Geiger, Leon M.

Howard, Ian

Jouda, Mazin

Korvink, Jan

Länge, Kerstin

MacKinnon, Neil

Rapp, Michael

Voigt, Achim

English

KITopen

KIT – The Research University in the Helmholtz Association1Institute of Microstructure Technology at KITOptimization of laser-induced OH• Radical Generation in WaterLeon M. Geiger1, Mazin Jouda1, Kerstin Länge1, Michael Rapp1, Achim Voigt1, Ian Howard1, Neil MacKinnon1, Jan G. Korvink1Contact: M. Sc. Leon GeigerKarlsruhe Institute of TechnologyInstitute of Microstructure Technology (IMT)76344 Eggenstein-Leopoldshafen, GermanyL.geiger@kit.eduwww.imt.kit.eduSampleMW Source DetectorModulationPhase−Detector2. Setup for EPR MeasurementsThe use of radicals to transfer spin polarization to nuclei is an important technique to increase the signal intensity of NMRmeasurements. The generation of OH• radicals is usually done with UV irradiation of hydrogen peroxide or sonication of water amongother techniques.[1] These methods require several tens of minutes and therefore suffer from the degradation of radical concentrationduring the generation process when spin-traps like DMPO (5,5-Dimethyl-1-pyrrolin-N-oxid) are used. Hence, the usable radicalconcentration is lower than the number of radicals produced during the generation process. Therefore, we investigated the laser-inducedradical generation dependent on pulse length and pulse energy.References:[1] P. Riesz. Et al. Free radical generation by ultrasound in aqueous and nonaqueous solutions. Environ Health Perspect. 1985 Dec;64:233-52. doi: 10.1289/ehp.8564233[2] J. Noack and A. Vogel, "Laser-induced plasma formation in water at nanosecond to femtosecond time scales: calculation of thresholds, absorption coefficients, and energy density," in IEEE Journal of Quantum Electronics, vol. 35, no. 8, pp. 1156-1167, Aug. 1999, doi:10.1109/3.777215.The influence of laser pulse lengthThe 10 mM DMPO-water solution shows a significant increase ofcaptured OH• radicals with increasing laser intensity. The samplesolution was irradiated for 1 min, directly followed by the EPRmeasurement. However, significantly less radicals are producedwhen the pulse length is increased. At 3 ps pulse length the EPRsignal intensity drop by the factor of 30. No signal can beacquired with 5 ps pulse length.How does the plasma effect radical generation?Even though multiphoton absorption in water starts from 109W/cm2, the EPR signal was measurable exceeding a laserintensity of 1013 W/cm2. With our laser setup, a 270 fs pulselength at the highest laser intensity resulted in the highestDMPO-OH• radical concentrations. This agrees well with theplasma generation in water theory.[2] With shorter pulses moreenergy is transformed into the plasma – and water dissociation –compared to longer pulses. Longer pulses are more likely to heatthe water instead of dissociating water molecules.The concept of laser-induced radical generation is an attractivemethod for spin polarization due to its speed and the absence ofadditive molecules. These preliminary results lay the foundationfor exploring laser-generated radicals in the context of NMRsignal hyperpolarization.1. Setup for OH•Radical Generation3. EPR−Spectra of DMPO−OH• Radicals

Optimization of laser-induced OH Radical Generation in Water

https://publikationen.bibliothek.kit.edu/1000161097/151088613

Optimization of laser-induced OH Radical Generation in Water

Abstract

Similar works

Full text

Available Versions

KITopen