Fluorescence lifetime studies of NO2. III. Mechanism of fluorescence quenching

Abstract

NO2 fluorescence, excited by fixed visible frequencies of a Nd-YAG laser, was measured as a function of time, pressure, and fluorescence wavelength. The low resolution fluorescence spectrum at all excitation wavelengths consists of strong banded features, assignable to ground state vibrational progressions, superimposed on an apparent continuum. The ratio of banded-to-continuum intensity decreases with increasing pressure, indicating that the continuum is partially of collisional origin at high pressures. There is also a red shift of fluorescence at high pressures, indicative of vibrational relaxation within the emitting state. A residual continuum is found under collision-free conditions (0.1–0.01 mtorr) and ascribed to violation of the ΔK=0 selection rules in the initially excited levels of the highly perturbed 2B2 state. The time dependent decay of fluorescence excited at the banded features contains a fast component kB and a weaker, long-lived component kC, identified with the underlying continuum. Rate constants of kB=5.9×10−10 and kC=1.1×10−10 cm3 sec−1 were measured for quenching by NO2. The slow process has been identified with stepwise vibrational quenching while the fast process is best interpreted as a collision induced change in the rotational quantum state of the initially excited state. Computer modelling was used to fit the pressure dependence of the banded-to-continuum intensity ratio and of the fluorescence red shift, using the measured low-pressure spectrum, the two measured quenching rate constants, and a single adjustable parameter Δεvib=1000±500 cm−1, the amount of vibrational energy transferred per vibrational–quenching collision. Quenching rate constants kB and kC were also measured for He, Ar, O2, N2, D2, H2, SF6, CO2, ND3, NH3, H2O and D2O. They range from 0.38 to 2.0 times gas kinetic for kB and from 0.041 to 0.52 times gas kinetic for kC.