Multi-photon ionization

To better understand the process of ionization of an atom in strong laser fields, it is convenient to resort to a simple quasi-static picture. In Fig. 4.6 

At low intensities (/ lO W/cm, 7 1) the electron has to absorb several photons before leaving the atom. This is the so-called multi-photon ionization regime. At higher intensities (/ 10 W/cm, 7 1) we enter the tunneling regime. If we further increase the strength of the laser field (/ 10 W/cm, 7 C 1), then the electron can simply pass over the barrier. 

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 

Similar experimental results were found for a variety of molecules. Furthermore, in these more complex systems, the coupling of the nuclear and the electronic degrees of freedom gives rise to new physical phenomena. As illustrative examples of such phenomena, we refer to the so-called ionization induced Coulomb explosion [71], and the production of even harmonics as a consequence of beyond-Born-Oppenheimer dynamics [72]. 

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The temi action spectroscopy refers to those teclmiques that do not directly measure die absorption, but rather the consequence of photoabsorption. That is, there is some measurable change associated with the absorption process. There are several well known examples, such as photoionization spectroscopy [47], multi-photon ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [, 51], vibrational predissociation spectroscopy [ ] and optothemial spectroscopy [53, M]. These teclmiques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

Laser Ionization Mass Spectrometry Laser Microprobe Mass Analysis Laser Microprobe Mass Spectrometry Laser Ionization Mass Analysis Nonresonant Multi-Photon Ionization... 

Surface Analysis by Laser Ionization Post-Ionization Secondary Ion Mass Spectrometry Multi-Photon Nonresonant Post Ionization Multiphoton Resonant Post Ionization Resonant Post Ionization Multi-Photon Ionization Single-Photon Ionization... 

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

Valuable findings on the electronic ground and excited states of clusters have been derived from laser-induced multi-photon ionization (MPl) investigations, such as laser-induced fluorescence (LIF) and REMPI. This latter technique is particularly promising since it enables mass selection of cluster species and their spectral and thermochemical characterization. The complex is excited from its electronic ground state from a photon and then ionized by a second photon of equal or different frequency, near threshold to avoid cluster fragmentation. ... 

Photoionization, where electrons are released by molecules following the absorption of energy from photons, has long been viewed as a non-radioactive means to ionize explosives in the vapor phase [39]. In recent years, two teams have sought to employ laser ionization with IMS for explosive determinations. A team at Implant Sciences Corporation has utilized a laser (or flash lamp) for sampling surfaces and for ionization of sample vapors in an IMS analyzer [40, 41]. In their approach, the sample is removed from a surface with an increased temperature from laser exposure. Gases (and presumably particulate matter) from over the surface are drawn into an IMS drift tube using a wall-free inlet vida supra). In the IMS drift tube, resonance multi-photon ionization by a laser is used to produce ions from the explosives. Their... 

REMPI Resonantly Enhanced Multi-Photon Ionization... 

Tunable laser spectroscopic techniques such as laser-induced fluorescence (LIF) or resonantly enhanced multi-photon ionization (REMPI) are well-established mature fields in gas-phase spectroscopy and dynamics, and their application to gas-surface dynamics parallels their use elsewhere. The advantage of these techniques is that they can provide exceedingly sensitive detection, perhaps more so than mass spectrometers. In addition, they are detectors of individual quantum states and hence can measure nascent internal state population distributions produced via the gas-surface dynamics. The disadvantage of these techniques is that they are not completely general. Only some interesting molecules have spectroscopy amenable to be detected sensitively in this fashion, e.g., H2, N2, NO, CO, etc. Other interesting molecules, e.g. 02, CH4, etc., do not have suitable spectroscopy. However, when applicable, the laser spectroscopic techniques are very powerful. 

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...

Here, I is the light intensity, a parameterizes the avalanche free-electron generation, and b(I) represents the Multi Photon Ionization (MPI) rate that is a highly nonlinear function of the intensity. The last term describes plasma recombination. 

Therefore, heterogeneous catalysts present a greater potential for the application of HT and Combinatorial methods, because they involve diverse compositional phases that are usually formed by interfacial reactions during their synthesis, which in turn produce a variety of structural and textural properties, often too vast to prepare and test by traditional methods. In this respect the HT and Combinatorial methods extend the capabilities of the R D cycle, which comprises the synthesis, the characterization of physicochemical properties and the evaluation of catalytic properties. The primary screening HT method gives the possibility of performing a rapid test of hundreds or thousands of compounds using infrared detection methods [27-29]. Alternatively, a detection method called REMPI (Resonance Enhanced Multi Photon Ionization) has been used, which consists of the in situ ionization of reaction products by UV lasers, followed by the detection of the photoions or electrons by spatially addressable microelectrodes placed in the vicinity of the laser beam [30, 31]. 

An important parameter for comparison with theory as well as for understanding many properties would be relative binding energies or stabilities. Unfortunately those are hard to assess in the gas phase. One of the few experiments to report thermodynamic binding energies between base pairs is the work by Yanson et al. in 1979, based on field ionization [25], Relative abundances of nucleobase clusters in supersonic beams are an unreliable measure of relative stability for a two reasons First, supersonic cooling is a non-equilibrium process and thus comparison with thermal populations is tenuous at best. Secondly, ionization probabilities may be a function of cluster composition. The latter is certainly the case for multi photon ionization, as will be discussed in detail below. 

Figure 10 Effects of multi-photon ionization rates on damage threshold fluences at A = 780 nm for various pulse durations [13]...

The recently introduced atmospheric-pressure laser ionization system (APLI) can be considered as a modification of APPI (Ch. 5.7.3). In APLI, the one-step photoionization of APPI is replaced by a two-photon process in resonantly-enhanced multi-photon ionization [148]. Enhanced response for polycyclic aromatic hydrocarbons (relative to APCI) was demonstrated. Molecular ions rather than protonated molecules are generated in APLI (cf. Ch. 6.5). 

Comparison of One-Photon and Multi-Photon Ionization in the He Buffer Gas... 

FIGURE 9.2 Comparison of TOF mass spectra obtained by one-photon ionization (IPI) and multi-photon ionization (MPI) of laser ablated graphite in the He buffer gas. 

MALDI MCPs MO MPI MWNT matrix-assisted laser desorption/ionization multi-channel plates molecular orbital multi-photon ionization multiwalled carbon nanotube... 

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