Spectral taxonomy: A semi-automated combination of chirped- pulse and cavity Fourier transform...

Spectral taxonomy: A semi-automated combination of chirped-pulse and cavity Fourier transform microwave spectroscopyKyle N. Crabtree, Marie A. Martin-Drumel, and Michael C. McCarthy

Harvard-Smithsonian Center for Astrophysics

Microwave spectroscopy at the CfA

Microwave spectroscopy of reactive molecules: astrochemistry, atmospheric chemistry, and exotic species

200 molecules detected in 20 yearsCations: HNNO+/NNOH+, H2NCO+

Anions: C4H-, C6H-, C8H-

Radicals: HOCO, HO3, C4H, HC3OExotic: Si3, HOON, CH2OO, HOCOH

Cavity FTMspectrometer

Fourier transform spectrometer (5-26 GHz, 1 MHz bandwidth)

Supersonic expansion discharge nozzle

High sensitivity, high resolution

Dilutegaseous

precursors -HV

CP-FTMW spectrometer

20 GHz, 50 GS/s scope

24 GS/sAWG

+38 dB

+30 dB

18.99 GHzPLDRO

10 MHz RbStandard

LPFilter

0.5-10.5 GHz;4 msec duration

StepAtten

0-70 dB

10 pulses; D 20 mst=0

t 250 ms

• Real-time phase scoring and correction• >1M co-averages routine

Linux DAQsoftware

18.5-8.5 GHz

200W7.5-18 GHz

TWTAData Validity

Monitor

Eccosorb−30 dB

Opportunities and goals Chemical profiling of

reactive gas mixtures with intermediates (Miller-Urey-like experiments)

Conformer, vibrational-state discrimination

Analysis challenges for U-lines: Combine CP and cavity FT spectroscopy

Illustration by Adrian J. Hunter, CC-BY-SA 3.0

Example: C2H2 + CS2 discharge150k gas pulses, 1.5M chirps: ~8.5h @ 5Hz1% C2H2, 1% C2S, 98% Ne1 kV discharge

Example: C2H2 + CS2 discharge

495 features with SNR > 3 Of the strongest 34 features (SNR > 50):

13 artifacts (LO; harmonics; scope spurs) 10 assigned (C3S, c-C3H2, SO2, HC3S, C5S, H2C3S, C2S) 11 unidentified

150k gas pulses, 1.5M chirps: ~8.5h @ 5Hz1% C2H2, 1% C2S, 98% Ne1 kV discharge

Chirped-pulse vs. cavityC

hir

ped

-pu

lse

8.5 h


hir

ped

-pu

lse

Cavit

y

8.5 h

11 h


hir

ped

-pu

lse

Cavit

y

8.5 h

11 h

8.5 h

9 s(0.0025 h)


hir

ped

-pu

lse

Cavit

y

8.5 h

11 h

8.5 h

9 s(0.0025 h)

Bandwidth Intensity accuracy Resolution Frequency accuracy Sensitivity

Bandwidth Intensity accuracy Resolution Frequency accuracy Sensitivity

Chirped-pulse + cavityC

hir

ped

-pu

lse

Cavit

y

Automated analysis with cavity

QtFTM: Cavity control software designed for programmatic, automated acquisition

Automated analysis with cavity

Automated analysis with cavity Import list of

frequencies from CP spectrum

Automatically tune cavity to each frequency

Integrate for a duration based on CP intensity

Determine how many lines are present, fit to instrument response function

Remeasure under different conditions (e.g. discharge turned off)

For each line found in CP spectrum, determine if line disappears when conditions change; classify:

Taxonometric classification

Discharge

Magneticfield

Gas 2present

Gas 1present

Buffer gasimpurities

Gas A cmplxor impurity

Gas

B c

mpl

x

or im

purit

y

Close

d-sh

ell

inte

rmed

/pro

d

Open-shell I/P

Example acquisition: Discharge test

Dischargeenabled

Dischargedisabled


DC DC DC DC DC DC

Taxonomy flowchart

Yes

Yes No

Yes No Yes No

Y N NY Y N Y N

Y N Y N Y N Y NY N Y N Y N Y N

Detected in cavity?

Discharge?

Magnetic?

CS2 only?

C2H2 only?

Taxonomy flowchart

Yes

Yes No

Yes No Yes No

Y N NY Y N Y N

Y N Y N Y N Y NY N Y N Y N Y N

Detected in cavity?

Discharge?

Magnetic?

CS2 only?

C2H2 only?

795 1328 170 0 0 0 0 0 0 3 0 0 0

28 17 102 13 0 0 3 0

45 115 0 3

160 3

163

Non-detections: 330/495 (past expt: 20%)


Discharge

Magneticfield

C2H2

present

CS2

present0

0

0

00

00

395

7

132817

0


Discharge

Magneticfield

C2H2

present

CS2

present0

0

0

00

00

395

7

132817

0

Next step: cross-correlation within groups using double resonance

MW-MW double resonance

fFTM

J = 0

J = 2

J = 1fFTM

MW-MW double resonance

fDR

fFTM

J = 0

J = 2

J = 1fFTM

fDR

DR cross correlation

Discharge

Magneticfield

C2H2

present

CS2

present0

0

0

00

00

395

7

132817

0

For each line, look for DR link at every other (unique) frequency# scans Nlines

2/2

DR cross correlation – C2H2 magnetic


No DR


Link!


8/17 lines: c-C3H2

Remainder: unassigned.No obvious linkages

DR cross correlation

Discharge

Magneticfield

C2H2

present

CS2

present0

0

0

00

00

395

7

132818

0

Only the 60 strongest lines are shown in this example

DR cross correlation – CS2 + C2H2 discharge

4: C5S4: C4S2: C3S n5=12: C3S n5=12: C13CCS

Remainder: unassigned.Some linkages are likely unassigned vibrational states of molecules above

Spectral taxonomy

Future: 2-26 GHz CP coverage

Spectral taxonomy: A semi-automated combination of chirped- pulse and cavity Fourier transform...

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