Optical field-ionization

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...

The photoionization threshold values listed in column 4 are all lower by some 0.002 to 0.005 eV (15-30 cm l). Similar differences were found in uranium and neptunium and remain unexplained. Electric fields from the ion optics (field ionization) and collisional effects are possible explanations. In all cases, the Rydberg convergence limits are the most accurate and they are the preferred values. 

Fig. 6. 58 Real-time observation of the optical field ionization of neon atoms with attosecond time resolution [748]...

Svanberg XUV spectra of optical-field ionized plasmas. Phys. Rev. E 51, 6016 (1995)... 

Using two pulsed tunable dye lasers, Na atoms in a beam are excited to an optically accessible ns or ml state as they pass between two parallel plates. Subsequent to laser excitation the atoms are exposed to millimeter wave radiation from a backward wave oscillator for 2-5 [is, after which a high voltage ramp is applied to the lower plate to ionize selectively the initial and final states of the microwave transition. For example, if state A is optically excited and the microwaves induce the transition to the higher lying state B, atoms in B will ionize earlier in the field ramp, as shown in Fig. 16.5. The A-B resonance is observed by monitoring the field ionization signal from state B at fB of Fig. 16.5 as the microwave frequency is swept. 

Microwave ionization of nonhydrogenic atoms, first thought to be a non-resonant process, has, by a series of experiments, been shown to be a resonant process. There is a clear parallel with optical experiments, where what had been thought to be non-resonant ionization was shown to be resonantly enhanced by Stark shifts due to the optical field [31]. As in the microwave experiments, all the... 

With two visible lasers, levels m) with excitation energies up to 6 eV can be reached. Optical frequency doubling of both lasers allows even the population of levels up to 12 eV. This makes the Rydberg levels of most atoms and molecules accessible to detailed investigations. The population of Rydberg levels of species M can be monitored either by their fluorescence or by detecting the ions or the electrons e that are produced by photoionization, field ionization, collisional ionization, or autoionization of the Rydberg levels. 

To illustrate atomic dynamics, which require subfemtosecond resolution. Fig. 6.58 shows the time-resolved field ionization of neon atoms in the optical field of a 5 fs laser pulse which consists of only three optical cycles within the pulse half width [750]. The whole process proceeds within about 6 fs, but one can clearly see peaks in the ionization probability at times of maxima in the optical field, which means that the time resolution is below 1 fs. 

Rapid ionization of a material by avalanche production of electrons caused by an intense optical field... 

Detection of microwave transitions via fluorescence becomes more and more difficult with increasing principal quantum number n of the level under study, since the oscillator strength of an optical transition from a level with n to a lower level decreases as n. The field ionization technique therefore favorably replaces fluorescence detection for large principal quantum numbers. Laser-microwave spectroscopy combinedwith field ionization was first realized by Gallagher et and by Fabre et aL Two-step... 

The inherent resolution of collinear-beam spectroscopy is still limited by the residual Doppler broadening. In beams with a broad velocity distribution the labeling of one velocity class by optical pumping, probed in a second Doppler-tuning zone, was exploited already before narrow Doppler widths were achieved. The complete elimination of the first-order Doppler effect in resonant two-photon absorption on Ne I has been discussed in Section 3.3, in connection with a precision measurement of the relativistic Doppler effect. A similar experiment was performed on In I, where the 29p Rydberg state was excited from 5p Pi/2 via 6s Si/2 and detected by field ionization. The linewidth caused by the laser jitter can be reduced to the transit-time limit of a few hundred kilohertz. 

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