Above-threshold ionization process

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 ... 

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. 

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. 

The above considerations need relativistic correction at v c, which may be performed in a straightforward manner. More importantly, Eq. (10) assumes that the ionization process is direct, i.e., once a state above the ionization potential is reached, ionization occurs with a certainty. Platzman [25] points out that in molecules, this is not necessarily so and superexcited states with energy exceeding the ionization potential may exist, which will dissociate into neutral fragments with a certain probability. For example, in water in the gas phase, ionization occurs with a sharp threshold at the ionization potential (I.P.) = 12.6 eV, but only with an efficiency of 0.4. Beyond the I.P., the ionization... 

In energy transfers above the ionization threshold, it is usual for several different ionization processes to occur with probabilities depending on E. The measured optical oscillator strength for absorption is thus a sum corresponding to a variety of different processes. Denoting this total optical oscillator (ionization potential) strength by df(0)/dE, we have, for... 

Characterization of the neutral processes is far more difficult, and little information is available at present. However, studies on some simple molecules (see Section V) have indicated that the ionization efficiency approaches unity quite rapidly as the energy loss increases above threshold, suggesting that, except where transitions to Rydberg states just below a new ionization threshold are significant, the dominant mode of energy loss in the far UV is by ionization often accompanied by molecular fragmentation. 

For a process like electron-ion recombination, the presence of resonances closely above the ionization threshold has a large impact on the recombination rate. A number of storage ring measurements, which can map out the resonances in great detail, have clearly shown that one cannot dismiss such resonances just because they would not be able to ionize in LS-coupling. 

At first sight, it seems surprising to observe competitive reactions within the same complex. However, it must be noticed that in the ionization processes the internal energy distribution within the ions can be broad since the cluster s geometries in the Sj excited state and the ionic state can be very different. This can be seen by the ionization threshold measurements which do not exhibit clear onsets. Therefore, the presence of competitive processes can be explained by different barrier heights for the different channels. When the ions are prepared below one barrier and above the other one, only one product will be observed. Due to this broad internal energy distribution, on average, many channels can be detected. Coincidence detection of the zero kinetic electron and the product ions... 

This wavelength corresponds closely to the threshold energy for the process, which is 140 nm. Since the efficiency for production of CS(A n) parallels the Rydberg structure in the CS2 absorption spectrum in the wavelength interval below 133.7 nm and above the ionization limit, it is suggested that the dissociation comes from these states rather than from a repulsive state and proceeds with high quantum efficiency. 

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