ULTRAVIOLET - CHIRPED PULSE FOURIER TRANSFORM MICROWAVE (UV-CPFTMW) DOUBLE-RESONANCE SPECTROSCOPY Brian C. Dian, Kevin O. Douglass, Gordon G. Brown, Jason J. Pajski, and Brooks H. Pate Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box , Charlottesville, VA Kevin O. Douglass

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 UV Scan V=0,J=3 V=0,J=2 S 1,J=3 MW Probe UVMW Timing Detect Ground State Depletion Transfer population before MW pulse Positive and negative peaks Coherence Method Destroy Coherence of molecular FID Negative peaks only UV MW Timing Detect Masakazu Nakajima, Yoshihiro Sumiyoshi, and Yasuki Endo, Rev. Sci. Instrum. 73, 165 (2002).

100 Shots: 20 s acquisition ~ 2  mol sample consumption Pure Rotational Spectrum of Suprane 20  s of FID Acquisition (80 kHz linewidth, FWHM) shots 20 μ s gate: 45 min. acquisition B-F Equivalent 0.1% Suprane in He/Ne Choose Your Sensitivity

100 Shots: 20 s acquisition ~ 2  mol sample consumption Pure Rotational Spectrum of Suprane 20  s of FID Acquisition (80 kHz linewidth, FWHM) 0.1% Suprane in He/Ne Choose Your Sensitivity ~500:1 S/N in 20 seconds Cavity has moved 5 MHz

Benzonitrile Multiplexed UV-CPFTMW UVMW Timing Detect UV Scan V=0,J=2 V=0,J=1 S 1,J= 2 MW Probe

Internal Reference Coherence Method time (  s) Intensity (V) FT gate 1 (laser off) Laser pulse FT gate 2 (laser on) Monitor: (FT gate 2*scale factor) - FT gate 1 Signal ~ 0 mV Equivalent to laser on – laser off for the same valve and MW pulse

Benzonitrile UV-CPFTMW (internal referenced coherence method) UV MW Timing Detect UV Scan V=0,J=2 V=0,J=1 S 1,J=2 MW Probe

UV-CPFTMW Double Resonance Spectroscopy Implemented both Ground State Depletion (GSD) and Dual-Gate Coherence Method of Endo Lower single-shot sensitivity for CP-FTMW spectroscopy requires higher number of spectrum averages than cavity spectrometer BUT gives multiplexed DR scans. Competitive sensitivity is reached when the CP-FTMW measurement reaches about 100:1 signal-to-noise ratio This limit is determined by the typical pulsed valve signal stability Comparisons between cavity FTMW and CP-FTMW spectrometers These comparisons between cavity and CP-FTMW spectrometer performance have been made obsolete by the development of a double-pulse method for laser-FTMW spectroscopy. Double-Pulse FTMW – Laser Spectroscopy A Background Free Detection Technique with Order-of-Magnitude Sensitivity Improvement

Narrowband FTMW cavity Spectrometer T.J. Balle and W.H. Flygare, Rev. Sci. Instrum. 52, 33 (1981). MW Synthesizer ν0ν0 ν0ν0 Free Induction Decay (30 MHz Carrier) 5 Gs/s Oscilloscope R.D. Suenram, J.U. Grabow, A. Zuban, and I. Leonov, Rev. Sci. Instrum. 70, 2127 (1999) 2 Gs/s AFG v MHz Single Sideband Pulsed 1 watt amp Dye laser Nd:YAG Continuum 10 Hz rep. rate 200 mJ/p 532 nm 5 mJ/p UV cm -1 bandwidth Front Panel Knob Control: 0.01 o Phase 1 mV / 1 V Amplitude

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)FIDFT

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 x- axis (lower rotational level excited): Laser Pulse “  / 2” “-  / 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): Laser Pulse “  / 2” “-  / 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 Information

Phenylacetylene UV-FTMW Background Free UV Scan V=0,J=2 V=0,J=1 S 1,J=2 MW Probe UVMW Timing DetectMW

Phenylacetylene UV-FTMW GSD vs. Background Free UV Scan V=0,J=2 V=0,J=1 S 1,J=2 MW Probe UVMW Timing DetectMW Background Free Previous Technique

Propyne IR-FTMW IR Scan V=0,J=1 V=0,J=0 V=1,J=1 MW Probe IRMW Timing Detect

Propyne IR-FTMW Background Free IR Scan V=0,J=1 V=0,J=0 V=1,J=1 MW Probe IRMW Timing DetectMW Imaginary FT (absorption) Real FT (dispersion)

Pyridine UV-FTMW Background Free UV Scan V=0,J=2 V=0,J=1 S 1,J=2 MW Probe UVMW Timing DetectMW Previous Technique Background Free

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 UV MW Timing Detect MW Pulse 1 MW Pulse 2 Adjustable phase and amplitude UV Laser 500 ns 50 ns Molecular FID FT WI02