NO 2 concentration profiles in shock-heated NO 2 /Ar mixtures were measured in the temperature range of 1350 -2100 K and pressures up to 380 atm using Ar ϩ laser absorption at 472.7 nm, IR emission at , and visible emission at 300 -600 nm. In the course of this study, the absorption coefficient of NO 2 at 472.7 nm was measured at temperatures from 300 K to 2100 K and pressures up to 75 atm. Rate coefficients for the reactions and were derived by comparing the measured and calculated NO 2 profiles. For reaction (1), the following low-and high-pressure limiting rate coefficients were inferred which describe the measured fall-off curves in Lindemann form within 15%:
The inferred rate coefficient at the low-pressure limit, k 1o , is in good agreement with previous work at higher temperatures, but the energy of activation is lower by 20 kJ/mol than reported previously. The pressure dependence of k 1 observed in the earlier work of Troe [1] was confirmed. The rate coefficient inferred for the high pressure limit, k 1ϱ , is higher by a factor of two than Troe's value, but in agreement with data obtained by measuring specific energydependent rate coefficients.
For the reactions (2a) and (2b), least-squares fits of the present data lead to the following Arrhenius expressions:
For reaction (2), the new data agree with previously recommended values of k 2a and k 2b , although the present study suggests a slightly higher preexponential factor for k 2a .