Transcript WH12_b.ppt
Amanda L. Steber, Brent J. Harris, Kevin K. Lehmann and Brooks H. Pate Department of Chemistry, University of Virginia, Charlottesville, VA 22904 Fourier Transform Millimeter Wave Spectroscopy Don’t detect against a light source High Power Double resonance capabilities FT Challenges? Low cost alternatives to current light source generation and digitizers Needs to be coherent over long time acquisitions High Dynamic Range detections Instrument Schematic Nutation Experiments p p/2 Time 25 ns resolution Fit to a bessel function Related to dipole moment R = E/ Spectrometer Sensitivity Signal Averaging in the Time domain Coherent light source Fill up the memory depth (160ms) Stack FT Steps for Frequency Determination: Determine experiment length (time) Determine frequency at synth Determine frequency at which you will have n*P Spectrometer Performance 18O13CS J=24-23 detected in natural abundance in 10,000 averages (10:1) 0.002% of the parent (50,000:1) Pressure: 15 mT Exact Resonance NOT Required J = 22-21 OCS FT Sync shape from FT Width of pulse dictates width of power envelop 250 ns =4 MHz OR 1 s = 1 MHz Verification/Analysis n1 n2 Double Resonance P+Δ P “p” pulse P+Δ P ΔP=2Δ ΔP=Δ P-Δ Signal ∝ ΔP ∝ Δ P-Δ P1 e E / kT P2 Double Resonance Modulation 60% modulation J=22-21 Selection of Two Color Pulse Duration Uses and Application Spectroscopy Verify assignments Verify a gas detection Reduce false positives Important in overlaps Application Acrolein 10,000 averages Pressure: 2 mTorr Potential Limitations Speed issues USB 2.0 – slow transfer speeds Must change the frequency of the synthesizer (35.2 ms) The amount of memory dictates how many averages can be collected For a measurement that is 5s, you can get 32,000 avgs Can’t do real time accumulation Conclusions Advances in technology allow for the use of low cost components Can be used to analyze/characterize a molecule Nutation experiment – helps determine correct pulse lengths and helps determine the dipole moment Averaging in the time domain – increases sensitivity Do NOT have to be on resonance Double Resonance – Spectrum Verification Acknowledgements Pate Lab NSF CCI (Center for Chemistry of the Universe) CHE-0847919 Dual Color Detuning Monitoring the J = 22- 21 transition of OCS while changing the frequency of the pulse to pump the J = 23-22 transition 120 kHz resolution Near optimal resolution occurs within 360 kHz of resonance