Spectroscopy basics

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INTRODUCTION TO
SPECTROSCOPIC METHODS
OF ANALYSIS
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WHAT IS SPECTROSCOPY?
The study of interactions between
ELECTROMAGNETIC RADIATIONS
and
MATTER
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TO UNDERSTAND SPECTROSCOPY WE
MUST UNDERSTAND ELECTROMAGNETIC
RADIATION
What is Electromagnetic Radiation?
 is a form of energy that has both Wave and
Particle Properties.
For example: Ultraviolet, visible, infrared,
microwave, radio wave.
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WAVE PROPERTIES
 EM radiation is conveniently modeled as waves
consisting of perpendicularly oscillating electric and
magnetic fields, as shown below.
x
y
z Electric Field
Magnetic Field
Direction of
propagation
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o At 90° to the direction of propagation is an oscillation in
the ELECTRIC FIELD.
o At 90° to the direction of propagation and 90° from the
electric field oscillation is the MAGNETIC FIELD
oscillation.
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WAVE PARAMETERS
We Use Symbols to Designate Various
Properties of Waves
  is the wavelength of the waves
 V is the frequency of the waves
 c is the speed of light
Time or Distance-
+
ElectricField
0
Amplitude (A)
Wavelength ()
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DEFINITIONS:
 Period (p) – the time required for one cycle to pass a fixed point in
space.
 Frequency (V) – the number of cycles which pass a fixed point in
space per second.
 Amplitude (A) – The maximum length of the electric vector in the
wave (Maximum height of a wave).
 Wavelength () – The distance between two identical adjacent points
in a wave (usually maxima or minima).
 Wavenumber () - The number of waves per cm in units of cm-1. 7
 Radiant Power ( P ) - The amount of energy reaching a given area
per second. Unit in watts (W)
 Intensity ( I ) - The radiant power per unit solid angle.
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DEFINITIONS:

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RELATIONSHIP BETWEEN THESE VARIABLES
Speed of light = Wavelength x Frequency
 c = V
  = c/V
 V = c/
For Electromagnetic Waves the Speed (c) is a Constant
c = 3.00 x 108 m/sec = 3.00 x 1010 cm/sec
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 This Constant Speed Means a Direct, Inverse
Relationship Between Wavelength and Frequency
 ∝ 1/V
 The Higher the Frequency the Shorter the
Wavelength . The Longer the Wavelength the
Lower the Frequency.
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THE RELATIONSHIP BETWEEN
FREQUENCY AND WAVELENGTH
Wavelength is inversely proportional to frequency
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800 nm
Infrared radiation
V = 3.75 x 1014 s-1
Ultraviolet radiation
V = 7.50 x 1014 s-1
PARTICLE PROPERTIES OF LIGHT:
PHOTONS
• Wave theory failed to explain phenomena associated with the
absorption and emission of radiation of radiant energy.
• Thus, EM is viewed as a stream of discrete particles, or wave
packets, of energy called photons.
• We can relate the energy of photon to its wavelength,
frequency and wavenumber by
E = hV V - frequency
= h c  - wavelength
 υ - wavenumber
= hcυ
 h – Planck’s constant =6.63x10-34 J·s 12

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THE ELECTROMAGNETIC SPECTRUM
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REGIONS OF THE UV, VISIBLE AND IR SPECTRUM
Region Wavelength Range
UV 180 – 380 nm
Visible 380 – 780 nm
Near-IR 0.78 – 2.5 μm
Mid-IR 2.5 – 50 μm
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PREFIXES FOR UNITS
Prefix Symbols Multiplier
giga- G 109
mega- M 106
kilo- k 103
deci- d 10-1
centi- c 10-2
milli- m 10-3
micro- µ 10-6
nano- n 10-9
pico- p 10-12
femto- f 10-15
atto- a 10-18 15
WAVELENGTH UNITS FOR VARIOUS SPECTRAL REGION
Region Unit Definition (m)
X-ray Angstrom unit, Å 10-10 m
Ultraviolet/visible Nanometer, nm 10-9 m
Infrared Micrometer, μm 10-6 m
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INTERACTION OF ELECTROMAGNETIC
RADIATION WITH MATTER
Infrared primarily acts to set molecules into vibration.
UV and visible light primarily acts to elevate electrons to higher energy levels.
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INTERACTION OF ELECTROMAGNETIC
RADIATION WITH MATTER
 The interaction of radiation with matter can cause redirection
of the radiation and/or transitions between the energy levels
of the atoms or molecules.
1. A transition from a lower level to a higher level with transfer
of energy from the radiation field to the atom or molecule is
called absorption.
2. A transition from a higher level to a lower level is called
emission if energy is transferred to the radiation field, or
nonradiative decay if no radiation is emitted.
3. Redirection of light due to its interaction with matter is called
scattering, and may or may not occur with transfer of energy,
i.e., the scattered radiation has a slightly different or the same
wavelength.
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TYPES OF SPECTRA
1. Absorption spectrum
2.Emission spectrum
 Absorption spectrum
 A plot of the absorbance as a function of wavelength
or frequency.
 Emission spectrum
 A plot of the relative power of the emitted radiation
as a function of wavelength or frequency.
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KIRCHOFF’S LAWS
 1st law: A luminous
solid or liquid, or a
sufficiently dense gas,
emits light of all
wavelengths and
produces a continuous
spectrum of radiation.
• 2nd law: A low-density hot
gas emits light whose
spectrum consists of a series
of bright emission lines which
are characteristic of the
chemical composition of the
gas.
• 3rd law: A cool thin gas
absorbs certain wavelengths
from a continuous spectrum,
leaving dark absorption lines
in their place superimposed
on the continuous spectrum.

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CONTINUOUS SPECTRUM
 Also called thermal
or blackbody
spectra
 Spectra of stars,
planets, moons
 Depends on
temperature
Ex: sunlight passing
through a prism
Hotter objects
Shift toward this end
Cooler objects
Shift toward this end
Shorter
wavelength
Longer
wavelength
Continuous Spectrum
continued

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DISCRETE SPECTRUM
ABSORPTION
Hot object
Cool,
thin gas
Spectra
Absorption
spectrum
Ex: stars,
planets w/
atmospheres,
& galaxies
Each element
has a unique
signature of
absorption
lines. That
pattern helps
scientists
identify the
element(s).
DISCREET SPECTRUM-
EMISSION
Spectra
Emission
spectrum
Thin,hot
gas
Again, the
pattern of the
lines
determines
the identity of
the element.

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ATOMIC vs MOLECULAR TRANSITIONS
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ATOMIC TRANSITION
 Atomic transitions are usually very discreet
changes of electrons from one quantum state to
another (energy levels, shells, spins, etc.).
 Only electronic transition is quantized.
 When an atom changes energy state, it absorbs or
emits energy equal to the energy difference
E = E1 – E0
 The wavelength or frequency of radiation absorbed
or emitted during a transition is proportional to E
 Transitions between electronic levels produce line
spectra. 26

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ATOMIC TRANSITION
 E0 – lowest energy electronic level or ground state
 E1, E2 – higher-energy electronic levels 27
MOLECULAR TRANSITION
 In molecules the electronic states are subdivided
into vibrational states.
 The energy of a band in a molecular absorption
spectrum is the sum of three different energy
components.
E = Eelectronic + Evibrational + Erotational
 Transitions between electronic-vibrational-
rotational states give rise to spectra that appear
to have bands. 28

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Vibrational
energy level
MOLECULAR TRANSITION
Energy
COMPONENTS OF INSTRUMENTS
FOR OPTICAL SPECTROSCOPY
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GENERAL DESIGN OF OPTICAL INSTRUMENTS
Absorption
Emission
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FIVE BASIC OPTICAL INSTRUMENT COMPONENTS
1)Source - A stable source of radiant energy at the desired wavelength (or  range).
2)Sample Holder - A transparent container used to hold the sample (cells,
cuvettes, etc.).
3)Wavelength Selector - A device that isolates a restricted region of the EM
spectrum used for measurement (monochromators, prisms, & filters).
4)Photoelectric Transducer - (Detector) Converts the radiant energy into a
useable signal (usually electrical).
5)Signal Processor & Readout - Amplifies or attenuates the transduced signal
and sends it to a readout device such as a meter, digital readout, chart recorder,
computer, etc.
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I. SOURCES OF RADIATION
• Generate a beam of radiation that is stable and has sufficient
power.
A. Continuum Sources
- emit radiation over a broad wavelength range and the intensity
of the radiation changes slowly as a function of wavelength.
This type of source is commonly used in UV, visible and IR
instruments.
• Deuterium lamp is the most common UV source.
• Tungsten lamp is the most common visible source.
• Glowing inert solids are common sources for IR instruments. 33
B. Line Sources
- Emit a limited number lines or bands of radiation at specific
wavelengths.
• Used in atomic absorption spectroscopy
• Types of line sources:
1) Hollow cathode lamps
2) Electrodeless discharge lamps
3) Lasers - Light amplification by stimulated emission of
radiation
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II. WAVELENGTH SELECTORS
• Wavelength selectors output a limited, narrow,
continuous group of wavelengths called a band.
• Two types of wavelength selectors:
A)Filters
B)Monochromators
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A. FILTERS
• Two types of filters:
1) Interference filters
2) Absorption Filters
B. Monochromators
• Wavelength selector that can continuously scan a broad range of
wavelengths
• Used in most scanning spectrometers including UV, visible, and IR
instruments.
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III. RADIATION TRANSDUCERS (DETECTORS)
• Early detectors in spectroscopic instruments were the human eye,
photographic plates or films. Modern instruments contain devices that
convert the radiation to an electrical signal.
Two general types of radiation transducers:
a. Photon detectors
b. Thermal detectors
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• Several types of photon detectors are available:
1. Vacuum phototubes
2. Photomultiplier tubes
3. Photovoltaic cells
4. Silicon photodiodes
5. Diode array transducers
6. Photoconductivity transducers
A. Photon Detectors
• Commonly useful in ultraviolet, visible, and near infrared instruments.
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• Three types of thermal detectors :
1. Thermocouples
2. Bolometers
3. Pyroelectric transducers
B. Thermal Detectors
• Used for infrared spectroscopy because photons in the IR region lack
the energy to cause photoemission of electrons.
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 Sample containers, usually called cells or cuvettes must have
windows that are transparent in the spectral region of interest.
 There are few types of cuvettes:
- quartz or fused silica
- silicate glass
- crystalline sodium chloride
QUARTZ OR FUSED SILICA
- REQUIRED FOR UV AND MAY BE USED IN VISIBLE REGION
SILICATE GLASS
- CHEAPER COMPARED TO QUARTZ. USED IN UV
CRYSTALLINE SODIUM CHLORIDE
- USED IN IR
IV.SAMPLE HOLDER (CONTAINER)
cuvette
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 SPECTROMETER
- is an instrument that provides information about the intensity of
radiation as a function of wavelength or frequency
 SPECTROPHOTOMETER
- is a spectrometer equipped with one or more exit slits and
photoelectric transducers that permits the determination of the
ratio of the radiant power of two beams as a function of
wavelength as in absorption spectroscopy.
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REGION SOURCE SAMPLE
HOLDER
DETECTOR
Ultraviolet Deuterium lamp Quartz/fused
silica
Phototube, PM
tube, diode array
Visible Tungsten lamp Glass/quartz Phototube, PM
tube, diode array
Infrared Nernst glower (rare earth
oxides or silicon carbide
glowers)
Salt crystals e.g.
crystalline
sodium chloride
Thermocouples,
bolometers
Types of source, sample holder and detector
for various EM region
SUMMARY
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