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 SciencesNew College of Florida5800 Bay Shore Road

Sarasota, FL 34243

GS

1

2

3

Chirped-Pulse FTMW SpectroscopyGoals: Pulses should span at least 5 GHz in 250 ns or less.

Pulses should be as clean as possible.

E(t) = E0 ei(0t + (t))

Electric Field:

Instantaneous Frequency:

= 0 + t, = sweep rate = (d/dt) (0t + (t))(t) = (/2) t2

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

T3/2 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 mTorrMany more molecules probed per repetition cycle

2) No moving partsElimination of pulsed nozzle – repetition rate limited by data processing

3) Static sample cellNo sample consumption; signal averaging can be pursued indefinitely

Instrument Schematic1) 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 ChoiceThis 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 106 – 107 candidates per run

Brute Force Automated Fitting Program

Allyl Chloride

• cis: mA = 1.5 D, mB = 1.4 D, Qvib = 4.0

• skew: mA = 2.0 D, mB = 1.0 D, Qvib = 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: http://web.mac.com/wcbailey/nqcc/

Hirota: Stark-modulated spectrometer, 8 – 35 GHz. Measurements done in dry ice. Peaks from 35Cl and 37Cl, 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 GHzAllyl chloride7 mTorr, 0 °C1.5x106 averages4 ms FIDs

Threshold Number

25:1 95

10:1 388

5:1 779

3:1 1331

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 ms FID.

Data are interpolated and splined. Peak centers are good to about 75 kHz.

Immediately Apparent FeaturesGS

ntors = 1

ntors = 2

ntors = 3

Data

Sim w/o hyperfine

16543.6: 303 ← 202

16546.3: 322 ← 221

16547.5: 321 ← 220

16553.0: 273 25 ← 264 22

Progression in 110 cm-1 mode(37Cl has a bit less intensity than ntors = 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.

MJ degeneracy means that rather high J transitions are prominent features.

33 33

8261 47

22

Data and Fits – Insets

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

35Cl skew Fit Summary35Cl skew GS ntors = 1 ntors = 2

A (MHz) 21669.64(8) 21782.72(9) 21892.71(11)

B (MHz) 2800.800(11) 2808.081(13) 2815.135(16)

C (MHz) 2714.182(11) 2719.015(12) 2723.301(15)

ΔJ (kHz) 1.77(12) 1.83(16) 1.90(19)

ΔJK

(kHz) -63.4(27) -64(4) -67(4)

ΔK (kHz) 948(15) 966(20) 996(24)

dJ (kHz) 0.1120(20) 0.139(14) 0.162(23)

dK (kHz) -38.9(8) -44(3) -48(5)

FJ (Hz) -0.07(4) -0.07(fixed) -0.07(fixed)

FJK (Hz) -3.4(15) -3.42(23) -3.41(29)

FKJ (Hz) 27(12) 28(9) 27(11)

FK (Hz) -240(130) -250(140) -240(180)

fK (Hz) -7.2(10) -6.5(18) -6.9(13)

Jmax 97 84 84

Nlines 445 366 314

sfit (kHz) 79.6 70.6 50.6

Hyperfine constants were same (within error) for all 35Cl skew states:

caa

-39.5(5) MHz

cbb

+3.5(4) MHz

ccc

+36.1(6) MHz

HirotaFrom 110-101, 18.954 GHz:

caa

-39.42 MHz

cbb

+3.45 MHz

ccc

+35.98 MHz

BaileyMP2/aug-cc-pVTZ:

caa

-39.23 MHz

cbb

+3.00 MHz

ccc

+36.23 MHz

35Cl cis and 37Cl skew35Cl cis GS Current Previous [1]

A (MHz) 13582.14(16) 13580.6(4)

B (MHz) 3816.633(17) 3816.7(4)

C (MHz) 3035.143(13) 3035.2(4)

ΔJK

(kHz) -40(9) –

ΔK (kHz) 1680(12) –

caa

(MHz) -18.4(4) -18.19 / -18.62*

cbb

(MHz) -17.98(22) -17.80 / -18.26*

ccc

(MHz) +36.4(5) +35.99 / +36.89*

Jmax 8

Nlines 30†

sfit (kHz) 84.2† Includes 13 transitions from [1].* MP2/aug-cc-pVTZ, from [2].

37Cl skew Current Previous [1]

A (MHz) 21576(82) 21593.4(4)

B (MHz) 2739.82(5) 2739.91(6)

C (MHz) 2655.73(5) 2655.68(6)

ΔJK

(kHz) -63(13) –

caa

(MHz) -31.6(8) -31.21*

cbb

(MHz) +3.1(10) 2.65*

ccc

(MHz) +28.5(13) 28.56*

Jmax 3

Nlines 34

sfit (kHz) 55.4

[1] Hirota, E., J. Mol. Spec., 35, 9 (1970).[2] W.C. Bailey’s NQCC site: http://web.mac.com/wcbailey/nqcc/

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 sim35Cl 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 35Cl skew GS and the first two torsionally excited states.

37Cl skew and 35Cl 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-R2 Award)

Noah Anderson (NCF ‘12)

Ian Finneran (NCF ‘11)

Pate lab members

Bill Bailey

35Cl skew Fits – ComparisonPrevious Results [1]

GS ntors = 1 ntors = 2 ntors = 3

A (MHz) 21669.1(3) 21784.4(11) 21896.1(16) 22008(3)

B (MHz) 2800.90(6) 2808.14(6) 2815.17(9) 2821.46(9)

C (MHz) 2713.99(6) 2718.81(6) 2723.11(9) 2726.51(9)

Current Results

A (MHz) 21669.64(8) 21782.72(9) 21892.71(11) 22009(8)

B (MHz) 2800.800(11) 2808.081(13) 2815.135(16) 2821.53(6)

C (MHz) 2714.182(11) 2719.015(12) 2723.301(15) 2726.53(6)

[1] Hirota, E., J. Mol. Spec., 35, 9 (1970).

35Cl skew ntors = 3 fit35Cl skew ntors = 3

A (MHz) 22009(8)

B (MHz) 2821.53(6)

C (MHz) 2726.53(6)

ΔJ (kHz) –

ΔJK

(kHz) -68(20)

Jmax 3

Nlines 21

sfit (kHz) 89.6