Presented here are tunable diode laser absorption spectroscopy measurements of the mixing volume of the NASA Ames Research Center 60-MW Interaction Heating Facility at its Medium Condition (3500 A, 5700 V, 520 g/s main air, 55 g/s add air). The mixing volume is located between the cathode of the heater and the nozzle. Absorbance measurements of an excited state of atomic oxygen, O(5S) at 777.2 nm, and atomic nitrogen, N(4P) at 868.0 nm, were non-intrusively measured at three centerline locations across the length of the mixing volume. Path-averaged temperature and specific enthalpy axial gradients were inferred from measured absorbance and facility supplied measurements of pressure and inlet flow composition through an assumption of chemical and thermal equilibrium. The lasers were scanned with triangle waves at a rate of 1 kHz that were then averaged to provide an effective sampling rate of 2 Hz for increased SNR. Results indicate a decrease in temperature and specific enthalpy along the mixing volume as expected. Minor disagreements between N* and O* inferred temperatures are presented, with N* and O* temperature inferences potentially diverging closer to the cathode. Measurements of absorbance at the Low Condition (1600 A, 2000 V, 70 g/s main air, 50 g/s add air) on the centerline closest to the nozzle, at an effective sampling rate of 2 kHz, were also analyzed to understand the high frequency behavior of the arc jet. Temperature changes in the mixing volume of greater than 100 K were observed on sub-millisecond time scales at a dominant modulation frequency.