Collisions deactivation

In complete equilibrium, the ratio of the population of an atomic or molecular species in an excited electronic state to the population in the groun d state is given by Boltzmann factor e — and the statistical weight term. Under these equilibrium conditions the process of electronic excitation by absorption of radiation will be in balance with electronic deactivation by emission of radiation, and collision activation will be balanced by collision deactivation excitation by chemical reaction will be balanced by the reverse reaction in which the electronically excited species supplies the excitation energy. However, this perfect equilibrium is attained only in a constant-temperature inclosure such as the ideal black-body furnace, and the radiation must then give -a continuous spectrum with unit emissivity. In practice we are more familiar with hot gases emitting dis-... 

Phosphorimetric methods have been used to determine such substances as nucleic acids, amino acids, and enzymes. However, this is not a widely used method since it cannot be run at room temperature. Measurements are usually performed with liquid nitrogen to prevent degradation due to collision deactivation. Fluorometric methods are used to determine both inorganic and organic species. Instruments used for measuring fluorescence and phosphorescence are fluorometers and spectrofluorometers, respectively. These instruments are similar to ultraviolet and visible spectrometers,... 

P13.3 On the assumption that every collision deactivates the molecule we may write... 

If the energy-rich radiation is absorbed as visible or ultraviolet light, then the energy is immediately assimilated and the molecule is converted into the excited state. The excited molecule either dissociates into free radicals or dissipates the energy by fluorescence, phosphorescence, or thermal collision deactivation. 

RRKM theory. At low pressures, the initially formed chemically activated cyclopropanes undergo decomposition, but at elevated pressures some collision deactivation is observed. ... 

Alternatively, in a photochemical reaction a molecule absorbs light. This results in it possessing more energy than an ordinary molecule and, consequently, more reactivity. However, collision of this activated molecule with another molecule may cause the former to lose its energy without reacting. The activated molecule is said to have undergone a deactivating collision and has itself become deactivated. 

In contrast to the bimoleciilar recombination of polyatomic radicals ( equation (A3.4.34)1 there is no long-lived intennediate AB smce there are no extra intramolecular vibrational degrees of freedom to accommodate the excess energy. Therefore, the fonnation of the bond and the deactivation tlirough collision with the inert collision partner M have to occur simultaneously (within 10-100 fs). The rate law for trimoleciilar recombination reactions of the type in equation (A3.4.47) is given by... 

The collision partners may be any molecule present in the reaction mixture, i.e., inert bath gas molecules, but also reactant or product species. The activation k and deactivation krate constants in equation (A3.4.125) therefore represent the effective average rate constants. 

An important example for the application of general first-order kinetics in gas-phase reactions is the master equation treatment of the fall-off range of themial unimolecular reactions to describe non-equilibrium effects in the weak collision limit when activation and deactivation cross sections (equation (A3.4.125)) are to be retained in detail [ ]. 

Rosser W A Jr, Sharma R D and Gerry E T 1971 Deactivation of vibrationally excited carbon dioxide (001) by collisions with carbon monoxide J. Chem. Phys. 54 1196-205... 

The lifetime of an analyte in the excited state. A, is short typically 10 -10 s for electronic excited states and 10 s for vibrational excited states. Relaxation occurs through collisions between A and other species in the sample, by photochemical reactions, and by the emission of photons. In the first process, which is called vibrational deactivation, or nonradiative relaxation, the excess energy is released as heat thus... 

Because of the relatively high population of the u" = 0 level the v" = 0 progression is likely to be prominent in the absorption spectrum. In emission the relative populations of the i/ levels depend on the method of excitation. In a low-pressure discharge, in which there are not many collisions to provide a channel for vibrational deactivation, the populations may be somewhat random. However, higher pressure may result in most of the molecules being in the v = 0 state and the v = 0 progression being prominent. 

What is that negative contribution We can follow the trajectories backward in time to find the well from which they originated. Of the number of trajectories initially moving from product to reactant, a fraction P is deactivated as reactant and a fraction 1 — f recross the TST due to inertial motion or frequent collisions. A fraction P( — P) will then be deactivated as product, and the remaining (1 — P)- will recross. And so on. The total fraction that is deactivated as product is... 

In this mechanism, conversion of the activated species A to the product P (the last reaction) competes with deactivating collisions of A with the species M (the second reaction). 

The rise in iso with pressure (and the resulting decrease in Or) can now be interpreted as arising from faster vibrational relaxation of the triplet from the point of crossover from the singlet with increasing numbers of molecular collisions. At lower pressures, where vibrationally deactivating collisions are few, the triplet has the possibility of crossing back over to the singlet level before deactivation has occurred. 

At low light intensities the probability of collision of two acceptor triplets is small thus the main process for deactivation of the acceptor triplet is... 

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