A photochemical study of nitrogen dioxide using a pulsed ruby laser

Abstract

The absorption of a single quantum of light at a wavelength of 6943A raises the N02 molecule to an excited state (2B2) where it can fluoresce, be electronically quenched to the ground state (2AI), or vibrationally deactivate within the excited state manifold prior to either of the above two processes. These various primary processes were studied by time resolution of the emission using a filter-photomultiplier combination and storing the data by photographing the resulting oscilloscope trace.

These processes were interpreted as beihg two-photon consecutive absorptions with a deactivating step prior to the second absorption. In the two-photon process the deactivating step is collisional, adding another order in the pressure dependence. In the three-photon process it is proposed that the deactivating step is an anti-Stokes Raman scattering of a laser photon offthe NO*2 intermediate. This maintains the pressure linearity and adds another order in laser intensity.

Multiphoton induced emission in the blue spectral region o (~4,000-4,400A)was observed at high pressures of N02 (~4 to 65 torr). The lifetime of the emission is compatible with an NO~ formed below the dissociation limit that quenches at an efficiency of about 10% the gas kinetic collision rate. The dependence of the emission intensity on laser intensity changes from third order at the lower pressures to a limiting value of two at the higher pressures. By considering the emission to be a sum of two and three photon processes it was shown that the two·photon process was proportional to the square of the pressure and the three photon process to be linearly dependent on pressure.

The dissociation of N02 by a two-step consecutive absorption process was also observed. The breaking of the ON-O bond followed by the reaction of the 0 atom with N02 leads to the production of molecular 02 which was measured mass spectrometrically. An estimate of the second absorption coefficient on the order of 3.3 cm-1atm-l was made. An attempt was made to observe isotope-selective formation of 02 but results were inconsistent probably due to the temperature instability of the available ruby laser.

At higher pressures of N02 the decay was markedly nonexponential, emission maxima actually occurring. The data is consistent with a consecutive stepwise vibrational deactivation mechanism wherein the detection system is more sensitive to the radiation from intermediate states than to the initially formed excited state. Analysis of the data, using different theoretical models for vibrational deactivation to relate the rate constants to one another, gave values for the vibrational deactivation rate constant in the order of 4 x l0-10 cm3/part-sec. This effect was observed only for addition of NO2 enhancement of the V -> T transfer and is probably a result of rate due to the chemical bond potential available to the N02-N02 pair. The vibrational deactivation rate constants for the other added gases must be much less than the corresponding electronic deactivation rate constants.

The processes, both physical and chemical, occurring as a result of the interaction between N02 and light from a high-power, pulsed ruby laser have been studied.

At low pressures an exponential decay was observed and Stern-Volmer plots were used to determine the radiative lifetime and electronic self-quenching rate constant for N02, as well as the electronic quenching rate constants for a series of added gases.