Relaxation collisional

A3.13.3.1 THE MASTER EQUATION FOR COLLISIONAL RELAXATION REACTION PROCESSES... 

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

Depending on the method of pumping, the population of may be achieved by — Sq or S2 — Sq absorption processes, labelled 1 and 2 in Figure 9.18, or both. Following either process collisional relaxation to the lower vibrational levels of is rapid by process 3 or 4 for example the vibrational-rotational relaxation of process 3 takes of the order of 10 ps. Following relaxation the distribution among the levels of is that corresponding to thermal equilibrium, that is, there is a Boltzmann population (Equation 2.11). 

Let us consider a laser oscillating at a single frequency (single-mode operation) and gas molecules inside the laser resonator which have absorption transitions at this frequency. Some of the molecules will be pumped by the laser-light into an excited state. If the relaxation processes (spontaneous emission and collisional relaxation) are slower than the excitation rate, the ground state will be partly depleted and the absorption therefore decreases with increasing laser intensity. 

Quantum-state decay to a continuum or changes in its population via coupling to a thermal bath is known as amplitude noise (AN). It characterizes decoherence processes in many quantum systems, for example, spontaneous emission of photons by excited atoms [35], vibrational and collisional relaxation of trapped ions [36] and the relaxation of current-biased Josephson junctions [37], Another source of decoherence in the same systems is proper dephasing or phase noise (PN) [38], which does not affect the populations of quantum states but randomizes their energies or phases. 

These highly rotationally excited molecules have many interesting properties, which opens up a number of possible avenues for study. In addition to the well-defined rotational energy, the extreme rotational excitation distorts the molecular frame, which can have an impact on dynamics and spectroscopy, and the large rotational spacing for these highly excited states makes them resistant to collisional relaxation or reorientation. 

First, collisional relaxation of rotation is fast. One to ten collisions are sufficient for rotation-translation energy transfer. At 1 torr at least one collision per microsecond will occur for both 02 and HC1. In contrast, rotational relaxation by radiation, when allowed, is very slow, of the order of 102 seconds. The absence of... 

It is also expected that reactive collisions may diminish the effects of collisional damping of the z-oscillation. An unreactive collision removes energy from the z-mode oscillation so that the ion contributes more signal current at its original cyclotron frequency whereas a reactive collision removes an ion from a reactant population giving a true indication of the loss from the original population. The loss rate from the reactant population for ions of z-oscillation, Az, is proportional to the density of reactant ions of amplitude Az. Thus, for very reactive ions, no change in sensitivity due to collisional relaxation is expected. 

The complexity of analysis depends on the relative values of collisional relaxation rates and the degree to which excited vibrational levels become populated thermally or under excitation conditions. For OH it is advantageous to pump into the electronic level to either the ground or first excited vibrational level. 

The simplest method consists of investigating the collisional depopulation of a laser excited rovibronic level, i.e. of measuring the rates and cross-sections of the collisional relaxation of its population bPo- The relaxation rate Tk of polarization moments bPQ of various rank K may be represented, in the case of isotropic collisions, as follows ... 

Fig. 3.21. Connection between the pumping parameter x-1 and the rate of collisional relaxation 7col in the fly-through region 7 < vp/ro (a), and the collisional region 7coi > vp/ro (b). a(jcoi) shows the calculated dependence of the exponent (3.33) on the collisional relaxation rate (c). The beam diameter is assumed to be 2ro = 3 mm.

Okunevich, A.I. (1981). Excited-state collisional relaxation by optical orientation of atoms with arbitrary electronic angular momentum,... 

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