1-Introduction to spectroscopy a basic idea

introduction to spectroscopy

1. Spectroscopy
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Dr. Jorphin Joseph
Assistant Professor
Department of Chemical oceanography
CUSAT

2. Light is a transverse wave
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3. Light is electromagnetic wave
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time
position

4. 2-D representation of electric vector
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Period(T)–thetime
requiredforonecycleto
passafixedpointin
space.
Frequency(ν)–the
numberofcycleswhich
passafixedpointin
spacepersecond.ν=1/T
Amplitude(A)–The
maximumlengthofthe
electricvectorinthe
wave(Maximumheight
ofawave).
Wavelength(λ)–The
distancebetweentwo
identicaladjacent
pointsinawave
(usuallymaximaor
minima).
Wavenumber(1/λ)-
Thenumberofwaves
percminunitsofcm
-1
.
Velocity(v)-The
distancetravelledby
waveinunittime.

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6. Effect of medium on a light wave
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8. Electromagnetic Spectrum
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9. Mathematical description of a wave
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10. Phase of a light wave
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Sine waves with different amplitudes and
with a phase different of 90 degree

11. Superimposition of waves
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If two plane-polarized waves overlap in space, the resulting electromagnetic disturbance is
the algebraic sum of the two waves.

12. Optical interference
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13. Periodicity or Beat
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14. Fourier Transformation
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15. Light –Matter interaction
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*We are ignoring scattering effects,
which occurs for less than 1 in 10,000 photons
electromagnetic radiation

16. Light–Matter interaction
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17. Consequences of Light-Matter Interaction
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Absorptioninduces
changesinmatter
aswellasinlight

18. Absorption
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Absorptionmethods:Radiationofincidentradiant
intensityI
0canbeabsorbedbytheanalyte,resultingina
transmittedbeamoflowerradiantintensityI.For
absorptiontooccur,theenergyoftheincidentbeam
mustcorrespondtooneoftheenergydifferencesshown
in(b).Theresultingabsorptionspectrumisshownin(c).
I
0 I

19. Interactions of different radiations with matter
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20. Major Absorption Spectroscopic Techniques
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21. Quantized energy levels in matter
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100-1000 kJ/mol1-100 kJ/mol0.01-1 kJ/mol

22. Quantized energy levels in matter-A comparison
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23. Quantized energy levels in matter-A comparison
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ThemoleculeMhaslowlyingrotationalstatesM
r
andvibrationalstatesM
v
.Electronic
excitedstatesM
*
canbereachedbyabsorptionoflightabove1000nm,innearinfrared,
Visible(VIS)andultraviolet(UV)spectra.Atshorterwavelengthsionizationisthemajor
process.Afewexamplesofbonddissociationenergiesinthisenergyrangearegiveninunits
ofkJmol
-1

24. Schematic of a grating spectrometer
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25. Idealized and actual spectrum
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•Source emissivity and the sensitivity of the detector vary with frequency. Thus the baseline-the no-sample condition-
is seldom horizontal, although matters can usually be arranged so that it is approximately linear.
•Sinceitisimpossibletomaketheslitsinfinitelynarrow,arangeoffrequencies,ratherthanjustasinglefrequency,
fallsonthedetectoratanygivenpositionofthegrating.Additionally,noenergytransitionsinatomsandmolecules
areabsolutelysharp,buttheyalwaysoccuroverarangeoffrequencies.Thesefactorscauseabroadeningofthe
spectralabsorption.
•Small peaks and troughs arising from randomly generated electrical signals in the detector, the amplifier, or the pen
recorder, usually collectively referred to as 'noise' on the spectrum.

26. Signal-to-noise ratio
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Sincealmostallmodernspectrometersusesomeformofelectronicamplificationtomagnifythesignalproducedbythedetector,every
recordedspectrumhasabackgroundofrandomfluctuationscausedbyspuriouselectronicsignalsproducedbythesourceordetector,or
generatedintheamplifyingequipment.Thesefluctuationsareusuallyreferredtoas'noise'.Inorderthatarealspectralpeakshouldshow
itselfassuchandbesufficientlydistinguishedfromthenoise,itmusthaveanintensitysomethreeorfourtimesthatofthenoisefluctuations
(ahighsignal-to-noiseratioofthreeorfour).Thisrequirementplacesalowerlimitontheintensityofobservablesignals.

27. Natural Spectral Line Widths
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28. Natural Line Widths
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×

29. Natural Line Widths
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×
×
×
×
×

30. Natural Line Widths
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31. Natural Line Widths
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32. Natural Line Widths
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33. Natural Line Widths
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34. Natural Line Widths
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35. Natural Line Widths
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36. Natural Line Widths
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37. Schematic of a grating spectrometer
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38. Resolving Power
08-09-2024 COD 38Resolving power and slit width
“the ability of an optical instrument to distinguish
between two points that are close together”

39. Resolving Power
•At(e)ofthisfigure,theabsorbanceisplottedagainstfrequency,togetherwiththeapproximate
positionsofstages(a),(b),(c),and(d).Itisquiteevidentthattheseparationbetweenthelineshas
disappearedundertheseconditions-thelinesarenotresolved.
•Itisequallyevidentthattheuseofamuchnarrowerslitwouldresultintheirresolution-theresolving
powerwouldbeincreased.Infact,providedtheslitwidthislessthantheseparationbetweenthelines,
thedetectoroutputwillshowaminimumbetweenthem.
•However,itmustberememberedthatanarrowerslitallowslesstotalenergyfromthebeamtoreach
thedetectorandconsequentlytheintrinsicsignalstrengthwillbeless.Therecomesapointwhen
decreasingtheslitwidthresultsinsuchweaksignalsthattheybecomeindistinguishablefromthe
backgroundnoise.Thusspectroscopyisacontinualbattletofindtheminimumslitwidthconsistentwith
acceptablesignal-to-noisevalues.
•Improvementsinresolvingpowermayarisenotonlyasaresultofobtainingbetterdispersionofthe
radiationbytheanalyserbutalsobyusingamoresensitivedetector.
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40. Difference in spectral shapes due to resolution
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Analysis of ethanol solution in benzene

41. Intensity of spectral lines
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Intensity of spectral lines

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Intensity of spectral lines

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Intensity of spectral lines

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Intensity of spectral lines

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Sensitivity-Population of States
EnergylevelsareobtainedfromtheresolutionoftheopportuneSchrödinger
equations.Bluearrowsdenotethetransitionsinvolvingachangeintheelectronic
state(fromlefttoright,thickertothinner:electronic,vibronicandrovibronic
transitions).Redarrowsdenotethetransitionsinvolvingdifferentvibrationalstates
(fromlefttoright:vibrationalandro-vibrationaltransitions).Greenarrowsdenote
thetransitionsonlyinvolvingrotationalenergylevels.Darkgreyarrowsdenotethe
transitionsbetweenenergylevelsobtainedfrommagneticfieldsplitting.
100-1000 kJ/mol
1-100 kJ/mol
0.01-1 kJ/mol
1-5×10
-5
kJ/mol
At298K(25°C),foraninstrumentoperatingat60MHzthere
are1,000,009nucleiinthelower(favoured)spinstatefor
every1,000,000thatoccupytheupperspinstate
NMR is less sensitive!

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Questions…!!