Waveguide Chirped-Pulse Fourier Transform Microwave (CP-FTMW) Spectrum of Allyl Chloride Erin B. Kent, Morgan N. McCabe, Maria A. Phillips, Brittany P. Gordon, and Steven T. Shipman Division of Natural Sciences New College of Florida 5800 Bay Shore Road Sarasota, FL GS 1 2 3

Chirped-Pulse FTMW Spectroscopy Goals:Pulses should span at least 5 GHz in 250 ns or less. Pulses should be as clean as possible. E(t) = E 0 e i(  0 t +  (t)) Electric Field: Instantaneous Frequency:  =  0 +  t,  = sweep rate  = (d/dt) (  0 t +  (t))  (t) = (  /2) t 2

Rotational Spectroscopy near 298 K High T spectra are complex! At 298 K, excited vibrational states and conformers are populated; in ground state, transitions involving J > 100 are commonly observed.

CP-FTMW at Elevated Temperatures T 3/2 scaling of rotational partition function! Population difference also small. Difficult to make up sensitivity loss vs. MB, but there are mitigating factors. 1) Sample pressures of 1 – 20 mTorr Many more molecules probed per repetition cycle 2) No moving parts Elimination of pulsed nozzle – repetition rate limited by data processing 3) Static sample cell No sample consumption; signal averaging can be pursued indefinitely

Instrument Schematic 1)250 ns sweep (0.1 – 4.9 GHz) generated by AWG and mixed with PLDRO. 2) Mixed sweep is amplified and sent into sample cell. 3) Molecular FID is amplified, downconverted, and detected with oscilloscope.

ISPs and Molecule Choice This talk is primarily the work of undergraduates in the lab over the month of January 2011, with a bit of additional work during the spring semester. Students created a list of ~40 molecules and optimized each with G03. Chose allyl chloride on basis of rotational constants, dipole moment, and easy availability ($32 / 500 mL from Aldrich). Initial assignments were made with a combination of ab initio results and an automated method of generating candidate rotational constants.

“Embarrassingly parallel”. Linear scaling with number of processors. Typical benchmark is about 500,000 per hour (4 core machine, 2.5 GHz) Original idea from Pate lab for fitting isotopomers in natural abundance. We implemented it in Python for a speed boost. Fitting done with SPFIT Generally evaluate 10 6 – 10 7 candidates per run Brute Force Automated Fitting Program

Allyl Chloride cis:  A = 1.5 D,  B = 1.4 D, Q vib = 4.0 skew:  A = 2.0 D,  B = 1.0 D, Q vib = 4.7 skew more stable by 376 cm -1 (G03, ZPC) [1] Hirota, E., J. Mol. Spec., 35, 9 (1970). [2] W.C. Bailey’s NQCC site: Hirota: Stark-modulated spectrometer, 8 – 35 GHz. Measurements done in dry ice. Peaks from 35 Cl and 37 Cl, cis and skew, ground and excited states. Bailey has performed high-level calculations to determine hyperfine constants, and they are in good agreement with Hirota’s results.

Allyl chloride from 8.7 – 18.3 GHz Allyl chloride 7 mTorr, 0 °C 1.5x10 6 averages 4  s FIDs ThresholdNumber 25: : : : Noise level at 0.2 units. Tallest peak has S:N of 124:1. Line density of ~1 peak per 7.5 MHz. (Of course, many peaks are blended…)

Noise Levels and Linewidths Unblended peaks have ~700 kHz FWHM, almost entirely due to 4  s FID. Data are interpolated and splined. Peak centers are good to about 75 kHz.

Immediately Apparent Features GS tors = 1 tors = 2 tors = 3 Data Sim w/o hyperfine : 3 03 ← : 3 22 ← : 3 21 ← : ← Progression in 110 cm -1 mode ( 37 Cl has a bit less intensity than tors = 2)

Data and Fits Assigned roughly 1200 transitions (many blends) to 7 species. Vast majority of the strong lines have been assigned.

Data and Fits – Insets Roughly half of assigned transitions are b-type P-branches. M J degeneracy means that rather high J transitions are prominent features

Data and Fits – Insets Uncertainties on hyperfine parameters are mainly determined by the fits of the a-type 3 ← 2 and 2 ← 1 transitions.

35 Cl skew Fit Summary 35 Cl skewGS tors = 1 tors = 2 A (MHz) (8) (9) (11) B (MHz) (11) (13) (16) C (MHz) (11) (12) (15) Δ J (kHz) 1.77(12)1.83(16)1.90(19) Δ JK (kHz) -63.4(27)-64(4)-67(4) Δ K (kHz) 948(15)966(20)996(24)  J (kHz) (20)0.139(14)0.162(23)  K (kHz) -38.9(8)-44(3)-48(5)  J (Hz) -0.07(4)-0.07(fixed)  JK (Hz) -3.4(15)-3.42(23)-3.41(29)  KJ (Hz) 27(12)28(9)27(11)  K (Hz) -240(130)-250(140)-240(180)  K (Hz) -7.2(10)-6.5(18)-6.9(13) J max 9784 N lines  fit (kHz) Hyperfine constants were same (within error) for all 35 Cl skew states:  aa -39.5(5) MHz  bb +3.5(4) MHz  cc +36.1(6) MHz Hirota From , GHz:  aa MHz  bb MHz  cc MHz Bailey MP2/aug-cc-pVTZ:  aa MHz  bb MHz  cc MHz

35 Cl cis and 37 Cl skew 35 Cl cis GSCurrentPrevious [1] A (MHz) (16) (4) B (MHz) (17)3816.7(4) C (MHz) (13)3035.2(4) Δ JK (kHz) -40(9)– Δ K (kHz) 1680(12)–  aa (MHz) -18.4(4) / *  bb (MHz) (22) / *  cc (MHz) +36.4(5) / * J max 8 N lines 30 †  fit (kHz) 84.2 † Includes 13 transitions from [1]. * MP2/aug-cc-pVTZ, from [2]. 37 Cl skewCurrentPrevious [1] A (MHz) 21576(82) (4) B (MHz) (5) (6) C (MHz) (5) (6) Δ JK (kHz) -63(13)–  aa (MHz) -31.6(8)-31.21*  bb (MHz) +3.1(10)2.65*  cc (MHz) +28.5(13)28.56* J max 3 N lines 34  fit (kHz) 55.4 [1] Hirota, E., J. Mol. Spec., 35, 9 (1970). [2] W.C. Bailey’s NQCC site:

Pushing Forward: 18 – 26 GHz We hope to extend our upper frequency limit to 26.5 GHz soon. Boltzmann factors are better and total bandwidth will approximately double. SPCAT sim 35 Cl skew GS

Summary and Future Work Build 18 – 26.5 GHz instrument and collect data! Further improvements on triples fitter to increase speed. Future work: We have significantly improved the fits on 35 Cl skew GS and the first two torsionally excited states. 37 Cl skew and 35 Cl cis are on par with prior work by Hirota. Data with better S/N will be needed to extend much further.

Acknowledgments New College of Florida (Start-up funding) Research Corporation (Cottrell College Science Award) ACS Petroleum Research Fund (UNI Award) National Science Foundation (MRI-R 2 Award) Noah Anderson (NCF ‘12) Ian Finneran (NCF ‘11) Pate lab members Bill Bailey

35 Cl skew Fits – Comparison Previous Results [1] GS tors = 1 tors = 2 tors = 3 A (MHz) (3) (11) (16)22008(3) B (MHz) (6) (6) (9) (9) C (MHz) (6) (6) (9) (9) Current Results A (MHz) (8) (9) (11)22009(8) B (MHz) (11) (13) (16) (6) C (MHz) (11) (12) (15) (6) [1] Hirota, E., J. Mol. Spec., 35, 9 (1970).

35 Cl skew tors = 3 fit 35 Cl skew tors = 3 A (MHz) 22009(8) B (MHz) (6) C (MHz) (6) Δ J (kHz) – Δ JK (kHz) -68(20) J max 3 N lines 21  fit (kHz) 89.6