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INTRODUCTION TO SPECTROSCOPY
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INTRODUCTION TO SPECTROSCOPY

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  • 1. INTRODUCTION TOSPECTROSCOPY
  • 2. 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.
  • 3. • 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 GELATINPHOTOGRAPHIC PLATE, INTERFEROMETER,BOLOMETERETC. 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.
  • 4. SINCE,WE ALL ARE FAMILIAR WITH “MATTER”AND THE “ELECTROMAGNETIC RADIATION”.SO,WITHOUT WASTING MUCH TIME,IT IS DEFINED AS THE STUDY OF THEINTERACTION OF MATTER &ELECTROMAGNETIC RADIATION.
  • 5. …. 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
  • 6. 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)
  • 7. •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.
  • 8. 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
  • 9. CONTINUOUS SPECTRA ARISE FROM DENSE GASESOF THEDISCRETE SPECTRA ARE THE OBSERVABLE RESULT ORSOLID OBJECTS WHICH RADIATE THEIR HEAT AWAYPHYSICS OF ATOMS.THROUGH THE PRODUCTION OF LIGHT. SUCH OBJECTSEMIT LIGHTTWO TYPES OF DISCRETE SPECTRA :THERE ARE OVER A BROAD RANGE OF WAVELENGTHS,THUS THE APPARENT SPECTRUM SEEMS SMOOTH ANDCONTINUOUS.• EMISSION (BRIGHT LINE SPECTRA) ,• ABSORPTION EMIT LIGHT IN A PREDOMINANTLY (BUT NOT STARS (DARK LINE SPECTRA) .COMPLETELY!) CONTINUOUS SPECTRUM.
  • 10. WHEN AN ATOM DROPS FROM EXCITEDENERGY LEVEL TO MOVES FROM LOWER STATE TO THEUPPER ENERGY THEY , THE WAVELENGTHSGROUND STATE,LEVEL EMIT A WAVE OF LIGHT OFCORRESPONDING TO TO THE ENERGY DIFFERENCEWAVELENGTH EQUALPOSSIBLE ENERGY TRANSITIONSWITHIN THAT ATOM WILL BE ABSORBED ANDBETWEEN THOSE TWO LEVELS. THIS ENERGYTHEREFOREAN OBSERVER WILL NOT SEE THEM. IN THIS WAY, A “DARK-CORRESPONDS TO A CERTAIN COLOUR, AND THUS WE ARELINE TO SEE AN “EMISSION SPECTRA”. THE CHANGE OFABLEABSORPTION SPECTRUM” IS BORN. EG.ENERGY IN AN ATOM GENERATES A PHOTON,WHICH ISTHEN EMITTED. EG.A hydrogen atom in the ground state is excited by a photon of exactly the `rightenergy 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.
  • 11. ABSORPTION SPECTROSCOPY• DEFINITION : ABSORPTION SPECTROSCOPY REFERS TO SPECTROSCOPICTECHNIQUES THAT MEASURE THE ABSORPTION OF RADIATION, AS A FUNCTIONOF 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”. ABSORPTIONSPECTROSCOPY IS PERFORMED ACROSS THE “ELECTROMAGNETIC SPECTRUM”.
  • 12. ATOMIC ABSORPTION SPECTROSCOPY• DEFINITION : ATOMIC ABSORPTION SPECTROSCOPY IS A TECHNIQUE USED TODETERMINE THE CONCENTRATION OF A SPECIFIC METAL ELEMENT IN ASAMPLE.• THE TECHNIQUE CAN BE USED TO ANALYZE THE CONCENTRATION OF OVER 70DIFFERENT METALS IN A SOLUTION.• PRINCIPLE : IT MAKES USE OF ABSORPTION SPECTROMETRY & IS HENCE,BASED ON “BEER-LAMBART’S LAW”.• INSTRUMENT : Atomic Absorption Spectrometer
  • 13. ATOMIC EMISSION SPECTROSCOPY• DEFINITION : IT IS THE QUANTITATIVE MEASUREMENT OF THE OPTICALRADIATION FROM EXCITED ATOMS, WHEN THEY FALL TO GROUND STATE, TODETERMINE ANALYTE CONCENTRATION.• THIS TECHNIQUE MAKES USE OF HIGH TEMPERATURE OF FLAME TO EXCITETHE ATOMS.• INSTRUMENT : Inductively-coupled Plasma Atomic Emission Spectrometer
  • 14. ATOMIC EMISSION SPECTROMETERExcited Wavelengthelectrons selector Excitation source Detector
  • 15. FLAME PHOTOMETRY• DEFINITION : FLAME PHOTOMETRY (MORE ACCURATELY CALLED FLAME• THE INTENSITY OF THE LIGHT EMITTED COULD BE DESCRIBED BY THEATOMIC 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×CNATOMS. 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 OFTEMPERATURE (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 ANDALSO SUBLIMATES AND ATOMIZES THE METAL AND THEN EXCITES A VALENCE• INSTRUMENT :ELECTRON TO AN UPPER ENERGY STATE. Photograph of a flame photometer
  • 16. FLAME PHOTOMETER ReadoutAerosol entersflame Photo-detector Fuel Lens Filter Air Discharge
  • 17. U.V., I.R., VIS. SPECTROPHOTOMETRY• U.V. SPECTROPHOTOMETRY : IT IS A BRANCH OF A.A.S/A.E.S IN WHICH ALLATOMS ABSORB/EMIT WAVELENGTH OF LIGHT CORRESPONDING TO U.V.REGION . IT IS USED IN QUANTIFYING PROTEIN AND DNA CONCENTRATION ASWELL 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 WHICHALL ATOMS ABSORB/EMIT WAVELENGTH OF LIGHT CORRESPONDING TO I.R.REGION. INFRARED SPECTROSCOPY OFFERS THE POSSIBILITY TO MEASUREDIFFERENT TYPES OF INTER ATOMIC BOND VIBRATIONS AT DIFFERENTFREQUENCIES .• VIS. SPECTROPHOTOMETRY : IT IS THE THIRD BRANCH OF A.A.S/A.E.S IN WHICHALL ATOMS ABSORB/EMIT WAVELENGTH OF LIGHT CORRESPONDING TOVISIBLE REGION.
  • 18. SPECTROPHOTOMETER
  • 19. FLUORIMETRY• DEFINITION : IT IS A TECHNIQUE IN WHICH THE AMOUNT OF SUBSTANCE IN ASAMPL CAN BE DETERMINED BY THE AMOUNT OF LIGHT EMITTED BY THEATOMS OF THAT SUBSTANCE.• THIS TECHNIQUE IS BASED ON THE PHENOMENON OF “FLUOROSCENCE”.• RELATION BETWEEN FLUOROSCENCE INTENSITY & ANALYTECONCENTRATION : F= K*(QE)*(Po)*[ 1- 10(A*B*C)]
  • 20. SPECTROGRAPH Focal Plane collimator camera detector Dispersing element SlitTelescope
  • 21. 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.