introduction to spectroscopy

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AmjadKhan Afridi
Introduction to Spectroscopy
What is Spectroscopy?
 Spectroscopy is the study of the interaction between matter and electromagnetic radiation
as a function of wavelength (λ). Or
 Spectroscopy is the use of the absorption, emission or scanning of electromagnetic
radiation by matter in order to qualitatively or quantitatively study of matter. The matter
can be atoms, molecules or solids.
 In fact, historically, spectroscopy referred to the use of visible light dispersed accordingly
to its wavelength e.g. by a prism.
 Later the concept was expanded
greatly to include any interaction
with radiative energy as a function
of its wavelength or frequency.
 Spectroscopic data is often
represented by an emission
spectrum, a plot of the response of
interest as a function of wavelength
or frequency.
 Spectral measurement devices are
referred to as spectrometers,
spectrophotometers, spectrographs or spectral analyzers.
History of Spectroscopy..
 The history of spectroscopy began in the 17th century.
 Advances in optics, specifically prisms, enabled systematic observations of the solar
spectrum.
 Isaac Newton first applied the word spectrum to describe the rainbow of colors that
combine to form white light.
 During the early 1800s, Joseph von Fraunhofer made experimental advances with
dispersive spectrometers that enabled spectroscopy to become a more precise and
quantitative scientific technique.
 Since then, spectroscopy has played and continues to play a significant role in chemistry,
physics and astronomy.

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Electromagnetic Radiation..
 Electromagnetic radiation—light—is a form of energy whose behavior is described by
the properties of both waves and particles.
 Some properties of electromagnetic radiation, such as its refraction when it passes from
one medium to another are explained best in Fig. 1 by describing light as a wave.
 Other properties, such as absorption and emission, are better described by treating light as
a particle.
 The exact nature of electromagnetic radiation remains unclear, as it has since the
development of quantum mechanics in the first quarter of the 20th century.
 Nevertheless, the dual models of wave and particle behavior provide a useful description
for electromagnetic radiation.
Wave Properties of Electromagnetic Radiation..
 Electromagnetic radiation consists of oscillating electric and magnetic fields that
propagate through space along a linear path and with a constant velocity.
 In a vacuum electromagnetic radiation travels at the speed of light, c, which is 2.997 92 ×
10e8 m/s.
 When electromagnetic radiation moves through a medium other than a vacuum its
velocity, v, is less than the speed of light in a vacuum.
 The difference between v and c is .sufficiently small (<0.1%)
 The oscillations in the electric and magnetic fields are perpendicular to each other, and to
the direction of the wave’s propagation.
Particle Properties of Electromagnetic Radiation..
 When matter absorbs electromagnetic radiation it undergoes a change in energy.

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 The interaction between matter and electromagnetic radiation is easiest to understand if
we assume that radiation consists of a beam of energetic particles called photons.
 When a photon is absorbed by a sample it is “destroyed,” and its energy acquired by the
sample.
 The energy of a photon, in joules, is related to its frequency by the following equation.
E = hv
where h is Planck’s constant, which has a value of 6.626 × 10–34 J . s.
What is Wavelength..?
 Forms of electromagnetic radiation like radio waves, light waves or infrared (heat) waves
make characteristic patterns as they travel through space.
 Each wave has a certain shape and length. The distance between peaks (high points) is
called wavelength.
Techniques in Spectroscopy..
1) Reflection of Light:
Reflection is a process whereby incoming EM radiations is reflected after
interaction with the surface of an object.
2) Scattering of Light:
Scattering is a random process whereby EM radiation is absorbed and
immediately re-emitted by particles or molecules.
3) Absorption of Light:
Absorption is a process whereby EM radiation is absorbed by particles or
molecules and converted to another form of energy.

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4) Emission Spectrum:
The emission spectrum of a chemical element or chemical compound is the
spectrum of frequencies of EM radiation emitted due to an atoms making a transition from a high
energy state to a lower energy state. The energy of the emitted photon is equal to the energy
difference between the two states.
5) Spectrum:
The data that is obtained from spectroscopy is called a spectrum. A spectrum is a
plot of the intensity of energy detected versus the wavelength of the energy.
Types of Spectroscopy..
1. ABSORPTION SPECTROSCOPY
 Absorption spectroscopy uses the range of electromagnetic spectra into which a
substance can be absorbed.
 “Absorption” is the phenomenon that occurs when a transition from a lower level to a
higher level takes place with transfer of energy from the radiation field to the atom or
molecule.
 When atoms or molecules absorb light, the incoming energy excites a structure (in energy
quanta) to a higher energy level. The type of excitation depends on the light wavelength.
Electrons are promoted to higher orbits by ultraviolet or visible light. Vibrations are
excited by infrared light and microwaves excite the rotations.

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 An absorption spectrum is a way to represent the absorption of light as a function of
wavelength. The spectrum of an atom or molecule depends on its energy-level structure,
and absorption spectra are useful for identifying compounds.
2. EMISSION SPECTROSCOPY
 “Emission” occurs during transition from a higher level to a lower level if energy is
transferred to the radiation field.
 When no radiation is emitted the phenomenon is called “nonradiative decay.”
 This type of spectroscopy relies on the range of electromagnetic spectra in which a
particular substance radiates.
 The substance first absorbs energy and then radiates (that is, emits) this energy as light.
 The excitement energy that is absorbed first can come from a number of different
sources, including collision (from high temperatures or other means), chemical reactions
or light.
 It also should be noted that atoms or molecules once excited to high-energy levels then
can decay to lower levels by emitting radiation.
 This is called emission or luminescence.
 When atoms are excited by a high-temperature energy source this light emission
commonly is called atomic or optical emission, and for atoms excited with light, it is
called atomic fluorescence.

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2. SCATTERING SPECTROSCOPY
 “Scattering” refers to light that is changed in direction (called redirection) from its
interaction with matter. It may or may not occur with energy transfer.
 This spectroscopy form measures certain physical properties by determining the amount
of light that a particular substance scatters at different wavelengths.
 It differs from other spectroscopy types primarily because of speed. The scattering
process is much faster than absorption or emission.
Spectrophotometry
 Spectrophotometry is the quantitative measurement of the reflection or transmission
properties of a material as a function of wavelength.

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 Spectrophotometry uses photometers that can measure a light beam's intensity as a
function of its wavelength known as spectrophotometers.
 Important features of spectrophotometers are spectral bandwidth, the percentage of
sample-transmission, the logarithmic range of sample-absorption, and sometimes a
percentage of reflectance measurement.
 A spectrophotometer is commonly used for the measurement of transmittance or
reflectance of solutions, transparent or opaque solids, such as polished glass, or gases.
Types of Spectrophotometr..
 There are two major classes of devices: single beam and double beam.
 A double beam spectrophotometer compares the light intensity between two light paths,
one path containing a reference sample and the other the test sample.
 A single-beam spectrophotometer measures the relative light intensity of the beam before
and after a test sample is inserted.
 Although comparison measurements from double-beam instruments are easier and more
stable, while single-beam instruments can have a larger dynamic range and are optically
simpler and more compact.

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UV-visible spectrophotometry..
 Ultraviolet–visible spectroscopy or ultraviolet-visible spectrophotometry (UV-Vis or
UV/Vis) refers to absorption spectroscopy in the ultraviolet-visible spectral region.
 This means it uses light in the visible and adjacent (near-UV and near-infrared [NIR])
ranges.
 The absorption or reflectance in the visible range directly affects the perceived color of
the chemicals involved.
Applications of UV/Visible Spectrophotometry..
 UV/Vis spectroscopy is routinely used in analytical chemistry for the quantitative
determination of different analytes, such as

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1. Transition Metal Ions
2. Highly Conjugated Organic Compounds
3. Biological Macromolecules.
 Organic compounds, also absorb light in the UV or visible regions of the electromagnetic
spectrum. The solvents for these determinations are often water for water-soluble
compounds, or ethanol for organic-soluble compounds.
IR spectrophotometry..
 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.
Applications of IR Spectrophotometry
 Infrared spectroscopy is a simple and reliable technique widely used in both organic and
inorganic chemistry, in research and industry.
 It is used in quality control, dynamic measurement, and monitoring applications such as
the long-term unattended measurement of CO2 concentrations in greenhouses and growth
chambers by infrared gas analyzers.
 It is also used in forensic analysis in both criminal and civil cases, for example in
identifying polymer degradation. It can be used in determining the blood alcohol content
of a suspected drunk driver.
 IR-spectroscopy has been successfully used in analysis and identification of pigments in
paintings and other art objects such as illuminated manuscripts.
Dr. Abdul Rehman
Notes: Instrumentation and Analytical Techniques
Chapter: Introduction to Spectroscopy
Semester:2nd
Date:25th
September, 2019
Made by Amjad Khan Afridi