June 21, 2012

Submillimeter Spectrum of Chloromethane:Analysis of the V3=1 Excited State

Presented by:

Alissa FisherAuburn University and

U.S. Army Aviation and Missile Research, Development, and Engineering Center

Presented to:

International Symposium on Molecular Spectroscopy

Approved for Public Release; distribution unlimited. Review completed by the AMRDEC Public Affairs Office 20 Apr 2012: FNS884Reference herein to any specific commercial, private or public products, process, or service by trade name, trademark, manufacturer, or otherwise, does not constitute or imply its endorsement, recommendation, or favoring by the United States Government

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CH3Cl SpectroscopyCH3Cl Spectroscopy

• Two isotopomers CH335Cl and

CH337Cl

• Methyl halide with a large permanent electric dipole moment

• Prolate symmetric top with threefold degeneracy

• Follows selection rules• ∆J= +-1 and ∆K=0

• Hyperfine splitting is visible• I = 3/2• quantum number F= I+J

• Does not occur naturally

**T. W. Pape. (1993). Collisional Processes in Methyl Chloride. (Doctoral Dissertation). Duke University.

J

µ

K

1000

En

erg

y (c

m-1)

V3

J’’

J’ K

K

F

F

0

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• V3=1 is associated with the stretching of the C-Cl bond

• Numerous studies of the ground state and V6 level exist

• V3 is 4kT above the ground state– Less intense by a

factor of e-4 based on Boltzmann statistics

Rotational Spectra of the V3=1 Energy Level

Rotational Spectra of the V3=1 Energy Level

Population fractions of the three most populated vibrational states.

Vibrational State

Energy(cm-1)

Degeneracy Population

Ground 0 1 95.553%

V3 732.88 1 2.677%

V6 1018.07 2 1.337%

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• Powerful source of CW THz radiation

• THz laser based on laser-induced rotational population inversions in GS, V6

• Inversion is collisionally quenched – Depopulating V6 rapidly

is critical to OPFIR laser performance

• Principal vibrational relaxation mechanism is from V6 to V3 to Ground

– DR Spectroscopy can measure the rate of this*

OPFIR LaserOPFIR Laser

* As presented by D. J. Phillips/ IR/THz Double Resonance Spectroscopy Energy Dynamics at Atmospheric Pressures/ /UAH. 2012.

1000

En

erg

y (c

m-1)

V3

0 Ground State

V6

1018 cm-1

0 cm-1

J10 K6

J11 K6

J12K6

J11 K7 l1

J12 K7 l1

J13 K7 l1

9R12

CO

2 L

aser

Lin

e

Relaxation pathway

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• Previous work– Thorough analysis of V3 with IR

spectroscopy which involves measured lines up to J=62-63, K=19*

• unable to resolve hyperfine• Use of ground state terms for the

higher order distortion constants

– Hyperfine constant eQq was fit with measured lines up to J=10-11, K=10 using microwave spectroscopy**

• Current work– Predictions off by a few MHz– Measurement from J=5-6, K=0

up to J=25-26, K=12– Higher order parameters

necessary for accurate prediction of line centers at high resolution

BackgroundBackground

Constant 112CH335Cl 12CH3

37Cl

B3/MHz 13177.6387 (12) 12975.8168 (13)

DJ/kHz 18.1302 17.6065

DJK /kHz 199.0172 193.684

HJ/Hz -0.013 -0.0108

HJK/Hz 0.246 0.29

HKJ/Hz 9.623 9.2

LJ/mHz - -

eQq/MHz -74.809 (45) -59.058 (45)

*M. Betrencourt / JOURNAL OF MOLECULAR SPECTROSCOPY 128,433-443 (1988)**G. Wlodarczak/ JOURNAL OF MOLECULAR SPECTROSCOPY 116,25 1-255 (1986)

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Experimental SetupExperimental Setup

• Virginia Diodes Inc. (VDI) frequency tunable Schottky diode source and detector

• Four multipliers spanning a region of 140 GHz to 750 GHz– WR – 5.1 (140-220 GHz) J5-7– WR – 3.4 (220-330 GHz) J8-11– WR – 1.5 (330-500 GHz) J12-18– WR – 2.2 (500-750 GHz) J19-27

Vacuum ChamberDetector

Methyl ChlorideVacuum Manifold

Terahertz Source

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• Data acquisition utilizes modulation spectroscopy and lock in detection

• To maximize resolution and signal strength– Measurements made at

pressure where Doppler and pressure broadening were equal

– Depth of modulation was set at Doppler width

• Experimental resolution is ± 50 kHz

Modulation SpectroscopyModulation Spectroscopy

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DataData

CH335Cl

J=23-24, K=12

CH337Cl

J=9-10, K=7

0

0.2

0.4

0.6

0.8

1.0

-0.2

-0.4

259.25 259.255 259.260 259.265

CH335Cl

J=9-10, K=9

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Transition FrequencyTransition Frequency

)32)(12)(12(2

)1()1()1(4/3

1)1(

23

JJII

JJIICC

JJ

KeQqQE

Q E

)K(JkkjL)KJ)((J)(JkjL)KJ)((J)(JjkL)J)((J)(JJjL

)K(JkjH )KJ)((J)(Jjk H) -J)((J)(Jj H

) J(Kjk - D)(JjD-

)B(Jv

612422221233231442414

41222222133231

222314

12

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ResultsResults

J transitions from 9 to 25 of the

K=9 levels

J transitions from 12 to 25 of the

K=12 levels

Deviation of measured line centers Vs. Theoretical line centers using older constants plotted with the deviation using the newer constants.

O-C

(kH

z)

O-C

(kH

z)

12 14 16 18 20 22 24-200

0

200

400

600

800

1000

1200

1400

1600

K12

literature

6 8 10 12 14 16 18 20 22 24 26

-100

-50

0

50

100

150

200

250

300

K8

literature

JLower

JLower

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AnalysisAnalysis

• SPFIT is used for fitting transitions and term values

Constant 112CH335Cl New 12CH3

37Cl (New)

B3/MHz 13177.6387 (12) 13177.64324 (19) 12975.8168 (13) 12975.8078(172)

DJ/kHz 18.1302 18.1372(10) 17.6065 17.6065(98)

DJK /kHz 199.0172 199.1587(23) 193.684 193.470(173)

HJ/Hz -0.013 -0.00756(23) -0.0108 -0.0435(204)

HJK/Hz 0.246 0.359(6) 0.29 .1525(250)

HKJ/Hz 9.623 9.46(10) 9.2 9.213(17)

LJ/Hz - 0.00000265(17) - 0.000000305(12)

eQq/MHz -74.809 (45) -74.857(29) -59.058 (45) -59.058(45)

Fits include J=0-1, K=0 up to J=25-26, K=12

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• V3=1 has been examined by a frequency tunable microwave source with Schottky diode multipliers and detectors in the submillimeter region.

• Doppler limited spectra was obtained for J <25 and K<12 for both Chlorine isotopomers.

• New hyperfine, quartic, and sextic rotational constants for the V3=1 were fitted

Special Thanks To:

DANE J. PHILLIPS, Kratos Defense and Security Solutions Digital Fusion, 4904 Research Dr., Huntsville, Al, 35805;

DENNIS G. WILSON, Massachusetts Institute of Technology;

ELIZABETH RHODES, University of Alabama Tuscaloosa;

HENRY O. EVERITT, Army Aviation and Missile RD&E Center, Weapon Sciences Directorate, Redstone Arsenal, AL, 35898.

ConclusionConclusion