Single photon ionization

Surface Analysis by Laser Ionization Post-Ionization Secondary Ion Mass Spectrometry Multi-Photon Nonresonant Post Ionization Multiphoton Resonant Post Ionization Resonant Post Ionization Multi-Photon Ionization Single-Photon Ionization... 

Despite the first prediction [34] of a measurable PECD effect being a few decades old, it is only in the last few years that experimental investigations have commenced. Practical experiments have needed to await advances in experimental technology, and improvements in suitable sources of circularly polarized radiation in the vacuum ultraviolet (VUV) and soft X-ray (SXR) regions needed for single-photon ionization have been been key here. In the meantime, developments in other areas, principally detectors, also contribute to what can now be accomplished. 

Figure 7. Time-of-flight mass spectra showing results of platinun cluster reactions with benzene. The lower trace is clean metal without reactant. The upper trace is with the pulsed addition of. 21 % benzene in heliun. The notation indicates the nmber of adducts on each metal cluster. The metal cluster are all two photon ionized, while the observed products are single photon ionized, hence the enhancement of the product over metal signals. Reproduced from Ref. 17.

The ionization of ammonia clusters (i.e. multiphoton ionization,33,35,43,70,71 single photon ionization,72-74 electron impact ionization,75 etc.) mainly leads to formation of protonated clusters. For some years there has been a debate about the mechanism of formation of protonated clusters under resonance-enhanced multiphoton ionization conditions, especially regarding the possible alternative sequences of absorption, dissociation, and ionization. Two alternative mechanisms63,64,76,77 have been proposed absorption-ionization-dissociation (AID) and absorption-dissociation-ionization (ADI) mechanisms see Figure 5. 

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

Milhlberger, F. Wieser, J. Ulrich, A. Zimmermann, R. Single Photon Ionization Via Incoherent VUV-Excimer Light Robust and Compact TOF Mass Spectrome-i t for Real-Time Process Gas Analysis. Anal Chem. 2002, 74, 3790-3801. 

Previous extensive studies have shown that the energy of 193-nm photons from ArF excimer lasers, E=6.42 eV, is sufficient to induce single-photon ionization of nucleic acid bases [44-47]. The energy delivered by a consecutive two-photon excitation of 2AP is E=1.11 eV this is the sum of the energy of the singlet excited state of 2AP (Eoo=3.74 eV) and the energy of a 308-nm... 

K. Kliinder, J.M. Dahlstrom, M. Gisselbrecht, T. Fordell, M. Swoboda, D. Guenot, et al., Probing single-photon ionization on the attosecond time scale, Phys. Rev. Lett. 106 (14) 143002. 

Single-photon ionization (SPI) in combination with mass-selective detection by TOF mass spectrometry has been used for the detection of explosives [13]. 

Fig. 7. Schematic diagram of the single-photon ionization-time-of-flight (SPI-TOF)-MS instrument. Reproduced with permission from Mullen et al. [13], Copyright 2006 American Chemical Society.

Figure 4. The MF PADs for single-photon ionization of ai and <22 symmetry orbitals of a model C3V molecule for light linearly polarized along different axes of the molecule (indicated in parentheses). Note that no photoionization can occur from the <22 orbital for light polarized along the z axis (molecular symmetry axis). The same dynamical parameters as for the calculations of the LF PADs shown in Fig. 3 were used. For further details see Ref. [55],...

Figure 5. A femtosecond pump-probe photoionization scheme for studying excited-state dynamics in DT. The molecule is excited to its S> electronic origin with a pump pulse at 287 nm (4.32 eV). Due to nonadiabatic coupling, DT undergoes rapid internal conversion to the lower lying Si state (3.6eV). The excited-state evolution is monitored via single-photon ionization. As the ionization potential is 7.29 eV, all probe wavelengths <417 nm permit single-photon ionization of the excited state.

Figure 7. Time-resolved mass spectrometry. AU-trcms-(2, 4, 6, 8) decatetraene was excited to its 5 2 electronic origin with a femtosecond pulse at A-pump — 287 nm. The excited-state evolution was probed via single-photon ionization using a femtosecond pulse at ApIObe = 235 nm. The time resolution in these experiments was 290 fs (0.3 ps). The parent ion CioH signal rises with the pump laser, but then seems to stay almost constant with time. The modest decay observed can be fit with a single exponential time constant of 1 ps. Note that this result is in apparent disagreement with the same experiment performed at Xprobe — 352 nm, which yields a lifetime of 0.4 ps for the S2 state. The disagreement between these two results can be only reconciled by analyzing the time-resolved photoelectron spectrum.

PI is single photon ionization El electron impact ionization and TPI two photon ionization. 

Figure 5 Time-of-flight mass spectrum of carbon clusters with single photon ionization (F2, 7.87 eV at 0.050 mJ/pulse) showing the relatively high abrmdance of the smaller fullerene ions C24, C2s, C32+, and Cso. (Reprinted with permission from D.M. Cox, K.C. Reichmann, and A. Kaldor, J Chem. Phys., 1988, 88, 1588. 1988, American Institute of Physics)...

Using acetic anhydride as an example, Feldmann et al. examined the feasibility and properties of a new method for studying IRMPE/D dynamics which involves single-photon ionization to monitor the time evolution of parent molecules and neutral intermediates. They discussed in some detail alternative detection schemes, such as mass spectrometry with ions formed by electron impact or MPI/REMPI, but decided upon single-photon ionization because of its wide applicability, its sensitivity, and its selectivity. VUV ionization ensures that the perturbation of the system is minimal as well as the extent of ion fragmentation. They used frequency-tripled 3rd harmonic (355 nm) radiation from a Nd-YAG laser as a source of 118.4 nm light. 

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... 

Cao, L. Muhlberger, F. Adam, T. et al. Resonance-enhanced multiphoton ionization and VUV-single photon ionization as soft and selective laser ionization methods for on-line time-of-flight mass spectrometry investigation of the pyrolysis of typical organic contaminants in the... 

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