Photoionization, resonance-enhanced

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... 

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. 

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. 

Summarizing the individual decay branches of the 4d5/2 -> 6p resonance, one finds that all final ionic states can also be reached by outer-shell photoionization, in (a) and (b) by main processes, and in (c)-(i) by discrete and continuous satellite processes. The effect of the resonance decay will then be a modification of these otherwise undisturbed direct outer-shell photoionization processes which turns out to be an enhancement in the present case. Therefore, these outer-shell satellites are called resonantly enhanced satellites. In this context it is important to note that outer-shell photoionization also populates other satellites, attached, for example, to electron configurations 5s25p4ns and 5s25p4nd. However, the parity of these satellites is even, while the decay branches (c)-(/) lead to odd parity. Therefore, both groups of final ionic states can be treated independently of each other (if configuration interaction in the continuum is neglected). 

Therefore, heterogeneous catalysts present a greater potential for the application of HT and Combinatorial methods, because they involve diverse compositional phases that are usually formed by interfacial reactions during their synthesis, which in turn produce a variety of structural and textural properties, often too vast to prepare and test by traditional methods. In this respect the HT and Combinatorial methods extend the capabilities of the R D cycle, which comprises the synthesis, the characterization of physicochemical properties and the evaluation of catalytic properties. The primary screening HT method gives the possibility of performing a rapid test of hundreds or thousands of compounds using infrared detection methods [27-29]. Alternatively, a detection method called REMPI (Resonance Enhanced Multi Photon Ionization) has been used, which consists of the in situ ionization of reaction products by UV lasers, followed by the detection of the photoions or electrons by spatially addressable microelectrodes placed in the vicinity of the laser beam [30, 31]. 

The recently introduced atmospheric-pressure laser ionization system (APLI) can be considered as a modification of APPI (Ch. 5.7.3). In APLI, the one-step photoionization of APPI is replaced by a two-photon process in resonantly-enhanced multi-photon ionization [148]. Enhanced response for polycyclic aromatic hydrocarbons (relative to APCI) was demonstrated. Molecular ions rather than protonated molecules are generated in APLI (cf. Ch. 6.5). 

Fifth Resonance-enhanced midtiphoton ionization (REMPI). In the most sophisticated form of photoionization we vary the wavelength of an excitation laser, such that a second photon (from the same or another laser) ionizes only when the first photon is resonant with a specific molecular level. Thus we obtain a combination of optical spectroscopy and mass spectrometry. 

The next stepping-stone to photoionization is finding the electronic levels of the neutral, because nonresonant ionization has rather low cross-sections that translate into poor ionization efficiencies along with high photon flux requirements. Resonant absorption of photons is more effective by several orders of magnitude [91]. Ideally, resonant absorption of the first photon leads to an intermediate state from where absorption of a second photon can forward the molecule into a continuum. This technique is known as 1 -i-1 resonance-enhanced multiphoton ionization (REMPI). From a practical point of view, the second photon should be, but not necessarily has to be, of the same wavelength (Fig. 2.20) [92]. Proper selection of the laser wavelengths provides compound-selective analysis at extremely low detection limits [90,91,93,94]. 

The mechanism of the multiphoton photoionization and fragmentation of polyatomic molecules by intense (>10 W/cm ) UV laser pulses was studied in detail by Boesl et al. (1980). The spectroscopic features of resonance-enhanced multiphoton excitation and ionization were reviewed by Ashfold and Howe (1994). 

However, this high variability in ionization efficiencies implies that LDl is a very selective ionization method. In some cases, this selectivity is advantageous, for example, when one wishes to observe the presence or concentration of one, known, select molecule of interest.However, in mass spectrometry, one usually is interested in detecting all molecules that are present, including unknown species. Thus, many methods were explored to ionize the molecules after they were desorbed by the laser. These methods are collectively known as laser desorption post-ionization methods, and the post-ionization techniques include electron impact (El, diagrammed in Figure 6.2), chemical ionization (Cl), photoionization (PI), resonant-enhanced multiphoton ionization (REMPI), and many others. 

Resonant photoemission spectroscopy can give valuable information about the atomic orbital component of the state in the spectra. Figure 10 shows a plot of area intensities of the surface state induced peak for TaC(lll) as a function of the exciting photon energy. In Fig. 10, a cross section of the Ta 5d band observed in the photoemission spectra for poly-Ta (48) is also shown. The photoionization cross section for the surface state on TaC(lll) is resonantly enhanced at hv of 40 and 50 eV, as in the case for the Ta 5d band in poly-Ta. These enhancements of the cross section are well explained by the resonance process that proceeds via photon-induced excitation,... 

Photoionization occurs via expulsion of an electron when a molecule absorbs the energy in one or more photons. Resonance-enhanced multiphoton ionization under supersonic jet conditions has great potential as a soft and highly selective ionization method. The resonance-enhanced process proceeds via the absorption of two photons in which the first photon excites the molecule to a resonant intermediate excited state, and the second photon ionizes the molecule. The ability to obtain energy-resolved ionization is a unique feature of resonance-enhanced photoionization. 

For pump-probe photoionization (PPI, Fig.l) the first laser pulse is tuned into resonance with the (vibrationless) electronic transition of the molecule, the second pulse is red-shifted in wavelength, so that the enhanced (1+1 ) photoion signal can be easily identified. When a time-of-flight mass spectrometer is used for detection the mass-selective photoion signal as a function of time delay can be recorded as the RCS spectrum of the electronically excited state, which is particularly useful for the specific investigation of molecular clusters. 

The excitation profile for two-color 1+1 photoionization of 2,2 -bithiophene seeded in a supersonic helium expansion showed that its two-photon photoionization is significantly enhanced by resonance with the Si state, but the observed dynamics clearly show that the dominant channels for photoionization involve long-lived triplet state into which Si decays (94JPC4990). To fit the observed temporal profiles, sequential decay of Si through two triplet states must be invoked. 

J.-Z. Tang, I. Shimamura, Mechanism of the enhancement of some high-lying resonance series in the photoionization spectra of excited helium, Phys. Rev. A 50 (1994) 1321. 

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