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Analytical SpectroscopyReviewλ = wavelength

ν = frequency

V = velocity = ν x λ

= 2.998 x 108 m/sec

= c (in a vacuum)

ν is determined by source and does not change as wavepropogates, but V can change as wave passes through matter, it becomes less than c. Therfore, λ also changes in matter.

Electromagnetic Spectrum & Quantum transitions

Nuclear Inner electron

Valence electron Molecular

vibrationsMolecular rotations

Nuclear spin

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Superposition of wavesPrinciple of superposition – when two or more wave disturbances occupy the same space at the same time, the resultant disturbance can be predicted by the sum of the individual disturbances.

Electric Field representation

y = A sin (2πνt + φ) = A sin (ωt + φ)

A = amplitude

ω = angular frequency

)sin(1

iin

iitotal tAY φω +∑=

=

Constructive interference

Destructive interference

Wavelength Selection

Separation of EM radiation into it’s constituent frequency components (or colours).

In analytical spectroscopy, we may have wavelength selection devices BEFORE sample or AFTER sample, depending on the type of spectroscopy.

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Basic Optical SchemesLight source 6 sample 6 wavelength selector 6 detector

Absorption

Emission – some luminescence, atomic emission,

Excited sample 6 wavelength selector 6 detector

Light induced Emission – fluorescence, phosphorecence, Raman…

Laser or other excitation light source

Excitation light source 6 sample 6 wavelength selector 6 detector

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Components of Optical SpectrometersLight Sources

Incandescent - made of resistive materials that are heated electrically

Arc - an electrical discharge is maintained through a gas or metalvapour. (continuous and line sources).

Continuous source - outputs a continuous intensity of radiation as a function of wavelength. Extensively used in molecular spectroscopy.

examples: deuterium lamp (UV) tungsten filament (VIS) Nernst Glower (IR) Xe arc (UV-VIS)

Arc

Filament

Deuterium

Glow bar

Arc with parabolic reflector

Spectral Irradiance Curves-Xe

Typical spectral irradiance of 150 W Xe Lamp.

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Spectral Irradiance Curves-Deuterium

Fig. 3 Typical spectral irradiance of 1000 W QTH Lamp.

Spectral irradiance of various Deuterium Lamps.

Spectral Irradiance Curves-Quartz Tungsten Halogen (QTH)

Typical spectral irradiance of 1000 W QTH Lamp.

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Line sources - have radiation at discrete quantized energies.

Mercury lamps - many discrete lines in UV and visible

Hollow cathode - for atomic spectroscopy

Lasers - a special light source in a class of it’s own, many different types and wavelengths

Fig. 1 Typical spectral irradiance of 200 W Hg Lamp.

Stimulated and Spontaneous Emission

M* 6 M + hν

Emission of photon from excited state species is random in time and direction

hν + M* 6 M + 2 hν

Emission of photon from excited state species is induced by first photon. Both photons that leave have the same direction, phase and frequency…coherent radiation!

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BlackbodiesBlackbody radiator absorbs all EM radiation without reflection (ie-black) but is in thermal equilibrium with surroundings…it’s rate of absorption is equal to rate of emission.

Planck’s Law (flux distribution)

112

/5

2

−=Φ kThc

b

ehc

λλ λπ

Wien’s Displacement Law (max λ)

TKnm

khc 6

max10897.2

5×

==λ

λ in nm, T in kelvins

Total radiation of black body

Stefan-Boltzmann constant

Kirchoffs Law

Real bodies are not "black", they do not absorb radiation with 100% efficiency at all wavelengths. Kirchoff's law states that if an object absorbs radiation with efficiency, ελ, , then it emits radiation with a fractional efficiency ελ of the blackbody emission at temperature, T.

Φλ (T) = ελ(T) Φλb(T)

The emission spectrum is known if we know the absorption spectrum and T

428

4432

45

0

1067.5

152

−−−

∞

=

==Φ=Φ ∫KWmx

TThckdtotal

σ

σπλλ

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Solar Radiation Spectra

Solar radiation flux at sea level is less than at the top of the atmosphere due to reflection by clouds, aerosols and absorption features by O2, O3, H2O and other trace gases.

The earth as a blackbody radiator

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LasersLight Amplification by Stimulated Emission of Radiation

Power source~100% ~95-99%

Lasing medium

Properties of Lasers

Monochromatic - single wavelength

Coherent - all photons appear to originate from a single point, electrical fields are all in phase.

Bright – spectral irradiance (intensity per unit wavelength) is much higher than conventional sources

Focussing – can be focussed to extremely small spots (λ/2)

Pumping and Population Inversions

Pumping – an external source provides energy to the lasing medium in order to achieve a "population inversion". A population inversion is necessary for a laser to operate. Pumps may include electrical discharge, optical energy from a flashlamp, or another laser (pump laser).

Population inversion – there more molecules or atoms are in the upper state (S2,) than in the lower state (S1) of the quantum transition. N2 > N1

x x x x x x x x x x x x

x x x x x x x x

S2 - State 2, E2 - Energy 2 N2 - # of molecules

S1 - State 1, E1 - Energy 1 N1 - # of molecules

Lasing transition – S2 S1

∆E = E2 – E1 = hν = hc/λ

N2 > N1

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2-Level Systemx x x x x x x x x x x x

x x x x x x x x

S2 , E2 , N2

σ2 1 cross section of stimulated emission

S1 , E1 , N1

σ1 2 cross section of absorption

Each absorption event causes a decrease in the radiant flux of photons, Φ, inside the lasing cavity. Each stimulated emission event causes an increase in Φ. From spectroscopic theory, it is known that σ1 2 = σ2 1.

The change in radiant flux, dΦ, inside a laser cavity along the axis starting from an arbitrary point, and travelling a distance dz is given by...

dΦ = + Φ N2 σ2 1 dz - Φ N1 σ1 2 dz = + Φ (N2 - N1) σ dz.

For lasing action to occur, the radiant flux of photons inside the cavity must be amplified ..ie- must be > 0. Therefore, (N2 - N1) > 0 OR N2 > N1 for lasing action.

If N2 > N1, dΦ > 0, population inversion exists, SE dominates, lasing action occurs.

If N2 = N1, dΦ = 0, no change in Φ (but considering other losses, NO lasing)

If N2 < N1, dΦ < 0, Φ decreases with z, absorption dominates, ...ie- Beers Law.

Boltzmann Considerations

Boltzmann Equation

Nj = number of molecules or atoms in state j

N0 = number of molecules or atoms in state 0 (ground state for example)

Pj = multiplicity of state j (2 electrons per state X number of levels with energy j)

P0 = multiplicity of state 0.

DE = energy difference between state j and 0 (Ej > E0) .

k = Boltzmann constant (1.28 x 10-23 J K-1)

T = temperature (Kelvin).

For 2 states with equal multiplicity, Nj / N0 < 1.0 at all times since T > 0K , and ∆E > 0. The best we can do is to heat a system to high T or to minimize the difference in energy between two states, but even if we do this, Nj/N0 only approaches 1.0, but is always less than 1. Therefore we cannot achieve a population inversion by heating a laser system....we must achieve a population inversion in some other way.

∆−=kTE

PP

NN jj exp

00

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3-Level and 4-Level Laser Systems

S1, E1

S0 , E0

fast non radiative transition

S2, E2

laser transitionpumpE

S2, E2

S0 , E0

fast non-radiative transition

S3, E3

laser transitionpumpE

S1, E1

fast transition

3-Level System

If σ0->2 > σ0->1, and if the 2 1 non-radiative transition is fast, then we can achieve a population inversion between state 1 and 0 by optically pumping at the E2 – E0 wavelength.

4-Level System

Since the laser transition in a 4-level system does not end up in the ground state, the population in the lowest state of the laser transition, S1 is very low. Thus it is even easier to obtain a population inversion, N2 > N1 in this type of system.

4-level systems are common. The first demonstrated laser, a ruby laser, was based upon a 3-level system

Types of LasersGas Lasers

He/Ne – a mixture of neutral gases in the laser cavity. the laser active gas is Ne. He gas is used for collisional pumping....see energy diagram. Lines at 632.8, 612.0 , 594.1, 543.5 nm. The red laser at 632.8nm is most common and most intense.

He/Cd – economical continuous wave laser source with lines at 442 and 325 nm. Cadmium is the active medium. heat is applied to a reservoir of Cd metal in order to vaporize the metal.

Ar+ - used extensively in Raman and laser induced fluorescence spectroscopy. Many laser lines available, most intense are at 488 and 514 nm.

Solid State Lasers - active medium is a host atom embedded in a solid state material.

Ruby – Cr III in Al2O3. pumped with a flash lamp; 3-level system.

Nd:YAG – Nd+3 in Yttrium Aluminum Garnet. 1064 nm IR laser. Very high power. these can be frequency doubled (532 nm) or tripled (355 nm). Used for commercial applications.

Dye Lasers – active lasing medium is a dye or mixture of dyes dissolved ain a solvent that is pumped through the lasing cavity. these lasers can be "tuned" to lase over a broad range of λ's. They are often pumped with a primary laser.

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Wavelength selectorsfilters, prisms, gratings and monochromators

these devices are used in all types of optical spectroscopy in order to select a specific range of wavelengths (bandwidth) from a much broader band of light.\

polychromatic wavelength monochromaticradiation selector radiation

Interference fi lter

silver films(semitransparent)

glass glassdielectric

Interference filters

through constructive interference, only a a narrow band of light is allowed to pass while the rest of the light is rejected (reflected).

Long Pass colored filters

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Interference Filters

Interference filters-cont'd

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Interference filters – cont'd 2

Monochromators

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Monochromators – prism vs. grating dispersion

Monochromator Function

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Gratings

θ = 45oC

Grating

λ = ??

Grating Selection# Intended Use Lines/m

mSpectral Range Blaze

WavelengthBest Efficiency(>30%)

1 UV 600 650 nm 300 nm 200-575 nm2 UV/VIS 600 650 nm 400 nm 250-800 nm3 VIS/Color 600 650 nm 500 nm 350-850 nm4 NIR 600 625 nm 750 nm 530-1100 nm5 UV/VIS 1200 300 nm holographic/UV 200-400 nm6 NIR 1200 200-270 nm* 750 nm 500-1100 nm7 UV/VIS 2400 100-140 nm* holographic/UV 200-500 nm8 UV 3600 50-75 nm holographic/UV 290-340 nm9 VIS/NIR 1200 200-300 nm* holographic/VIS 400-800 nm10 UV/VIS 1800 100-190 nm* holographic/UV 200-635 nm11 UV/VIS 1800 120-160 nm* holographic/VIS 320-800 nm12 UV/VIS 2400 50-120 nm* holographic/VIS 250-575 nm*13 UV/VIS/NIR 300 1700 nm** 500 nm 300-1100 nm14 NIR 600 625 nm 1000 nm 650-1100 nm

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Grating Efficiency

Grating –example

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Grating Monochmomator –

performance

Slits

19

resolution

Photon Detectors

20

Phototube

Photomultiplier tube (PMT)

21

Photodiodes

Fiber Optics

22

Photodiode Array

CCD Signal Collection

23

24

Jablonski Energy Level Diagram

25

relaxation mechanisms

beers law

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derivation of beers law

deviations from beers law

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deviations from beers law 2

Absorbance 1ε

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Absorbance 2

absorbance 3

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absorbance instruments

absorbance instruments 2

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luminescence

fluorescence

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fluorescence 2

Fluorescence3

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Excitation, Fluorescence and Phosphorescence of Phenanthrene

excitation

fluorescence phosphorescence

Schematic of a generic fluorescence instrument

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Filter Fluorometer

Excitation filterEmission filter

Spectrofluorometer

ORPDA or CCD

multichanneldetector

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Excitation and Fluorescence Spectra of Anthracene

Excitation Spectra – similar to absorbance spectra. Obtained by measuring luminescence at a fixed wavelength while scanning the excitation monochromatorwavelength.

Fluorescence (emission) Spectra – obtained by holding excitation wavelength constant while scanning the emission monochromator

Factors affecting fluorescence

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FAF2

Fluoresc vs abs

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Phosphorescence

Phos instrument

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Chemiluminescence 1

Chemilum 2

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Quiz 5Quizzes – worth 6%, best 3 out of 5.....2% each.For those who need a final quiz, it will be held on Monday April 4. Two times, 2 versions...QUIZ 5A - 12:30 - 1:00 QUIZ 5B – 2:30 – 3:00

Please meet at my office (PS 301) 5 minutes before quiz time, the location to write will be announced at that time.

Topic – Luminescence & Atomic Spectroscopy.

For those who do well, the quiz could be worth 4%, 2% applied to quiz mark, 2% applied to your last test mark (if you do better on the quiz than your test.

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