Collisional electron release

Using a modulated photon beam and a detector to measure only ionization produced at the modulation frequency, Lee and Mahan were able to show from the phase shift involved that both (48) and (49) occur. The associative ionization reaction (48) was found in cesium to occur for absorbed photons whose energy was within 0.70 eV of the atomic ionization potential, that is, for the states of cesium in which the excited electron is in the 8P, 9P, lOP,. .. levels. However, reaction (49), which we call collisional electron release, is observed only for cesium atoms in the 12P, 13P, 14P,... levels, and thus has a threshold energy that lies only within about 0.2 eV from the atomic ionization potential. Similar results were observed for rubidium and potassium, namely that collisional electron release occurs in addition to associative ionization, but does so only for states of the atoms lying much closer to the ionization continum than is the case for the latter process. 

Collisional-electron-release reactions of long-lived, high-lying states of noble-gas atoms and molecular hydrogen with a variety of molecules have been reported by Hotop " and Hotop and Niehaus/ In these experiments, apparatus similar to that shown schematically in Fig. 4 was used, with the intensity of the ions X being measured as a function of the energy of the exciting electrons. In all cases, the excitation... 

The appearance potentials for the collisional-electron-release reactions indicate that the states involved are all very high-lying levels that are within a few tenths of an eV of the ionization continuum. In the photon initiation in Cs, Rb, and K, these are known to be the nP optically allowed states, but in other systems, more detailed identification has not been made. 

The interaction of even simple diatomic molecules with strong laser fields is considerably more complicated than the interaction with atoms. In atoms, nearly all of the observed phenomena can be explained with a simple three-step model [1], at least in the tunneling regime (1) The laser field releases the least bound electron through tunneling ionization (2) the free electron evolves in the laser field and (3) under certain conditions, the electron can return to the vicinity of the ion core, and either collisionally ionize a second electron [2], scatter off the core and gain additional kinetic energy [3], or recombine with the core and produce a harmonic photon [4]. 

Electrons can be released from negative ions, either by photodetachment or by collisional detachment ... 

A molecule can store energy in the electronic, vibrational, rotational, and translational degrees of freedom. However, the probability that energy can accumulate in these degrees of freedom and can appear in the form of chemical laser emission differs considerably. Fig. 1 shows the usual form of the reaction profile for an exothermic reaction. It is apparent that a product molecule which has just been released from the activated complex is at some distance from its equilibrium state. It contains excess energy which can in principle be given off in two ways, namely by radiative or collisional processes. There is always competition between these two types of processes. The luminescence quantum yield r]a (4) will be different, depending on the type of excitation. 

The mechanism by which a vibralionally excilcd species relaxes to the nearest electronic stale involves a transfer of its excess energy to other atoms in the system through a series of collisions. As noted, this process lakes place at an enormous speed. Relaxation from one electronic stale to another can also occur by collisional ttaiisfer of energy, but the rate of this process is slow enough that relaxation by photon release is favorctl. 

The several reactions of Tables 3.8 and 3.9 which exhibit a strongly pressure-dependent photon yield apparently conform to a simple mechanistic explanation. A large fraction of the energy released is divided in a nearly statistical fashion among the various accessible internal product states. These states include high vibrational states of the ground electronic state and somewhat lower vibrational states of several electronically excited states. Communication between these electronic and vibrational states is maintained by rapid collisional processes. At low pressures the primary contributions to the photon yield come from directly excited states with relatively short... 

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