State-changing collisions molecular energy transfer

State-changing collisions molecular energy transfer 

Thermal equilibrium in the gas phase can be characterized by a constant (time-independent) fraction of molecules in any given energy level. At a temperature rthe relative population in the ith energy level is given by the Boltzmann distribution 

Here Ni is the number of molecules in level i and N is the total number of molecules. As usual, the degeneracy gi is the number of possible quantum states 

In principle, the system can also relax to equilibrium ty light emission. Unless the circumstances are unusual, emission of IR photons is too slow a process to significantly compete with collisional relaxation. An extreme case is that of molecules in outer space where the density is very low so the time between collisions is exceedingly long. Another unusual situation is a chemical laser. Section 9.0.3, where the presence of a high density of photons stimulates the emission of other photons. 

Following a transient disturbance the gas relaxes into an equilibrium distribution. One can follow this relaxation process in terms of the changes in the population of the different energy levels. Such changes are the manifestations of the energy-transferring molecular collisions. For example, a vihrationally excited HCl molecule can lose its excess energy in a collision with a vihrationally cold DCl molecule 

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State-changing collisions molecular energy transfer... 

ABSTRACT. Laser and molecular beam techniques allow detailed study of many dynamical properties of single reactive collisions. The chemical scope of these methods is now very wide and includes internal state preparation of reactants, change of collision energies, state detection of products, and thus determination of state-to-state reaction rates. The great impact of laser spectroscopy on knowledge in the field of structure, molecular energy transfer and the mechanism of elementary chemical reactions is illustrated by two selected examples, i.e. studies in which laser-induced fluorescence (LIF) has been used to determine the specific impact parameter dependence of the Ca + HF -> CaF(X) + H reaction and the product state distributions for the reaction of metastable Ca with SF5. 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

Molecular systems exist in discrete quantum states, the study of which lies in the realm of molecular structure and wave mechanics. Transitions between quantum states occur either by absorption or emission of radiation (spectroscopy) or by collisional processes. There are two main types of collisional transitions which are important in chemical physics these are first, reactive processes in which chemical rearrangement takes place (reaction kinetics), and secondly collisions in which the energy distribution is changed without overall chemical reaction. It may therefore be concluded that the energy transfer processes discussed here are of fundamental importance in all molecular systems, and that the subject, like molecular structure, is enormously varied and complex. 

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