Determination of the vibrational energy distribution in N 2 and H 2O formed in the NH 2+NO reaction by cars and IR spectroscopy

Abstract

The reaction NH 2 +NO→N 2 +H 2 O * (1) was studied directly in a flow reactor at room temperature. NH 2 -radicals were generated by exciplex laser photolysis of NH 3 at 193 nm. The NH 2 radicals and N 2 product molecules were monitored with high spectral resolution by CARS spectroscopy. The reaction product H 2 O * was observed by spectral and time resolved infrared fluorescence. From the measured CARS intensity-time profiles of the NH 2 -radical compared with calculated normalized concentration-time profiles a rate constant k 1 =1,0·10 13 cm 3 mol −1 s −1 at 295 K is obtained. From the time resolved infrared fluorescence signals of the ( ν 1 , ν 3 )-stretching levels of vibrationally excited H 2 O * in the 2,7 μm range a rate constant for reaction (1) at 295 K of k 1 =(7,6±2,5)·10 12 cm 3 mol −1 s −1 is derived. By comparison of the experimental spectra in the 2,7 μm range with a simulation of the spectral shape envelope for H 2 O formed in reaction (1) a vibrational temperature of T v ( ν , ν 3 )=10 4 K is obtained. CARS spectra originating from the N 2 -vibrational levels v =0 and v =1 taken 5 μs after photolysis show a much lower vibrational temperature for the N 2 -product. The measured vibrational population distributions in the products were compared with statistical predictions.