Collisional excitation

While monomolecular collision-free predissociation excludes the preparation process from explicit consideration, themial imimolecular reactions involve collisional excitation as part of the unimolecular mechanism. The simple mechanism for a themial chemical reaction may be fomially decomposed into tliree (possibly reversible) steps (with rovibronically excited (CH NC) ) ... 

Duncan M A, Bierbaum V M, Ellison G B and Leone S R 1983 Laser-induced fluorescence studies of ion collisional excitation in a drift field rotational excitation of N in He J. Chem. Phys. 79 5448-56... 

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. 

The excess energy (liv — 13.6 eV) of the ionizing photons supplies heat to the ionized gas, which is cooled chiefly by the emission of collisionally excited lines... 

Intensities of collisionally excited lines relative to hydrogen lines depend on the ionic abundance and on the balance between excitation by electron collisions and de-excitation by both electron collisions and radiation. The emission rates per unit volume are given respectively by ... 

At higher densities, the population factor in Eq. (3.65) ceases to be proportional to the collisional excitation rate, but is rather given (in a two-level approximation) by... 

Fluorescence and collisional excitation, arising primarily from the metastability of the 23S level (see Fig. 4.9), in which consequently a high population accumulates which can cause additional emission from lines such as X 4471, X 5876 by either collisional excitation or radiative transfer effects following absorption of higher lines in the 23S — n3P series. The singlet line X 6678 can also be enhanced by collisional excitation from 23S. The collisional effects can be calculated from the known electron temperature and density, and are quite small at... 

Lastly, we mention one more excitation mechanism that has been observed in molecules. It is well-established that following strong field ionization in atoms and molecules, under certain conditions, the ionized electron can be driven back to the ion core where it can recombine to produce high-harmonic radiation, induce further ionization, or experience inelastic scattering. However, there is also the possibility of collisional excitation. Such excitation was observed in [43] in N2 and O2. In both molecules, one electron is tunnel ionized by the strong laser field. When the electron rescatters with the ion core, it can collisionally ionize and excite the molecular ion, creating either N + or Ol+ in an excited state. When the double ion dissociates, its initial state can... 

CgH (n = 6, 7, 8). A novel collision-induced isomerization of CgH7 (10a), which has a sttained allenic bond, to (lOyS) has been reported to occur upon SIFT injection of (10a) at elevated kinetic energies (KE) and collision with helium. In contrast, radical anions (9) and (11) undergo electron detachment upon collisional excitation with helium. Bimolecular reactions of the ions with NO, NO2, SO2, COS, CS2, and O2 have been examined. The remarkable formation of CN on reaction of (11) with NO has been attributed to cycloaddition of NO to the triple bond followed by eliminative rearrangement. 

Collision induced dissociation the dissociation of ions after collisional excitation... 

Let the rate constant for collisional excitation to any particular vibrational level v be denoted ke (v). Then... 

Optical excitation differs from collisional excitation in a fundamental way the exciting photon is absorbed by the target atom. As a result, specifying the energy... 

Thermal unimolecular reactions usually exhibit first-order kinetics at high pressures. As pointed out originally by Lindemann [1], such behaviour is found because collisionally energised molecules require a finite time for decomposition at high pressures, collisional excitation and de-excitation are sufficiently rapid to maintain an equilibrium distribution of excited molecules. Rice and Ramsperger [2] and, independently, Kassel [3] (RRK), realised that a detailed theory must take account of the variation of decomposition rate of an excited molecule with its degree of internal excitation. Kassel s theory is still widely used and is valid for the chosen model of a set of coupled, classical, harmonic oscillators. 

Recently, however, emission from the (2,1) and (3,2) levels has been observed (Zuckerman et al., 1971). These levels radiatively decay in a very short time to the lower levels (see energy diagram). Thus collisional excitation of these levels would require molecular densities of probably 107 of 108 cm-3, so that these transitions should be ideal for investigating the dense cores of black clouds. 

In this chapter, we shall describe the basic theories of molecular energy transfer in nonreactive collisions, up to their present state of development. We shall then discuss the various experimental techniques of measuring collisional excitation or deexcitation probabilities. Finally, we will list some experimental results in both diatomic and polyatomic systems. 

Most, though not all, studies of collisional excitation transfer in alkali atoms have dealt with the 2P1/2 and 2Pa/i resonance substates. The experimental procedure usually involves the excitation of one 2P fine-structure state... 

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