De-excitation, collisional

The time constant r, appearing in the simplest frequency equation for the velocity and absorption of sound, is related to the transition probabilities for vibrational exchanges by 1/r = Pe — Pd, where Pe is the probability of collisional excitation, and Pd is the probability of collisional de-excitation per molecule per second. Dividing Pd by the number of collisions which one molecule undergoes per second gives the transition probability per collision P, given by Equation 4 or 5. The reciprocal of this quantity is the number of collisions Z required to de-excite a quantum of vibrational energy e = hv. This number can be explicitly calculated from Equation 4 since Z = 1/P, and it can be experimentally derived from the measured relaxation times. 

As the pressure increases, collisional de-excitation to produce the alternate reaction product Si2H6 begins to dominate. Thus bimol approaches its high-pressure limiting value bimoi,oo which drops off as p l. 

Collisional de-excitation is called quenching because it competes with spontaneous emission, and if significant the fluorescent signal will be reduced, or quenched. Chemical decay can also be important in some circumstances. 

The same types of products are obtained from alkanes and alkenes in the liquid phase as are obtained in the gas phase. As might be expected from the possibility of more rapid collisional de-excitation, there is a marked decrease in those products which result from decomposition of an excited species (Lee and Rowland, 1962) such as the hot radical produced by tritium addition to a double bond. 

A classical expression for the cross section for collisional de-excitation of He(2 P) is also derived from the formula by Eq. (16). However, the autoionization widths r(R) for Penning ionization by resonant atoms are not identical to the empirical form of Eq. (18) for electron exchange. Instead, a direct transition due to a dipole-dipole interaction is proposed to govern this Penning ionization [126,139,140,143], that is. 

A more rapid transfer of energy from dust to gas may exist through absorption of dust radiation in the rotational transitions of water molecules followed with collisional de-excitation by H2 With numerical calculations of the H2O excitation equilibrium, we have verified that this process is generally faster than direct H2... 

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