex
advantage": The information at all frequencies
is collected simultaneously, improving both
speed 
and signal-to-noise ratio.
Another 
is 
called "Jacquinot's Throughput Advantage":
A dispersive measurement requires detecting
much lower light levels than an FTIR measurement.[3]
There are other advantages, as well as 
some disadvantages,[3] but virtually all modern
infrared spectrometers are FTIR instruments.
