Are you getting the concept? If the average irradiance from the Sun impinging normally on a surface...

Are you getting the concept?Are you getting the concept?If the average irradiance from the Sun impinging normally on a If the average irradiance from the Sun impinging normally on a surface just outside the Earth’s atmosphere is 1400 W/msurface just outside the Earth’s atmosphere is 1400 W/m22, what is , what is the resulting pressure (assuming complete absorption)? How the resulting pressure (assuming complete absorption)? How does this pressure compare with atmospheric pressure (~ 10does this pressure compare with atmospheric pressure (~ 1055 N/mN/m22)?)?

<<PP(t)>(t)>TT = I/c = I/c

= (1400 W/m= (1400 W/m22)/(3.00 x 10)/(3.00 x 1088 m/s) = 4.7 x 10 m/s) = 4.7 x 10-6-6 W/m W/m··ss = 4.7 x 10= 4.7 x 10-6-6 N/m N/m22

This pressure is less than 5 x 10This pressure is less than 5 x 10-9-9% of atmospheric pressure.% of atmospheric pressure.When accounting for the surface area of the EarthWhen accounting for the surface area of the Earth(5.11 x 10(5.11 x 101414 m m22), this provides 2.4 x 10), this provides 2.4 x 1055 tons of force. tons of force.

Reminder: 1 W = 1 J/s = 1 NReminder: 1 W = 1 J/s = 1 N··m/s = 1 kgm/s = 1 kg··mm22/s/s33

Photon EmissionPhoton EmissionE. Hecht, E. Hecht, OpticsOptics, 1998., 1998.

• atom in ground stateatom in ground state• atom excited by high T or collision, stays in excited quantum atom excited by high T or collision, stays in excited quantum

state for 10state for 10-8-8 or 10 or 10-9-9 sec sec• atom returns to ground state, emitting a photonatom returns to ground state, emitting a photon

Frequency of emitted light is associated with the quantized Frequency of emitted light is associated with the quantized atomic transition (atomic transition (E = hE = h))

Photon RadiationPhoton Radiation

Figure 5-16 Partial energy-level diagram for a fluorescentFigure 5-16 Partial energy-level diagram for a fluorescentorganic molecule.organic molecule.

Skoog and Leary, Skoog and Leary, Principles of Instrumental AnalysisPrinciples of Instrumental Analysis, 1992., 1992.

Are you getting the concept?Are you getting the concept?Many streetlights are sodium discharge lamps. The emitted Many streetlights are sodium discharge lamps. The emitted orange light is due to the sodium D-line transition: orange light is due to the sodium D-line transition:

What is the energy level spacing (in eV) for What is the energy level spacing (in eV) for the 3p the 3p → 3s transition?→ 3s transition?

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

2. Light Sources2. Light Sources

3. Lasers3. Lasers

Optical SourceOptical SourceCharacteristicsCharacteristics

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.

Continuum SourceContinuum Source Line SourceLine Source

Continuum + Line SourceContinuum + Line Source


Incandescent LampIncandescent Lamp

1. Glass bulb (or "envelope") 2. Low pressure inert gas 3. Tungsten filament 4. Contact wire (goes to foot) 5. Contact wire (goes to base) 6. Support wires 7. Glass mount/support 8. Base contact wire 9. Screw threads 10. Insulation 11. Electrical foot contact

www.wikipedia.org

Black-body RadiationBlack-body Radiation

In an ideal Black body:In an ideal Black body:

(() = 1, ) = 1, (() = 0, T() = 0, T() = 0) = 0

Because a black body is at thermal equilibrium, emission Because a black body is at thermal equilibrium, emission must equal absorption. must equal absorption.

Thus, black bodies are perfect absorbers and the most Thus, black bodies are perfect absorbers and the most efficient emitters possible.efficient emitters possible.

There are no ideal black bodies.There are no ideal black bodies.

Spectral Distribution of Emission is Characteristic of Spectral Distribution of Emission is Characteristic of the Temperature of the Blackbodythe Temperature of the Blackbody

As T increases, As T increases, maxmax decreases. decreases.

Donald McQuarrie, Donald McQuarrie, Quantum ChemistryQuantum Chemistry, University , University Science Books, Mill Valley, CA, 1983.Science Books, Mill Valley, CA, 1983.

www.wikipedia.org

Rayleigh – Jeans LawRayleigh – Jeans Law

4b ckT2

B 4

b ckT2 B

bBbB

Spectral RadianceSpectral Radiance (Jm(Jm-3-3ss-1-1))

The Ultraviolet CatastropheThe Ultraviolet Catastrophe

Approximate Blackbody ExpressionsApproximate Blackbody Expressions

Wien’s LawWien’s Law

kThceh

/

5

2b c2

B kThceh

/

5

2b c2

B

www.wikipedia.org

Resolved (inadvertently) Resolved (inadvertently) in 1900 by Max Planck.in 1900 by Max Planck.

Assumed atoms could Assumed atoms could only absorb or emit only absorb or emit discrete amounts of discrete amounts of energy.energy.

Planck’s Radiation Law:Planck’s Radiation Law:

1 - e

1

c

8 U

/kTh3

3b

h

1 - e

1

c

8 U

/kTh3

3b

h

Donald McQuarrie, Donald McQuarrie, Quantum ChemistryQuantum Chemistry, University , University Science Books, Mill Valley, CA, 1983.Science Books, Mill Valley, CA, 1983.

bUbU

Spectral Energy DensitySpectral Energy Density (Jcm(Jcm-3-3HzHz-1-1))

1

12B

/5

2

kThc

b

e

hc 1

12B

/5

2

kThc

b

e

hc

Wien’s Displacement LawWien’s Displacement Law

T

nmK 10 2.897

6

m

Eugene Hecht, Eugene Hecht, OpticsOptics, 1998., 1998.

Differentiate Planck’s Differentiate Planck’s Law with respect to Law with respect to and and set equal to zero to find set equal to zero to find mm (wavelength of (wavelength of

maximum irradiance):maximum irradiance):

Stefan-Boltzman LawStefan-Boltzman Law

MMbb = = TT44

= 5.6697 = 5.6697 10 10-12-12 W W··cmcm-2-2··KK-4-4

Integrate Planck’s Law to Integrate Planck’s Law to find the total emittance of find the total emittance of a black body:a black body:

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

Suppose that we measure the emitted exitance from a small Suppose that we measure the emitted exitance from a small hole in a furnace to be 22.8 W/cmhole in a furnace to be 22.8 W/cm22. Compute the internal . Compute the internal temperature of the furnace.temperature of the furnace.

Non-Ideal Sources – “Gray Bodies”Non-Ideal Sources – “Gray Bodies”

bw B T B bw B T B

Spectral radianceSpectral radiance Spectral radiance of a Spectral radiance of a true black bodytrue black body

Spectral Emissivity, Spectral Emissivity, :: Ratio of the spectral radiance Ratio of the spectral radiance of a true source to that of a black bodyof a true source to that of a black body

Accounts for Accounts for < 1 < 1

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

= = (())

Corrections for Non-Ideal SourcesCorrections for Non-Ideal Sources

bw B T B bw B T B

TTww(() is the transmission factor of ) is the transmission factor of

the source envelopethe source envelope

www.edmundoptics.comwww.edmundoptics.com

Corrections for Non-Ideal SourcesCorrections for Non-Ideal Sources

Color Temperature (T)Color Temperature (T)

bw B T B bw B T B

T in is an adjustable parameterT in is an adjustable parameter

T is the temperature that the atoms experienceT is the temperature that the atoms experience

bBbB


Calculate the spectral radiance of a tungsten lamp at 500 Calculate the spectral radiance of a tungsten lamp at 500 nm with a color temperature of 2700 K, nm with a color temperature of 2700 K, = 0.40, and T = 0.40, and T = =

0.92 in J/m0.92 in J/m33s.s. Recall: k = 1.38 x 10Recall: k = 1.38 x 10-23-23 J J··KK-1-1

Describing a Real SourceDescribing a Real Source

1) Adjust T in to line up 1) Adjust T in to line up maxmax

2) The ratio of:2) The ratio of:

3) Measure T3) Measure Tww(() to ) to

calculate calculate (())

bBbB

T B

Bb

T B

Bb


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


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


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:

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