Experimental and theoretical study of the temperature and pressure dependences of the recombination reactions O+NO2(+M) -> NO3(+M) and NO2+NO3(+M) -> N2O5(+M)

Abstract

The recombination reactions O+NO2(+M)--> NO3(+M) and NO2+NO3(+M)-->N2O5(+M) were studied at temperatures of 300 and 400 K, and at pressures of the bath gas M=N-2 between 1 and 900 bar. Oxygen atoms were generated by laser flash photolysis of N2O at 193 nm, NO3 radicals were monitored by light absorption at 578 nm. The measured fall-off curve of the reaction O+NO2(+M)--> NO3(+M) could be well represented by limiting low pressure rate constants k(3,0)=(1.3 +/-0.3)x10(-31) (T/300 K)(-1.5) [N-2] cm(6) molecule(-2) s(-1), limiting high pressure rate constants k(3,infinity)=(2.3 +/-0.2)x10(-11) (T/300 K)(0.24) cm(3) molecule(-1) s(-1), and fall-off broadening factors of F-c=0.71 exp(-T/1700 K). The derived results are consistent with earlier relative rate measurements. Theoretical modeling of k(3,0), F-c and k(3,infinity) led to consistency with the experimental data. There is strong evidence that, besides the electronic ground state NO3((2)A'), the first excited electronic state NO3(E-2') also contributes to the observed recombination reaction. The measured fall-off curve of the reaction NO2+NO3(+M)-->N2O5(+M) was represented by limiting low pressure rate constants k(5,0)=3.6x10(-30)(T/300 K)(-5.0) [N-2] cm(6) molecule(-2) s(-1), limiting high pressure rate constants k(5,infinity)=(1.9 +/-0.3)x10(-12)(T/300 K)(0.2) cm(3) molecule(-1) s(-1) and F-c=0.38 exp(-T/4900 K). A theoretical analysis of these values is also presented.