Continuum ionization

Figure B2.5.16. Different multiphoton ionization schemes. Each scheme is classified according to the number of photons that lead to resonant intennediate levels and to the ionization continuum (liatched area). Adapted from [110].

E.M. Thurman, 1. Ferrer, and D. Barcelo, The ionization-continuum diagram a concept for selection of APCl and ESI conditions for HPLC/MS of pesticides , in 17th Montreux Symposium on Liquid Chromatography/Mass Spectrometry, Montreux, Switzerland, November 8-10, 2000 , p. 31 (2000). 

The extension of Eqs. (12)—(16) to a standard pump-probe experiment, where the final state is a (dissociation or ionization) continuum is straightforward. It requires only that we replace the pulse that spans two vibrational eigenstates by a shorter one that spans several eigenstates. 

The simplest version of REMPI uses a two photon (1-1-1) process, namely resonant two photon ionization (R2PI). In this, the species M is first promoted from its electronic ground state So to the electronic excited state Si via a resonant absorption step. Then, the non-resonant absorption of a second photon takes the species into the ionization continuum. If the frequencies of the excitation and ionization photons are equal, the process is named one color R2PI (lcR2PI), otherwise two colors R2PI (2cR2PI) (Fig. 1). 

Figure 4. Schematic of the potential energy curves of the relevant electronic states The pump pulse prepares a coherent superposition of vibrational states in the electronic A 1 EJ state at the inner turning point. Around v = 13 this state is spin-orbit coupled with the dark b 3n state, causing perturbations. A two-photon probe process transfers the wavepacket motion into the ionization continuum via the (2) llg state [7].

Figure 5. Wavepacket motion of the 39,39 K2 A state interrogated by (a) a two-photon probe pulse via the (2) 1 IIg state and (i>) a one-photon probe process into the ionization continuum (c, d) corresponding Fourier transforms, indicating a stronger second harmonic for the one-photon probe process [7].

In the upper parts of Figs. 6a-c the time-dependent Rabi frequencies of both laser pulses are shown for different delays. In all cases the dump laser pulse has a higher Rabi frequency than the pump laser pulse and twice its duration. Note that the Rabi frequency is proportional to the laser held strength and therefore to the square root of the pulse intensity. In the lower part the population dynamics for the three different pulse sequences is shown. The part of population transferred to the ionization continuum is indicated by a strong line. 

The main result of these first detailed experimental studies of the AI mechanism is that, even at thermal velocity, the system R -H follows a potential curve F ( R) that diabatically crosses an infinite number of potential curves dissociating into R+H, H being hydrogen in a Rydberg state, and continues to be well defined in the ionization continuum. We might mention that results on AI are also interesting because AI is the reverse reaction to the important process of dissociative recombination of molecular ions. 

Fig. 9.3 Diamagnetic Zeeman effect in Ba (a) zero-field, (b) o polarization with resonances extending into the ionization continuum of (a) (from ref. 2).

Fig. 8.9. Schematic energy diagrams illustrating recombination mechanisms. The ionization continuum is shown shaded, (a) Spontaneous radiative capture, reaction (8.11) (b) stimulated radiative capture by irradiation with laser light, reaction (8.12) (c) three-body recombination in which the excess energy is removed by an extra positron, reaction (8.14).

For autoionization following inner-shell excitation such a classification neglects the competing and interfering process of direct photoionization leading to the same final state. It would be more appropriate to consider autoionization as a resonance feature embedded in the ionization continuum of main and satellite photoprocesses. 

For a correct treatment of Auger electron emission as a resonance in the double ionization continuum, one has to start with the transition rate P ... 

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