Multiphoton ionization applications

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

The large variability in elemental ion yields which is typical of the single-laser LIMS technique, has motivated the development of alternative techniques, that are collectively labeled post-ablation ionization (PAI) techniques. These variants of LIMS are characterized by the use of a second laser to ionize the neutral species removed (ablated) from the sample surface by the primary (ablating) laser. One PAI technique uses a high-power, frequency-quadrupled Nd-YAG laser (A, = 266 nm) to produce elemental ions from the ablated neutrals, through nonresonant multiphoton ionization (NRMPI). Because of the high photon flux available, 100% ionization efflciency can be achieved for most elements, and this reduces the differences in elemental ion yields that are typical of single-laser LIMS. A typical analytical application is discussed below. 

The advancement of the application of lasers in combination with the molecular beam technique has made a great impact in the understanding of primary photodissociation processes. For state-specific detection of small fragments, laser-induced fluorescence, multiphoton ionization, and coherent laser scattering have provided extremely detailed information on the dynamics of photodissociation. Unfortunately, a large number of interesting... 

A number of other laser spectroscopic techniques are of interest but space does not permit their discussion. A few specialized methods of detecting laser absorption worthy of mention include multiphoton ionization/mass spectrometry (28), which is extremely sensitive as well as mass selective for gas-phase systems optically detected magnetic resonance (29) laser intracavity absorption, which can be extremely sensitive and is applicable to gases or solutions (30) thermal blooming, which is also applicable to very weak absorbances in gases or liquids (31) and... 

Mach-Zehnder interferometer, 144 Medical applications, 153 Metal-insulator transitions, 52 Monte Carlo procedure, 135 Multi-energy X-ray imaging, 131 Multilayer targets, 131 Multiphoton absorption, 85 Multiphoton ionization, 82 Multiple filamentation, 91, 92 Multipulse techniques, 152... 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

In practice, for application to ambient air, efficient photoionization requires the use of pulsed lasers and multiphoton absorption methods. The terms multiphoton ionization, or MPI, and resonance-enhanced multiphoton ionization, or REMPI, are used to describe these processes. 

S.-I. Chu, WP. Reinhardt, Intense field multiphoton ionization via complex dressed states Application to the H atom, Phys. Rev. Lett. 39 (19) (1977) 1195. 

Lindner J, Grotemeyer J, Schlag EW (1990). Applications of multiphoton ionization mass spectrometry Small protected nucleosides and nucleotides. Int J Mass Spectrom Ion Proc 100 267-285. 

The spectroscopic methods are based on time-resolved pump-probe schemes where the collision-free regime is usually attained by using low pressure conditions. Application of various linear and non-linear laser techniques, such as LIF (laser-induced fluorescence), REMPI (resonant-enhanced multiphoton ionization) and CARS (coherent antistokes Raman spectroscopy) have provided detailed information on the internal states of nascent reaction products [58]. Obviously, an essential prerequisite for the application of these techniques is the knowledge of the spectroscopic properties of the products. 

The chapter is organized as follows. The principle of the coupled-channel method is reviewed in detail in Section 2. The results are discussed in connection to higher order terms in Section 3. The application to multiphoton ionization is described in Section 4. Comparisons with measurements are provided in Section 5. A simple model for the electronic energy loss is... 

Czech Republic). Development and application of semipermeable membrane devices (SPMDs) as environmental dosimeters for PCB contaminants in water, air, sediment, and soil is the subject of ongoing research by Huckins and Petty at Columbia Environmental Research Center in Missouri. Also at the Columbia Environmental Research Center, C. Orazio et al. are developing analytical methods for determining PCBs in environmental matrices. A reliable method for continuous monitoring of PCBs in incinerator stack gas emissions using resonance-enhanced multiphoton ionization spectroscopy in conjunction with time-of-flight mass spectroscopy (REMPI/TOFMS) is the topic of current research by... 

In addition to laser fluorescence excitation, several other laser spectroscopic methods have been found to be useful for the state-selective and sensitive detection of products of reactive collisions resonance-enhanced multiphoton ionization [58], coherent anti-Stokes Raman scattering [M], bolometric detection with laser excitation [30], and direct infrared absorption [7]. Several additional laser techniques have been developed for use in spectroscopic studies or for diagnostics in reacting systems. Of these, four-wave mixing [ ] is applicable to studies of reaction dynamics although it does have a somewhat lower sensitivity than the techniques mentioned above. 

In summary the discussion includes formalism and analysis contributing to the understanding of the nature of unstable states, as well as indicative theoretical and numerical examples from applications to atoms and molecules, and related comparisons with other methods, concerning prototypical problems of autoionization, predissociation, series of isolated and overlapping resonances, structure and spectroscopy of doubly and multiply excited states, multiphoton ionization, field-induced polarization, etc. 

The essentials of the SSEA for numerically compufed energy-normalized N-elecfron wavefunctions were published in 1994 by Mercouris ef al. [54], The firsf application was not only to the multiphoton ionization of H (whose specfrum is known exactly analytically), as a test case, but also to the multiphoton detachment of the four-electron Li negative ion, with two free channels, Li ls 2s S and ls 2p P°. Li (or Be) is the first system of fhe Periodic Table for which the proper description of the zero-order electronic structure requires a multiconfigurational Hartree-Fock (MCHF) description. In the context of the review of the SSEA, we also discuss briefly the formulation of the problem in terms of the full atom-EMF interaction,Vext(f), which is computationally convenient as well as necessary for certain problems involving, say, off-resonance coupling of Rydberg states, for which use of just the electric dipole term is inadequate [55-57]. 

C.A. Nicolaides, Th. Mercouris, Multiphoton ionization of negative ions in the presence of a dc-field. Application to Li , Chem. Phys. Lett. 159 (1989) 45. 

ABSTRACT. Photoelectron and mass spectral analyses have become important adjuncts to the recently developed field of multiphoton ionization spectroscopy. Each of the methods are discussed and the significant enhancement in capability provided by their joint application is illustrated by their use in the spectroscopic study of free radicals, molecular ions and transient excited states of molecules. 

A major advance in the utility of laser spectroscopy came as a result of the development of multiphoton ionization MPI as a means of detection of multiphoton absorption by molecules [1]. The resonance encountered as the n-photon energy of a scanning laser becomes coincident with that of a molecular excited state is evidenced by a large increase in ionization rate. Since single ionization events can be detected with near unit efficiency, this results in a very sensitive means of detecting weak multiphoton absorption. MPI is a more widely applicable method than laser induced fluorescence since it can be used for non-emitting states. 

Muhlberger, F., Zimmermann, R., Kettrup, A. (2001) A Mobile Mass Spectrometer for Comprehensive On-line Analysis of Trace and Bulk Components of Complex Gas Mixtures Parallel Application of the Laser-based Ionization Methods VUV Single-photon Ionization, Resonant Multiphoton Ionization, and Laser-induced Electron Impact Ionization. Anal. Chem. 73 3590-3604. 

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