Laser photoionization

Lykke K R and Kay B D 1990 State-to-state inelastic and reactive molecular beam scattering from surfaces Laser Photoionization and Desorption Surface Analysis Techniquesvo 1208, ed N S Nogar (Bellingham, WA SPIE) p 1218... 

Wight C A and Armentrout P B 1993 Laser photoionization probes of ligand-binding effects in multiphoton dissociation of gas-phase transition-metal complexes ACS Symposium Series 530 61-74... 

Figure 1. Schematic illustration of the laser-vaporization supersonic cluster source. Just before the peak of an intense He pulse from the nozzle (at left), a weakly focused laser pulse strikes from the rotating metal rod. The hot metal vapor sputtered from the surface is swept down the condensation channel in dense He, where cluster formation occurs through nucleation. The gas pulse expands into vacuum, with a skinned portion to serve as a collimated cluster bean. The deflection magnet is used to measure magnetic properties, while the final chaiber at right is for measurement of the cluster distribution by laser photoionization time-of-flight mass spectroscopy.

Reactions (19)-(21) represent the dissociation of benzene and reactions (22)-(26) represent the detection of fragments by VUV laser photoionization. The line-shape images resulted from these reactions. 

Powis, I. Baer, T. Ng, C.-Y. (Eds.). High Resolution Laser Photoionization and Photoelectnm Studies, John Wiley Chicester, 1995. 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

Laser photoionization Another ionization method with great potential for ambient air applications is... 

Cass, G. R M. H. Conklin, J. J. Shah, J. J. Huntzicker, and E. S. Macias, Elemental Carbon Concentrations Estimation of and Historical Data Base, Atmos. Environ., 18, 153-162 (1983). Castaldi, M. J., and S. M. Senkan, Real-Time, Ultrasensitive Monitoring of Air Toxics by Laser Photoionization Time-of-Flight Mass Spectrometry, J. Air Waste Manage. Assoc., 48, 77-81... 

Letokhov, V. S., Laser Photoionization Spectroscopy, Academic Press, San Diego, 1987. 

H. Lefebvre-Brion. in High Resolution Laser Photoionization and Photoelectron Studies, I. Powis, T. Baer, and C. Y. Ng, Eds., Wiley Series in Ion Chemistry and Physics, Wiley, Chichester, 1995, p. 171. 

Figure 1. (a) Schematic diagram of a laser photoion projection microscope and (b) spectrally selective multistep photoionization scheme for absorbing centers (color centers, molecular chromophores, etc.) by ultrashort laser pulses. 

However, the main important potentialities of femtosecond laser photoionization in future lie in combination with photoion microscopy. 

Oser, H., M.J. Coggiola, S.E. Young, D.R. Crosley, V. Hafer, and G. Grist. 2007. Membrane introduction/ laser photoionization time-of-flight mass spectrometry. Chemosphere 67 1701-1708. 

Osorio et al. [134] performed TOF-MS measurements of TNT and RDX on soil surfaces. They used tunable UV radiation from a 130 fs laser to monitor the kinetic energy distribution of N0/N02 photofragments released by the dissociation of TNT and RDX. Analysis of the kinetic energy distribution of the photofragments revealed differences in the processes for NO and NOz ejection in different substrates. Mullen et al. [135] detected triacetone triperoxide (TATP) by laser photoionization. Mass spectra in two time regimes were acquired using nanosecond (5 ns) laser pulses at 266 and 355 nm and femtosecond (130 fs) laser pulses at 795, 500, and 325 nm. The major difference observed between the two time regimes was the detection of the parent molecular ion when femtosecond laser pulses were employed. 

The experimental conditions under which clusters are produced and detected must be carefully understood and considered when proposing explanations. A chemical or a photophysical explanation may be equally viable when only a limited amount of information is available. Fully understanding the chemistry requires being able to differentiate between the effects of laser photoionization, vaporization, and chemical reaction. More tools are needed to be able to study the structure and bonding of clusters and cluster-adduct complexes. 

LEI has been applied successfully to the trace determination of T1 [674] for certification purposes, and for combinations with laser evaporation and all other atomization techniques represents a powerful approach to detection. Laser photoionization and galvanic detection have been applied to hollow cathode dark space diagnostics [675]. Photoionization is produced to measure the dark space widths of linear field distributions directly. A theoretical model has been developed and its predictions verified with experimental findings for a uranium hollow cathode discharge operated in neon or xenon. Variations in the ground-state densities of sputtered neutrals have also been measured. 

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