Multiphoton methods

Figure B2.3.8. Energy-level sehemes deseribing various optieal methods for state-seleetively deteeting ehemieal reaetion produets left-hand side, laser-indueed fluoreseenee (LIF) eentre, resonanee-enlianeed multiphoton ionization (REMPI) and right-hand side, eoherent anti-Stokes Raman speetroseopy (CARS). The ionization oontinuiim is denoted by a shaded area. The dashed lines indieate virtual eleetronie states. Straight arrows indieate eoherent radiation, while a wavy arrow denotes spontaneous emission.

Chu S I 1991 Complex quasivibrational energy formalism for intense-field multiphoton and above-threshold dissociation—complex-scaling Fourier-grid Hamiltonian method J. Chem. Phys. 94 7901... 

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. 

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,... 

Luntz and co-workers have recently carried out an impressive study that follows in the spirit of the Eley-Rideal work.44 Specifically, laser-assisted recombination of N-atoms desorbing to form gas-phase N2 on Ru(0001) was investigated. Experimental measurements of state-selectively detected N2 recoiling from the surface recombination event were obtained using resonance enhanced multiphoton ionization and ion time-of-flight methods. In this way translational energy distributions of individual rovibrational states could be obtained experimentally. In addition, N2-vibrational population distributions could be derived. 

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... 

Fig. 1.14. Multiphoton excitation scheme in Hj showing the laser-induced fluorescence detection method...

The focus of this chapter has been on the synthesis of new catalysts by parallel and combinatorial methods. Another aspect important to the development of new catalysts by these methods is the screening of these large libraries. We will not attempt to cover this topic comprehensively but do feel it is necessary to summarize some of the approaches that have been taken. Methods for screening libraries can be divided into both serial and parallel methods. Generally, the serial methods are adaptations of standard methods that allow for rapid individual analysis of each member of a library. Serial approaches for the analysis of libraries can be as simple as use of an auto sampler on a GC or HPLC system or as advanced as laser-induced resonance-enhanced multiphoton ionization of reaction products above the head-space of a catalyst (16) or microprobe sampling MS (63). The determination of en-antioselectivity in catalysis is a particular problem. Reetz et al. (64) reported the use of pseudoenantiomers and MS in the screening of enantioselective catalysis while Finn and co-workers (65) used diastereoselective derivatization followed by MS to measure ee. 

The procedure described, involving the variation of the laser energy, has some advantages relative to the alternative method of using several solutions with different transmittances. First, it provides a check for multiphoton effects simply by analyzing the quality of the linear correlations obtained. It should be stressed that the excellent correlations in figure 13.7 are typical, that is, correlation factors are usually better than 0.9995. Second, the method requires considerably less sample (only one solution is needed). Third, the analysis of experimental data is also conceptually simpler, because no normalization is required. 

Selected entries from Methods in Enzymology [vol, page(s)] Overview, 226, 119, 147 absorption apparatus, 226, 131 apparatus, 226, 152 detectors, 226, 126 detector systems, 226, 125 excitation source, 226, 121 global analysis, 226, 146, 155 kinetic applications, 226, 134 heme proteins, 226, 142 multiphoton effects, 226, 141 nanosecond time-resolved recombination, 226, 141 quantum yields, 226, 139 singular value decomposition, 226, 146, 155 spectral dynamics, 226, 136 time delay generators, 226, 130. 

Xu, B., Gunn, J. M., Dela Cruz, J. M., Lozovoy, V. V, and Dantus, M. 2006. Quantitative investigation of the multiphoton intrapulse interference phase scan method for simultaneous phase measurement and compensation of femtosecond laser pulses. J. Opt. Soc. Am. B Opt. Phys. 23(4) 750-59. 

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. 

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. 

Guizard, S D. Chapoulard, M. Horani, and D. Gauyacq, Detection of NO Traces Using Resonantly Enhanced Multiphoton Ionization A Method for Monitoring Atmospheric Pollutants, Appl Phys. B, 48, 47f-477 (1989). 

Fig. 15.14 Analytical techniques for time-resolved headspace analysis. An electronic nose can be used as a low-cost process-monitoring device, where chemical information is not mandatory. Electron impact ionisation mass spectrometry (EI-MS) adds sensitivity, speed and some chemical information. Yet, owing to the hard ionisation mode, most chemical information is lost. Proton-transfer-reaction MS (PTR-MS) is a sensitive one-dimensional method, which provides characteristic headspace profiles (detailed fingerprints) and chemical information. Finally, resonance-enhanced multiphoton ionisation (REMPI) TOFMS combines selective ionisation and mass separation and hence represents a two-dimensional method. (Adapted from [190])...

---