SEEING IS BELIEVING: An 11 GHz molecular beam rotational spectrum (7.5 – 18.5 GHz) with 100 kHz resolution in 15  s measurement time Brian C. Dian, Kevin O. Douglass, Gordon G. Brown, Scott M. Geyer and Brooks H. Pate Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box , Charlottesville, VA 22904

Ground State Rotational Spectrum of Cyclopropanecarboxaldehyde Trans Cis Cis Trans Trans Cis Q-Branch 100:1 S/N 10  s gate~ 100 kHz resolution

Trans Cis Cis Trans Trans Cis Q-Branch 100:1 S/N 10  s gate~ 100 kHz resolution Full Acquisition in SINGLE Valve Pulse (10  s)!!!

Chirped Pulse (Linear Frequency Sweep) Excitation Chirped Pulse Instantaneous Frequency:  = sweep rate Need: 11,000 GHz/1  s Synth: 300MHz/1ms Use arbitrary waveform generator as the frequency source Need sweep rates that are 10 5 times faster than standard synthesizers

11 GHz Chirped Pulse Frequency Sweep in TimeFrequency Sweep in Frequency Signal recorded at -60 dB from 3kW TWT

Time-Frequency Transform 11 GHz Chirped Pulse Essentially single frequency excitation at any point in time Spurious field (Harmonics, Carrier/Mix feed through etc.) suppression -10 dB Power suppression: -20 dB

Modifications to Balle-Flygare Instrument Design Modifications: Three Elements 1)Broadband Excitation… MW Synthesizer 30 MHz ν0ν0 v MHz Fourier Transform Free Induction Decay (30 MHz Carrier) Single Sideband ν0ν0 Digitizer (8 Ms/s) T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52, 33 (1981)

Modifications to Balle-Flygare Instrument Design Modifications 1)Broadband Excitation: replace cavity mirrors with horns (TA07) 2)Broadband Excitation…

Modifications to Balle-Flygare Instrument Design Modifications 1)Broadband Excitation: replace cavity mirrors with horns (TA07) 2)Broadband Excitation: replace synthesizer with arbitrary waveform generator 3)Broadband Detection… 30 MHz v MHz Fourier Transform Free Induction Decay (30 MHz Carrier) Single Sideband Digitizer (8 Ms/s) Arbitrary Waveform Generator Linear Sweep: 7.5 – 18.5 GHz Chirped Pulse Frequency Sweep 4 GS/s 2 GHz Bandwidth x8

Modifications to Balle-Flygare Instrument Design Modifications 1)Broadband Excitation: replace cavity mirrors with horns (TA07) 2)Broadband Excitation: replace synthesizer with arbitrary waveform generator 3)Broadband Detection: upgrade digitizer (40 Gs/s Digitizer) Oscilloscope Free Induction Decay Arbitrary Waveform Generator 4 GS/s 12 GHz Oscilloscope (40 Gs/s) 11.5 GHz FID acquisition and Fourier transform FID Downconversion using a Single Frequency Mix 0.5– Linear Sweep: 7.5 – 18.5 GHz Chirped Pulse Frequency Sweep x8

11 GHz CP-FTMW Spectrometer Pulse Monitor 12 GHz Oscilloscope (40 Gs/s) Free Induction Decay 0.5– 11.5 GHz Arbitrary Waveform Generator Chirped Pulse Frequency Sweep 4 GS/s x GHz TWT Amplifier FID acquisition and Fourier transform 9.9 GHz PDRO 2 GHz Bandwidth Multi-Nozzle Sample Feed GHz PDRO IR Multipass

The Need for a Broadband FTMW Spectrometer Using Rotational Spectroscopy to Study Molecular Dynamics –Our need is driven by the use of a pulsed laser to record IR excited rotational spectra. –Need fast acquisition times to minimize laser drift Suprane Chirped Pulse Delivers –10 4 reduction in power requirements –Signal scales w/ 1/(sqrt(  )) –Allow frequency multiplication (Increase Initial Bandwidth)

Pulse Requirements for Efficient Polarization of Sample (  ~ 1D) Transform Limited Pulse –11 GHz Bandwidth ~ 100 ps pulse vs. 1  s B-F –No Cavity: Q=1 vs. Q=10,000 B-F –Power Gain for  /2 pulse: –Excitation Efficiency Scales as (Pulse Energy)/  For B-F power equivalent at 11 GHz For a Transform Limited Pulse

Pulse Requirements for Efficient Polarization of Sample (  ~ 1D) Chirped Pulse –11 GHz Bandwidth ~ 1  s pulse vs. 1  s B-F –No Cavity: Q=1 vs. Q=10,000 B-F –Power Gain for  /2 pulse: –Excitation Efficiency Scales as (Pulse Energy)/  For B-F power equivalent at 11 GHz For a Chirped Pulse (Pulsed TWT up to 3 kW)

1 GHz100 MHz10 MHz Chirped Pulse vs Transform Limited Pulse Phase Control: Chirped pulse excitation* Chirped pulse Signal α 1/SQRT(bandwidth) Transform limited pulse Signal α 1/bandwidth * J.C. McGurk, T.G. Schmalz, and W.H. Flygare, J. Chem. Phys. 60, 4181 (1974). CP= TLP=

Chirped Pulse Generation: x8 Scheme x4 x2 7.5 – 18.5 GHz To Experiment Chirped Pulse Frequency Sweep – MHz Phase locked oscillator 9.9 GHz 10 MHz Rb Oscillator 4 GHz Arb. Waveform Generator Dual Channel Independent Trigger Laser/TWT Trigger Scope Trigger 0-80 dB Programmable Atten. Single Sideband Filter Sweep – GHz Sweep – GHz 19.8 GHz High Power TWT Amplifier x2 Low Power Loop

Ground State Rotational Spectrum: Suprane A types B types C types µA = 1.48* µB = µC = A * (MHz) B (MHz) C (MHz) *Unpublished data: Suenram, R. D., Lovas, F C and 18 O Lines assigned in Broadband Spectrum Excellent Intensities Across entire Spectral range 10,000 Avg. (45 min. acquisition time, 20  s Gate).1% Suprane in He/Ne

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 ms gate: 45 min. acquisition B-F Equivalent 0.1% Suprane in He/Ne Choose Your Sensitivity

trans cis Cyclopropanecarboxaldehyde IR-induced MW transitions Application to Excited State Rotational Spectroscopy Data collection time: CP-FTMW ~ 1 hour Traditional FTMW: ~ 10,000 hours (1.2 yrs) See More: Dynamics Apps: TI09 and TI10 On Technique: FC03 and FC04

Summary We have built a GHz FTMW spectrometer utilizing a chirped excitation pulse (G.G.Brown TA08) Single shot acquisition times of 15 to 25  s. S/N comparable to Balle-Flygare (K.O.Douglass TA07) Dynamic range covers 5 orders of magnitude (K.O.Douglass TA07)

Summary Phase stability (~ 5 ps time jitter) allows averaging in time-domain (>90% of single- shot signal after 10,000 averages) Fast acquisition times are ideally suited to coupling to laser techniques (K.O.Douglass TA09; B.C.Dian TA10) Spectrometer is compatible with double and triple resonance techniques (K.O.Douglass FC03;B.C.Dian FC04)

Acknowledgements Pate Lab Group Members Funding: NSF Chemistry SELIM Program NSF MRI Program (with Tom Gallagher, UVa Physics) University of Virginia John D. and Catherine T. Macarthur Foundation The Jeffress Trust

Phase Control time (µs) Intensity (V) phase+0 -  /4 -  /2 - 3  /4 -  - 5  /4 - 3  /2 - 7  /4 - 2  time (µs) Intensity The phase of the arbitrary waveform generator sets the phase of the molecular signal. Phase stability to ~ 5 ps. Propyne 1 0 – 0 0 (Single Freq) Phase shifts by  /4 increments Cyclopropane-Carboxaldehyde (Multiple Frequencies) Phase shifts 0 and 

Ground State Rotational Spectrum: OCS * * * Isotopic Species% Error OCS OC 34 S O 13 CS OC 33 S- 3/2<--3/ /2<--3/ /2<--3/ O 13 C 34 S OCS- 5/2<--5/ /2<--5/ /2<--5/ OC 36 S OC 34 S O 13 C 33 S- 3/2<--3/ /2<--3/ /2<--3/ O 13 CS

Ground State Rotational Spectrum: OCS Noise Limit avg: pk-pk: ~ 850 nV RMS: ~ 550 nV Signal-to-Noise avg: pk-pk: ~ 91000:1 RMS: ~ :1 Detection Limit: 18 O 13 CS % Natural Abundance O 13 C 33 S % 3/2  3/ % 3/2  5/ % 1/2  3/ % Succesive OCS slides blowing up scale x10

Application to Excited State Rotaional Spectra

Ground State Rotational Spectrum: OCS Detection Limit: 18 O 13 CS % Natural Abundance O 13 C 33 S % 3/2  3/ % 3/2  5/ % 1/2  3/ % * * *

Ground State Rotational Spectrum: Suprane A types B types C types µA = 1.48* µB = µC = A * (MHz) B (MHz) C (MHz) *Unpublished data: Suenram, R. D., Lovas, F.

Ground State Rotational Spectrum: Suprane Add Q-Branch Title