Fluorescence and vibrational relaxation of nitric oxide studied by kinetic spectroscopy

Abstract

The first excited vibrational level of the ground electronic states of nitric oxide was popu­lated above its equilibrium value by flash photolysis of nitric oxide + inert gas mixtures, under isothermal conditions. Electronic excitation NO 2 II ( v = 0) + hv → NO 2 Ʃ ( v = 0, 1, 2) was followed either by fluorescence NO 2 Ʃ ( v = 0, 1, 2) → NO 2 II ( v = 0, 1, 2...) + hv , or by quenching NO 2 Ʃ ( v = 0, 1, 2) + M → NO 2 II( v = 0, 1, 2...) + M , causing a non-equilibrium population of the vibrational levels of the ground electronic states. Subsequently, the reactions NO 2 II ( v = 1) + M → NO 2 II ( v = 0) + M and NO 2 II ( v = 1) + NO 2 II ( v = 0) → 2NO 2 II ( v = 1) caused a decay of the vibrationally excited molecules with time; this was followed in absorption by kinetic spectroscopy. Because of the rapidity of the last reaction, bands of NO2 II with v >1 were usually observed only in the fluorescence spectrum. In mixtures of 1 to 5 mm of NO with a large excess of nitrogen or krypton, the con­centration of NO2 II ( v = 1) produced by the flash was of the order of 10-1 mm pressure, i. e. about the same concentration which is present in one atmosphere pressure of NO at room temperature. The absolute concentration of NO2 II ( v = 1) was measured accurately by plate photometry, high pressures of NO being used for calibration. The recorded probabilities of vibrational relaxation, P1-0, for NO2 II ( v = 1), and radii for electronic quenching, σ e , by NO, N 2 , CO, H 2 O and CO 2 , are P 1-0 σ e (Å) NO 3.55 x 10 -4 14 N 2 4 x 10 -7 ≤ 2x 10 -2 CO 2.5 x 10 -5 0.6 H 2 O 7 x 10 -3 30 CO 2 1.7 x 10 -4 5 With the use of an analytic form for the flash duration, the entire rise and fall of the concentration of excited species was quantitatively interpreted. A very small fraction of the NO was decomposed by the flash, due either to absorption of radiation below 1900 Å or by reaction of metastable NO molecules with each other or with ground state molecules. Abnormal effects were observed in NO+ H 2 +inert gas mixtures and chemical reaction occurred.