I. GALLI, S. BARTANLINI, S. BORRI, P. CANCIO, D. MAZZOTTI, P.DE NATALE, G. GIUSFREDI Molecular Gas Sensing Below Parts Per Trillion: Radiocarbon-Dioxide Optical Detection Presented By: Jessica McNutt March 5, 2012

Overview The authors developed a technique, saturated absorption ringdown spectroscopy (SCAR), for detecting trace amounts of radiocarbon present in a sample of carbon dioxide gas. They were able to detect radiocarbon concentrations at 43 parts-per-quadrillion levels. This technique provides a unique sensitivity, wide dynamic range, compact, low cost, tabletop apparatus for radiocarbon tracing applications. The setup provides wide and continuous mid-IR coverage which would allow for the detection of other rare molecules.

Cavity Ringdown Spectroscopy γ g = gas decay rate, γ c = cavity decay rate

Experimental Setup The intracavity DFG (using a difference-frequency- generation process) generates a signal by combining an external cavity diode laser and Nd:YAG laser in a nonlinear crystal.The intracavity DFG The DFG has a range of 3850 – 4540 nm. The setup is contained within the cavity of a Ti:Sapphire laser.

Numerical Model for Computing γ g Rate Equation for the saturation parameter: At each IR frequency, a SCAR-decay curve is fit to the data (via 4 th order Runge-Kutta algorithm for numerical integration of the above rate equation) to extrapolate values for each decay rate. Where,

SCAR Radiocarbon Dioxide Spectra of P(20) Rovibrational Transition (4.5 μm) Temperature = 195K, Pressure = mbar to achieve strong absorption signal and initial saturation level, G 0, of MHz redshifted scan width chosen to optimize S/N and acquisition time. Calculated a natural abundance of 1.24(10) ppt for the “2010” sample compared to a true value of 1.235(14) ppt.

Radiocarbon-Dioxide Detection Linearity and Sensitivity Linear behavior of measured radiocarbon dioxide verus trace content in carbon dioxide gas. Age range from “ancient” (virtually zero radiocarbon present) to All data results from 32-scan average over 1 hr. Fit line is within one standard deviation for all data. Inset: concentration measurements plotted as a function of age.

Conclusion The authors have presented a technique for the optical detection of radiocarbon containing carbon dioxide well below natural abundance. The technique is roughly 1 order of magnitude less accurate than accelerator mass spectrometry, but much cheaper and easier to use. This method could be used to detect other molecules, or potentially isotopes, with highly absorbing transition states.

Intracavity DFG back