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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
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
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
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
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
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
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
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
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
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
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
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
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):):
Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, Reading, MA, 1998., Addison-Wesley, Reading, MA, 1998.
Stimulated EmissionStimulated Emission
jjij
d
dnnUB
t-
st
jji
j
d
dnnUB
t-
st
Einstein Coefficient for Einstein Coefficient for Stimulated Emission:Stimulated Emission:
Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, Reading, MA, 1998., Addison-Wesley, Reading, MA, 1998.
OverallOverall
iijjjijjii nn
d
dn
UB nUB A t
iijjjijjii nn
d
dn
UB nUB A t
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
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
Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, Reading, MA, 1998., Addison-Wesley, Reading, MA, 1998.
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
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
Three - Level SystemThree - Level System
No saturationNo saturation
Not very efficientNot very efficient
Better for pulsed mode operationBetter for pulsed mode operation
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
The ruby laser is a The ruby laser is a three – level laserthree – level laser
Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, Reading, MA, 1998., Addison-Wesley, Reading, MA, 1998.
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
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
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
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
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:
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
bnn thij
2
)/1ln()( 21
bnn thij
2
)/1ln()( 21
Eugene Hecht, Eugene Hecht, OpticsOptics, Addison-Wesley, Reading, MA, 1998., Addison-Wesley, Reading, MA, 1998.
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