Waveguide Chirped-Pulse FTMW Spectroscopy

Steven T. Shipman,1 Justin L. Neill,1 Brett Kroncke,1

Brooks H. Pate,1 and P. Groner 2

1 University of Virginia2 University of Missouri – Kansas City

Overview

• Waveguide Chirped Pulse FTMW Spectroscopy

• Instrument Performance

• MW-MW and IR-MW Double Resonance

• Future Directions

Room Temperature Spectroscopy

Room temperature measurements have been an important part of rotational spectroscopy.

A number of groups have performed room or near-room temperature measurements using waveguide FTMW or other techniques (Wilson-Hughes Stark spectroscopy, mm-wave).

The majority of MW spectroscopy today is performed with pulsed jets that allow for the study of weakly-bound clusters and reactive species.

Still, there are many things to explore at room temperature:

• Rotational spectra of vibrationally excited states• Isomerization kinetics under thermal conditions

• Effects of thermal energy on IVR rates

Why Return to Room Temperature Now?

CP-FTMW technique offers an enormous multiplex advantage which can be used to perform complex measurements rapidly.

These include the potential for fully 2D-FTMW spectroscopy as well as efficient double resonance measurements.

Current design is on par with previous implementations (50 MHz BW at 30 kHz rep. rate vs. 9 GHz BW at 80 Hz); the bottleneck is oscilloscope data processing speeds.

Also: broadband detection is identical in spirit to previous implementations – no Q-factor advantage being lost as in broadband molecular beam work!

Diplexed Spectrometer Design

This design covers 9 GHz of bandwidth (double-ridged waveguide).

Pulse generation is diplexed to get maximal spectral purity from AWG.

Detection circuit is diplexed to take advantage of parallel processing capabilities of the digital oscilloscope.

Repetition rates of 80 Hz (10 GS/s, 2 s FID) can currently be achieved.

Tradeoffs to Consider

Waveguide pressureMore molecules – larger signals at short times but faster decays lead to a poorer ultimate resolution.

FID durationRepetition rate is set by # of points to digitize; higher resolution means less averaging. Also, noise decreases as sqrt(# of points).

In practice, transition density (# of lines / MHz) will determine this for larger molecules.

All data were taken with a 2 s FID at 80 Hz in an 8 m waveguide. Pressures adjusted to maximize signal over the full FID (except laser work, optimized for first 200 ns).

Pure rotational: MeOH, acetone, and acetaldehyde – 10 mTorrLaser-WG DR: MeOH – 35 mTorr

The Actual Implementation

Methanol at 10 mTorr and 298 K

This spectrum represents 1.7 hours of data collection.

Linewidth is due to optimized pressure for a 2 s FID.

10 s FIDs are achievable, for a cost in repetition rate (~ 9 Hz).

Previously observed methanol line positionsAssignment WG Freq.

(MHz)

Prev. Obs.

(MHz)

Calc. Freq.

(MHz)

WG – Prev. (kHz)

WG – Calc. (kHz)

9-19 – 8-27 9936.206 9936.200 9936.203(14) 6 3

432 – 523 9978.663 9978.669 9978.703(15) -6 -40

431 – 524 10058.276 10058.316 10058.281(15) -40 -5

202 – 3-13 12178.599 12178.593 12178.561(15) 6 38

165 12 – 174 13 12229.354 12229.400 12229.335(30) -46 19

514 – 515 12511.203 12511.000 12511.228(12) 203 -25

16-2 15 – 15-3 13 16395.720 16395.740 16395.861(26) -20 -141

615 – 616 17513.331 17513.341 17513.372(16) -10 -41

213 18 – 204 16 17910.841 17910.820 17910.982(70) 21 -141

22 kHzStd. Dev.: 64 kHz

Xu, L.-H. and Lovas, F.J., J. Phys. Chem. Ref. Data 26, 17 (1997).

Newly observed methanol line positions:Torsional ground state

Assignment WG Freq.

(MHz)

Calc. Freq.

(MHz)

Dev.

(kHz)

182 16 – 182 17 10782.611 10782.506(79) 105

192 17 – 192 18 13267.524 13267.400(96) 124

16-3 14 – 172 15 13775.614 13775.553(41) 61

143 11 – 150 15 14905.241 14905.222(32) 19

254 22 – 245 19 15214.137 15214.619(160) -482

254 21 – 245 20 15642.393 15642.781(163) -388

202 18 – 202 19 16143.626 16143.438(116) 188

Xu, L.-H. and Lovas, F.J., J. Phys. Chem. Ref. Data 26, 17 (1997).

Std. Dev.: 267 kHz

12 new observations (7 torsional ground state, 5 torsional excited state)

Intensities are qualitatively correct.

Also – 10 previously seen 13CH3OH transitions presentin natural abundance.

Joint Time-Frequency Analysis of the Broadband FID

We can segment the FID into 50 ns slices and FT each slice, giving information on how a given frequency component evolves with time.

J=24 vt = 0

J=20 vt = 1

J=16 vt = 0

Variability in Collisional Decay Rates

These are simultaneous measurements, so identical pressures are guaranteed.

There’s a lot of detailed analysis to do!

MeOH (10 mTorr)500k shots

MW – MW Double Resonance

Broadband approach takes advantage of multiplexed detection channels so that a DR measurement only needs to be performed once.

404

505

303

202212

313

414

515Initial chirped pulse polarizes rotational transitions over a large bandwidth.

A second narrowband pulse selectively pumps a single transition. This destroys all coherences between levels connected to pumped pair, giving significant intensity modulation in detected FID.

High Line Densities - Acetaldehyde

Acetaldehyde, though simple, has an extremely high line density at 298 K.

Double resonance measurements can help to pick apart the spectrum.

MW-MW DR on Acetaldehyde

Pumping one transition alters connected transitions (across diplex bands) while leaving unconnected transitions untouched.

Currently power-starved; next generation will use a 10 W SSA. Should be able to consistently achieve > 75% modulations.

Still Higher Line Density - Acetone

x7.5

A conservative peak count gives 1165 distinct transitions from 9.1 – 18 GHz: average line separation of 7.6 MHz.

Clean MW-MW DR measurements will be a problem if density increases by much (200 ns pulses only select a 5 MHz region).

Full 2D-FTMW will eventually be required.

IR – MW Double Resonance

Addition of an IR laser allows one to obtain rotationally-resolved infrared spectra.

Technique exploits small population difference between rotational levels at 298 K.

Not limited to IR (though will want sapphire instead of mica for UV).

J’+1

J’v

v+1 J’’

IR-Cavity vs. IR-Waveguide (MeOH)

Scaled to 2954.38

Some laser power drift on IR-Waveguide, but overall S/N within about a factor of 3.However, at the same time, we measured other bands…

MANY improvements can be made – this was a first attempt!

See RA04 for more!

IR-Waveguide DR – MeOH, J = 16 - 15

Phase information is preserved, so IR transitions can be sorted by upper or lower rotational level.

Doubled features (~0.05 cm-1 spacing) are due to imperfect laser mode.

IR-Waveguide DR – MeOHJ = 17 – 16 and J = 21 - 20

These have all been ground state MW transitions.We also see upper state transitions appear…

2D IR-MW Upper State Spectra

Ground state transitions have been removed from these plots; all spots represent upper state rotational transitions induced by the IR laser.

(MW transitions have been artificially broadened to 50 MHz for visualization purposes.)

Time-Frequency Analysis During IR Scan:Ground State Transition

Measured decay constant of 362(11) ns is on par with increased pressures used for the laser scan.

Time-Frequency Analysis During IR ScanLaser-Induced Transition

The same decay constant as in GS is observed within fit uncertainty.

Still a lot of work to do just on this data set!

Future Directions

• How far can we go in number of heavy atoms and Qvib at RT?

• More IR-MW work on MeOH, make improvements to setup.

Short Term:

• Implement full 2D-FTMW techniques to deal with line density

• A nearly infinite number of possible directions…

Long Term:

Acknowledgements

Special Thanks: Tom Fortier and Tektronix

The Pate LabLeonardo Alvarez-Valtierra

Matt MuckleJustin Neill

Sara Samiphak

CollaboratorsZbigniew KisielAlberto Lesarri

David Perry

FundingNSF Chemistry CHE-0616660NSF CRIF:ID CHE-0618755

Newly observed methanol line positions:Torsional excited state

Assignment WG Freq.

(MHz)

Calc. Freq.

(MHz)

Dev.

(kHz)

141 13 – 141 14 10041.034 10041.182(111) -148

151 14 – 151 15 11463.477 11463.701(143) -224

161 15 – 161 16 12977.550 12977.595(187) -45

171 16 – 171 17 BLENDED 14582.130(245) –

20-1 19 – 21-2 19 15303.305 15304.866(630) -561

181 17 – 181 18 16275.881 16276.508(317) 373

Xu, L.-H. and Lovas, F.J., J. Phys. Chem. Ref. Data 26, 17 (1997).

Std. Dev.: 337 kHz

Previously observed 13CH3OH line positions

Assignment WG Freq.

(MHz)

Prev. Obs.

(MHz)

Calc. Freq.

(MHz)

WG – Prev. (kHz)

WG – Calc. (kHz)

1019 – 928 9153.487 9153.500 9153.485(30) -13 2

184 14 – 175 13 9651.420 9651.480 9651.486(40) -60 -66

122 11 – 111 10 9786.479 9786.400 9786.407(50) 79 72

204 16 – 213 18 11359.222 11359.230 11359.169(130) -8 53

255 21 – 246 18 12346.580 12346.720 12347.739(1360) -140 -1159

255 20 – 246 19 12349.727 12349.840 12350.793(1360) -113 -1066

1037 – 1129 12918.117 12918.070 12918.089(30) 47 28

515 – 606 14300.383 14300.350 14300.308(20) 33 75

202 – 3-13 14782.256 14782.270 14782.212(20) -14 44

236 18 – 227 15 15193.156 15193.626(1160) -470

236 17 – 227 16 15193.643(1160) -487

122 10 – 111 11 15667.571 15667.550 15667.472(70) 21 99

615 – 616 16682.089

(BLENDED)

16682.760 16682.748(20)

Xu, L.-H. and Lovas, F.J., J. Phys. Chem. Ref. Data 26, 17 (1997).

Torsionally excited states are highlighted.

MW-MW Double Resonance

First (chirped) pulse polarizes rotational transitions over a large bandwidth.

A second narrowband pulse pumps a single transition. This destroys coherences with connected levels, giving intensity modulations in the detected FID.

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