1 Introduction to Spectroscopic methods Spectroscopy: Study of interaction between radiation (or...

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Introduction to Spectroscopic methodsIntroduction to Spectroscopic methods

Spectroscopy:Spectroscopy:

Study of interaction between Study of interaction between radiation (or other radiation (or other forms of energy)forms of energy) and and matter matter (a branch of (a branch of science). science).

Spectrometry:Spectrometry:

Analytical methodsAnalytical methods based on atomic and based on atomic and molecular spectroscopymolecular spectroscopy

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Types of Analytical Types of Analytical SpectroscopySpectroscopy

AbsorptionAbsorption Fluoresence and PhosphoresenceFluoresence and Phosphoresence Emission (atomic with flames, Emission (atomic with flames,

arcs, sparks, and palsmas)arcs, sparks, and palsmas) Chemilumenesence and Chemilumenesence and

BiolumenesenceBiolumenesence ReflectionReflection

The Electromagnetic The Electromagnetic SpectrumSpectrum

= c / E = h

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What about E?What about E?

= c / E = h

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Kinds of Kinds of SpectroscopySpectroscopy

Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

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LIGHTLIGHT

Electro-Electro-magnetic magnetic radiationradiation

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Light as a WaveLight as a Wave


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Frequency = Frequency = Velocity of propagation = Velocity of propagation = vv = = Speed of light in a vacuum = c = 3.00 x 10Speed of light in a vacuum = c = 3.00 x 1088 m/s m/sWavenumber (reciprocal of Wavenumber (reciprocal of ) = ) = = = kk = = //vv


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Effect of the Medium on a Light WaveEffect of the Medium on a Light Wave

• Frequency remains the same.Frequency remains the same.

• Velocity and Wavelength change.Velocity and Wavelength change.


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Mathematic Description of a WaveMathematic Description of a Wave

Y = A sin(Y = A sin(t + t + ))

A = Amplitude A = Amplitude

= angular frequency = 2= angular frequency = 2 = =v2v2

= phase angle= phase angle

Y = A sin(2Y = A sin(2t + t + ))


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Mathematic Description of a WaveMathematic Description of a Wave


Sine waves with different amplitudes and with a phase different of 90 degree

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

CoherenceCoherence: : When two waves have an initial phase When two waves have an initial phase difference of zero or it is constant for a long time they difference of zero or it is constant for a long time they are considered coherent.are considered coherent.

Superposition of WavesSuperposition of Waves

Y = AY = A11sin(2sin(211t + t + ) + ) + AA22sin(2sin(222t + t + ) +…….) +…….

Optical Optical Interference:Interference: The interaction of two or more The interaction of two or more light waves yielding an irradiance that is not equal to light waves yielding an irradiance that is not equal to the sum of the irradiances.the sum of the irradiances.

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Optical InterferenceOptical Interference

Constructive InterferenceConstructive Interference1) Have identical frequency1) Have identical frequency

2)2)22 – – 11 = = = = m2m2

Destructive InterferenceDestructive Interference1) Have identical frequency1) Have identical frequency

2)2)22 – – 11 = = = (2m+1) = (2m+1)

Figure 3-4 – Ingle and Crouch, Figure 3-4 – Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

22 – – 11 = 180 deg or integer = 180 deg or integer

of multiple of 360 deg. of multiple of 360 deg.

22 – – 11 = 0, or 360 deg or = 0, or 360 deg or

integer of multiple of 360 deg. integer of multiple of 360 deg.

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Superposition of sinusoidal wave: (a) A1 < A2, (1 - 2) = 20º, 1 = 2;(b) A1 < A2, (1 - 2) = 200º, 1 = 2


16Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007Douglas A. Skoog, et al. Principles of Instrumental Analysis, Thomson, 2007

Superposition of tw sinusoidal wave of different frequencies but identical amplitudes.

Should be

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Diffraction: The Bending of Light as It Diffraction: The Bending of Light as It Passes Through an Aperture or Around Passes Through an Aperture or Around

a Small Objecta Small Object

Fraunhofer Fraunhofer DiffractionDiffractionNarrow SlitNarrow SlitDiffractionDiffraction


Diffraction is a consequence of interference

18Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, Reading, MA, 1998., Addison-Wesley, Reading, MA, 1998.

Diffraction increases as Diffraction increases as aperture size aperture size

Diffraction of Waves in a LiquidDiffraction of Waves in a Liquid

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Diffraction Pattern From Diffraction Pattern From Multiple SlitsMultiple Slits


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CF = BC sin = nn is an integer called order of interference

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Coherent RadiationCoherent Radiation

Conditions for coherent of

two sources of radiation are:

1.Identical frequencies and wavelength

2.Phase relationship remains constant with time

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Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, , Addison-Wesley, Reading, MA, 1998.Reading, MA, 1998.

Conservation LawConservation Law

TT = 1 = 1

= Fraction Absorbed= Fraction Absorbed

= Fraction Reflected= Fraction Reflected

TT = Fraction = Fraction TransmittedTransmitted

What happens when light What happens when light hits a boundary between hits a boundary between two media?two media?

RefractionRefraction: : change in direction change in direction of radiation as it passes from of radiation as it passes from one medium to another with one medium to another with different densitydifferent density

Physics of RefractionPhysics of Refraction

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Refractive index (Refractive index (nn))

the velocity (v) of EM radiation the velocity (v) of EM radiation depends on the medium depends on the medium through which it travelsthrough which it travels

nnii = c/v = c/vii (>1). (>1). the ratio of the velocity in vacuum the ratio of the velocity in vacuum

over the velocity in the medium over the velocity in the medium nn depends on the frequency of depends on the frequency of

the lightthe light

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RefractionRefraction

nn11sinsin11 = = nn22sinsin22

Snell’s LawSnell’s Law

vv22sinsin1 = v1 = v11sinsin22

Douglas A. Skoog, Douglas A. Skoog, et al.et al. Principles of Instrumental Analysis, Principles of Instrumental Analysis, ThomsonThomson, , 20072007

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RefractionRefraction

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Transmission: The Refractive IndexTransmission: The Refractive Index

Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, Reading, MA, 1998., Addison-Wesley, Reading, MA, 1998.

vn

c v

nc

n is wavelength (frequency) n is wavelength (frequency) dependent.dependent.

In glass n increases as In glass n increases as decreases.decreases.

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Dispersion and PrismsDispersion and Prisms

iv

c iv

c

DispersionThe variation in refractive index of a substance with wavelength or frequency

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Dispersion and PrismsDispersion and Prisms


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A ray of single-wavelength incident on a prismA ray of single-wavelength incident on a prism

12 3

: angle of deviation

Cai® 2007

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A ray of white-wavelength incident on a prismA ray of white-wavelength incident on a prism

RB

White light

Cai® 2007

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Reflection of RadiationReflection of Radiation

212

212

0 )(

)(

nn

nn

I

I r

I0: intensity of incident lightIr: reflected intensity

For monochromatic For monochromatic light hitting a flat light hitting a flat surface at 90surface at 9000

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Reflection of RadiationReflection of Radiation

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Specular reflection: Reflection of light from a Specular reflection: Reflection of light from a smooth surfacesmooth surface

Diffuse reflection: Reflection of light from a rough Diffuse reflection: Reflection of light from a rough surfacesurface

Smooth or rough surface ???????

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Reflection

Refraction

M1

M2

37Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

at different interfacesat different interfacesReflectance Reflectance is is the fraction of the incident radiant energy refelcted.the fraction of the incident radiant energy refelcted.

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Scattering of RadiationScattering of Radiation

Rayleigh scatteringRayleigh scattering Molecules or aggregates of molecules Molecules or aggregates of molecules

smaller than smaller than Scattering by big moleculesScattering by big molecules

Used for measuring particle sizeUsed for measuring particle size Raman ScatteringRaman Scattering

Involves quntized frequency changesInvolves quntized frequency changes

The fraction of radiation transmitted at all angles from its original path

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Serway, Physics, 4th edition, 1996

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Light as ParticlesLight as Particles

c Eh

h

c

Eh

h


hh = Planck Constant = 6.63 = Planck Constant = 6.63 10 10-34-34 Js Js

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The Photoelectric EffectThe Photoelectric Effect

Vo: Stopping voltage (the negative voltage at which the photocurrent is zero)

eV0 = h -

: work needed to remove e-


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Cut-off

Current is Current is proportional to the proportional to the intensity of the intensity of the radiationradiation

VV00 depends on the depends on the frequency of the frequency of the radiation and the radiation and the chemical composition chemical composition of the coatingof the coating

VV00 depends on depends on the the chemical composition chemical composition of the coating on the of the coating on the photocathodephotocathode

VV00 independent of the independent of the intensity of the intensity of the radiationradiation


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Energy States of Energy States of ChemicalChemical

Quantum theory by Quantum theory by PlanckPlanck (1900) (1900)

Black body radiationBlack body radiation Atoms, ions , and Atoms, ions , and

molecules exist in molecules exist in discrete statesdiscrete states

Characterized by Characterized by definite amounts of definite amounts of energyenergy

Changes of state Changes of state involve absorption or involve absorption or emission of energyemission of energy

EE11-E-E00 = = hh = = hhc/c/

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Interaction of Radiation Interaction of Radiation and Matterand Matter

Emission and Chemiluminescence Process

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Absorption Process

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Photoluminescence method (Fluorescence and phosphorescence)

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Inelastic Scattering in Raman Spectroscopy

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Emission of RadiationEmission of Radiation

Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

EmissionEmission XX** X + h X + h

Excitation needs energy!

•Particle bombardment (e-)

•Electrical currents (V)

•Fluorescence

•Heat

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Emission: Saltwater in Emission: Saltwater in a flamea flame

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Line SpectraLine Spectra

Individual atoms, well separated, in a gas phase

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Band SpectraBand Spectra

Small molecules and radicals

Vibrational levels

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Continuum SpectraContinuum Spectra Produced when solid are heated to Produced when solid are heated to

incandescence. incandescence. Blackbody Radiation (Thermal Radiation) Blackbody Radiation (Thermal Radiation)

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Blackbody RadiationBlackbody Radiation A A blackbodyblackbody is a theoretical is a theoretical

object, (i.e. object, (i.e. emissivityemissivity = 1.0), = 1.0), which is both a perfect absorber which is both a perfect absorber and emitter of radiation. and emitter of radiation. Common usage refers to a source Common usage refers to a source of infrared energy as a of infrared energy as a "blackbody" when it's emissivity "blackbody" when it's emissivity approaches 1.0 (usually e = 0.99 approaches 1.0 (usually e = 0.99 or better) and as a "graybody" if or better) and as a "graybody" if it has lower emissivity. it has lower emissivity.

Important sources of infrared, Important sources of infrared, visible, and long wavelength UV visible, and long wavelength UV for analytical instrumentsfor analytical instruments

http://www.electro-optical.com/bb_rad/bb_rad.htm

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Wien’sWien’sDisplacement LawDisplacement Law

T

nmK 10 2.897

6

max

T

nmK 10 2.897

6

max


Stefan-Boltzman LawStefan-Boltzman Law

P = P = TT44

= 5.6697 = 5.6697 10 10-12-12 Wcm Wcm-2-2KK-4-4

Blackbody RadiationBlackbody Radiation

Both max and radiation power (P) are related to TEMPERATURE and current!

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Continuum SourceContinuum Source Line SourceLine Source

Continuum + Line SourceContinuum + Line Source

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Al + Mg

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Ranges of Common SourcesRanges of Common Sources

Douglas A. Skoog and James J. Leary, Douglas A. Skoog and James J. Leary, Principles of Instrumental Principles of Instrumental AnalysisAnalysis, Saunders College Publishing, Fort Worth, 1992., Saunders College Publishing, Fort Worth, 1992.

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Optical Source CharacteristicsOptical Source Characteristics


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Nernst GlowerNernst GlowerRare earth oxides formed into a Rare earth oxides formed into a cylinder (1-2 mm diameter, cylinder (1-2 mm diameter, ~20mm long).~20mm long).

Pass current to give:Pass current to give:T = 1200 – 2200 K.T = 1200 – 2200 K.

Ingle and Crouch, Ingle and Crouch, Spectrochemical Spectrochemical AnalysisAnalysis

Douglas A. Skoog and James J. Leary, Douglas A. Skoog and James J. Leary, Principles of Instrumental Principles of Instrumental AnalysisAnalysis, Saunders College Publishing, Fort Worth, 1992., Saunders College Publishing, Fort Worth, 1992.

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GlobarGlobar

Silicon Carbide Rod (5mm diameter, 50 mm long).Silicon Carbide Rod (5mm diameter, 50 mm long).

Heated electrically to 1300 – 1500 K.Heated electrically to 1300 – 1500 K.

Positive temperature coefficient of resistancePositive temperature coefficient of resistance

Electrical contact must be water cooled to prevent arcing.Electrical contact must be water cooled to prevent arcing.


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Tungsten FilamentTungsten Filament


Heated to 2870 K.Heated to 2870 K.

Useful Range: 350 – 2500nmUseful Range: 350 – 2500nm

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Tungsten / HalogenTungsten / Halogen

Iodine added.Iodine added.

Reacts with gaseous W near the quartz wall to form WIReacts with gaseous W near the quartz wall to form WI22..

W is redeposited on the filament.W is redeposited on the filament.

Gives longer lifetimesGives longer lifetimes

Allows higher temperatures (~3500 K).Allows higher temperatures (~3500 K).

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Intensity Spectrum of Intensity Spectrum of the Tungsten-Halogen the Tungsten-Halogen

LampLamp

• Weak intensity in UV range• Good intensity in visible range• Very low noise• Low drift

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Arc LampsArc Lamps


Electrical discharge is Electrical discharge is sustained through a gas or sustained through a gas or metal vapor.metal vapor.

Continuous emission due to Continuous emission due to rotational/vibrational energy rotational/vibrational energy levels and pressure levels and pressure broadening.broadening.

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HH22 or D or D22 Arc Lamps Arc Lamps


DD22 + E + Ee-e- D D22* * D’ + D” + h D’ + D” + h

Energetics:Energetics: EEe-e- = E = EDD22** = E = ED’D’ + E + ED”D” + h + h

Useful Range: 185 – 400 nm.Useful Range: 185 – 400 nm.

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Intensity Spectrum of Intensity Spectrum of the Xenon Lampthe Xenon Lamp

• High intensity in UV range• High intensity in visible range• Medium noise

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Hg Arc LampHg Arc Lamp

Continuum + Line SourceContinuum + Line Source

High Power Source.High Power Source.

Often used in photoluminescence.Often used in photoluminescence.


67Douglas A. Skoog and James J. Leary, Douglas A. Skoog and James J. Leary, Principles of Instrumental Principles of Instrumental AnalysisAnalysis, Saunders College Publishing, Fort Worth, 1992., Saunders College Publishing, Fort Worth, 1992.

Hollow Cathode Discharge Tube.Hollow Cathode Discharge Tube.

Apply ~300 V across Apply ~300 V across electrodes.electrodes.

ArAr++ or Ne or Ne++ travel toward the travel toward the cathode.cathode.

If potential is high enough If potential is high enough cations will sputter metal off cations will sputter metal off the electrode.the electrode.

Metal emits photons at Metal emits photons at characteristic atomic lines as characteristic atomic lines as the metal returns to the the metal returns to the ground state.ground state.

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Hollow Cathode Discharge Tube.Hollow Cathode Discharge Tube.

Line Widths are typically 0.01 – 0.02 Line Widths are typically 0.01 – 0.02 Å.Å.


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Absorption of Absorption of RRadiationadiation

Is a quantized process???Is a quantized process??? The energy absorbed is released, although The energy absorbed is released, although

not necessarily all as light energy (e.g. heat)not necessarily all as light energy (e.g. heat) Results in excitation of a molecule to a Results in excitation of a molecule to a

higher energy statehigher energy state E= E E= E electronicelectronic + E + E vibrationalvibrational + E + E rotationalrotational

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Absorption of Absorption of RadiationRadiation

Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

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Atomic absorption

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Rotational energy levels associated with each vibrational level not shown

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Relaxation Resonance fluorescence

F = A

Non- Resonance fluorescence F A

Stokes shift F > A

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Quantitative Aspects of Quantitative Aspects of Spectrochemical Spectrochemical MeasurementsMeasurements

Radiation power PRadiation power P The energy of the a beam of The energy of the a beam of

radiation that reaches a given radiation that reaches a given area per secondarea per second

S =kPS =kP

S is an electrical signalS is an electrical signal Dark currentDark current

Response of the detector in the Response of the detector in the absence of radiationabsence of radiation

S =kP + S =kP + kkdd

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Quantitative Aspects of Quantitative Aspects of Spectrochemical Spectrochemical MeasurementsMeasurements

TransmittanceTransmittance T = P/PT = P/Po o (definition)(definition)

PPoo - incident light power - incident light power P - transmitted light powerP - transmitted light power

%T = P/P%T = P/Po o x 100 %x 100 % AbsorbanceAbsorbance

A = - log T A = - log T (definition)(definition) Beer’s Law Beer’s Law (physical law applicable under (physical law applicable under

certain conditions)certain conditions) A = A = b c b c (basis of quantitation)(basis of quantitation)

- molar absorptivity (L mol- molar absorptivity (L mol-1-1 cm cm-1-1)) b - pathlength (cm)b - pathlength (cm) c - concentration (mol Lc - concentration (mol L-1-1))

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Non-radiative Non-radiative relaxationrelaxation

Vibrational Relaxation:Vibrational Relaxation: A molecule can give off some of its energy from A molecule can give off some of its energy from

absorbed light (usually uv-vis) by jumping to a lower absorbed light (usually uv-vis) by jumping to a lower energy vibrational state.energy vibrational state.

The excess energy is used to make the conversion. No The excess energy is used to make the conversion. No light is given off.light is given off.

Internal Conversion:Internal Conversion: The molecule transitions to a lower energy electronic The molecule transitions to a lower energy electronic

state without giving off light. state without giving off light. Excess energy is used to covert the molecule from one Excess energy is used to covert the molecule from one

electronic state to another.electronic state to another. Poorly understoodPoorly understood

External conversion:External conversion: The molecule gives off energy to an external source, The molecule gives off energy to an external source,

such as by collision with another similar molecule or such as by collision with another similar molecule or solvent molecule. This is called solvent molecule. This is called ““quenchingquenching””

Intersystem Crossing:Intersystem Crossing: The molecule goes from a singlet to triplet excited The molecule goes from a singlet to triplet excited

state and uses up energy changing the spin of an state and uses up energy changing the spin of an electron.electron.

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