Collision effect lifetime

Natural linewidths are broadened by several mechanisms. Those effective in the gas phase include collisional and Doppler broadening. Collisional broadening results when an optically active system experiences perturbations by other species. Collisions effectively reduce the natural lifetime, so the broadening depends on a characteristic impact time, that is typically 1 ps at atmospheric pressure ... 

It is necessary to distinguish between the effective lifetime, tc, which allows for collisional deexcitation, and the collision-free radiative lifetime of the excited species, r0. These two lifetimes are simply related through the reactive collision frequency (kn) ... 

The effective lifetime of an excited molecular level is tesip = 5 mbar) = 8 X 10" s and Tesip = 1 mbar) = 12 x 10" s for molecules with the mass M = 43 AMU in a gas cell with argon buffer gas at T = 500 K. Calculate the radiative lifetime, the collision-quenching cross section, and the homogeneous linewidth Av( >). 

From the Stem-Volmer plots, Matsuzaki and Nagakura found the collision-free lifetime to become shorter in the presence of a magnetic field and they explained the reduction in fluorescence lifetime and the decrease in fluorescence intensity by an enhancement of ISC (due to the perturbation of the excited singlet state by triplet states). The influence of the magnetic field on the emission spectrum, which consists of a banded structure and a continuum, has not been examined and hence the magnetic effect on the continuum emission is unknown. 

As soon as collisions start to occur on a scale comparable to the radiative lifetime, the evolution of the upper state population is affected. The lifetime of a transition is apparently shortened. This shortening can be associated with the rate of quenching collisions and one arrives at an effective lifetime equation... 

Measuring the effective lifetime as a function of the exciting radiation intensity and also its dependence on the density A b of collision partners (Stem-Vollmer plot) allows one to determine the three transition probabilities separately (Sect. 11.3). 

An optically excited sodium atom Na(3F) with a spontaneous lifetime r(3F) = 16ns is placed in a cell filled with lOmbar nitrogen gas at a temperature of T = 400K. Calculate the effective lifetime Teff (3F) if the quenching cross section for Na(3F)-N2 collisions is = 4 x 10 cm. ... 

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