Multiphoton activation

Figure 15. Changes in reactant and product ion yields resulting from cw infrared laser multiphoton activation of the Co(2-pentene)+ adduct. Data from reference 6.

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

In our laboratory we have utilized multiphoton infrared laser activation of metal ion-hydrocarbon adducts to probe the lowest energy pathways of complex reaction systems (6). Freiser and co-workers have utilized dispersed visible and uv radiation from conventional light sources to examine photochemical processes involving organometallic fragments... 

The activation step can alternatively be performed without gas by means of infrared multiphoton dissociation (IRMPD) or electron capture dissociation (BCD) (Chap. 2.12.2). Both IRMPD and BCD, solely require storage of the ions during their excitation by photons or electrons, respectively. It is one of the most charming properties of FT-ICR-MS/MS that even the accurate mass of the fragment ions can be determined. [216,217]... 

Hashimoto, Y. Hasegawa, H. Yoshi-nari, K. Collision-Activated Infrared Multiphoton Dissociation in a Quadrapole Ion Trap Mass Spectrometer. Anal. Chem. 2003, 75,420-425. 

Gauthier, J.W. Trautman, T.R. Jacobson, D.B. Sustained Off-Resonance Irradiation for Collision-Activated Dissociation Involving FT-ICR-MS. CID Technique That Emulates Infrared Multiphoton Dissociation. Anal. Chim. Acta 1991,246,211-225. 

FT-ICR instruments are also capable of performing MS" experiments. The most popular method of ion activation is sustained off-resonance irradiation (SORI), where ions are excited to a larger cyclotron radius using rf energy, undergo collisions with a neutral gas pulsed into the cell and dissociate. Other methods are available, including infrared multiphoton dissociation (IRMPD)65 and electron capture dissociation (ECD)66 which is of particular value in glyco-peptide analysis (Section VIA). 

Examples of studies on multiphoton absorption processes and nonlinear second-and third-harmonic generation processes will be discussed along with some possible radiative and nonradiative processes. The selection rules for multiphoton absorption will be mentioned in Section 7.3, and molecular examples will be shown along with their correlating photophysical properties in Section 7.4. The effect of some parameters relating to second-order activity along the lanthanide... 

Gauthier, J.W., Trautman, T.R. and Jacobson, D.B. (1991) Sustained off-resonance irradiation for collision-activated dissociation involving Fourier transform mass spectrometry. Collision-activated dissociation technique that emulates infrared multiphoton dissociation. 

Two other ion activation methods were developed to replace the gas molecules as targets by laser beams (photodissociation or infrared multiphoton dissociation IRMPD) or by electron beams (electron capture dissociation ECD). These two methods can be applied to ions that are trapped during their excitations by photons or electrons, respectively. Thus, they are most often used with ion trap or ICR analysers because the residence time and the interaction time are longer. 

The fragmentation of peptides can also be obtained by FTICR instruments. Besides the most commonly used activation method, namely CID, the activation can alternatively be performed without gas by infrared multiphoton dissociation (IRMPD) and electron capture dissociation (ECD). These methods fragment peptide ions in the ICR cell by emitting a laser beam or electron beam, respectively. 

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