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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Spontaneous emission ： ウィキペディア英語版 Spontaneous emission Spontaneous emission is the process by which a quantum system such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a lower energy (e.g., the ground state) and emits quanta of energy. Light or luminescence from an atom is a fundamental process that plays an essential role in many phenomena in nature and forms the basis of many applications, such as fluorescent tubes, older television screens (cathode ray tubes), plasma display panels, lasers, and light emitting diodes. Lasers start by spontaneous emission, and then normal continuous operation works by stimulated emission.〔 〕 ==Introduction== If a light source ('the atom') is in the excited state with energy $E\_2$, it may spontaneously decay to a lower lying level (e.g., the ground state) with energy $E\_1$, releasing the difference in energy between the two states as a photon. The photon will have angular frequency $\backslash omega$ and energy $\backslash hbar\; \backslash omega$ (= $h\backslash nu$, where $h$ is the Planck constant and $\backslash nu$ is the frequency): :$E\_2\; -\; E\_1\; =\; \backslash hbar\; \backslash omega,$ where $\backslash hbar$ is the reduced Planck constant. The phase of the photon in spontaneous emission is random as is the direction in which the photon propagates. This is not true for stimulated emission. An energy level diagram illustrating the process of spontaneous emission is shown below: If the number of light sources in the excited state at time $t$ is given by $N(t)$, the rate at which $N$ decays is: :$\backslash frac\; =\; -A\_\; N(t),$ where $A\_$ is the rate of spontaneous emission. In the rate-equation $A\_$ is a proportionality constant for this particular transition in this particular light source. The constant is referred to as the ''Einstein A coefficient'', and has units $s^$.〔R. Loudon, The Quantum Theory of Light, 3rd ed. (Oxford University Press Inc., New York, 2001).〕 The above equation can be solved to give: :$N(t)\; =N(0)\; e^=\; N(0)\; e^,$ where $N(0)$ is the initial number of light sources in the excited state, $t$ is the time and $\backslash Gamma\_$ is the radiative decay rate of the transition. The number of excited states $N$ thus decays exponentially with time, similar to radioactive decay. After one lifetime, the number of excited states decays to 36.8% of its original value ($\backslash frac$-time). The radiative decay rate $\backslash Gamma\_$ is inversely proportional to the lifetime $\backslash tau\_$: :$A\_=\backslash Gamma\_=\backslash frac\{\backslash tau\_\{21\}\}.$ 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Spontaneous emission」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.