Photoionization application

Y. Matsuda, M. Hachiya, A. Fujii, and N. Mikami, Stimulated Raman spectroscopy combined with vacuum ultraviolet photoionization Application to jet cooled methanol clusters as a new vibrational spectroscopic method for size selected species in the gas phase. Chem. Phys. Lett. 442, 217 219 (2007). 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Hepburn J W 1995 Generation of coherent vacuum ultraviolet radiation applications to high-resolution photoionization and photoelectron spectroscopy Laser Techniques in Chemistry vol 23, ed A B Myers and T R Rizzo (New York Wley) pp 149-83... 

The discrete line sources described above for XPS are perfectly adequate for most applications, but some types of analysis require that the source be tunable (i.e. that the exciting energy be variable). The reason is to enable the photoionization cross-section of the core levels of a particular element or group of elements to be varied, which is particularly useful when dealing with multielement semiconductors. Tunable radiation can be obtained from a synchrotron. 

W. Haag and C. Wrenn, Handbook of Theory and Applications of Direct-reading Photoionization Detectors (PIDs), RAE Systems Inc., Sunnyvale, CA (2002). 

Molecular Fluorescence Spectroscopy Photometric Titrations Analytical Applications of Interferometry Vol. 9 Ultraviolet Photoelectron and Photoion Spectroscopy... 

The most important techniques belonging to this class are electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and, more recently, atmospheric pressure photoionization (APPI). At present the latter does not have applications in cultural heritage, so it will be not described here. 

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. 

The two most common LG/MS interfaces used for routine quantitative analyses are APCI and APT electrospray. The principles of these techniques in direct infusion analyses have been described earlier (see Sections 3.3 and 3.4). As APTelectrospray has a broader application profile, its use is more widespread than APCI. Other configurations including El, atmospheric pressure photoionization (APPl), and thermospray interfaces with liquid chromatographs are available but are less commonly used for high throughput or routine analysis. 

Bos, S. J., Leeuwen, S. M., and Karst, U. (2006). Erom fundamentals to applications recent developments in atmospheric pressure photoionization mass spectrometry. Anal. Bioanal. Chem. 384, 85-99. 

Laser photoionization Another ionization method with great potential for ambient air applications is... 

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. 

For molecules containing atoms of high electron affinity the photocurrent at the first threshold may be due to a dissociation into ions. This phenomenon was already demonstrated in the early 1930 s by Terenin and Popov28 for TIHal vapors, which split into T1+ + Hal - as a primary photoprocess. Such was the first instance of the application of mass spectrometry to the study of the photoionization of gases. A similar process has been later shown by Morrison et al.8 for Br2 and I2, the first threshold, corresponding to a pre-ionization, accompanied by the dissociation into Hal+ + Hal-. 

Some of the earliest applications of MQDT dealt with vibrational and rotational autoionization in H2 [21-25]. One concept that emerged from these studies is that of complex resonances [26], which are characterized by a broad resonant distribution of photoionization intensity with an associated rather sharp fine structure. These complex resonances cannot be characterized by a single decay width they are the typical result of a multichannel situation where several closed and open channels are mutually coupled. The photoionization spectrum of H2 affords a considerable number of such complex resonances. 

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