Spectroscopy ionization

Ionization spectroscopy monitors the absorption of photons on the molecular transition Ej Ej by detecting the ions or electrons, produced by some means, while the molecule is in its excited state Ej.  

The ionizing photon may come either from the same laser which has excited the level E, or from a separate light source, which can be another laser or even an incoherent source (see Fig.8.13). 

If the excited level E is not too far from the ionization limit, the molecule may be ionized by thermal collisions with other atoms. If Ej lies above the ionization limit of the collision partners A, Penning ionization becomes an efficient process [8.30] which proceeds as 

If the excited level lies close below the ionization limit, the molecule M (E ) can be ionized by an external electric dc field. This method is particularly efficient if the excited level is a long-lived highly excited Rydberg state. The required minimum electric field can be readily estimated from Bohr s atomic model which gives a good approximation for atomic levels with large principal quantum number n. The ionization potential for the outer electron at a mean radius r from the nucleus is determined by the Coulomb field of the nucleus shielded by the inner electron core. 

Example For levels 10 meV below the ionization limit, (8.20) gives E lO V/m for the ionizing external field. However, because of the quantum mechanical tunnel effect the fields required for complete ionization are even lower. 

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Meier C and Engel V 1995 Pump-probe ionization spectroscopy of a diatomic molecule sodium molecule as a prototype example Femtosecond Chemistry Proc. Berlin Conf Femtosecond Chemistry (Berlin, March 1993) (Weinheim Verlag Chemie)... 

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. 

James A M, Kowaiczyk P, Langlois E, Campbell M D, Ogawa A and Simard B 1994 Resonant two photon ionization spectroscopy of the molecules Nb, and Nb2 J. Chem. Rhys. 101 4485... 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

Sputter-Initiated Resonance Ionization Spectroscopy Surface Analysis by Resonant Ionization Spectroscopy Time-of-Flight Mass Spectrometer... 

REMPI resonance-enhanced multiphoton ionization spectroscopy... 

Mons M, Piuzzi F, Dimicoli I, Gorb L, Lesczynski J (2006) Near-UV resonant two-photon ionization spectroscopy of gas phase guanine evidence for the observation of three rare tautomers. J Phys Chem A 110 10921-10924... 

The direct proof that H is present in certain centers in Ge came from the substitution of D for H, resulting in an isotopic energy shift in the optical transition lines. The main technique for unraveling the nature of these defects, which are so few in number, is high-resolution photothermal ionization spectroscopy, where IR photons from an FTIR spectrometer excite carriers from the ls-like ground state to bound excited states. Phonons are used to complete the transitions from the excited states to the nearest band edge. The transitions are then detected as a photocurrent. 

As for silicon, secondary ion mass spectrometry (SIMS) is the most widely used profiling analysis technique for deuterium diffusion studies in III-V compounds. Deuterium advantageously replaces hydrogen for lowering the detection limit. The investigations of donor and acceptor neutralization effects have been usually performed through electrical measurements, low temperature photoluminescence, photothermal ionization spectroscopy (PTIS) and infrared absorption spectroscopy. These spectroscopic investigations will be treated in a separated part of this chapter. 

Resonance ionization spectroscopy is a photophysical process in which one electron can be removed from each of the atoms of a selected type. Since the saturated RIS process can be carried out with a pulsed laser beam, the method has both time and space resolution along with excellent (spectroscopic) selectivity. In a recent article [2] we showed, for example, that all of the elements except helium, neon, argon, and fluorine can be detected with the RIS technique. However, with commercial lasers, improved in the last year, argon and fluorine can be added to the RIS periodic table (see figure 2). 

Charge Neutralization and Resonance Ionization Spectroscopy with Amplification (RISA)... 

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