The passively Q-switched of TEA-$CO_2$ laser is obtained by using the 30 cm long cell of $SF_6$ gas as saturable absorber.
When the gain switched pulse has FWHM 105 nsec, the power 0.48 MW on the transition line 10P(20), the power of the Q-switched pulse is 0.85 MW and FWHM is 58 nsec, which is close to the theoretical Value of 45 nsec.
The optimum pressure of $SF_6$ gas for Q-switching of 10P(20) line is 20 torr, and the peak power becomes smaller as the fill gas pressure becomes higher or less than 20 torr, and the mode beating signals grow as the pressure becomes higher.
At the 9R (12) line, Q-switching is not occured. The reason may be due to the small absorption cross section of $SF_6$ molecule at 9R(12)-wauelength.