em radiation sources 1. fundamentals of em radiation 2. light sources 3. lasers

EM Radiation SourcesEM Radiation Sources1. Fundamentals of EM Radiation1. Fundamentals of EM Radiation

2. Light Sources2. Light Sources

3. Lasers3. Lasers

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 KT = 1200 – 2200 K

Can operate in air (no need for Can operate in air (no need for glass/quartz enclosure)glass/quartz enclosure)

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 AnalysisPrinciples of Instrumental Analysis, , Saunders College Publishing, Fort Worth, 1992.Saunders College Publishing, Fort Worth, 1992.

GlobarGlobar

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

Heated electrically to 1300 – 1500 KHeated electrically to 1300 – 1500 K

Positive temperature coefficient of resistancePositive temperature coefficient of resistance

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

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Tungsten FilamentTungsten Filament


Heated to 2870 K in vacuum or Heated to 2870 K in vacuum or inert gasinert gas

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

Tungsten / Halogen LampTungsten / Halogen Lamp

II22 or Br or Br22 added 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 filamentW is redeposited on the filament

Gives longer lifetimesGives longer lifetimes

Allows higher temperatures (~3500 K) and thus higher Allows higher temperatures (~3500 K) and thus higher apparent brightnessapparent brightness

Arc LampsArc Lamps


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

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

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 nmUseful Range: 185 – 400 nm

Hg Arc LampHg Arc Lamp

Continuum + line sourceContinuum + line source

High power sourceHigh power source

Often used in photoluminescenceOften used in photoluminescence


Douglas A. Skoog and James J. Leary, Douglas A. Skoog and James J. Leary, Principles of Instrumental AnalysisPrinciples of Instrumental Analysis, ,

Saunders College Publishing, Fort Worth, 1992.Saunders College Publishing, Fort Worth, 1992.

Hollow Cathode Discharge TubeHollow Cathode Discharge Tube

Apply ~300 V across electrodesApply ~300 V across electrodes

ArAr++ or Ne or Ne++ travel toward the travel toward the cathodecathode

If potential is high enough If potential is high enough cations will sputter metal off the cations will sputter metal off the electrodeelectrode

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

Hollow Cathode Discharge TubeHollow Cathode Discharge Tube

Line widths are typically 0.01 – 0.02 Line widths are typically 0.01 – 0.02 Å FWHMÅ FWHM


Light-Emitting DiodesLight-Emitting Diodes

Operate with 30-60 mW of power - ~80% efficiencyOperate with 30-60 mW of power - ~80% efficiencyLong lifetimes, stable outputLong lifetimes, stable output

www.wikipedia.orgwww.wikipedia.org

Are you getting the concept?Are you getting the concept?

List one light source with each of the following characteristics.List one light source with each of the following characteristics.

Common IR source:

Spans UV – IR:

Standard household/office lighting:

Lights quickly to full brightness:

Common atomic absorbance source:

Common photoluminescence source:

EM Radiation SourcesEM Radiation Sources1. Fundamentals of EM Radiation1. Fundamentals of EM Radiation

2. Light Sources2. Light Sources

3. Lasers3. Lasers

What is a laser?What is a laser?

www.laserglow.comwww.laserglow.com

LLight ight AAmplification by mplification by SStimulated timulated EEmission of mission of RRadiationadiation

OverallOverall


Stimulated AbsorptionStimulated Absorption

Einstein Coefficient for Absorption BEinstein Coefficient for Absorption B ijij (J (J-1-1 cm cm33):):

with Uwith U: energy density of the field at the: energy density of the field at the

appropriate frequency appropriate frequency (J cm (J cm-3-3 Hz Hz-1-1))

-dnidt

BijUni

-dnidt

BijUni

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

Spontaneous EmissionSpontaneous Emission

jjij

d

dnn A

t-

sp

jji

j

d

dnn A

t-

sp

Einstein Coefficient AEinstein Coefficient Ajiji for for

Spontaneous Emission (sSpontaneous Emission (s-1-1):):


Stimulated EmissionStimulated Emission

jjij

d

dnnUB

t-

st

jji

j

d

dnnUB

t-

st

Einstein Coefficient for Einstein Coefficient for Stimulated Emission:Stimulated Emission:


OverallOverall

iijjjijjii nn

d

dn

UB nUB A t

iijjjijjii nn

d

dn

UB nUB A t


For an ideal black body, the rate of absorption For an ideal black body, the rate of absorption and emission must be balanced:and emission must be balanced:

BBijijUUnnii = A = Ajijinnjj + B + BjijiUUnnjj

Rearrange:Rearrange:

UB A

UB

n

n

i

j

jiji

ij

UB A

UB

n

n

i

j

jiji

ij

Are you getting the concept?Are you getting the concept?

Determine the population ratio for atoms/molecules in two Determine the population ratio for atoms/molecules in two energy states spaced by 1 eV at T = 300 K:energy states spaced by 1 eV at T = 300 K:

Recall: h = 6.63 x 10-34 Js k = 1.38 x 10-23 J/K 1 eV = 1.6 x 10-19 J

nj

ni

We know:We know:

Set equal and solve for USet equal and solve for Uvv::

UB A

UB

n

n

i

j

jiji

ij

UB A

UB

n

n

i

j

jiji

ij

kTe /h-

i

j ji n

n kTe /h-

i

j ji n

n

1 - e

1

c

8 U

/kTh3

3b

h

1 - e

1

c

8 U

/kTh3

3b

h

1 - e

1 U

/kThb

ij

ji

B

A 1 - e

1 U

/kThb

ij

ji

B

A

Looks similar to Planck’s Radiation Law:Looks similar to Planck’s Radiation Law:

3

3h8

cB

A

ij

ji 3

3h8

cB

A

ij

ji

Spectral Energy DensitySpectral Energy Density

Population InversionPopulation Inversion

Goal: More atoms or molecules in the upper energy level Goal: More atoms or molecules in the upper energy level than the lower energy level.than the lower energy level.

Heating the lasing medium will not work:Heating the lasing medium will not work:

nnjj = n = niiee-(E-(Ejj-E-E

ii)/kT)/kT

Must selectively excite atoms/molecules to particular energy Must selectively excite atoms/molecules to particular energy levels. Most common approaches:levels. Most common approaches:

*light*light*electricity*electricity

Optical PumpingOptical Pumping

Intense light source at hIntense light source at h(e.g. flash lamp)(e.g. flash lamp)

Excites to a metastable state to achieve population inversionExcites to a metastable state to achieve population inversion

With fast flashing, initial photons start chain reactionWith fast flashing, initial photons start chain reaction


Electrical DischargeElectrical Discharge

Accelerated eAccelerated e-- and ions excite atoms/molecules and ions excite atoms/molecules into higher energy statesinto higher energy states

Common in gas lasersCommon in gas lasers


Three - Level SystemThree - Level System

No saturationNo saturation

Not very efficientNot very efficient

Better for pulsed mode operationBetter for pulsed mode operation


The ruby laser is a The ruby laser is a three – level laserthree – level laser


Commercial ruby laserCommercial ruby laseroperates with efficiency ~ 1%operates with efficiency ~ 1%

Four - Level SystemFour - Level System

More efficient than 3-levelMore efficient than 3-level

Laser transition does not involve Laser transition does not involve ground state or most highly ground state or most highly excited stateexcited state

Easier to achieve population Easier to achieve population inversioninversion



The He – Ne laser is a The He – Ne laser is a four – level laserfour – level laser

He* + Ne → He + Ne* + ΔE

Resonance Cavity and GainResonance Cavity and Gain


Gain = degree ofamplification basedon positive feedback

GainGain

Gain (G) = eGain (G) = e(n(njj-n-n

ii)b)b

= transition cross-section= transition cross-sectionb = length of active mediumb = length of active medium

Oscillation begins when:Oscillation begins when:

gain in medium = losses of systemgain in medium = losses of system

1122GG22 = 1 = 1

Threshold population inversion:Threshold population inversion:


bnn thij

2

)/1ln()( 21

bnn thij

2

)/1ln()( 21


Light Amplification in Light Amplification in Resonance CavityResonance Cavity

Highly collimated beamHighly collimated beam

Typically ~mm beam width, Typically ~mm beam width, ~mrad divergence~mrad divergence

A typical photon travels A typical photon travels about 50 times forward and about 50 times forward and backward within the cavitybackward within the cavity

em radiation sources 1. fundamentals of em radiation 2. light sources 3. lasers

Documents

e e d

e d e d h e e

d d h d

spectrochemical analysis

e d e d h useful range

nm slide

lasers slide

d d h energetics