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Figure 28-15 Waves of incoherent light (a) and coherent light (b) are shown
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Section 282 The Quantum Model of the Atom
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Figure 28-16 During spontaneous emission, an electron in an atom drops from the excited state, E2, to the ground state, E1, by spontaneously emitting a photon with energy, hf (a) During stimulated emission, an excited atom is struck by a photon with energy E2 E1 The atom drops to the ground state and emits a photon Both the incident and emitted photon have the same energy: Ephoton E2 E1 (b)
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Stimulated Emission Ephoton hf
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Ephoton Ephoton hf Ephoton E1 E1 Before After
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Spontaneous and stimulated emission What happens when an atom is in an excited state After a very short time, it normally returns to the ground state, giving off a photon of the same energy that it absorbed, as shown in Figure 28-16a This process is called spontaneous emission In 1917, Einstein considered what happens to an atom already in an excited state that is struck by a photon with an energy equal to the energy difference between the excited state and the ground state He showed that the atom, by a process known as stimulated emission, returns to the ground state and emits a photon with an energy equal to the energy difference between the two states The photon that caused, or stimulated, the emission is not affected The two photons leaving the atom not only will have the same frequency, but they will be in step, or coherent, as shown in Figure 28-16b Either of the two photons can now strike other excited atoms, thereby producing additional photons that are in step with the original photons This process can continue and produce an avalanche of photons, all of the same wavelength and all having their maxima and minima at the same times For this process to occur, certain conditions must be met First, there must be other atoms in the excited state Second, the atoms must remain in the excited state long enough to be struck by a photon Third, the photons must be contained so that they are able to strike other excited atoms In 1959, a device called a laser was invented that fulfilled all the conditions needed to produce coherent light The word laser is an acronym that stands for light amplification by stimulated emission of radiation An atom that emits light when it is stimulated in a laser is said to lase Atom excitation The atoms in a laser can be excited, or pumped, as outlined in Figure 28-17 An intense flash of light with a wavelength shorter than that of the laser can be used to pump the atoms The shorter wavelength, higher energy photons produced by the flash collide with and excite the lasing atoms When one of the excited atoms decays to a lower energy state by emitting a photon, the avalanche of photons begins This results in the emission of a brief flash, or pulse, of laser light Alternatively, the lasing atoms can be excited by collisions with other atoms In the helium-neon lasers often seen in science classrooms, an electric discharge excites the helium atoms These excited helium atoms collide with the neon atoms, pumping them to an excited state and causing them to lase The laser light resulting from this process is continuous rather than pulsed
28 The Atom
Lasing The photons emitted by the lasing atoms are Collision occurs Excited state contained by confining the lasing atoms within a glass tube that has parallel mirrors at each end One of the Stimulated mirrors is more than 999% reflective and reflects nearly emission Lasing all of the light hitting it, whereas the other mirror is partransition tially reflective and allows only about 1 percent of the light hitting it to pass through Photons that are emitted in the direction of the ends of the tube will be reflected back into the gas by the mirrors The reflected photons strike more atoms, releasing more photons with each pass Pumping between the mirrors As the process continues, a high process intensity of photons builds The photons that exit the tube through the partially reflecting mirror produce Return to the laser beam Figure 28-18 shows a laser being used in ground state a laboratory Because all the stimulated photons are emitted in step with the photons that struck the atoms, laser light is coherent light The light is also all of one wavelength, or monochromatic, because the transition of electrons between only one pair of energy levels in one type of atom is involved The parallel mirrors used in the laser Ground state result in the emitted laser light being highly directional Figure 28-17 When a photon In other words, laser light does not diverge much as it strikes an excited atom, it travels Because a typical laser beam is very small, often only about 2 mm stimulates the atom to emit a in diameter, the light is very intense Many solid, liquid, and gas second coherent photon and to substances can be made to lase Most substances produce laser light at make a transition to a lower state only one wavelength The light from some lasers, however, can be tuned, or adjusted, over a range of wavelengths
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