Lecture4 - Lecture 4 6. Creation and Detection of Light 7....

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Unformatted text preview: Lecture 4 6. Creation and Detection of Light 7. Propagation of light and interaction of light with matter 6. Creation and detection of light 6.1 Creation Linearly accelerated charges: Acceleration leads to bend electric Feld lines At a given point in space this means that the electric Feld is changing this causes a magnetic Feld we have a wave or uniformly moving particles the Feld lines are not bend. The different density of Felds lines become only important for velocities close to the speed of light. In this picture the charge is accelerated between t o and t 1 The picture shows the E-feld at t=t 2 Between t o and t 1 we have transversal compenents oF the feld which are reduced much slower (1/r) than the radial components (1/r 2 ) For long times they will dominate.They are bent, change in time and will thereFore have a magnetic feld associated with it:: pulse oF a wave Charged particle on a circular orbit We have radial acceleration which will lead to irradiation oF a electromagnetic wave or velocities close to c we get very intense, highly polarized and directed radiation: Synchrotron irradiation See e.g.: Electric Dipole Irradiation Irradiation intensity is maximum perpendicular to the movement oF the dipole- + Harmonic oscillation oF two charges relative to each other: Time dependent dipole moment: p = p " cos # t p = p 2 " 4 32 # 2 c 3 $ sin % r 2 Molecular Oscillator Their vibrational energy levels are governed by quantum mechanic. In frst approximation they can be described as a harmonic oscillator (parabolic potential) E n = n + 1 2 " # $ % h & Their emission will consist oF lines The electromagnetic spectrum Why do we need to distinguish the different spectral ranges ? Interaction with matter is different Different creation Different detection But from the theoretical point of view there is no difference We will limit ourselves to the UV-VIS-IR region Types of light sources Blackbody radiation: broad spectrum Optical transitions of particular elements: line spectrum excitation Relaxation heat blackbody radiation emission " " Light sources For more details see: Pedrotti-Pedrotti: Introduction to Optics See also Some typical spectra See` also 6.2 Detection of light Photons cannot be seen. We can observe them through the process they induce during their creation or annilation Photons begin and end on charged particles Detector types Transducers: Photon --> Electronic information Chemical reaction In modern detectors a very high quantum efFciency can be achieved. Their sensitivity is only limited by the thermal background irradiation and the noise of the electronic Photons create heat Photons create electrons or carriers Photomultipliers Photo-electric effect + Multiplication of the created electrons...
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Lecture4 - Lecture 4 6. Creation and Detection of Light 7....

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