A pulsed ∼2 mJ Fe:ZnSe laser tunable around ∼4.3 μm is used to optically pump mixtures of CO2 and He to create gain at 10 μm. A conventional low-pressure CO2 laser operating on both regular (001-100) and sequence (002-101) bands is used to study the gain dynamics of the optically pumped CO2 amplifier. Time-resolved measurements of the CO2 asymmetric stretching mode vibrational temperature, T3, as well as the translational temperature, T, are made. The measured T3 value of ∼2500 K is much higher than that typically measured in discharge pumped CO2 lasers. High gain coefficients ∼30%/cm in the optically pumped active medium are attributed to the efficient storage of energy in the asymmetric stretching mode and the selective population of the upper laser level. The measured optical-to-optical energy conversion efficiency of ∼30% for 10 μm lasing at sub-atmospheric pressures is close to the theoretical quantum limit of 40% and, thus, supports our claim of gain dynamics optimization. It is concluded that a joule-class 4.3 μm pump laser will be required for the amplification of sub-picosecond 10 μm pulses in a multi-atmosphere optically pumped CO2 active medium.