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Since the atoms have some temperature, they are moving at some velocity, causing what is known as Doppler broadening. When an atom is moving towards or away from the laser beam, it sees photons of a higher or lower frequency as being the correct frequency to excite the electrons—so, different frequencies of light interact with different \textbf{velocity classes} of atoms. In the atom frame of reference, only photons with the correct frequency are absorb ed, but in the lab frame of reference, the Doppler shift added to the frequency of the laser is the frequency of light that the atoms absorb. We measure in the lab frame, so we see a broadened curve.   The\textbf{ zero velocity class} (atoms with zero velocity) is the one which will absorb light at the exact frequency of the transition in the lab frame. Due to this broadening, the absorption curve is a convolution between a Gaussian and a Lorentzian, with the Gaussian dominating, so we end up fitting our data to a negative Gaussian. Gaussian.\textbf{ An example of a fit to a negative Gaussian is shown in Fig.~\ref{fig:NegativeGaussianExample}. }  The effects of Doppler broadening can be ignored when using a technique known as saturated absorption spectroscopy. In saturated absorption spectroscopy, two counter-propagating beams interact with the rubidium cell. Both beams interact with separate velocity classes, except that both can interact with the zero velocity class. However, one beam (known as the pump beam) is much stronger than the other (known as the probe beam). Since the pump beam is so much stronger, is interacts with the atoms much more strongly than the probe beam, and so the atoms in the zero velocity class are excited by the pump beam but not the probe beam.