A Test of Femtosecond Laser Desorption for Improved Neutral Production with the Laser Desorption Resonance Ionization Mass Spectrometer (LDRIMS), 15-R8067

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Principal Investigators
F. Scott Anderson
Keith Nowicki
Chris Anderson

Inclusive Dates:  06/01/09 – 07/01/09

Background - SwRI researchers tested a femtosecond (fs) laser desorption system to determine if it was capable of increasing the accuracy of elemental and isotopic measurements in a laser desorption resonance ionization mass spectrometer (LDRIMS). The LDRIMS instrument is capable of making extremely sensitive measurements of a wide range of difficult to measure isotopes in a portable and real-time mode. The team is currently focused on precise measurement of strontium (Sr), as precise measurement of the isotopic ratio of Sr enables the dating of rocks on Mars and other planets, a high priority goal for NASA. An fs-laser desorption system in principle could improve the LDRIMS measurement accuracy by mitigating effects in the plume of plasma formed under the current nano-second laser desorption system. Femtosecond laser desorption produces neutrals and ions; unlike other techniques, LDRIMS can measure both materials simultaneously. As neither of the other two institutions with a RIMS instrument within this country (Argonne National Lab and Atom Sciences) is currently exploring fs-desorption physics, SwRI is uniquely positioned to make dramatic advances in theoretical fs-physics using the LDRIMS instrument.

Approach -  Amplitude-Laser Inc. agreed to loan SwRI an S-Pulse fs-laser for two weeks, as well as assistance with set up and testing. It was anticipated that the results would allow a better understanding of the advantages an fs-laser might provide for LDRIMS, establish the required laser power level, and clarify the potential for miniaturization. Finally, it would provide initial data for a NASA or NSF proposal to purchase an fs-laser.

Accomplishments - After integrating the laser, data for both ns and fs desorption were obtained. Following data analysis, the project team determined that there was no apparent effect of pulse length on the precision or accuracy of the result. No combination of tuning was found that improved fs-desorption. This suggests that while fs-desorption may improve ion production, for neutral production it is inferior. This is a critical finding because it will be significantly simpler to produce a miniature ns-laser desorption system for a spaceflight geochronology experiment. Furthermore, it is of fundamental physical importance that fs-laser desorption does not improve neutral production. While this negative result was somewhat disheartening, as the literature strongly supports an improvement for the production of ions, it is now definitively clear that further research effort, proposal production, and instrument testing (all part of SwRI's long-term plan) on this option are not needed. This result is an important part of the publication describing LDRIMS development, illustrating that fs-desorption does not aid neutral production for LDRIMS instruments.

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