Nonresonant multiphoton ionization

This article discusses why one would choose nonresonant multiphoton ionization for mass spectrometry of solid surfaces. Examples are given for depth profiling by this method along with thermal desorption studies. 

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. 

Talbert S. Stein and Kilter E. Kauppila Nonresonant Multiphoton Ionization of Atoms, J. Morellec, D. Normand, and G. Petite Classical and Semiclassical Methods in Inelastic Heavy-Particle Collisions, A. S. Dickinson and D. Richards... 

Using a wavelength of 415 nm, the positive ions were mainly detected when there was a nonzero time delay between the pump and probe laser pulses, confirming that sequential processes of detachment and ionization are involved in the creation of the cations. Remarkably, more than 90% of the cluster cations were detected as trimers, showing that with ultrashort laser pulses, nonresonant multiphoton ionization with very little fragmentation is indeed possible. Nevertheless, small fragment peaks are detectable. 

Fig. 10.1 Energy-level diagram of multiphoton ionization of molecules (a) resonance two-photon ionization (b) resonance-enhanced multiphoton ionization (REMPI) and (c) nonresonance multiphoton ionization (MPI).

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... 

Consider first some of the factors affecting the design of such laser schemes. Ground electronic state based laser enhancement schemes [216, 3 366] rely on the induction of nuclear dipole moments to aid in promoting a desii reaction [30, 367], For example, the use of infrared (IR) radiation has been propoS to overcome reaction barriers on the ground electronic state [30, 367]. However proposal requires powers on the order of terawatts per centimeter sipis (TW/cm2). At these powers nonresonant multiphoton absorption, which irtvar leads to ionization and/or dissociation, becomes dominant, drastically reducin, yield of the reaction of interest. 

Among the possibilities available to postionize sputter-ejected neutral surface particles, electron impact ionization has been employed in a variety of experimental approaches. More recently, photoionization by resonant or nonresonant multiphoton absorption processes has been established as another very effective technique in SNMS. Other processes as Penning ionization or charge exchange play only a minor role in postionization for SNMS. 

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 VUV intensity generated by nonresonant conversion methods is sufficient for most investigations in linear (absorption or fluorescence) spectroscopy. Other applications (like multiphoton excitation and ionization or photodissociation) require more powerful light pulses. 

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