INTRODUCTION
TO
SPECTROSCOPY

HISTORY
• THE BEAUTIFUL PHENOMENON OF “RAINBOW” WAS
THE FIRST DISPERSED SPECTRUM.
• 1665 - NEWTON TOOK THE FIRST & MOST
IMPORTANT STEP TOWARDS THE DEVELOPMENT
OF SPECTROSCOPY.
• 1752 - THOMAS MELVILL GAVE THE FIRST
DESCRIPTION OF LABORATORY EMISSION
SPECTRUM.
• 1802 - THOMAS YOUNG SHOWED THAT THE
RANGE OF WAVELENGTH IN VISIBLE SPECTRUM
EXTENDS FROM 424-675 NM.
• FRAUNHOFFER RULED THE FIRST GLASS
TRANSMISSION GRATING.
• 1848 - FOUCAULT’S WORK INDICATED A RELATION
BETWEEN EMISSION & ABSORPTION SPECTRA.

• 1859 - G.R. KIRCHOFF STATED THAT “RATIO OF EMISSIVE
POWER TO THE ABSORPTIVITY FOR THERMAL RADIATION
IS CONSTANT FOR SAME WAVELENGTH & TEMPERATURE”.
• G.R. KIRCHOFF & R.BUNSEN EMERGED AS THE “FATHER OF
MODERN SPECTROSCOPY”.
• NEW DEVELOPMENTS SUCH AS DRY GELATIN
PHOTOGRAPHIC PLATE, INTERFEROMETER,BOLOMETER
ETC. CAME IN THE TWENTIETH CENTURY.
• INFRARED,MICROWAVE,SUBMILLIMETER,RADIO-
FREQUENCY,U.V.,X-RAY,GAMMA –RAY REGIONS CAME INTO
EXISTENCE WITH THE HELP OF SPETROSCOPY.
• SPECTROSCOPY PLAYED A GREAT ROLE IN THE FORMULA-
TION OF QUANTUM MECHANICS & RELATIVISTIC THEORY
IN THE TWENTIETH CENTURY.

SINCE,WE ALL ARE FAMILIAR WITH “MATTER”
AND THE “ELECTROMAGNETIC RADIATION”.
SO,WITHOUT WASTING MUCH TIME,
IT IS DEFINED AS THE STUDY OF THE
INTERACTION OF MATTER &
ELECTROMAGNETIC RADIATION.

…. REVIEW OF SOME BASICS
• c= x
• Angular resolution: = 1.22 / D radians
206,265” in a radian
• E=h
• F = L / 4 d2
• Important constants :
G = 6.67 x 10-8 (c.g.s)
c = 3 x 1010 cm/sec,
k = 1.38 x 10-16
h = 6.626 x 10-27
mH ~ mproton = 1.67 x 10-24 grams
me = 0.91 x 10-27 grams
eV = 1.602 x 10-12 erg
Luminosity of Sun = 4 x 1033 erg/sec
Mass of the Sun = 2 x 1033 grams

THE PHYSICS OF EM RADIATION
• Light:
- = c = 2.998 x 1010 cm/s (in vacuum)
- E=h Photon energy (erg)
1 erg sec-1 = 10-7 Watt
h = 6.626 x 10-27 (c.g.s)
1 eV = 1.602 x 10-12 erg
- p =E/c=h/ Photon momentum
- = h / p = h / m v de Broglie wavelength
Planck Function: B (T)
• Emission, absorption, continua
• Wave no. : Reciprocal of wavelength (in cm)

•SPECTROSCOPY : STUDY OF INTERACTION OF MATTER AND
ELECTROMAGNETIC RADIATION.
• SPECTROMETRY : AN ANALYTICAL TECHNIQUE IN WHICH
EMISSION (OF PARTICLE/RADIATION) IS
DISPERSED ACCORDING TO SOME
PROPERTY OF THE EMISSION AND THE
AMOUNT OF DISPERSION IS MEASURED.
EG. MASS SPECTROMETRY.
• SPECTROPHOTOMETRY : A QUANTIFIABLE STUDY OF
ELECTROMAGNETIC SPECTRA.
• SPECTROGRAPHY : ANOTHER NAME FOR SPECTROSCOPY.

TYPES OF SPECTROSCOPY
• Electromagnetic Waves: Emission, absorption
Visual, near-IR., FIR, Radio, UV/X-ray, gamma-ray
- Solids, liquids, gasses, plasmas
- Emission, absorption
- Spectral line, molecular bands, continua:
- Thermal (~LTE, blackbody, grey-body):
- Non-thermal (masers, synchrotron, …)
- Electronic, vibrational, rotational transitions.
- Effects of B (Zeeman), E ( Stark), motion (Doppler),
pressure (collisions), natural life-time (line widths)
- Radiative Transfer (optical depth)
Other types (not covered in this course):
• NMR
• Raman
• Phosprescence / Fluorecence
• Astro-particle

CONTINUOUS SPECTRA ARISE FROM DENSE GASESOF THE
DISCRETE SPECTRA ARE THE OBSERVABLE RESULT OR
SOLID OBJECTS WHICH RADIATE THEIR HEAT AWAY
PHYSICS OF ATOMS.
THROUGH THE PRODUCTION OF LIGHT. SUCH OBJECTS
EMIT LIGHTTWO TYPES OF DISCRETE SPECTRA :
THERE ARE OVER A BROAD RANGE OF WAVELENGTHS,
THUS THE APPARENT SPECTRUM SEEMS SMOOTH AND
CONTINUOUS.
• EMISSION (BRIGHT LINE SPECTRA) ,
• ABSORPTION EMIT LIGHT IN A PREDOMINANTLY (BUT NOT
STARS (DARK LINE SPECTRA) .
COMPLETELY!) CONTINUOUS SPECTRUM.

WHEN AN ATOM DROPS FROM EXCITEDENERGY LEVEL TO
MOVES FROM LOWER STATE TO THE
UPPER ENERGY THEY , THE WAVELENGTHS
GROUND STATE,LEVEL EMIT A WAVE OF LIGHT OF
CORRESPONDING TO TO THE ENERGY DIFFERENCE
WAVELENGTH EQUALPOSSIBLE ENERGY TRANSITIONS
WITHIN THAT ATOM WILL BE ABSORBED AND
BETWEEN THOSE TWO LEVELS. THIS ENERGYTHEREFORE
AN OBSERVER WILL NOT SEE THEM. IN THIS WAY, A “DARK-
CORRESPONDS TO A CERTAIN COLOUR, AND THUS WE ARE
LINE TO SEE AN “EMISSION SPECTRA”. THE CHANGE OF
ABLEABSORPTION SPECTRUM” IS BORN. EG.
ENERGY IN AN ATOM GENERATES A PHOTON,WHICH IS
THEN EMITTED. EG.
A hydrogen atom in the ground state is excited by a photon of exactly the `right'
energy needed to send it to level 2, absorbing the level 1, in the process.
An excited Hydrogen atom relaxes from level 2 to photon yielding a photon.
This results in a dark absorption line.
bright emission line.

ABSORPTION SPECTROSCOPY
• DEFINITION : ABSORPTION SPECTROSCOPY REFERS TO SPECTROSCOPIC
TECHNIQUES THAT MEASURE THE ABSORPTION OF RADIATION, AS A FUNCTION
OF FREQUENCY OR WAVELENGTH, DUE TO ITS INTERACTION WITH A SAMPLE.
• THE INTENSITY OF THE ABSORPTION VARIES AS A FUNCTION OF FREQUENCY,
AND THIS VARIATION IS THE “ABSORPTION SPECTRUM”. ABSORPTION
SPECTROSCOPY IS PERFORMED ACROSS THE “ELECTROMAGNETIC SPECTRUM”.

ATOMIC ABSORPTION SPECTROSCOPY
• DEFINITION : ATOMIC ABSORPTION SPECTROSCOPY IS A TECHNIQUE USED TO
DETERMINE THE CONCENTRATION OF A SPECIFIC METAL ELEMENT IN A
SAMPLE.
• THE TECHNIQUE CAN BE USED TO ANALYZE THE CONCENTRATION OF OVER 70
DIFFERENT METALS IN A SOLUTION.
• PRINCIPLE : IT MAKES USE OF ABSORPTION SPECTROMETRY & IS HENCE,
BASED ON “BEER-LAMBART’S LAW”.
• INSTRUMENT :
Atomic Absorption Spectrometer

ATOMIC EMISSION SPECTROSCOPY
• DEFINITION : IT IS THE QUANTITATIVE MEASUREMENT OF THE OPTICAL
RADIATION FROM EXCITED ATOMS, WHEN THEY FALL TO GROUND STATE, TO
DETERMINE ANALYTE CONCENTRATION.
• THIS TECHNIQUE MAKES USE OF HIGH TEMPERATURE OF FLAME TO EXCITE
THE ATOMS.
• INSTRUMENT :
Inductively-coupled Plasma Atomic Emission Spectrometer

ATOMIC EMISSION SPECTROMETER
Excited Wavelength
electrons selector
Excitation source
Detector

FLAME PHOTOMETRY
• DEFINITION : FLAME PHOTOMETRY (MORE ACCURATELY CALLED FLAME
• THE INTENSITY OF THE LIGHT EMITTED COULD BE DESCRIBED BY THE
ATOMIC EMISSION SPECTROMETRY) IS A BRANCH OF ATOMIC SPECTROSCOPY
“SCHEIBE-LOMAKIN EQUATION”:
IN WHICH THE SPECIES EXAMINED IN THE SPECTROMETER ARE IN THE FORM OF
I=K×CN
ATOMS. THE ATOMS UNDER INVESTIGATION ARE EXCITED BY LIGHT.
WHERE, C : CONCENTRATION OF ELEMENT,
• THE TECHNIQUE CAN BE USED FOR QUALITATIVE AND QUANTITATIVE
K : PROPORTIONALITY CONSTANT,
DETERMINATION OF SEVERAL CATIONS, ESPECIALLY FOR METALS THAT ARE
N : N ~1 (AT LINEAR PART OF CALIBRATION CURVE)
EASILY EXCITED TO HIGHER ENERGY LEVELS AT A RELATIVELY LOW FLAME
THEREFORE ,THE INTENSITY OF
TEMPERATURE (MAINLY NA, K, RB, CS, CA, BA, CU).
EMITTED LIGHT IS DIRECTLY PROPORTIONAL TO
CONCENTRATION.
• PRINCIPLE : IT MAKES USE OF A FLAME THAT EVAPORATES THE SOLVENT AND
ALSO SUBLIMATES AND ATOMIZES THE METAL AND THEN EXCITES A VALENCE
• INSTRUMENT :
ELECTRON TO AN UPPER ENERGY STATE.
Photograph of a flame photometer

FLAME PHOTOMETER
Readout
Aerosol enters
flame
Photo-detector
Fuel Lens
Filter
Air
Discharge

U.V., I.R., VIS. SPECTROPHOTOMETRY
• U.V. SPECTROPHOTOMETRY : IT IS A BRANCH OF A.A.S/A.E.S IN WHICH ALL
ATOMS ABSORB/EMIT WAVELENGTH OF LIGHT CORRESPONDING TO U.V.
REGION . IT IS USED IN QUANTIFYING PROTEIN AND DNA CONCENTRATION AS
WELL AS THE RATIO OF PROTEIN TO DNA CONCENTRATION IN A SOLUTION .
• I.R. SPECTROPHOTOMETRY : IT IS ALSO A BRANCH OF A.A.S/A.E.S IN WHICH
ALL ATOMS ABSORB/EMIT WAVELENGTH OF LIGHT CORRESPONDING TO I.R.
REGION. INFRARED SPECTROSCOPY OFFERS THE POSSIBILITY TO MEASURE
DIFFERENT TYPES OF INTER ATOMIC BOND VIBRATIONS AT DIFFERENT
FREQUENCIES .
• VIS. SPECTROPHOTOMETRY : IT IS THE THIRD BRANCH OF A.A.S/A.E.S IN WHICH
ALL ATOMS ABSORB/EMIT WAVELENGTH OF LIGHT CORRESPONDING TO
VISIBLE REGION.

SPECTROPHOTOMETER

FLUORIMETRY
• DEFINITION : IT IS A TECHNIQUE IN WHICH THE AMOUNT OF SUBSTANCE IN A
SAMPL CAN BE DETERMINED BY THE AMOUNT OF LIGHT EMITTED BY THE
ATOMS OF THAT SUBSTANCE.
• THIS TECHNIQUE IS BASED ON THE PHENOMENON OF “FLUOROSCENCE”.
• RELATION BETWEEN FLUOROSCENCE INTENSITY & ANALYTE
CONCENTRATION :
F= K*(QE)*(Po)*[ 1- 10(A*B*C)]

SPECTROGRAPH
Focal Plane collimator camera
detector
Dispersing element
Slit
Telescope

SPECTROGRAPH OVERVIEW
• Slit & Decker:
Restrict incoming light
Spatial direction vs. Spectral direction
• Collimator & Camera:
Transfer image of slit onto detector.
• Grating:
Disperse light: dispersion => spectral resolution
• What determines spectral resolution & coverage?
- Slit-width
- Grating properties: Ngrooves , order number
- Camera / collimator magnification (focal length ratio)
- Detector pixel size and number of pixels.