Vibrationally Inelastic Collisions

K. L. Saenger, N. Smith, S. L. Dexheimer, C. Engleke, and D. E. Pritchard. Role of initial rotation on vibrationally inelastic collisions enhancements and specificity in level to level rate constants for Li. J. Chem. Phys., 79 4076-4084 (1983). 

Moran and coworkers107 127 have studied vibrationally inelastic collisions of CO+ with Ar and satisfactory vibrational assignments were made. 

It is relevant to note that studies of rotational relaxation in the collisions of electronically excited diatomic molecules and other atoms or molecules also indicate the absence of any strong constraints to the change in rotational quantum number. The most extensive data are for colhsions of B I2 and other atoms and molecules, from the work of Steinfeld and collaborators. For a given collision partner, say He, the rotational energy distributions generated by vibrationally inelastic collisions are... 

The site specificity of reaction can also be a state-dependent site specificity, that is, molecules incident in different quantum states react more readily at different sites. This has recently been demonstrated by Kroes and co-workers for the Fl2/Cu(100) system [66]. Additionally, we can find reactivity dominated by certain sites, while inelastic collisions leading to changes in the rotational or vibrational states of the scattering molecules occur primarily at other sites. This spatial separation of the active site according to the change of state occurring (dissociation, vibrational excitation etc) is a very surface specific phenomenon. 

In (a), an ion and a gas atom approach each other with a total kinetic energy of KE, + KEj. After collision (b), the atom and ion follow new trajectories. If the sum of KE, + KEj is equal to KE3 + KE4, the collision is elastic. In an inelastic collision (b), the sums of kinetic energies are not equal, and the difference appears as an excess of internal energy in the ion and gas molecule. If the collision gas is atomic, there can be no rotational and no vibrational energy in the atom, but there is a possibility of electronic excitation. Since most collision gases are helium or argon, almost all of the excess of internal energy appears in the ion. 

Subsequent to the formation of a potentially chemiluminescent molecule in its lowest excited state, a series of events carries the molecule down to its ground electronic state. Thermal deactivation of the excited molecule causes the molecule to lose vibrational energy by inelastic collisions with the solvent this is known as thermal or vibrational relaxation. Certain molecules may return radia-tionlessly all the way to the ground electronic state in a process called internal conversion. Some molecules cannot return to the ground electronic state by internal conversion or vibrational relaxation. These molecules return to the ground excited state either by the direct emission of ultraviolet or visible radiation (fluorescence), or by intersystem crossing from the lowest excited singlet to the lowest triplet state. 

All the above-mentioned experiments dealt with vibrational excitation of molecules by infrared laser lines. Inelastic collision processes in excited electronic states of molecules can be investigated in a similar way by means of visible or ultraviolet laserlines. 

A chemically pumped CO2 laser oscillating at 10 p was reported by GrossIn this system vibrationally excited COj molecules are produced by inelastic collisions with vibrationally excited DF which was formed by ultraviolet photolysis of a F2O-D2 mixture with a Xe flashlamp, producing free fluorine atoms which could react with Dj... 

Activation by inelastic collisions is also called thermal activation. After a large number of collisions, a distribution over internal (rotational and vibrational) states will be established as given by the Boltzmann distribution at the given temperature. 

Recent developments [150-156] in molecular-beam methods provide techniques by which inelastic collisions involving vibrational and rotational energy transfer may be studied directly at a specified value of incident particle velocity and scattering angle. These techniques deserve special attention because of their great potential and their fundamental nature. 

Schottler and Toennies [152] have employed a velocity-selected Li+ beam to study inelastic collisions giving rise to vibrational excitation in H2. Although individual vibrational levels were not completely resolved in these experiments, it was demonstrated that as the incident kinetic energy of the ions was... 

The interaction of an electron with a molecule is described as a collision or impact, although the electron is so small that there is no collision in the usual sense of the word. The collision process may be termed elastic (the electron is merely deflected), inelastic (energy is transferred from the electron to the molecule), and superelastic (energy is transferred from the molecule to the electron). Electron-impact ionization is an example of an inelastic collision. The energy imparted to a molecule during an inelastic collision can lead to rotational, electronic, and vibrational excitation with or separate from ionization. Further, multiple-electron excitation can occur followed by autoionization, and the latter process has been shown to lead to a substantial fraction of total ionized species in many cases (S. Meyerson et al., 1963). Thus, an electron of energy 20 eV may lead to any of the above excitations of a molecule. The gas pressures used in a mass spectrometer and the density of electrons in the electron-beam are such that multiple electron-molecule interactions leading to ionization are improbable. 

---