Steven T. Shipman, 1 Leonardo Alvarez-Valtierra, 1 Justin L. Neill, 1 Brooks H. Pate, 1 Alberto Lesarri, 2 and Zbigniew Kisiel 3 Design and performance of a direct digital chirped- pulse Fourier transform microwave (CP-FTMW) spectrometer operating from 2 – 8 GHz 1 University of Virginia 2 Universidad de Valladolid 3 Polish Academy of Sciences I II

Overview Chirped-Pulse Fourier Transform Microwave Spectroscopy (CP-FTMW) Iodobenzene and its Ne complex Interlude: Strawberry aldehyde (C 12 H 14 O 3 ) MW-MW Double Resonance Low frequency Stark measurements

True Broadband Rotational Spectroscopy Two issues to resolve: Generating broad (in frequency), short (in time) pulses Getting these pulses in and out of the spectrometer Arbitrary Waveform Generator MW Synthesizer Replacement (Image from Standard Gain Horn Mirror Replacement (Image from The Goal: Acquire a multi-GHz spectrum with every valve shot to gain a multiplex advantage for use with other techniques

Chirped Pulse Excitation (Linear Frequency Sweep) Chirped Pulse Instantaneous Frequency:  = sweep rate Need: 11,000 MHz/1  s Synthesizer: 300 MHz/1ms Use arbitrary waveform generator as the frequency source J.C. McGurk, T.G. Schmalz, and W.H. Flygare, “Fast passage in rotational spectroscopy: Theory and experiment”, J. Chem. Phys. 60, 4181 (1974).

2 – 8 GHz Direct Detect CP-FTMW Spectrometer Horns 2.5 – 7.5 GHz, 15 dBi gain Amplifier 300 W TWTA 4 W SSA Repetition rate 5 Hz (20 GS/s, 20  s FID) Freq. accuracy Better than 4 kHz Note: hard to completely isolate from 2.4 GHz wireless band. 20 Gs/s Oscilloscope

2 – 8 GHz Pulse spectrograms Pre-amplification Post-amplification The 300 W TWTA output has some 2 nd and 3 rd harmonic character. Solid state amplifiers are cleaner, but at the expense of peak power.

2 – 8 GHz Iodobenzene Spectrum Dorosh, O. et al., J. Mol. Spec. 246 (2007), atm of He/Ne buffer gas flowing over liquid reservoir. This spectrum took about 3 days to acquire.

2 – 8 GHz Iodobenzene Spectrum Dorosh, O. et al., J. Mol. Spec. 246 (2007), 228. J = 4 – 3 inset

Iodobenzene 13 C Isotopomers

Ne-Iodobenzene Cluster The quadrupole tensor of the cluster is indicative of an 11.43(2)º rotation of the a-axis with respect to the C-I bond.

Equivalent Sensitivity to 8 – 18 GHz CP-FTMW Data in each region scaled to match noise in overlap, giving signals in good agreement with SPCAT. See RH06 for more!

4 W vs. 300 W Amplifier 300 W TWTA for 4  s = 1200  J delivered to sample 4 W SSA for 20  s = 80  J delivered to sample Theoretical ratio = sqrt(1/15) = W SSAs are much cheaper than 300 W TWTAs! 300 W TWTA, 4  s 4 W SSA, 20  s Scale TWTA by to match SSA

Spectrum of a Large Molecule: Strawberry aldehyde (C 12 H 14 O 3 ) I II IIIIV 1 nozzle, heated to 120°C Future: Use multiple nozzles in 2-8 and 8-18 GHz. Assigned 5 th, haven’t gotten its complement yet.

Strawberry Aldehyde Rotational Constants IIIIIIIV A (MHz) (6) (3) (6) (13) B (MHz) (3) (12) (6) (4) C (MHz) (24) (12) (9) (4)  J (kHz) 0.069(3)0.0144(3)0.3083(10)0.0178(11)  JK (kHz) (14) (18)-0.75(3)-0.091(5)  K (kHz) 0.078(17)0.905(19)0.553(22)0.96(7)  J (kHz) (18) (14)0.125(5)0.0031(5)  K (kHz) (7)-0.271(25)-0.091(23)0.41(15) # lines OMC (kHz)

MW-MW Double Resonance Scheme 20 Gs/s Oscilloscope Multiple isolators needed to keep reflected TWTA power from blowing up SSA!

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

MW-MW Double Resonance on Iodobenzene

MW-MW DR quickly establishes level connectivity, greatly facilitating assignments.

Low Frequency Stark Spectra (WF12) Inhomogeneity leads to weak / missing lines, but there are many usable lines. The best fit used 329 lines at 4 fields to give a dipole moment of (68) D. Field strengths were calibrated with TFP (2.319 D), which was in turn calibrated with OCS in the 8 – 18 GHz spectrometer.

Future Directions Start taking data with multiple heated nozzles Stark effect measurements on strawberry aldehyde conformers Short Term: Laser ablation sources for larger molecules Construct a 2 – 18 GHz direct detect spectrometer Long Term:

Acknowledgements Special Thanks: Tom Fortier and Tektronix The Pate Lab Leonardo Alvarez-Valtierra Matt Muckle Justin Neill Sara Samiphak Collaborators David Pratt (Pittsburgh) Rick Suenram (UVa) Lu Kang (Union College, KY) Nick Walker (University of Bristol, UK) Funding NSF Chemistry CHE NSF CRIF:ID CHE

Iodo and Ne-Iodo Constants Iodo20Ne-Iodo22Ne-Iodo A (MHz) (33) (214) (145) B (MHz) (194) (126) (85) C (MHz) (183) (112) (66)  J (kHz) (43)1.1910(68)1.335(54)  JK (kHz) 0.202(38)18.292(44)15.59(43)  K (kHz) Not included16.39(284)Not included  J (kHz) Not included0.2985(52)0.3490(196)  K (kHz) Not included13.16(35)12.44(306)  aa (MHz) (45) (43) (29)  bb (MHz) (11)801.06(17)776.55(49)  cc (MHz) (16)978.21(24)978.29(64)  ab (MHz) (76)597.74(39) # of lines Dorosh, O. et al., J. Mol. Spec. 246 (2007), 228.

Stark Fit Histogram

Iodobenzene K1 Stark Anomalies

MW-MW Double Resonance on Iodobenzene

Extension to Very Low Frequency 0.8 – 2.0 GHz CP-FTMW (Suprane) MHz MHz