19 Lasers cont

19 Lasers cont - Plan for Today 1 Lasers 2 Molecules 1 2 3...

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Physics 13 Lecture 19 Prof. H. Gallagher 1 Plan for Today 1. Lasers 2. Molecules 1. Vibrational modes 2. Rotational excitations 3. Spectroscopy 3. Nuclei HW 9 has been posted and is due next Tues., Nov. 25 at 5:00.
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Physics 13 Lecture 19 Prof. H. Gallagher 2 Lasers There are a number of ways in which radiation can interact with the energy levels of atoms and molecules. 1.spontaneous emission (A’ --> A + γ ) 2.Absorption ( γ +A --> A’) 3.induced (or stimulated) emission ( γ +A’ --> γ + γ + A) Photons in stimulated emission travel in the same direction, with identical energy, and are in phase. If we had a large number of atoms in the excited state A’ we could use this process in a cascade to produce a large number of monoenergetic, coherent (i.e. in phase) photons. How do we get lots of atoms into an appropriate excited state?
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Physics 13 Lecture 19 Prof. H. Gallagher 3 He-Ne laser partially silvered mirror The operation of a laser requires more electrons in the excited state than the ground state, which would absorb the photons. This situation is called “population inversion”. images from http://commons.wikimedia.org/wiki/Image:Laser.svg , http://en.wikipedia.org/wiki/Population_inversion#Four-level_lasers
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Physics 13 Lecture 19 Prof. H. Gallagher 4 Example A small HeNe laser produces a light beam with an average power of 3.5 mW and a diameter of 2.4 mm. a) How many photons per second are emitted by the laser? b) What is the amplitude of the electric field of the light wave? Compare this result with the electric field at a distance of 1 m from an incandescent light bulb that emits 100 W of visible light.
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Physics 13 Lecture 19 Prof. H. Gallagher 5 Molecules As we bring atoms together to form molecules the electronic states become molecular states. Knowing the wavefunctions for atomic and molecular states allows us to determine the lowest energy states and thus what kinds of bonds are possible. Two extremes: Covalent bonds: The equal sharing of electrons between two atoms in a single molecule (H 2 ). Ionic bonding: Electron is essentially removed from one atom and moved to the second atom. The two ions can form a molecule due to their Coulomb attraction.
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Physics 13 Lecture 19 Prof. H. Gallagher 6 Molecular Spectroscopy In addition to their electronic states, molecules have additional ways in which they can absorb or emit energy.
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