Are you getting the concept? If the average irradiance from the Sun impinging normally on a surface just outside the Earth’s atmosphere is 1400 W/m 2, what is the resulting pressure (assuming complete absorption)? How does this pressure compare with atmospheric pressure (~ 10 5 N/m 2 )? T = I/c T = I/c = (1400 W/m 2 )/(3.00 x 10 8 m/s) = 4.7 x W/m·s = (1400 W/m 2 )/(3.00 x 10 8 m/s) = 4.7 x W/m·s = 4.7 x N/m 2 = 4.7 x N/m 2 This pressure is less than 5 x % of atmospheric pressure. When accounting for the surface area of the Earth (5.11 x m 2 ), this provides 2.4 x 10 5 tons of force. Reminder: 1 W = 1 J/s = 1 N·m/s = 1 kg·m 2 /s 3

Photon Emission E. Hecht, Optics, atom in ground stateatom in ground state atom excited by high T or collision, stays in excited quantum state for or secatom excited by high T or collision, stays in excited quantum state for or 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 atomic transition (  E = h )

Photon Radiation Figure 5-16 Partial energy-level diagram for a fluorescent organic molecule. Skoog and Leary, Principles of Instrumental Analysis, 1992.

Are you getting the concept? Many streetlights are sodium discharge lamps. The emitted orange light is due to the sodium D-line transition: What is the energy level spacing (in eV) for the 3p → 3s transition?

EM Radiation Sources 1. Fundamentals of EM Radiation 2. Light Sources 3. Lasers

Optical Source Characteristics Ingle and Crouch, Spectrochemical Analysis Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.

Continuum Source Line Source Continuum + Line Source Ingle and Crouch, Spectrochemical Analysis

Incandescent 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

Black-body Radiation In an ideal Black body:  ( ) = 1,  ( ) = 0, T( ) = 0 Because a black body is at thermal equilibrium, emission must equal absorption. Thus, black bodies are perfect absorbers and the most efficient emitters possible. There are no ideal black bodies.

Spectral Distribution of Emission is Characteristic of the Temperature of the Blackbody As T increases, max decreases. Donald McQuarrie, Quantum Chemistry, University Science Books, Mill Valley, CA,

Rayleigh – Jeans Law  Spectral Radiance (Jm -3 s -1 ) (Jm -3 s -1 ) The Ultraviolet Catastrophe Approximate Blackbody Expressions Wien’s Law

Resolved (inadvertently) in 1900 by Max Planck. Assumed atoms could only absorb or emit discrete amounts of energy. Planck’s Radiation Law: Donald McQuarrie, Quantum Chemistry, University Science Books, Mill Valley, CA,  Spectral Energy Density (Jcm -3 Hz -1 ) (Jcm -3 Hz -1 )

Wien’s Displacement Law Eugene Hecht, Optics, Differentiate Planck’s Law with respect to and set equal to zero to find m (wavelength of maximum irradiance): Stefan-Boltzman Law M b =  T 4  =  W·cm -2 · K -4 Integrate Planck’s Law to find the total emittance of a black body:

Are you getting the concept? Suppose that we measure the emitted exitance from a small hole in a furnace to be 22.8 W/cm 2. Compute the internal temperature of the furnace.

Non-Ideal Sources – “Gray Bodies” Spectral radiance Spectral radiance of a true black body

Spectral Emissivity,  : Ratio of the spectral radiance of a true source to that of a black body Accounts for  < 1 Eugene Hecht, Optics, Addison-Wesley, Reading, MA,  =  ( ) Corrections for Non-Ideal Sources

T w ( ) is the transmission factor of the source envelope Corrections for Non-Ideal Sources

Color Temperature (T) T in is an adjustable parameter T is the temperature that the atoms experience

Are you getting the concept? Calculate the spectral radiance of a tungsten lamp at 500 nm with a color temperature of 2700 K,  = 0.40, and T  = 0.92 in J/m 3 s. Recall: k = 1.38 x J · K -1

Describing a Real Source 1) Adjust T in to line up max 2) The ratio of: 3) Measure T w ( ) to calculate  ( ) Ingle and Crouch, Spectrochemical Analysis

Nernst Glower Rare earth oxides formed into a cylinder (1-2 mm diameter, ~20mm long) Pass current to give: T = 1200 – 2200 K Can operate in air (no need for glass/quartz enclosure) Ingle and Crouch, Spectrochemical Analysis Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.

Globar Silicon Carbide Rod (5mm diameter, 50 mm long) Heated electrically to 1300 – 1500 K Positive temperature coefficient of resistance Electrical contact must be water cooled to prevent arcing Ingle and Crouch, Spectrochemical Analysis

Tungsten Filament Ingle and Crouch, Spectrochemical Analysis Heated to 2870 K in vacuum or inert gas Useful Range: 350 – 2500nm

Tungsten / Halogen Lamp I 2 or Br 2 added Reacts with gaseous W near the quartz wall to form WI 2 W is redeposited on the filament Gives longer lifetimes Allows higher temperatures (~3500 K) and thus higher apparent brightness

Arc Lamps Ingle and Crouch, Spectrochemical Analysis Electrical discharge is sustained through a gas or metal vapor Continuous emission due to rotational/vibrational energy levels and pressure broadening

H 2 or D 2 Arc Lamps Ingle and Crouch, Spectrochemical Analysis D 2 + E e-  D 2 *  D’ + D” + h D 2 + E e-  D 2 *  D’ + D” + h Energetics: E e- = E D 2 * = E D’ + E D” + h E e- = E D 2 * = E D’ + E D” + h Useful Range: 185 – 400 nm

Hg Arc Lamp Continuum + line source High power source Often used in photoluminescence Ingle and Crouch, Spectrochemical Analysis

Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, Hollow Cathode Discharge Tube Apply ~300 V across electrodes Ar + or Ne + travel toward the cathode If potential is high enough cations will sputter metal off the electrode Metal emits photons at characteristic atomic lines as the metal returns to the ground state

Hollow Cathode Discharge Tube Line widths are typically 0.01 – 0.02 Å FWHM Ingle and Crouch, Spectrochemical Analysis

Light-Emitting Diodes Operate with mW of power - ~80% efficiency Long lifetimes, stable output

Are you getting the concept? 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: