Measurements of cold lithium atoms in a magneto-optical trap

We present measurements of cold $^7$Li atoms in a magneto-optical trap based on an amplified external cavity diode laser at 671~nm. The temperature of the atoms is measured to be 550 $\pm$ 70 $\mathrm{ \mu K}$ using a time-lapsed absorption imaging and cloud radius measurement technique. This temperature is confirmed using the release-and-recapture method with a trap diameter of $d=1.9~cm$. We recorded the frequency spectrum of the cold atoms at approximately 1.5~mK on the D2 unresolved lines ($2S_{1/2}\, F\!=\!2 \rightarrow 2P_{3/2}\, F'$) in $^7$Li using fluorescence imaging. The spectroscopic line width of the $F=2\rightarrow F'=3$ transition is approximately 11 MHz full-width at half-maximum, consistent with temperature measurements and a Voigt spectral line shape with natural line width $\gamma/2\pi=5.87~\mathrm{MHz}$. We observed line shape effects due to probe laser beam polarization in the presence of an equal irradiance re-pumping beam on the $F=1\rightarrow F'$ transition, which included a peak shift of approximately 1~MHz.