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

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:

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

What is a laser? Light Amplification by Stimulated Emission of Radiation

Overall Ingle and Crouch, Spectrochemical Analysis

Stimulated Absorption Einstein Coefficient for Absorption B ij (J -1 cm 3 ): with U : energy density of the field at the appropriate frequency (J cm -3 Hz -1 ) with U : energy density of the field at the appropriate frequency (J cm -3 Hz -1 ) Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.

Spontaneous Emission Einstein Coefficient A ji for Spontaneous Emission (s -1 ): Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.

Stimulated Emission Einstein Coefficient for Stimulated Emission: Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.

Overall Ingle and Crouch, Spectrochemical Analysis

For an ideal black body, the rate of absorption and emission must be balanced: B ij U n i = A ji n j + B ji U n j Rearrange:

Are you getting the concept? Determine the population ratio for atoms/molecules in two energy states spaced by 1 eV at T = 300 K: Recall: h = 6.63 x Js k = 1.38 x J/K 1 eV = 1.6 x J njnj nini

We know: Set equal and solve for U v : Looks similar to Planck’s Radiation Law: Spectral Energy Density

Population Inversion Goal: More atoms or molecules in the upper energy level than the lower energy level. Heating the lasing medium will not work: n j = n i e -(E j -E i )/kT Must selectively excite atoms/molecules to particular energy levels. Most common approaches: *light*electricity

Optical Pumping Intense light source at h  (e.g. flash lamp) Excites to a metastable state to achieve population inversion With fast flashing, initial photons start chain reaction Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.

Electrical Discharge Accelerated e - and ions excite atoms/molecules into higher energy states Common in gas lasers Ingle and Crouch, Spectrochemical Analysis

Three - Level System No saturation Not very efficient Better for pulsed mode operation Ingle and Crouch, Spectrochemical Analysis

The ruby laser is a three – level laser Eugene Hecht, Optics, Addison-Wesley, Reading, MA, Commercial ruby laser operates with efficiency ~ 1%

Four - Level System More efficient than 3-level Laser transition does not involve ground state or most highly excited state Easier to achieve population inversion Ingle and Crouch, Spectrochemical Analysis

The He – Ne laser is a four – level laser He* + Ne → He + Ne* + ΔE

Resonance Cavity and Gain Ingle and Crouch, Spectrochemical Analysis Gain = degree of amplification based on positive feedback

Gain Gain (G) = e  (n j -n i )b  = transition cross-section b = length of active medium Oscillation begins when: gain in medium = losses of system  1  2 G 2 = 1 Threshold population inversion: Ingle and Crouch, Spectrochemical Analysis

Eugene Hecht, Optics, Addison-Wesley, Reading, MA, Light Amplification in Resonance Cavity Highly collimated beam Typically ~mm beam width, ~mrad divergence A typical photon travels about 50 times forward and backward within the cavity