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 Virginia 2 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 pressure More molecules – larger signals at short times but faster decays lead to a poorer ultimate resolution. FID duration Repetition 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 mTorr Laser-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 positions AssignmentWG Freq. (MHz) Prev. Obs. (MHz) Calc. Freq. (MHz) WG – Prev. (kHz) WG – Calc. (kHz) – (14) – (15) – (15) – (15) – (30) – (12) – (26) – (16) – (70) 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 AssignmentWG Freq. (MHz) Calc. Freq. (MHz) Dev. (kHz) – (79) – (96) – (41) – (32) – (160) – (163) – (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 13 CH 3 OH transitions present in 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 v t = 0 J=20 v t = 1 J=16 v t = 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 Initial 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 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 = 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 – MeOH J = 17 – 16 and J = 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 Scan Laser-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 Lab Leonardo Alvarez-Valtierra Matt Muckle Justin Neill Sara Samiphak Collaborators Zbigniew Kisiel Alberto Lesarri David Perry Funding NSF Chemistry CHE NSF CRIF:ID CHE

Newly observed methanol line positions: Torsional excited state AssignmentWG Freq. (MHz) Calc. Freq. (MHz) Dev. (kHz) – (111) – (143) – (187) – BLENDED (245) – – (630) – (317)373 Xu, L.-H. and Lovas, F.J., J. Phys. Chem. Ref. Data 26, 17 (1997). Std. Dev.:337 kHz

Previously observed 13 CH 3 OH line positions AssignmentWG Freq. (MHz) Prev. Obs. (MHz) Calc. Freq. (MHz) WG – Prev. (kHz) WG – Calc. (kHz) – (30) – (40) – (50) – (130) – (1360) – (1360) – (30) – (20) – (20) – (1160) – (1160) – (70) – (BLENDED) (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