Mass-spectroscopy photoionization

Figure 1. Schematic illustration of the laser-vaporization supersonic cluster source. Just before the peak of an intense He pulse from the nozzle (at left), a weakly focused laser pulse strikes from the rotating metal rod. The hot metal vapor sputtered from the surface is swept down the condensation channel in dense He, where cluster formation occurs through nucleation. The gas pulse expands into vacuum, with a skinned portion to serve as a collimated cluster bean. The deflection magnet is used to measure magnetic properties, while the final chaiber at right is for measurement of the cluster distribution by laser photoionization time-of-flight mass spectroscopy.

Photoelectron spectroscopy of free radicals has been utilized for detection of radicals. It can be via resonance photoexcitation and photoionization (e.g. ZEKE) or non-resonance photoionization (e.g. single-photon VUV photoionization). The photoelectron spectroscopy of free radicals has been reviewed in 1994 by Chen.5 A recent review on mass spectrometry, photoelectron spectroscopy, and photoionization of free radicals by Sablier and Fujii is available.72 It is worthwhile to point out that mass spectrometry by photoionization offers some advantage for the detection of radicals, in comparison with the conventional mass spectroscopy by electron-impact... 

MS = mass spectroscopy PID = photoionization detector ppbv = parts per billion by volume... 

Total photoabsorption Total photoionization Photoelectron spectroscopy Photoionization mass spectrometry Electron-energy-loss spectroscopy, dipole (e, e) Dipole (e, 2e) or (e, e + ion) (from sums of partials) Electron energy loss-ejected electron coincidence, dipole (e, 2e) Electron-ion coincidence, dipole (e, e + ion)... 

In this section we discuss the more important experimental results for continuum oscillator strengths measured by electron spectroscopy that have been reported up to mid 1978. The discussion is divided on the basis of target species rather than the type of experiment since this stresses the interrelation and complementary nature of many of the experiments. As the experimental work is far from complete in many cases, only a limited picture of the overall breakdown processes is available at present. In particular, a very limited amount of work has been reported for inner shells. More data are generally available for mass fragmentation (photoionization mass spectrometry) than for partial ionization cross sections (photoelectron spectroscopy). 

The molecules and their first vertical ionization energies listed (Table 3) supplement those already presented as examples. The parent silylene has been generated by the SiFLt + 2F — SiH2 + 2HF reaction and investigated by photoionization mass spectroscopy, which yields the adiabatic onset value quoted73. Silyl derivatives such as C(SiH3)4 have been thermally decomposed to detect new routes to amorphous,... 

Recently in our laboratory we have initiated a program to study the photoabsorption processes of metal vapors throughout the UV and EUV region. Our research interests are (1) to obtain the absolute cross section measurement of atomic and molecular metal vapors, (2) to study the photoionization processes of molecular metal species, and (3) to study the photodissociation processes of molecular metal ions. Several experimental methods such as the heat-pipe absorption spectroscopy, photoionization mass spectroscopy, and electron-ion coincidence technique, will be used in the study. This report summarizes our first experiment using heat-pipe absorption spectroscopy. 

The very first fluorescence spectra of jet-cooled exciplexes indicated the existence of two types of ground-state van der Waals adducts. For instance, the anthracene-dimethylaniline system displayed two types of cluster bands in the fluorescence excitation spectrum broad ( 150 cm ) and structureless, leading to typical ex-ciplex emission, and narrow (1 cm ), leading to resonance-type emission [10, 20]. It was assumed that they are due to different 1 1 adducts, distinguished by different geometries. Recently, laser-based techniques were developed that allow the discrimination of different species. One is hole-burning spectroscopy and another— mass-selected photoionization. 

Y. Kato, T. Wakabayashi, and T. Momose, Preferential formation of neutral Cio upon laser vaporized graphite in He gas as studied by photoionization mass spectroscopy with 10.5 eV photons, J. Chem. Phys. 2003,118, 5390-5394. 

Fig. 22. Size distribution of carbon clusters obtained by laser vaporization and measurement by photoionization time-of-flight mass spectroscopy. From Ref. 353.

Modem mass spectrometry, photoionization techniques, and ion cyclotron resonance spectroscopy revolutionized gas-phase basidty determination. These techniques have permitted extremely accurate absolute gas-phase basidties to be obtained for a few molecules via thermodynamic cycles using electron affinity and ionization energy data. - Absolute gas-phase basicities for even a few molecules provide valuable references for determination of GB values for bases for which direct GB determination is problematic. The mathematical approach is conceptually straightforward. Consider the general GB equations for Bj and B2 ... 

In contrast to the stable hydrocarbons, where the standard enthalpy of formation is based on one or another of the direct calorimetrical methods, values for radicals come from all sorts of very difficult measurements ranging from photoionization mass spectroscopy to measurements of reaction rates. It is no surprise that the results are more contentious and less accurate. In Table 1.3, the uncertainty ranges can be seen to be typically an order of magnitude greater than those for stable hydrocarbon values except where the Active Thermochemical Tables can help. 

The ionization energies determined by photoelectron-photoion coincidence spectroscopy are plotted as a function of the cube root of the reciprocal number of atoms x in the cluster. The latter is determined by in situ mass spectroscopy of the ionized clusters. 

The recent femtosecond experimental data on electron solvation in water (Gaudel et al., 1984, 1987) and observations in a mass spectroscopy on the formation of electrons stabilized in molecular beam clusters (Arbruster et al., 1984) has rekindled extensive interest on the microscopic details of the dynamics and structure of e in particular. Since the appearance of the visible spectrum of e has now been observed from the two photon photoionization of pure water (e.g., no dopant molecules or ions were present) we must focus on what responses can be Induced from the medium on this timescale. From our present databank, it is evident that following the instantaneous electronic polarizability (linear and nonlinear) response to the moving charge and/or field, it is the librational responses that must be the key motion. We assume, for the moment, that the lifetime of the autoionizing level in H2O is not a significant factor. As we have discussed elsewhere (Kenney-Wallace,... 

See also Laser Applications in Electronic Spectroscopy Laser Spectroscopy Theory Light Sources and Optics Multiphoton Excitation in Mass Spectrometry Muitiphoton Spectroscopy, Applications Optical Frequency Conversion Pharmaceuticai Appiications of Atomic Spectroscopy Photoelectron Spectroscopy Photoionization and Photodissociation Methods in Mass Spectrometry Pyroiysis Mass Spectrometry, Methods Time of Flight Mass Spectrometers X-Ray Spectroscopy, Theory Zero Kinetic Energy Photo-eiectron Spectroscopy, Appiications Zero Kinetic Energy Photoelectron Spectroscopy, Theory. 

See also Fragmentation in Mass Spectrometry Gas Phase Appiications of NMR Spectroscopy ion Dissociation Kinetics, Mass Spectrometry ion Energetics in Mass Spectrometry Photoionization and Photodissociation Methods in Mass Spectrometry Time of Fiight Mass Spectrometers. 

Figure 12 Mass-selected photoion yield curve of C2H2 + in the vicinity of the ionization threshold. Three steps are observed owing to direct ionization from the vibrational ground state of the neutral to v = 0,1 and 2 of the C-C stretch of the ion. Autoionization structure is superimposed upon the steplike vibrational structure. Reproduced with permission of Academic Press from Berkowitz J (1979) Photoabsorption, Photoionization, and Photoeiectron Spectroscopy, Chapter VI.

The most interesting new results characterizing the reactivity of naked clusters have been obtained with unsupported clusters by mixing pulsed cluster beams with pulsed flows of various reactant molecules and analyzing the products by photoionization mass spectroscopy (see section 2.1.). As far as supported clusters are concerned, we will focus our attention on a new class of low-nuclearity bimetallic clusters formed by the decomposition of organometallic complexes. 

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). 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

The rationalization of mass spectrometric investigations of nitro compounds has benefited significantly from numerous studies applying techniques adopted from photochemistry, such as photodissociation, photoionization and photoelectron photoion coincidence spectroscopy. 

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