Collisional activation and deactivation

The simple three-step mechanism of dissociation and recombination, given in sections 1.2.2 and 1.2.3, must be generalized to obtain a quantitative description of dissociation and recombination rates. Collisional activation and deactivation steps, (1.4) and (1.5), and the final dissociation of highly excited molecules (1.6), never proceed in a unique way. Instead, many different individual reaction paths exist which contribute to the overall reaction. This will be illustrated briefly in the following by looking at the fate of one particular molecule. 

Some applications of the kinetic data on clustering reactions are presented in subsequent sections. One application should be pointed out here, namely that to the development of kinetic theory. The existing theory of third-order association reactions and collisional activation and deactivation was developed on the basis of experiments with neutral species. However, the systems involving neutrals for which reliable data are available are quite limited. The state of the theory is also not quite satisfactory. The large variety of systems observable in ion-molecule studies and particularly the wide pressure and temperature variation which is experimentally feasible should provide a new pool of data and thus stimulate further development of the theory. 

Until now, it is virtually impossible to evaluate the function k(E, E ) for polyatomic molecules. For this reason, the theory of collisional activation and deactivation is to a considerable extent based on hypotheses concerning the general properties of the function k(E, E ). The two alternative hypotheses substantially simplifying the microscopic kinetic equations are the strong-collision hypothesis and that of stepladder activation and deactivation [336, 339, 486]. 

In reality, more complicated expressions apply because of the multi-step character of the reaction ki and k i depend on the energy E and the angular momentum (quantum number J), and the collisional energy transfer is a multi-step process with activations and deactivations, to be characterized by a master equation. It has become customary to consider "strong collisions" first and to introduce "weak collision effects" afterwards. For strong collisions, equation (6) takes the form... 

Rates of Gas-Phase Reactions. Reaction rates have been reported for only a few CVD gas-phase reactions, and most reports are primarily for the silane system. Because of the high temperatures and low pressures used in CVD, the direct use of reported gas-phase rate constants must be done with care. In addition to mass-transfer and wall effects, process pressure may be another factor affecting reaction rates. Process pressure affects major CVD processes, such as the deposition of Si from SiH4 and GaAs from Ga(CH3)3, reactions that involve unimolecular decomposition. The collisional activation, deactivation, and decomposition underlying these reactions can be summarized qualitatively by the following reactions (139, 140) ... 

The fall-off of the unimolecular rate constant as a function of pressure arises because of the way in which the competition between reaction and deactivation depends on pressure. The rate of reaction is equal to /t2[A ], and so its pressure dependence follows the pressure dependence of [A ], given by Eq. (8). As the pressure is reduced, the importance of k2 in the denominator relative to A i[M] increases, and so the steady-state concentration [A ] is reduced. A useful way of looking at this is that [A ] is depleted by reaction, and is lower than its Boltzmann value, A i[A]/fc i. This reactive depletion is most important at low pressures, where the collisional activation is too slow to replenish it. 

According to Eq. (1.7), the fall-off plot depends not only on the value of the unimolecular rate constant, /cj, but also on the collisional activation, fc, and deactivation, k.1, rate constants. The bimolecular reactions both complicate and aid the analy-... 

Sensitized Fluorescence. In this type of fluorescence, an atom emits radiation after collisional activation by a foreign atom that was excited previously by absorbing resonance radiation, but which has not yet been deactivated again. An example is the sensitized fluorescence of thallium atoms in a gas mixture containing a high pressure of mercury vapor and a low pressure of thallium vapor. When irradiated at the 253.65-nm mercury line, the thallium atoms emit at 377.57 and 535.05 nm. This type of fluorescence requires a higher concentration of foreign atoms than can be obtained in flame cells, but presumably it could be observed in nonflame cells. 

Multifrequency Quantum Rice-Ramsperger-Kassel (QRRK) is a method used to predict temperature and pressure-dependent rate coefficients for complex bimolecular chemical activation and unimolecular dissociation reactions. Both the forward and reverse paths are included for adducts, but product formation is not reversible in the analysis. A three-frequency version of QRRK theory is developed coupled with a Master Equation model to account for collisional deactivation (fall-off). The QRRK/Master Equation analysis is described thoroughly by Chang et al. [62, 63]. 

The first factor gives the rate constant for the collisional activation which initiates unimolecular reaction. The second factor in brackets is the fraction of A which then reacts by step (1.6). As processes (1.5) and (1.6) compete, this fraction may be less than unity. At low pressures collisional deactivation is much rarer than reaction, <5C the fraction of dissociated A becomes 1 and... 

The most important effect of pressure is the collisional deactivation of the reaction complex and collisional activation of the reactants. The collisional deactivation of the reaction complex can lead to a change of the reaction products in a very large number of cases. Given below are several reactions without and with collisional deactivation ... 

There is an urgent need for an analysis of bottleneck properties in collisional activation-deactivation processes, which is more general than equation (8.4), although (8.4) will probably be useful initially in characterising the position and the severity of the bottleneck. Whether or not there are other patterns of transition probabilities, not exhibiting bottle-... 

If the molecule lives longer than the time between deactivating collisions, [M] ki, most of the collisionally activated molecules will also be deactivated by collisions. Then the concentration of energy-rich molecules and hence the rate of formation of products is independent of the pressure of the bath gas. 

It is useful to interpret Eqs. (2.6)-(2.9). The dissociation rate at low pressure is equal to /ci[M], the rate of collisional activation [Eq. (2.6)]. At high pressures, the collisional activation/deactivation processes establish an equilibrium ratio of AB and AB, described by the rate-coefficient ratio ki/k-i, and the unimolecular dissociation process of AB, reaction (2), becomes rate determining [Eq. (2.8)]. In recombination at low pressures, association and redissociation of AB are much more frequent than collisional stabilization, such that an equilibrium between A, B, and AB is established, as described by the rate-coefficient ratio fc 2//c2 Collisional stabilization of AB, reaction (-1), is then rate determining [Eq. (2.7)]. At high pressures, collisional stabilization is so frequent that the rate of association of A and B, reaction (2), determines the recombination rate [Eq. (2.9)]. 

The chemical-activation step is between one and two orders of magnitude faster than the subsequent collisional deactivation of vibrationally excited O2. Finally, the population of individual vibrational levels v" of O2 is probed tluough LIF in the Schiunann-Runge band Oi X E") after exciting the oxygen... 

Gas-phase photolysis of diazoethane results in mixtures of ethylene, acetylene, and cis- and frans-2-butene. A mechanism involving the initial formation of ethylidene followed by formation of activated ethylene [which is collisionally deactivated or decomposes to produce acetylene and hydrogen— Eqs. (11.26(b,c,d)] or alternate attack on diazoethane to produce 2-butene [Eq. 11.26(e)] is proposed ... 

As mentioned earlier, practically all reactions are initiated by bimolecular collisions however, certain bimolecular reactions exhibit first-order kinetics. Whether a reaction is first- or second-order is particularly important in combustion because of the presence of large radicals that decompose into a stable species and a smaller radical (primarily the hydrogen atom). A prominent combustion example is the decay of a paraffinic radical to an olefin and an H atom. The order of such reactions, and hence the appropriate rate constant expression, can change with the pressure. Thus, the rate expression developed from one pressure and temperature range may not be applicable to another range. This question of order was first addressed by Lindemann [4], who proposed that first-order processes occur as a result of a two-step reaction sequence in which the reacting molecule is activated by collisional processes, after which the activated species decomposes to products. Similarly, the activated molecule could be deactivated by another collision before it decomposes. If A is considered the reactant molecule and M its nonreacting collision partner, the Lindemann scheme can be represented as follows ... 

The possibility of deactivation of vibrationally excited molecules by spontaneous radiation is always present for infrared-active vibrational modes, but this is usually much slower than collisional deactivation and plays no significant role (this is obviously not the case for infrared gas lasers). CO is a particular exception in possessing an infrared-active vibration of high frequency (2144 cm-1). The probability of spontaneous emission depends on the cube of the frequency, so that the radiative life decreases as the third power of the frequency, and is, of course, independent of both pressure and temperature the collisional life, in contrast, increases exponentially with the frequency. Reference to the vibrational relaxation times given in Table 2, where CO has the highest vibrational frequency and shortest radiative lifetime of the polar molecules listed, shows that most vibrational relaxation times are much shorter than the 3 x 104 /isec radiative lifetime of CO. For CO itself radiative deactivation only becomes important at lower temperatures, where collisional deactivation is very slow indeed, and the specific heat contribution of vibrational energy is infinitesimal. Radiative processes do play an important role in reactions in the upper atmosphere, where collision rates are extremely slow. 

In this reaction, triplet methylene inserts in the ethene molecule to produce an energized cyclopropane molecule, which may then either isomerize to propene or may be collisionally deactivated to give cyclopropane. Molecules may also be activated photochemically. Although most photochemical processes involve more than one electronic state of the molecule, it is possible in some cases to produce molecules in their electronic ground states with high vibrational excitation, and these may subsequently isomerize or dissociate. A typical example is the photoexcitation of cycloheptatriene followed by its isomerization to toluene. 

It has been generally recognized since about 1920 that thermal unimolecu-lar reactions are activated by intermolecular collisions. The key idea is that the chemical reaction does not follow immediately after energization, but competes with collisional deactivation of the energized molecule. This idea, and the associated mechanism, were suggested independently by Lindemaim [4] and Christiansen [5]. In the first step of this mechanism a molecule A is energized to A by collision with a bath gas molecule M. 

The temperature obtained here is for the tangential height of 21 km and agrees with that of the U.S. Standard Atmospheric Supplements, 1966, as compiled in the "Handbook of Geophysics and Space Environments". This means, that this altitude, the 02( g) molecules are in thermal equilibrium with ground state N2 molecules the collisional deactivation is faster than activation of oxygen photochemical processes (12). Thus, we were able to confirm that the present spectrometer measures the effective spectrum necessary to determine rotational temperatures with a satisfactory accuracy. The rotational temperatures of the O2 ( F ) molecule, at other altitudes were similarly obtained, the final report will be presented elsewhere (13). 

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