Photoionization

Photoionization at 193 nm in oxygenated solution of poly(C) causes strand breakage with high efficiency, half of which occurs at times 4 ms, the other half with a half-life of 7 ms (Melvin et al. 1996 Table 11.8). This kinetic behavior is very different from what is seen after -OH-attack and points to the direct involvement of the Cyt radical cation. In poly(U), the (biphotonic) photoionization shows similar results (Table 11.8). With poly(A), the formation of strand breaks is 20-times less efficient as compared to poly(C) (Table 11.8), and this is in agreement with the above conclusion that the A-+ or A- do not cause strand breakage to any major extent. 

In poly(U), the slow component shows all the kinetic properties of strand break formation by the -OH-adducts (Schulte-Frohlinde et al. 1985). Thus, it is reasonable to assume that the fast component has been induced by the rapid transformation of the Ura radical cation into the C(2 ) radical (for a mechanistic discussion of the analogous Cyd system see Aravindakumar et al. 2003 and Chap. 10.2). The branching (60% fast to 40% slow) maybe due to a competition of C(2 )OH and water for the uracil radical cation. 

In the 193-nm photolysis ofpoly(U), only low yields ofbase release compared to photoionization have been observed (Gurzadyan and Corner 1994). Only the prompt base release has been determined, and the question must be posed whether these values would also increase substantially upon heating as has been observed after -OH-attack (see above). 

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 resonance-enhanced multiphoton ionization (REMPI, also commonly referred to as resonance ionization—RI) near-UV photons can be used for ionization [60]. When 

One method to study energy-selected ions is threshold ionization, in which ions with precisely defined energy contents are produced. These ions can then be used to study unimolecular fragmentation, ion-molecule reactions, van der Waals clusters, and hydrogen-bonded clusters [62]. 

Since different lamps and windows are available, this detection method can often be selective for only some of the components present in the sample. Its ability to detect small concentrations is especially good for aromatic hydrocarbons and inorganics. It is a nondestructive detector. 

The usual qualitative analysis procedure, then, is to establish the conditions for the experiment, perhaps by trial and error in one s own laboratory or by matching conditions outlined in a given procedure, that would resolve all compounds that may potentially be in the unknown. The idea is to match the retention time data, either ordinary retention time or the selectivity, whichever is appropriate, for standards (pure samples) with that for the unknown. The analyst can then proceed to match the retention time data for the unknown to those of the pure samples to determine which substances are present in the unknown (Experiment 40). 

One important caution, however, is that there may be more than one component with the same retention time (no separation), and thus further experimentation may be required. For example, when working with a complex mixture whose components are perhaps not all known, it may be necessary to change the experimental conditions to determine whether a given peak is due to one component (known) or more (e.g., one known and one unknown). Changing the stationary phase may prove useful. Such a change would produce a chromatogram with completely different retention times and probably a different order of elution. Thus two components that were co-eluted before may now be separated, evidence for which would be a different peak size for the known component. 

Let us consider an example of the first case. If the ions are formed in the excited state, the natural characteristic of anisotropy of the process consists of the polarization of their fluorescence. This method was first applied by Poliakoff, Zare et al. [315] in the reaction 

The authors of [315] applied linearly polarized synchrotron radiation (45-66 nm) for ionization, which corresponds to photon energy from 18.76 eV (threshold) + 0.7 eV up to 27 eV. The measured V values, as dependent on photon energy, changed correspondingly from 0.052 down to approximately half the value, which made it possible to determine the value of r within the range 0.4-0.7. Further improvement of the experiment and refinement of the theoretical description was carried out in [179]. Accounting for the hyperfine and spin-rotational interaction effect made it possible to refine the photoionization channel relation r, which yielded values of 0.2-0.4 for photon energies between threshold and 32 eV. 

The molecular weight aud aualytical applications mentioned for the field ion source can be extended to include this source. Further, proper choice of wavelength may be made so that only single components in a mixture are ionized. For this technique to be successful, of course, the ionization potentials must be separated by 2 to 3 eV otherwise, the various components might interfere with each other. 

Physically, the PI source resembles the electron bombardment source, with a light source and monochromator replacing the heated filament and electron trap. Since there is no heated filament, the PI source has the further advantage that decompositions promoted by heat are eliminated. 

PI mass spectra usually are dominated by molecular radical cations M+ or, less often, by protonated molecules [M + H]+. 

High-pressure mass spectrometry has been studied for many years by workers interested in ion-molecule reactions. Ionization of the substance under scrutiny is affected by reactions between the molecules of the compound and a set of ions that serve as ionizing reactants. The reactant ions are formed by subjecting a gas to electron bombardment at pressures of about 1 mm. The reactant gas is extensively ionized and, because of the high pressure, undergoes ion molecule reactions with itself and with the sample molecules that are present at relatively low concentrations (less than 1 percent. This process in a mass spectrometer is termed chemical ionization CCI). 

If the reactant gas is methane, the most abundant ions formed are CH5+ and C2H5+, which then become the reactant ions. On collision with the sample molecules, these ions donate a proton to form what is termed a quasi-molecular ion [M + H]+ at a mass 1 Da higher than the molecular weight (Equation 4.4). 

Alternatively, tunable lasers that prodnce longer-wavelength photons (e.g., UV, visible, or infrared radiation) can be employed in a two-step process known 

A precondition for PI is that the analyte molecule must be present in the gas phase. Solid samples must be desorbed or vaporized prior to photoionization. Thermal vaporization [8] or laser desorption (LD) [9] are convenient means to convert the solid samples into a gas-phase plume. A combination of LD and laser ionization has been used successfully to analyze organic contantinants in water and soils [9]. A proper choice of the wavelength will exclude ionization of the bulk components of a real-world sample (e.g., N2, O2, CO2, or water). 

The excited molecules can be ionized by absorption of a second photon, i.e.. 

The ionizing photon may come either from the same laser which has excited the level Ej or from a separate light source, which can be another laser or even an incoherent source (Fig. 6.19a). 

A very efficient photionization process is the excitation of high-lying Rydberg levels above the ionization limit (Fig.6.19b) which decay by autoionization into lower levels of the ion M  

The absorption cross-section for this process is generally much larger than that of the bound-free transition described by (6.30a) (Sect. 10.4.2). 

The excited molecule may also be ionized by a nonresonant two-photon process (Fig.6.19c) 

---

The above fomuilae for the absorption spectrum can be applied, with minor modifications, to other one-photon spectroscopies, for example, emission spectroscopy, photoionization spectroscopy and photodetachment spectroscopy (photoionization of a negative ion). For stimulated emission spectroscopy, the factor of fflj is simply replaced by cOg, the stimulated light frequency however, for spontaneous emission... 

Above approximately 80 km, the prominent bulge in electron concentration is called the ionosphere. In this region ions are created from UV photoionization of the major constituents—O, NO, N2 and O2. The ionosphere has a profound effect on radio conmumications since electrons reflect radio waves with the same frequency as the plasma frequency, f = 8.98 x where 11 is the electron density in [147]. The... 

Moseley J T 1985 Ion photofragment spectroscopy Photodissociation and Photoionization ed K P Lawley (New York Wiley)... 

Lykke K R and Kay B D 1990 State-to-state inelastic and reactive molecular beam scattering from surfaces Laser Photoionization and Desorption Surface Analysis Techniquesvo 1208, ed N S Nogar (Bellingham, WA SPIE) p 1218... 

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. 

Wight C A and Armentrout P B 1993 Laser photoionization probes of ligand-binding effects in multiphoton dissociation of gas-phase transition-metal complexes ACS Symposium Series 530 61-74... 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

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). 

Similarly, tire cross sections for radiative recombination (RR) and for photoionization (PI), the forward and reverse directions of... 

Figure B2.3.7. Schematic apparatus of crossed molecular beam apparatus with synclirotron photoionization mass spectrometric detection of the products [12], To vary the scattering angle, the beam source assembly is rotated in the plane of the detector. (By pemrission from AIP.)...

Yang X, Lin J, Lee Y T, Blank D A, Suits A G and Wodtke A M 1997 Universal crossed molecular beams apparatus with synchrotron photoionization mass spectrometric product detection Rev. Sc/. Instrum 68 3317-26... 

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... 

Herrmann A, Leutwyler S, Schumacher E and Woste L 1978 On metal-atom clusters IV. Photoionization thresholds and multiphoton ionization spectra of alkali-metal molecules Hel. Chim. Acta 61 453... 

Yang S and Knickelbein M B 1990 Photoionization studies of transition metal clusters ionization potentials for Fe... 

Koretsky G M and Knickelbein M B 1997 Photoionization studies of manganese oiusters ionization potentiais for Mn-,to Mn, J. Chem. Phys. 106 9810... 

Wang L S 2000 Photodetachment photoelectron spectroscopy of transition metal oxide species Photoionization and Photodetaohment Advanced Series in Physical Chemistry 10, ed C Y Ng (Singapore World Scientific)... 

The photoionization process with which we shall be concerned in both UPS and XPS is that in Equation (8.4) in which only the singly charged is produced. The selection mle for such a process is trivial - all ionizations are allowed. 

The lines of primary interest ia an xps spectmm ate those reflecting photoelectrons from cote electron energy levels of the surface atoms. These ate labeled ia Figure 8 for the Ag 3, 3p, and 3t7 electrons. The sensitivity of xps toward certain elements, and hence the surface sensitivity attainable for these elements, is dependent upon intrinsic properties of the photoelectron lines observed. The parameter governing the relative iatensities of these cote level peaks is the photoionization cross-section, (. This parameter describes the relative efficiency of the photoionization process for each cote electron as a function of element atomic number. Obviously, the photoionization efficiency is not the same for electrons from the same cote level of all elements. This difference results ia variable surface sensitivity for elements even though the same cote level electrons may be monitored. 

Fig. 9. Scofield s x-ray photoionization cross-section relative to that for C electrons as a function of atomic number (19).

Fig. 10. Comparison of Scofield s calculated x-ray photoionization cross-sections relative to that for F 1 electrons and experimental values (22) as a...

C. L. Wilson, Comprehensive Analytical Chemisty Ultraviolet Photoelectron and Photoion Spectroscopy Auger Electron Spectroscopy Plasma Excitation in SpectrochemicalAnalysis, Vol. 9, Elsevier Science, Inc., New York, 1979. 

The near-ir spectmm of ethylene oxide shows two peaks between 1600—1700 nm, which are characteristic of an epoxide. Near-ir analyzers have been used for verification of ethylene oxide ia railcars. Photoionization detectors are used for the deterrnination of ethylene oxide ia air (229—232). These analyzers are extremely sensitive (lower limits of detection are - 0.1 ppm) and can compute 8-h time-weighted averages (TWAg). 

In Surface Analysis by Laser Ionization (SALI), a probe beam such as an ion beam, electron beam, or laser is directed onto a surfiice to remove a sample of material. An untuned, high-intensity laser beam passes parallel and close to but above the sur-fiice. The laser has sufficient intensity to induce a high degree of nonresonant, and hence nonselective, photoionization of the vaporized sample of material within the laser beam. The nonselectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species. SALI spectra accurately reflect the surface composition, and the use of time-of-flight mass spectrometers provides fast, efficient and extremely sensitive analysis. 

The spectra of Figure 3 illustrate two further points. All the C Is peaks in Figure 3a are of equal intensity because there are an equal number of each type of C atom present. So, when comparing relative intensities of the same atomic core level to get composition data, we do not need to consider the photoionization cross section. Therefore, Figure 3c immediately reveals that there is four times as much elemental Si present as Si02 in the Si 2p spectrum. The second point is that the chemical shift range is poor compared to the widths of the peaks, especially for the solids in Figures 3b and 3c. Thus, not all chemically inequivalent atoms can be distin-... 

J. H. Scofield. J. Electron Spect. 8,129, 1976. This is the standard quoted reference for photoionization cross sections at 1487 eV. It is actually one of the most heavily cited references in physical science. The calculations are published in tabular form for all electron level of all elements. 

Si 2p line, at about 100 eV BE, is also easily accessible at most synchrotron sources but cannot, of course, be observed using He I and He II radiation. On the other hand, the Zn 3d and Hg 4f lines can be observed quite readily by He I radiation (see Table 1) and the elements identified in this way. Quantitative analysis using relative peak intensities is performed exactly as in XPS, but the photoionization cross sections a are very different at UPS photon energies, compared to A1 Ka energies, and tabulated or calculated values are not so readily available. Quantitation, therefore, usually has to be done using local standards. 

---