拉曼光譜原理 Theory of Raman

自發拉曼效應是光子將分子從基態激發到一個虛擬的能量狀態,當激發態的分子放出一個光子後並返回到一個不同於基態的旋轉或振動狀態,在基態與新狀態間的能量差會使得釋放光子的頻率與激發光的不同。如果最終振動狀態的分子比初始狀態時能量高,所激發出來的光子頻率則較低,以確保系統的總能量守衡。這一個頻率的改變被稱為史托克位移。如果最終振動狀態的分子比初始狀態時能量低,所激發出來的光子頻率則較高,這一個頻率的改變被稱為反史托克位移。史托克位移的拉曼散射強度較大,這是因為大部分分子將在室溫下的基態中找到。

The spontaneous raman effect is the incident photon exciting the molecule from ground state into a virtual energy state, When the virtual energy state emit a photon and relax back down to the ground state is different from a rotating or vibrating state, the difference in energy between the ground state and the virtual energy state leads to a shift in the emitted photon’s frequency away from the excitation wavelength.
If the final vibrational state of the molecule is more energetic than the initial state, the inelastically scattered photon will be shifted to a lower frequency for the total energy of the system to remain balanced. This shift in frequency is designated as a Stokes shift. If the final vibrational state is less energetic than the initial state, then the inelastically scattered photon will be shifted to a higher frequency, and this is designated as an anti-Stokes shift. Stokes shift is the larger raman scattering, Due to the fact that most molecules will be found in the ground state at room temperature.