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The vibrational relaxation of NO(X 2 n) in the v = 0, I, and 2 states in collisions with Ar, NO, N0 2 , and electronic ground state 0 and CI atoms has been studied in shock waves. 0 and CI atoms were produced in the thermal dissociation ofN0 2 or N 2 0 at T"",2700 K, and ofCINO at T"",1700 K, respectively. Both atoms were found to be very efficient catalysts for vibrational energy transfer to NO. The relaxation rates observed suggest a mechanism involving bound (N0 2 )* and (CINO)* collision complexes.

The thermal dissociation of N02, CINO, 03, and N20 in the low pressure range was studied in shock wave experiments. The measurements were made by using the different bath gases He, Ne, Ar, Kr, Xe, N2, 02, C02, CF4, and SF6. The results show that at high temperatures all of these gases, M, become "weak" colliders, i.e., their collision efficiencies dc(M) < 1. Comparison of relative collision efficiencies shows that dc(He)/dc(Ar) is smaller than one at low temperatures and larger than one at high temperatures. Including results of low temperature recombination data, the temperature dependencies of the average energies (AE) transferred per collision were found to fall essentially into the range (AE) « T°±0-5 (300 K < T < 2000 K).

The thermal decomposition of nitric acid highly diluted in Ar has been studied in shock waves at 890 < T < 1200 K and 8.0 · 10−6 < [Ar] < 3.2 · 10−4mol cm−3. Concentration profiles of HNO3, NO2, and NO3 have been monitored. The rate constants of the unimolecular decomposition of HNO3 and the subsequent reaction OH + HNO3 can be determined directly. The results for the unimolecular decay and low temperature data of the combination reaction of HO and NO2 have been combined to derive expressions for the rate constants in the range between room temperature and 1200K. A reaction mechanism which accounts for the observed concentration profiles is discussed.

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