Laser-induced Mg production from magnesium oxide using Si-based agents and Si-based agents recycling

We observed direct desorption and ionization of angiotensin II and bovine insulin from a frozen polyacrylamide gel without the addition of an exogenous matrix, using picosecond pulses from a tunable, mid-infrared free-electron laser tuned to strong absorption bands of the gel. At 5.7, 5.9, 6.1 and 6.3 microm we were able to desorb and ionize both analyte molecules, with the strongest analyte signal generated at 5.9 microm. However, no analyte signal was observed at 5.5 microm. Consistent with a previous report, we did not observe ions of either polypeptide at 2.9 microm, in spite of strong overall absorption. We discuss the implications of this wavelength-dependent ionization, including possible ablation mechanisms and energy partitioning between competing vibrational modes.

The mechanisms responsible for matrix-assisted laser desorption/ionization (MALDI) are far from being well understood, particularly where infrared laser irradiation is used to initiate the process. We measured the emission yields and kinetic energy distributions of positive ions emitted from 2,5-dihydroxybenzoic acid loaded with angiotensin II in a standard MALDI preparation during irradiation with an infrared free-electron laser tuned to 2.94 microm. As the laser intensity is scanned through the MALDI threshold, we see a marked change in the energy distributions of the matrix ion. Above threshold, the energy distributions of both analyte and matrix cations are constant over a broad range of laser intensities. This behavior does not appear to be consistent with any extant model of the MALDI mechanism.

We examine the formation of a fluorescent plume during pulsed laser irradiation of wide band gap materials at sub-band gap photon energies. We show that fluences near the threshold for plume fluorescence in MgO do not produce adequate electron densities for significant laser-plume interactions via inverse bremsstrahlung processes. At a wavelength of 248 nm, 30 ns pulse width, and fluences well below the onset of detected plume fluorescence, we observe intense Mg+ emissions and electron temperatures in excess of 1 eV. The onset and growth of plume fluorescence closely parallels the neutral emission intensities, suggesting that the lack of plume fluorescence below the fluence threshold is due to the lack of gas phase neutral Mg. Time-resolved measurements of the atomic line emissions show peaks well after the peak laser intensity, arguing against direct laser-plume interactions. We propose that electron acceleration involves electrostatic interactions between photoelectrons and photoelectronically emitted ions. The unusual temporal evolution of the atomic line emissions can be attributed to the time dependence of the spatial overlap between neutral particles and the accelerated electrons that collisionally excite the atomic lines.

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