Field ionization resonance tunneling

Fig. 2.7 Diagram showing how resonance field ionization occurs. When an image gas atom is field ionized, the tunneling electron may be reflected right back to the atom. Field ionization is enhanced if the atomic level lines up with an energy level formed between the metal surface and the potential barrier of the applied field, as shown in the figure. The potential barrier is approximately triangular in shape.

Fig. 2.5 An ion kinetic energy distribution of field desorbed He ions taken with a pulsed-laser time-of-flight atom-probe. In pulsed-laser stimulated field desorption of field adsorbed atoms, atoms are thermally desorbed from the surface by pulsed-laser heating. When they pass through the field ionization zone, they are field ionized. Therefore the ion energy distribution is in every respect the same as those in ordinary field ionization. Beside the sharp onset, there are also secondary peaks due to a resonance tunneling effect as discussed in the text. The onset flight time is indicated by to, and resonance peak positions are indicated by arrows. Resonance peaks are pronounced only if ions are collected from a flat area of the...

Vertical Resonance peaks in field ionization distance to xc Z-piezo for relative distance Resonance tunneling peaks for absolute distance calibration... 

Figure 2.1 shows the ionization mechanisms for atoms in high intensity laser fields. Non-resonant multiphoton ionization (NRMPI) is expected at an irradiation intensity of around 1013 W cm 2. Optical field ionization (OFI), which comprises tunneling ionization (TI) and barrier suppression ionization (BSI), occurs at an intensity above 1014Wcm 2. The original Coulomb potential is distorted enough for the electron to either tunnel out through or escape over the barrier. The threshold intensity of BSI for atoms can be estimated by (2.1) [14] ... 

Fig. 2.1. Ionization mechanisms at different irradiation intensities are non-resonant multi-photon ionization (NRMPI) intensity 1013 Wcm-2, and optical field ionization (OFI), including tunnel (TI) and barrier suppression ionization (BSI) intensity 1013 - 1014 Wcm-2...

For weak fields of amplitude <electron through the Coulomb barrier and the survival probability P t) of the ground state follows approximately an exponential law. The order of magnitude of the width T of the resonance is given approximately by the quasi-classic theory [37] ... 

These states are formed inside the continuous spectra of the total Hamiltonian and are responsible for phenomena such as resonances in electron scattering from atoms or molecules, autoionization, predissociation, etc. Furthermore, in this work we also consider as unstable states those states that are constructs of the time-independent theory of the interaction of an atom (molecule) with an external field which is either static or periodic, in which case the effect of the interaction is obtained as an average over a cycle. In this framework, the "atom - - field" state is inside the continuous (ionization or dissociation) spectrum, and so certain features of the problem resemble those of the unstable states of the field-free Hamiltonian. The probability of decay of these field-induced unstable states corresponds either to tunneling or to ionization-dissociation by absorption of one or more photons. 

The results are summarized as follows. For atoms with at least one non-zero quantum defect, there are many states whose ionization is caused mainly by mixing of bound and free parabolic states by the non-hydrogenic core electrons. Above the classical saddle point, the ionization rate of a particular atomic resonance rises rapidly with field to a fixed plateau value and then levels off. The plateau ends abruptly by beginning a sharp rise at a field value where the ionization process is dominated by purely hydrogenic tunneling to the continuum. 

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