Ionization schemes

Isotope shifts for most elements are small in comparison with the bandwidth of the pulsed lasers used in resonance ionization experiments, and thus all the isotopes of the analyte will be essentially resonant with the laser. In this case, isotopic analysis is achieved with a mass spectrometer. Time-of flight mass spectrometers are especially well-suited for isotopic analysis of ions produced by pulsed resonance ionization lasers, because all the ions are detected on each pulse. 

A versatile Laser-SNMS instrument consists of a versatile microfocus ion gun, a sputtering ion gun, a liquid metal ion gun, a pulsed flood electron gun, a resonant laser system consisting of a pulsed Nd YAG laser pumping two dye lasers, a non-resonant laser system consisting of a high-power excimer or Nd YAG laser, a computer-controlled high-resolution sample manipulator on which samples can be cooled or heated, a video and electron imaging system, a vacuum lock for sample introduction, and a TOF mass spectrometer. 

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Figure B2.5.16. Different multiphoton ionization schemes. Each scheme is classified according to the number of photons that lead to resonant intennediate levels and to the ionization continuum (liatched area). Adapted from [110].

Fig. 16.3 ionization scheme of niflumic acid (CAS-RN 4394-00-7). The macro- (pfCJ and microprotonation (p(t ) schemes of the compound are shown. The logP value is determined by partition of neutral species (dubbed as NO) only. The microconstants and partition coefficients of each microspecies as well as the nature and the concentration of... 

Fig. 6. Angular distribution and fits for the (a) v = 3 and (b) v 0 peak, in the 255 nm energy distribution. The two curves in each figure represent angular distributions from different 0(1D2) ionization schemes.

The possible profiles of intensity versus the delay between the pump and probe photons for the several potential processes operative in the mechanism of ionization are shown in Figures 6a, b, and c. Several different laser schemes have been employed to investigate the reaction dynamics pertaining to the formation of protonated ammonia clusters through the A and C states (the latter possibly including the B state as well). The ionization schemes employed in the present study are shown in Figure 7. 

Figure 7. Several of the ionization schemes employed in various pump-probe experiments of ammonia clusters the energy levels correspond to those of the ammonia monomer. The upper hatched region denotes the ionization limit. Taken with permission from ref. 68.

The Raman spectrum of a solution of GlFgO in anhydrous HF shows no bands due to GIF3O but only those of GIF2O+ in agreement with the following ionization scheme 38) ... 

Various mechanisms can be envisaged for the conversion of substituted acetonitriles into dithiazolium salt 145, yet no firm evidence has been given. The first step could be the chlorination of acetonitrile by sulfur monochloiide, as demonstrated for acetonitrile itself (1985CB1632) and for phenylacetonitrile (1939ZOK1329). This could be followed by sulfur monochloride addition to the cyano group, cyclization and ionization (Scheme 75). 

Figure 11.18 illustrates the principles of application of REMPI to NO (discussed in more detail later). The electronically excited states of NO are shown in Fig. 11.18a and some potential ionization schemes in Fig. 11.18b (Pfab, 1995). Pulsed tunable lasers with wavelengths from 190 to 1000 nm and spectral resolutions of 0.1 cm 1 are readily available. To ionize NO, the absorption of two, three, or four photons is needed. The first photon excites the NO into an intermediate state from which it is ionized using a second or, in some cases, two more photons. The transitions are described as an (n + m) transition, where n is the...

Fig. 1. Two-color 3-photon ionization scheme. Fig. 2. Three-photon ionization photoelectron CHD is excited by a Ha, photon to thel B2 spectrum, obtained in the scheme of Fig.l,...

With both the Fu and the Denmark catalysts it can be assumed that catalysis is effected by formation of a highly electrophilic silicon cation D from tetrachlorosi-lane and the nucleophilic catalyst C, i.e. by attack of the pyridine N-oxide or of the phosphoramide O-atom on silicon, followed by ionization (Scheme 13.38). The latter cation can then activate the epoxide toward nucleophilic attack by the chloride ion. Exchange of the product silane for another molecule of tetrachlorosilane completes the catalytic cycle [75],... 

Figure U. Classification of resonance ionization schemes for atomic ion formation (38). and cOp denote photons of frequency 1 and 2, respectively 2co denotes a photon that has been generated by frequency-doubling.

Laser-enhanced ionization (LEI) is one of a family of laser-induced ionization techniques which have been exploited for analytical spectrometry. The laser-induced ionization schemes which are important for flame spectrometry are illustrated in Fig. 1. 

Fig. 1. Laser-induced ionization schemes in flames. LEI-laser-enhanced ionization, DLI-dual laser ionization, MPI-multiphoton ionization...

Since the second laser ionizes the excited atom in DLI, this step may be accomplished by an off-resonant photon (see Fig. 1). If a nitrogen laser-pumped dye laser provides the resonant photon, a fraction of the nitrogen laser beam can conveniently ionize the atom from the laser-excited state 14,15). It is theoretically possible to photoionize every atom in the periodic table except helium and neon using five RIS ionization schemes involving stepwise and/or multiphoton excitation4). Presumably these... 

For the reaction of ozone with alkanes under superacidic conditions, two mechanistic pathways may be considered. The first is the formation of an alkylcarbenium ion via protolysis of the alkane prior to quenching of the ion by ozone (Scheme 6.26, route a). Alkylcarbenium ions may also be generated via initial transformation of the alkane to an alcohol followed by protonation and ionization (Scheme 6.26, route b). There have already been a number of reports of ozone reacting with alkanes to give alcohols and ketones. °" In both cases, intermediary alkylcarbenium ions would then undergo nucleophilic attack by ozone as mentioned earlier. 

Increasing the temperature induces breakage of the bridges between the octahedrons and shifts the reaction (10.10) to the left or/and lowers the n value. A similar self-ionization scheme probably occurs for pure molten Zrp4. 

The data of Figure 6.7 indicate that the stability of the maximum oxidation state, 6+, increases in the group, while that of the 5+, 4+ and 3+ states decreases. In contrast to expectations, the 4+ state of Sg will be less stable than that of Mo and W. This fact can be explained by the step-wise ionization scheme of Sg (see Figure 10 from Pershina et al. (1999)) showing that the 4+ state of Sg has the 6d2 electronic configuration, by analogy with Mo and W, and not the 7s2. Since the 6d orbitals of Sg... 

Fig. 4 shows selected energy levels of the He /He systems. The Is3s4s state of He is situated just below the He (3 S) threshold. This state, which is excited with laser (O, rapidly autodetaches via the 2 Sks and 2 Pk.p channels. Following the decay, the residual He atom will be left in either the or 2 P excited states. Two different laser frequencies and a" were applied separately in the resonance ionization scheme used to monitor the population of the 2 8 and 2 states. The frequency 0)2 was chosen to induce a transition between the 2 S and the 24 P states of the He atom, when photodetachment into the 2 Sks channel was studied. The frequency co" induced a resonance transition between the 2 P and 26 D states of He, when photodetachment into the 2 Pkp channel was studied. The population of both the high lying Rydberg states were efficiently depleted by the electric field of the second quadrupole deflector and He+ ions thus produced were recorded as a function of frequency of laser m,. The output of laser w, was attenuated to avoid... 

Figure 3. Two variants of the two-color two-photon tagging scheme used in the H(D) atom photofragment translational spectroscopy technique. Early implementations used the two-color two-photon threshold ionization scheme shown on the left, but in all recent high-resolution work the second photon has been chosen so as to excite a high n Rydberg state lying at an energy just below the ionization limit.

In a series of recent investigations [189-193], Bondybey, Miiller-Dethlefs, and co-workers have demonstrated that PFI-PE spectra for molecular species can be obtained with good sensitivity by using one-color, nonresonant, two-photon (N2P) excitation. In many cases, the N2P-PFI-PE spectra are surprisingly similar to those acquired by the single VUV photon ionization scheme [190]. The N2P-PFI scheme is very attractive because of its high sensitivity and the availability of commercial pulsed-dye lasers with an output range of 200-400 nm, as required for the ionization of most polyatomic species at their thresholds. 

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