Photoionization multiphoton ionization

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... 

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. 

Multiphoton ionization MPI Photoionization Atomic and molecular ions Resonance-enhanced MPI is highly selective Trace analysis... 

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. 

Finally, solute radical ions can be generated by light-induced, one-photon or multiphoton ionization of their parent compounds (Chaps. 5 and 16). This approach is particularly useful in the ultrafast studies of short-lived, unstable radical ions that aim to unravel their solvation, recombination, reaction, and vibrational relaxation dynamics of the primary charges (see, e.g., Chap. 10). Whereas the time scale of radiolytic production of secondary ions is always limited by the rate with which the primary species reacts with the dispersed parent molecules, light-induced charge separation can occur in <100 fsec. There are many studies on photoionization of solute molecules in liquid solutions we do not intend to review these works. 

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. 

The photochemistry of Zn+—(CH4) and Zn —(CH3OH) complexes has been studied in detail In that context, the possibility of the formation of a metal-hydroxo insertion complex [HO—Zn—CH3]+ (the isomer of [Zn—(CH30H)]+) has also been discussed . In other series of studies, the mechanism of dimethylzinc zinc monoethyl cation, diethylzinc diethylzinc dimer and dipropylzinc photolysis has been investigated by photoionization techniques. It was the study of Borsella and Larciprete that first observed different gas phase photodissociation mechanisms for Mc2Zn and Et2Zn by using one- and two-color multiphoton ionization combined with TOE MS. 

Figure 1. Versions of photoionization spectroscopy wherein not only the dependence of the multiphoton ionization efficiency on the laser wavelength is subject to measurement, but also the mass spectrum of photons and energy spectrum of photoelectrons (a) energy-level diagram (b) collision of a neutral particle with laser photons.

A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

The H2 molecule is a system for which quite recently it has been possible to measure in unprecedented detail state-selected vibrationally and rotation-ally resolved photoionization cross sections in the presence of autoionization [27-29]. The technique employed has been resonantly enhanced multiphoton ionization. The theoretical approach sketched above has been used to calculate these experiments from first principles [30], and it has thus been possible to give a purely theoretical account of a process involving a chemical transformation in a situation where a considerable number of bound levels is embedded in an ensemble of continua that are also coupled to one another. The agreement between experiment and theory is quite good, with regard to both the relative magnitudes of the partial cross sections and the spectral profiles, which are quite different depending on the final vibrational rotational state of the ion. 

Intense laser sources have been used to induce ionization or fluorescence of products in several kinds of photolytic experiments. Welge and his colleagues have used these sources to detect NO by multiphoton ionization (MPI) of the product. They have also used lasers that have been extended to the VUV region by various nonlinear mixing schemes as photoionization sources for mass spectrometric detection of reaction products. 

As long as one deals with small clusters, beam analysis is possible by combining spectroscopy with expansion modeling. It is possible to use, for example, the soft ionization methods to obtain a better idea of the relative concentration of different clusters in the beam. Soft ionization can be achieved either by direct photoionization or by applying the multiphoton ionization methods (see Cheshnovsky and Leutwyler as a recent example). This technique does not solve completely the fragmentation problem, since if the positively charged species formed is not stable, it will fall apart. However, combining it with sp>ectroscopy, satisfactory results can be obtained. 

Photoionization time-of-flight mass spectrometry is used almost exclusively in all experiments described in this review. The ionizing laser sources have included excimer lasers for photon energies up to 7.87 eV and tunable ultraviolet sources up to 6.5 eV. In some early studies, multiphoton ionization was used, but it has become quite clear that this usually results in dissociative ionization. Such effects have been observed in many systems, ranging from Si49,5o ( 51 jQ jjjg transition metals. Thus single-photon ionization has... 

Laser-induced multiphoton ionization spectroscopy has been applied to the parent, methyl-, and chloro-substituted pyrazines <85ANC29ll). The ions produced by laser photoionization in the supersonic jet are mass-analyzed using a time of flight mass spectrometer, in which the spectra obtained reflect the absorption of the n-n transition. Mass-resolved excitation spectroscopy by laser ionization in the infrared region has also been applied to the conformational analysis of a-alkyl substituted pyrazines <92JA5269). Dynamic and structural properties of electronically excited states of pyrazine have been elucidated from high resolution laser spectroscopy with MHz resolution <88JST(173)201). 

The term action spectroscopy refers to those techniques that do not directly measure the 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 [42], multiphoton ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [50, 51], vibrational predissociation spectroscopy [52] and optothermal spectroscopy [53, M]- These techniques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

Photoionization and Electron Detachment.—Photoelectron spectroscopy is not within the scope of this volume, but some papers on photoionization which may be of some interest are listed here. Several theoretical treatments of photoionization in atoms and small molecules have appeared.213 Photoionization in the rare gases,214 atomic hydrogen,215 alkali-metal and alkaline-earth atoms,21 magnesium atoms,217 group II atoms,218 mercury atoms,219 molecular hydrogen,220 several polyatomic molecules,221 and solvated electrons 222 has been discussed. Multiphoton ionization in the rare gases,223 benzene,224 and in molecular crystals has also been discussed.225 ... 

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