Because here what we have is some atom that we're studying, in the case, it's going to be a gas, and we hit it with a photon that has some incident energy.

因为这里我们要研究的是一些原子,在这种情况下，一般是气态的，然后我们令一个,具有一定初始能量的光子打上去。

麻省理工公开课 - 化学原理课程节选