24-01-2013, 10:59 AM
Spectroscopy its Application
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Introduction
Spectroscopy
Spectroscopy is the study of matter
and its properties by investigating
light, sound, or particles that are
emitted, absorbed or scattered by the
matter under investigation.
Spectroscopy may also be defined as
the study of the interaction between
light and matter. Historically, spectroscopy referred to a branch of science in
which visible light was used for theoretical studies on the structure of matter and
for qualitative and quantitative analyses. Recently, however, the definition has
broadened as new techniques have been developed that utilize not only visible
light, but many other forms of electromagnetic and non-electromagnetic
radiation: microwaves, radiowaves, x-rays, electrons, phonons (sound waves)
and others. Impedance spectroscopy is a study of frequency response in
alternating current.
Spectroscopy is often used in physical and analytical chemistry for the
identification of substances through the spectrum emitted from them or absorbed
in them. A device for recording a spectrum is a spectrometer. Spectroscopy can
be classified according to the physical quantity which is measured or calculated
or the measurement process.
Spectroscopy is also heavily used in astronomy and remote sensing. Most large
telescopes have spectrographs, which are used either to measure the chemical
composition and physical properties of astronomical objects or to measure their
velocities from the Doppler shift of spectral lines.
Physical Quantity Measured
The type of spectroscopy depends on the physical quantity measured. Normally, the
quantity that is measured is an amount or intensity of something.
· The intensity of emitted electromagnetic radiation and the amount of absorbed
electromagnetic radiation are studied by electromagnetic spectroscopy (see also
cross section).
· The amplitude of macroscopic vibrations is studied by acoustic spectroscopy and
dynamic mechanical spectroscopy.
What is Spectroscopy?
Spectroscopy pertains to the dispersion of an object's light into its component
colors (i.e. energies). By performing this dissection and analysis of an object's
light, astronomers can infer the physical properties of that object (such as
temperature, mass, luminosity and composition).
But before we hurtle headlong into the wild and woolly field of spectroscopy, we
need to try to answer some seemingly simple questions, such as what is light?
And how does it behave? These questions may seem simple to you, but they have
presented some of the most difficult conceptual challenges in the long history of
physics. It has only been in this century, with the creation of quantum mechanics
that we have gained a quantitative understanding of how light and atoms work.
You see, the questions we pose are not always easy, but to understand and solve
them will unlock a new way of looking at our Universe.
The Nature of Light
To understand the processes in astronomy that generate light, we must realize
first that light acts like a wave. Light has particle-like properties too, so it's actually
quite a twisted beast (which is why it took so many years to figure out). But right
now, let's just explore light as a wave.
Picture yourself wading around on an ocean beach for a moment, and watch the
many water waves sweeping past you. Waves are disturbances, ripples on the
water, and they possess a certain height (amplitude), with a certain number of
waves rushing past you every minute (the frequency) and all moving at a
characteristic speed across the water (the wave speed). Notice the distance
between successive waves? That's called the wavelength.
General Types of Spectra
Typically one can observe two distinctive classes of spectra: continous and
discrete. For a continuous spectrum, the light is composed of a wide, continuous
range of colors (energies). With discrete spectra, one sees only bright or dark
lines at very distinct and sharply-defined colors (energies). As we'll discover
shortly, discrete spectra with bright lines are called emission spectra, those with
dark lines are termed absorption spectra.
Continuous Spectra
Continuous spectra arise from dense gases or solid objects which radiate their
heat away through the production of light. Such objects emit light over a broad
range of wavelengths, thus the apparent spectrum seems smooth and
continuous. Stars emit light in a predominantly (but not completely!) continuous
spectrum. Other examples of such objects are incandescent light bulbs, electric
cooking stove burners, flames, cooling fire embers and... you. Yes, you, right this
minute, are emitting a continuous spectrum -- but the light waves you're
emitting are not visible -- they lie at infrared wavelengths (i.e. lower energies,
and longer wavelengths than even red light). If you had infrared-sensitive eyes,
you could see people by the continuous radiation they emit!
Absorption Line Spectra
On the other hand, what would happen if we tried to reverse this process? That
is, what would happen if we fired this special photon back into a ground state
atom? That's right, the atom could absorb that `specially-energetic' photon and
would become excited, jumping from the ground state to a higher energy level. If
a star with a `continuous' spectrum is shining upon an atom, the wavelengths
corresponding to possible energy transitions within that atom will be absorbed
and therefore an observer will not see them. In this way, a dark-line absorption
spectrum is born.