Ionization multiphoton techniques

The large variability in elemental ion yields which is typical of the single-laser LIMS technique, has motivated the development of alternative techniques, that are collectively labeled post-ablation ionization (PAI) techniques. These variants of LIMS are characterized by the use of a second laser to ionize the neutral species removed (ablated) from the sample surface by the primary (ablating) laser. One PAI technique uses a high-power, frequency-quadrupled Nd-YAG laser (A, = 266 nm) to produce elemental ions from the ablated neutrals, through nonresonant multiphoton ionization (NRMPI). Because of the high photon flux available, 100% ionization efflciency can be achieved for most elements, and this reduces the differences in elemental ion yields that are typical of single-laser LIMS. A typical analytical application is discussed below. 

Recently, laser multiphoton ionization of solutes has been used. Defining the threshold of ionization, iJth, can be a problem in some of these methods. A recent multiphoton technique, utilizing femtosecond laser pulses, appears to give quite accurate thresholds [56]. In this work, a conductivity spectrum is measured at visible wavelengths and a sharp drop in current occurs as the mechanism changes from -photon excitation to (n+ l)-photon excitation, where n is typically 3 to 4. The threshold is defined by fitting the current to an analytic function that defines the midpoint of this transition. Eth is then n times the energy at which the midpoint occurs. The thresholds are sensitive to Vg and could be used for determination of this quantity. 

Multiphoton techniques can also be used for ionization. Figure 6.2d shows a simple scheme for the multiphoton ionization (MPI) of benzene using three photons. 

Spectroscopy and Time-of-Flight Analysis. Multiphoton techniques are now widely used to obtain information on bound states that cannot be obtained by singlephoton absorption spectroscopy. Because multiphoton processes are nonlinear, it is not practical to monitor the absorption of the laser beam intensity. Thus, a more common approach is to use laser induced fluorescence (LIF), where the absorption threshold for electronic states is obtained from the onset of fluorescence radiation. Alternatively (as we have indicated above) absorption spectra can be obtained by detecting ions produced by resonant ionization. 

Another example of a teclmique for detecting absorption of laser radiation in gaseous samples is to use multiphoton ionization with mtense pulses of light. Once a molecule has been electronically excited, the excited state may absorb one or more additional photons until it is ionized. The electrons can be measured as a current generated across the cell, or can be counted individually by an electron multiplier this can be a very sensitive technique for detecting a small number of molecules excited. 

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

For ion TOF measurement a probe laser was used to ionize reaction products in the reaction zone. The (1 + F) resonance-enhanced multiphoton ionization (REMPI) method was adapted for H-atom detection. The necessary vacuum ultraviolet (VUV) radiation near 121.6 nm (for Lyman-a transition) can readily be generated by a frequency-tripling technique in a Kr cell.37 The sensitivity of this (1 +1 ) REMPI detection scheme is extremely high owing to the large absorption cross-section of Lyman-a transition,... 

The advancement of the application of lasers in combination with the molecular beam technique has made a great impact in the understanding of primary photodissociation processes. For state-specific detection of small fragments, laser-induced fluorescence, multiphoton ionization, and coherent laser scattering have provided extremely detailed information on the dynamics of photodissociation. Unfortunately, a large number of interesting... 

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. 

A number of other laser spectroscopic techniques are of interest but space does not permit their discussion. A few specialized methods of detecting laser absorption worthy of mention include multiphoton ionization/mass spectrometry (28), which is extremely sensitive as well as mass selective for gas-phase systems optically detected magnetic resonance (29) laser intracavity absorption, which can be extremely sensitive and is applicable to gases or solutions (30) thermal blooming, which is also applicable to very weak absorbances in gases or liquids (31) and... 

Mach-Zehnder interferometer, 144 Medical applications, 153 Metal-insulator transitions, 52 Monte Carlo procedure, 135 Multi-energy X-ray imaging, 131 Multilayer targets, 131 Multiphoton absorption, 85 Multiphoton ionization, 82 Multiple filamentation, 91, 92 Multipulse techniques, 152... 

A technique that has been used in laboratory studies for oxides of nitrogen and shows promise for field measurements is resonance-enhanced multiphoton ionization (REMPI) (Guizard et al., 1989 Lemire et al., 1993 Simeonsson et al., 1994). For example, Akimoto and co-workers (Lee et al., 1997) have reported a REMPI system in which a (1 + 1) two-photon absorption of light at 226 nm by NO results in ionization (vide supra). They report a detection limit of 16 ppt in their laboratory studies. Other oxides of nitrogen such as NOz and HN03 can also photodissociate in the... 

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. 

We have employed this phase-sensitive pump-probe technique to further investigate the multiphoton ionization of Na2 with 618-nm femtosecond pulses as discussed in the previous paragraph and have observed the interference of the A E and 2 Tlg wavepackets created by the first pulse and those created by the second pulse in the Na2+ signal. The amplitude of the high-frequency oscillations in the Na2+ signal was obtained as a function of pump-probe delay by filtering the transient with the laser frequency. It is shown in Fig. 8 (top). Below the averaged Na2+ transient of Fig. 4 is... 

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. 

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