ULTRAVIOLET CHIRPED PULSE FOURIER TRANSFORM MICROWAVE UV CPFTMW
- Slides: 31
ULTRAVIOLET - CHIRPED PULSE FOURIER TRANSFORM MICROWAVE (UV -CPFTMW) DOUBLE-RESONANCE SPECTROSCOPY Kevin. C. O. Dian, Douglass Brian Kevin O. Douglass, Gordon G. Brown, Jason J. Pajski, and Brooks H. Pate Department of Chemistry, University of Virginia, Mc. Cormick Rd. , P. O. Box 400319, Charlottesville, VA 22904
Introduction • UV – Chirped Pulse FTMW spectroscopy – Measure entire 7. 5 – 18. 5 GHz MW spectrum as laser is actively scanned • UV – Cavity FTMW spectroscopy – Enhanced sensitivity when monitoring single line – Multiple MW pulse techniques: background free
Laser-FTMW Double-Resonance Ground State Depletion Timing UV MW Detect Transfer population before MW pulse Positive and negative peaks S 1, J=3 Coherence Method Timing Destroy Coherence of molecular FID MW UV Detect Negative peaks only UV Scan V=0, J=3 MW Probe V=0, J=2 Masakazu Nakajima, Yoshihiro Sumiyoshi, and Yasuki Endo, Rev. Sci. Instrum. 73, 165 (2002).
Pure Rotational Spectrum of Suprane 20 ms of FID Acquisition (80 k. Hz linewidth, FWHM) Choose Your Sensitivity 100 Shots: 20 s acquisition ~ 2 mmol sample consumption 10000 shots 20 μs gate: 45 min. acquisition B-F Equivalent 0. 1% Suprane in He/Ne
Pure Rotational Spectrum of Suprane 20 ms of FID Acquisition (80 k. Hz linewidth, FWHM) Choose Your Sensitivity ~500: 1 S/N in 20 seconds Cavity has moved 5 MHz 0. 1% Suprane in He/Ne 100 Shots: 20 s acquisition ~ 2 mmol sample consumption
Benzonitrile Multiplexed UV-CPFTMW Timing UV MW Detect S 1, J=2 UV Scan V=0, J=2 MW Probe V=0, J=1
Internal Reference Coherence Method 1. 5 Intensity (V) 1. 0 0. 5 0. 0 -0. 5 -1. 0 -1. 5 0 2 4 6 time (ms) FT gate 1 (laser off) Laser pulse FT gate 2 (laser on) 8 10 Monitor: (FT gate 2*scale factor) - FT gate 1 Signal ~ 0 m. V Equivalent to laser on – laser off for the same valve and MW pulse
Benzonitrile UV-CPFTMW Timing (internal referenced coherence method) MW Detect UV Detect S 1, J=2 UV Scan V=0, J=2 MW Probe V=0, J=1
UV-CPFTMW Double Resonance Spectroscopy • Implemented both Ground State Depletion (GSD) and Dual-Gate Coherencebetween Method ofcavity Endo and CP-FTMW These comparisons spectrometer performance have been made • obsolete Lower single-shot sensitivity for of CP-FTMW spectroscopy by the development a double-pulse requires number of spectroscopy. spectrum averages than cavity method forhigher laser-FTMW spectrometer BUT gives multiplexed DR scans. Comparisons between cavity FTMW and CP-FTMW spectrometers • Double-Pulse Competitive sensitivity is reached when the CP-FTMW – Laser Spectroscopy measurement reaches about 100: 1 signal-to-noise ratio A Background Free Detection Technique with • This limit is determined by the typical pulsed valve signal Order-of-Magnitude Sensitivity Improvement stability
Narrowband FTMW cavity Spectrometer Front Panel Knob Control: Continuum Nd: YAG 10 Hz rep. rate 200 m. J/p 532 nm 0. 01 o Phase 1 m. V / 1 V Amplitude MW Synthesizer ν 0 v 0 + 30 MHz Pulsed 1 watt amp Dye laser 0. 025 cm-1 2 Gs/s AFG Single Sideband ν 0 5 m. J/p UV bandwidth 5 Gs/s Oscilloscope (30 MHz Carrier) Free Induction Decay T. J. Balle and W. H. Flygare, Rev. Sci. Instrum. 52, 33 (1981). R. D. Suenram, J. U. Grabow, A. Zuban, and I. Leonov, Rev. Sci. Instrum. 70, 2127 (1999)
Bloch Vector Model for a Resonant Double-Pulse MW Excitation Scheme “ / 2” “- / 2” - “- / 2” pulse used to counteract M-dependence of transition moment
Demonstration of Double-Pulse MW Excitation MW Pulse(s) FID FT
Bloch Vector Model for a Resonant Double Pulse MW Excitation Scheme “ / 2” Laser Pulse “- / 2” How do we describe the interaction of the laser pulse with the coherent superposition of rotational levels created by the first MW pulse?
The Effects of Selective Laser Excitation Pulse With the laser ON RESONANCE, the Bloch vector rotates about the xaxis (lower rotational level excited): “ / 2” Laser Pulse “- / 2” With laser excitation, the second pulse leaves the laser-induced population change in the x-y plane for background free detection.
Implications of the Mechanism • For resonant laser excitation, there is a 180 o phase shift for laser excitation of the lower and upper rotational levels (phase sensitive detection). • Off-resonance the Bloch vector rotates around the pseudo-vector: This gives rise to a phase shift in the FID as the laser is scanned across a resonance. The technique measures the susceptibility of the laser transition giving both the real (dispersion) and imaginary (absorption) components via the FTMW spectrum.
The Effects of Selective Laser Excitation Pulse With the laser ON RESONANCE, the Bloch vector rotates about the -)x-axis (upper rotational level excited): “ / 2” Laser Pulse ( “- / 2” With laser excitation, the second pulse leaves the laser-induced population change in the x-y plane for background free detection.
Phenylacetylene Phase Information R(3) R(4)
Phenylacetylene Phase Shift Across Resonance
Phenylacetylene Phase Shift Across Resonance
Phenylacetylene Phase Shift Across Resonance
Phenylacetylene Phase Information
Phenylacetylene UV-FTMW Background Free Timing MW UV MW Detect S 1, J=2 UV Scan V=0, J=2 MW Probe V=0, J=1
Phenylacetylene UV-FTMW GSD vs. Background Free Timing MW UV MW Detect S 1, J=2 UV Scan Background Free Previous Technique V=0, J=2 MW Probe V=0, J=1
Propyne IR-FTMW Timing IR MW Detect V=1, J=1 IR Scan V=0, J=1 MW Probe V=0, J=0
Propyne IR-FTMW Background Free Timing MW IR MW Detect Imaginary FT (absorption) Real FT (dispersion) V=1, J=1 IR Scan V=0, J=1 MW Probe V=0, J=0
Pyridine UV-FTMW Background Free Timing MW UV MW Detect S 1, J=2 UV Scan Previous Technique Background Free V=0, J=2 MW Probe V=0, J=1
Conclusions • UV – Chirped-Pulse FTMW Spectroscopy Demonstrated – Ability to monitor multiple transitions (conformers) simultaneously • UV – Cavity FTMW – Increased sensitivity for measuring single transition – Double MW pulse technique for zero-background laser scanning
Acknowledgements Pate Lab Group Members Funding: • NSF Chemistry • NSF MRI Program (with Tom Gallagher, UVa Physics) • John D. and Catherine T. Macarthur Foundation • SELIM Program • University of Virginia
2 Pulse Background Free Technique Timing 50 ns 500 ns MW Detect UV Detect 500 ns MW Pulse 2 MW Pulse 1 UV Laser Adjustable phase and amplitude Molecular FID FT WI 02
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