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To understand Doppler broadening, consider a vapor cell at room temperature. Since the atoms have some temperature, they are moving at some velocity whose distribution (ignoring the natural Lorentzian profile) can be exactly described by the Maxwell Boltzmann distribution. 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. This atom is either not moving, or is moving perpendicular to the laser beam. To be exact, the atoms velocity in the direction of the laser beam has to be within the Lorenztian profile of the transition. An example of a fit to a negative Gaussian is shown in Fig.~\ref{fig:NegativeGaussianExample}. Fig.~\ref{fig:NegativeGaussian}.  The effects of Doppler broadening can be removed when using a technique known as saturated absorption spectroscopy. In saturated absorption spectroscopy, two counter-propagating beams interact with the rubidium cell. While not on resonance, both beams interact with separate velocity classes. On resonance, both lasers 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, it 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. If we monitor the transmission of the probe beam, the result is an increase in the transmission for the zero velocity class. To summarize, off resonance the transmission of the probe beam is exactly like Doppler spectroscopy since the pump and probe beam interacts with different velocity classes. As the two laser beams pass through resonance, they interact with the same atoms causing a decrease in absorption for the probe beam. Since this interference only happens for the zero velocity class atoms, the shape of the increase in transmission is the Lorenztian profile.