Single-photon ionisation

There are other processes for charge generation these include two-photon absorption and single photon ionisation of singlet and triplet excitons. These processes are proportional to the square and higher powers of the light intensity and usually somewhat weaker than the processes described in the previous paragraph. 

The method works particularly well for ATI spectra excited by circularly polarised light. The reason for this is as follows an atom which absorbs N photons then acquires Nh units of angular momentum. The emerging electron is then subject to a repulsive centrifugal barrier (see chapter 5) and does not therefore penetrate into the core. Consequently, most atomic effects are suppressed, and a final state representation as a Volkov wavefunction is a reasonable approximation. This is also why intensity suppression occurs near threshold in this case the effect is very similar to delayed onset in single-photon ionisation to continua of high angular momentum. 

Laser desorption/Iaser post-ionisation (two-shot LDI) can display either non-specific detection (using non-resonant single-photon ionisation) or species-specific detection (using resonance-enhanced multiphoton ionisation). By exercising both of these options, complicated mixtures can be analysed for surface species. As efficiency of mass spectral analysis is greatly enhanced by ionising the... 

The physical principles involved in L MS are either non-resonant single-photon ionisation or (non)-resonant multiphoton ionisation. In two-step laser desorption/non-resonant single-photon ionisation... 

Zewail acknowledged early on that he was inspired to work in the dynamics area by amongst others, George Porter s development of fast reaction techniques, viz. Flash Photolysis which is reported elsewhere in this volume. In the early experiments outlined in the present paper, three detection techniques were employed time-correlated single photon counting, with 30-50 ps time resolution streak camera detection of fluorescence, with 10 ps resolution, and multiphoton ionisation with resolution determined by the pulse width of the laser, 1 or 15 ps. 

Centrifugal barriers have a profound effect on the physics of many-electron atoms, especially as regards subvalence and inner shell spectra. One aspect not discussed above is how energy degeneracies arising from orbital collapse can lead to breakdown of the independent electron approximation and the appearance of multiply excited states. Similarly, we have not discussed multiple ionisation (the ejection of several electrons by a single photon) enhanced by a giant resonance. Both issues will be considered in chapter 7. 

Fig. 9.3. The Q-reversal effect in multiphoton ionisation. Note that Q stands for the generalised asymmetry parameter in multiphoton spectroscopy, as opposed to q in single-photon spectroscopy. The choice of parameters corresponds to REMPI of K (see text - after J.-P. Connerade and A.M. Lane [385]).

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