Light sources. Excited gas atoms are the primaty source of radiation in combustion and discharge lamps. lamps is not continuous!

Transcription

1 Light sources Excited gas atoms are the primaty source of radiation in combustion and discharge lamps. Continuous spectrum: black body radiation Characteristic spectrum: emission lines Absorption lines The spectrum of discharge lamps is not continuous!

2 Atom models: Thomson and Rutherford Thomson model raisins in a cake Rutherford experiment

3 The Rutherford model m m The mass and charge are concentrated in the core.

4 The Bohr model of hydrogen atom Problems of the Rutherford s model: - electron should fall on the core synchrotron radiaton - does not explain why atomic spectra are not continous

5 Bohr model of hydrogen atom Balmer the lines in hydrogen spectrum Rydberg R H = m 1 Lyman ultraviolet spectrum n=2,3,4... Paschen, Brackett, Pfund, Humphrey series - infrared n =1,2,3... n>n

6 Bohr model principles 1. Electron moves on circular orbit around the atom. The electron energy is constant it does not emit radiation. 2. The electron can stay on orbits, for which the orbital angular momentum is equal the multiplicity of h/2p 3. Emission and absorption occurs, when the electron jumps between allowed orbits. The frequency of the radiation is related to the energy: E = h L n n h 2 n- quantum number

7 Bohr model electron energy L m u n e r n 2 n m u e n Ze rn r n n h 2 E( n) E ( n) E ( n) E( n) p m e Z n 0 2 e k 4 1 n=1 the basic state n= ionized state The wavelength is related to the quantum numbers: m e e c 2 1 n 2 1 m 2 R 1 R m e M R H

8 Bohr model hydrogen spectrum

9 Gaseous spectra Helium Neon Argon Krypton Xenon Mercury Sodium

10 Discharge lamps 1. Emission of electrons from the electrode (called a cathode) 2. These electrons knock out the electrons from nobel gas atoms 3. Avalanche ionization of gas, conductivity increases. 4. Electrons or excited atoms collide with mercury atoms. 5. In mercury, the electrons jump to higher energy levels 6. These electrons return to lower levels with emission of a photon 7. The UV light is converted to visible light in the lamp coating Electrodes of tungsten, either coated with a layer which decreases the work function ( cold cathode) or in the form of a incadescent wire ( hot cathode). Pressure of 0.3 bar - noble gas and mercury vapor. Fluorescent coating Power supply with a ballast. Initially, the lamp has a high resistance decreases upon ingition. The ballast decreases the current of the ignited lamp.

11 Discharge bulbs Inductor Starter (i.e. bimetallic)

12 Neon lights 1.Bending of a glass tube 2. Welding of electrodes on both ends 3. Removal of impurities under vacuum and using high current 4. Covering of electrodes with layer of BaO 2 low exit function, cold cathode 5. Evacuation of gas to 1/100 bar 5. Refill with noble gas or mercury vapor. Power supply: 3-15 kv, up to 100 ma.

13 HID lamps HID- High Intensity Discharge Ignition: ionization of xenone, the color correponds to xenon emission line. Heating: metal salt inside the bulb evaporate and are ionized increase of current. Continous work: the lamp reaches the target color. Powered by AC current, 85 V RMS, frequency of about 400 Hz. United States Department of Transportation Federal Highway Administration

14 Flame and its color Smoke unburnt carbon black. Gaseous burning: oxidation of smaller molecules C Burning of carbon black C. So called reducing flame. Still not enough oxygen to burn the fuel. Production of soot (carbon black) not enough oxygen to burn the fuel. Partial burning of gaseous fuel to carbon oxide and dioxide. Remaining fuel thermally decomposes to smaller molecules.

15 Frank-Hertz experiment

16 The characteristic spectrum of X-rays Moseley law lines K Shielding by the K shell

17 Luminescence Luminescence emission of light not related to thermal excitation. Also called cold light sources Only a portion of atoms emits light! Kinds and sources of luminescence: Chemiluminescence chemical reactions Electroluminescence electrical current Photoluminescence incident light Fluorescence Phosphorescence Mechanoluminescence mechanical stimulation Scyntillation ionizing radiation Thermoluminescence increase of temperature N. Rakov et al., Optical Materials Express 11 (2013) 1803

18 Chemiluminescence and bioluminescence Photo: David Mülheims

19 Photoluminescence Emission of light after absorption of light (infrared to UV) Fluorescence instant emission (time 10 8 s) Phosphorescene emission with delay (up to hours) By Maxim Bilovitskiy - Own work, CC BY-SA 4.0,

20 Phosphors Discharge lamps: calcium halofosphoranes activated by manganese - the emission centers related to manganese Phosphor material converts UV light to visible light.

21 Phosphors Cathodoluminescence excitation by electron beam CRT TV receivers, oscilloscopes

22 Electron shells Electron shells K,L,M,N,O,P,Q 2n 2 electrons on a shell

23 The periodic table