Above threshold ionization

L. Woste You showed that the above-threshold ionization process always ends at the bottom of the ionic state when exciting the system with femtosecond pulses. So, going to higher laser powers, you observe the consecutive onset of multiphotonic processes. What happens when you cross the double-ionization barrier Is the same true for doubly charged clusters ... 

G. Gerber The observed molecular ATI (above-threshold ionization) in Na2 occurs for laser intensities above 10l2W/cm2. The observation that no additional fragmentation channels (except the 2EJ channel) open up in Na2+ might explain why in femtosecond cluster experiments additional ion fragmentation channels do not show up. Within the femtosecond interaction time even under ATI conditions the lowest Franck-Condon factor (FCF-allowed) vibrational levels are the only ones observed. At even higher intensities when multiple ionization of clusters occurs, the situation can be different. This has not yet been investigated in detail. 

The resulting phenomena of above-threshold dissociation (ATD) resemble those of above-threshold ionization (ATI) see Ref. 34. 

Eight- and nine- 532-nm photon above threshold ionization 87... 

At larger radii, eight-photon, above-threshold ionization (ATI), and a small amount of nine-photon (two-photon ATI) are observed. These are labeled as 8hv and 9hv in Figure 8A. These signals are very weak and the intensity is distributed over many vibrational levels. Within the first abovethreshold ionization signal (corresponding to eight-photon ionization) one... 

Eight- and Nine- 532-nm Photon Above Threshold Ionization. We now... 

FIGURE 4. Energy level schemes for (a) non-resonant MPI, (b) resonantly enhanced MPI, (2+1) REMPI, (c) MPI involving two continua and (d) above-threshold ionization,... 

Fig. 1. Schematic diagram showing above threshold ionization (ATI) and high harmonic generation (HHG)...

Above threshold ionization of H in the presence of a dc field was studied by Mercouris and Nicolaides (83) using the MEMPT method. Telnov and Chu... 

The continuous parts of the spectra ("continua") of atomic and molecular systems were traditionally thought of as incoherent sinks that result in "rate-like" processes and irreversible decay. While this view may sometimes be true for confinua whose coupling matrix element varies relatively slowly with energy ("flat" continua), experiments of fhe last two decades have demonstrated coherent behavior in many laser-mediated processes associated with continua. The "dressing" of confinua by light was shown to cause coherent phenomena, such as induced transparency, nonexponential decay and recurrences, and "above-threshold" ionization and dissociation processes, involving optical transitions within continua. 

B.J. Dalton, M.E.StJ. Dutton, Laser-induced continuum structures and above-threshold ionization, J. Mod. Opt. 41 (1994) 1099. 

Projection onto scattering states provides additional information concerning the continuous spectrum, such as the understanding of above threshold ionization" (ATI) in the presence of a dc field [183] or of angular distributions of the emitted electron. 

The continuous spectrum is also present, both in physical processes and in the quantum mechanical formalism, when an atomic (molecular) state is made to interact with an external electromagnetic field of appropriate frequency and strength. In conjunction with energy shifts, the normal processes involve ionization, or electron detachment, or molecular dissociation by absorption of one or more photons, or electron tunneling. Treated as stationary systems with time-independent atom - - field Hamiltonians, these problems are equivalent to the CESE scheme of a decaying state with a complex eigenvalue. For the treatment of the related MEPs, the implementation of the CESE approach has led to the state-specific, nonperturbative many-electron, many-photon (MEMP) theory [179-190] which was presented in Section 11. Its various applications include the ab initio calculation of properties from the interaction with electric and magnetic fields, of multiphoton above threshold ionization and detachment, of analysis of path interference in the ionization by di- and tri-chromatic ac-fields, of cross-sections for double electron photoionization and photodetachment, etc. 

S. Dionissopoulou, Th. Mercouris, A. Lyras, C.A. Nicolaides, Strong laser-field effects in hydrogen High-order above-threshold ionization and photoelectron angular distribution, Phys. Rev. A 55 (1997) 4397. 

Similarly, intense lasers make it possible to study the early stages in the multiphoton ionization of molecules through multiply excited states. Above-threshold ionization gives rise to a quasi-free electronic wavepacket state, and the recombination (collision) of such an electronic wavepacket with the remaining cation species results in the high harmonic... 

We can clearly consider a laser to be strong when its intensity becomes comparable to (4.113). In this regime, perturbation theory is no longer applicable, and the theorist has to resort to non-perturbative methods. When approaching these high intensities, a wealth of non-linear phenomena appear, like multi-photon ionization, above threshold ionization (ATI), high harmonic generation, etc. 

The measured energy spectrum of the outgoing photo-electrons is called the above threshold ionization (ATI) spectrum [69]. As the electron can absorb more photons than necessary for escaping the atom, an ATI spectrum will consist of a sequence of equally spaced peaks at energies... 

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