Chapter 30 Light Emission

Excitation and de-excitation of electrons

Simple model of the atom Electron absorbs some energy which causes it to “jump” to a higher energy level. This is called excitation. E2 Energy E1 Nucleus

de-excitation The electron, preferring to be in the lower energy, immediately drops back down to the lower energy level. Energy E2 E1

In order to conserve energy, a photon (discrete bundle of energy) is emitted. Photon Energy E1 Photon Energy = E2 – E1 Because of the behavior of matter and energy at the atomic scale, which is governed by Quantum Mechanics, the energy of the photon is equal to the difference in the energies of the two energy levels and is also proportional to its frequency.

That is: Ephoton = hf where h = Planck’s constant. f = frequency of the photon. Depending on the element, only certain excitations and de-excitations (or transitions) are allowed for the electrons. The cumulative result is that an atom of a particular element will only have light emissions of certain frequencies.

The combination of all the allowed transitions produces an emission spectrum for that particular element. No two elements have the same emission spectrum, so the emission spectrum can be used to identify the element in question.

Emission Spectrum (Chaisson/McMillan) Astronomy

Emission Lines for the Hydrogen Atom (Chaisson/McMillan) Astronomy

Emission Spectra of Some Common Elements (Chaisson/McMillan) Astronomy

Emission Spectrum Emission spectra are emitted by atoms in a gaseous state where the atoms are so far apart that interactions between them are negligible. i.e. (each atom behaves as an isolated system.)

Incandescence Hot Matter in condensed states (solid or liquid or dense gas) nearly always emits radiation with a continuous spectrum. This is called Incandescence.

Continuous Spectrum This results in a much larger number of possible transitions with corresponding frequencies of radiant energy, producing a continuous spectrum. When the atoms are in a condensed state, the electrons can make transitions not only within the energy levels of their own atom, but also between the levels of neighboring atoms.

Continuous Spectrum (Chaisson/McMillan) Astronomy

The continuous spectrum emitted by an ideal surface that is completely nonreflecting is called blackbody radiation. Example: A Star. The predominant frequency of radiation (the peak frequency) is proportional to the temperature of the emitter: f  Temperature

Blackbody (Planck) Curves (Chaisson/McMillan) Astronomy

Absorption Spectrum When we view light from an incandescent source through a spectroscope, we see a continuous spectrum. (Chaisson/McMillan) Astronomy

If a gas is placed between the source and the spectroscope, we see an absorption spectrum - the inverse of emission spectrum. (Chaisson/McMillan) Astronomy

Example Emission and Absorption Spectra of Sodium (Chaisson/McMillan) Astronomy

Fluorescence Fluorescence occurs when UV light is absorbed by the electrons of an atom and visible light is emitted upon de-excitation.

Excitation E2 UV light E1 Nucleus

De-excitation to intermediate level Visible Light E1 Nucleus

Fluorescent Lamps e- Fluorescence High-speed electron Hg atom UV Photon Phosphor Coating Fluorescence

Phosphorescence Atoms or molecules are excited by incident visible light and remain in an excited state for a long time before they de-excite. This allows phosphorescent materials to glow in the dark.

Phosphorescent Dove

Glow in the Dark Dove

Incoherent and Coherent light White light from an incandescent light bulb is called incoherent light. The light is composed of different colors and the waves are out of phase with each other.

If we pass the white light through a filter, we could get only one color (monochromatic) light which would still be out of phase. (incoherent red light)

Monochromatic, Coherent light By getting all the red light in phase, we would have monochromatic, coherent light. This is the kind of light emitted by a LASER.

Monochromatic, Coherent Red light

Lasers Light Amplification Stimulated by Emission Radiation of

High Voltage is applied to electrodes. Electrical discharge excites He atoms. Excited He atoms transfer energy to Ne atoms via collisions. Neon atoms can stay in a prolonged excited state (“metastable state”) Eventually, there are more neon atoms in the metastable state than in lower energy states.

A population inversion now exists. As some neon atoms de-excite from the metastable state, red photons are emitted. As more neon atoms de-excite from the metastable state, more red photons are emitted. As these red photons pass other excited neon atoms, they too are stimulated into emitting a red photon of the exact same frequency and in phase with stimulating radiation.

By placing mirrors on each end of the cavity or tube, the light emission is amplified. With one end having a partially reflective mirror, eventually the monochromatic, coherent light is emitted from the LASER. LASER DEMO

End of Chapter 30