Negative ions, vibrational relaxation

The effective vibrational relaxation of the formed negative ion with excess energies due to the presence of a built-in third-body molecule in the vdW molecule. 

The chain of reactions can be accelerated by vibrational excitation of molecules. The typical reaction time is about 0.1 ms and is much faster than ion-ion recombination (1-3 ms), which determines the termination of the chain. As the negative cluster size increases, the probability of reactions with the vibrationally excited molecules decreases because of an effect of vibrational-translational (VT) relaxation on the cluster surface. When the particle size reaches a critical value (about 2 nm at room temperature) the cliain reaction of cluster growth becomes much slower and is finally stopped by the ion-ion recombination process. The typical time for 2 nm particle formation by this mechanism is about 1 ms at room temperature. A critical temperature effect on particle growth is partially due to VT relaxation, which depends exponentially on translational gas temperature according to the Landau-Teller effect (Section 2.6.2). Even a small increase of gas temperature results in a reduction of the vibrational excitation level and decelerates the cluster growth. 

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