Infrared-Laser Induced Fluorescence and Instrumental Design Michael W. Morton Geoffrey A. Blake Division of Geology and Planetary Science California Institute of Technology Pasadena, CA 91125

Stable Isotope Studies Applications include Atmospheric chemistry Geochemistry Bio-geochemistry  Planetary Exploration Specific examples Atmospheric photochemistry Climate studies Biological influences

Instrumentation Mass Spectrometers Utilize mass differences in isotopes, Great for laboratory studies, but not in situ Limitations in planetary exploration: Size, weight Remote operation Sample preparation Sample destruction Isotopomers/ – N 15 NO 15 NNO NN 17 O Isotopologues in situ Stable Isotope Ratio Spectrometer ? Utilize spectroscopic differences in isotopes

A Brief Look at Photoacoustic Detection Figures courtesy of – Miklos, A. & P, Hess (2001). Am. Inst. Phys., 72, 1938, 1951.

Photoacoustic Considerations Indirect absorption technique signal strength depends on Beer’s law and cell geometry Cell volume and sample size Non-selective detection of mixed samples HDO - D 2 O Short path length Good sensitivity

Sensitivity of PA Detection Show high laser power spectrum with small peak detection cm -1

IR-LIF Technique Small instrument size Remote operation Little to no sample preparation non-destructive Isotopomer resolution Small sample size Figure courtesy of – Geoff Blake

Instrumental Setup chopper PA cell dewar detector filter wheel lock-in amplifier scope PC wavemeter PC

HDO Transitions 1 – OD stretch cm -1  – bend – OH stretch Pump v1+v3 near 1550 nm (can use telecommunications diode lasers). cm

HDO Vapor Spectrum S/N ~ 90 ~1 pMole HDO in volume imaged onto detector.

Signal and Noise Analysis 1 Why is the signal ~independent of pressure in a pure gas?

Signal and Noise Analysis S/N a √ t out to ~10 sec Microphonic peak (vibrations)

Signal and Noise Analysis Figure from – Bernath, P.F. (1995). Oxford University Press, Inc., 8. Limited by warm window?

Future Work Characterize noise H 2 O quenching of HDO emissions FM modulation instead of chopper wheel New integrated sample cell on the order of < cm 3 Measure isotopic fractionation of water Low pressure studies < 100 mtorr New detector, lasers

Acknowledgements Professor Geoff Blake Vadym Kapinus Dr. Pin Chen Dr. Keith Matthews The Blake Group