Above threshold ionization

Using a 10-μm pulse to ionize xenon the authors demonstrate that by the simple procedure of changing the laser polarization, the electron temperature (drift energy) can be determined anywhere between 50 and 800 eV. If they were to change the ionizing laser wavelength, the drift velocity would change even further. Finally, the ionization potential of the atom or ion can also be considered a variable thus giving even greater flexibility in selecting electron temperatures. This control over the electron temperature of an underdense inertially confined plasma has many important implications. Plasmas produced by multiphoton ionization are not in thermal equilibrium. Using practical lasers, plasmas can be produced where the electron temperature is much less than the ionization potential of the ion from which the last electron was extracted. Such situation should lead to transient (x-ray) gain during the recombination phase of the plasma. Even ground state lasers are predicted. In the other extreme, energetic electrons produced using long wavelength lasers and high ionization potential ion will represent an extremely high specific power deposition. The authors point out other implications of multiphoton ionization for plasma physics. New nonlinear phenomena are introduced by the process of plasma formation. The presence of even relatively low levels of radiation can inhibit the recombination process. It should be possible to produce energetic ions by removing a number of molecular electrons in rapid succession. This process, known as Coulomb explosion, will require pulses with a duration of the order of the vibrational time of the molecule. Using large molecules or clusters, microplasmas can be produced which will exhibit some of the characteristics of laser-solid target interactions and some of the characteristics of gaseous targets