Quenching collisions kinetics

Hence quenching collisions per excited atom by the foreign gas M per second, Z, can be expressed by the quenching cross section a1 (cm2 molec ) in analogy with the gas kinetic collision cross section (Note that a is used before to designate the absorption cross section see Section I —8.1.)... 

By decreasing the detection bandwidth as much as possible, consistent with maintaining a good signal to noise ratio, a limiting condition can be approximated for which the quenching summation varies in a simple manner from flame to flame. In the limit in which only one transition is monitored from the v J state populated by the laser, almost every vibrational or rotational relaxation from that state is an effective quenching collision. Under these conditions the quench summation term approximates to a gas kinetic quench rate. 

Various kinds of information can be expected from the high pressure combustion and flame experiments Reaction kinetics data for conditions of very high collision rates. Results about combustion products obtained at high density and with the quenching action of supercritical water, without or with flame formation. Flame ignition temperatures in the high pressure aqueous phases and the ranges of stability can be determined as well as flame size, shape and perhaps temperature. Stationary diffusion flames at elevated pressures to 10 bar and to 40 bar are described in the literature [12 — 14]. 

On the other hand, if the rate constant for the quenching step exceeds that expected for a diffusion-controlled process, a modification of the parameters in the Debye equation is indicated. Either the diffusion coefficient D as given by the Stokes-Einstein equation is not applicable because the bulk viscosity is different from the microviscosity experienced, by the quencher (e.g. quenching of aromatic hydrocarbons by O, in paraffin solvents) or the encounter radius RAb is much greater than the gas-kinetic collision radius. In the latter case a long-range quenching... 

The initial kinetic energy may be relatively well defined. For quenching by N2, which is typical for the heavier molecules, we have the initial kinetic energy of the collision system defined to in=150 meV 100 meV (FWHM) at To=300°K and (125 60) meV at ro=80°K, which is already better than that for thermal conditions and may be improved drastically by also using a supersonic molecular beam. 

The Ar(3P(, Pt) levels are 11.623 and 11.827 eV, respectively, above the ground (1S) level. The lifetimes are 8.4 and 2.0 nsec (33), respectively. The Ar(3P,1 Pj) states are formed by absorption of the Ar resonance lines at 1067 and 1048 A. In the 1 to 100 mtorr concentration range the lifetime of Ar(3P, P() atoms is of the order of 10 /tsec [Hurst et al. (494)], which is 1000 times as long as that of isolated atoms because of imprisonment of resonance radiation. If the ionization potential ofa molecule is below 11.6 eV, it is possible to increase the photoionization yield (sensitize) by adding Ar to the sample. The increase of the ionization yield is caused by collisional energy transfer between Ar(3P, Pi) atoms and the molecule before the excited atoms return to the ground state by resonance emission. Yoshida and Tanaka (1065) have found such an increase in the Ar propane, and Ar-ammonia mixtures when they are excited by an Ar resonance lamp. Boxall et al. (123) have measured quenching rate constants for Ar(3P,) atoms by N2) 02, NO, CO, and H2. They are on the order of the gas kinetic collision rate. 

Rate constants for quenching can be compared with those predicted by the collision theory of chemical kinetics. According to this theory, a rate constant, k, is given by... 

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