Spectral taxonomy: A semi-automated combination of chirped- pulse and cavity Fourier transform microwave spectroscopy Kyle 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 years Cations : HNNO + / NNOH +, H 2 NCO + Anions : C 4 H -, C 6 H -, C 8 H - Radicals : HOCO, HO 3, C 4 H, HC 3 O Exotic : Si 3, HOON, CH 2 OO, HOCOH

Cavity FTM spectrometer  Fourier transform spectrometer (5-26 GHz, 1 MHz bandwidth )  Supersonic expansion discharge nozzle  High sensitivity, high resolution Dilute gaseous precursors - HV

CP-FTMW spectrometer 20 GHz, 50 GS / s scope 24 GS / s AWG +38 dB +30 dB GHz PLDRO 10 MHz Rb Standard LP Filter GHz ; 4  sec duration Step Atten 0-70 dB 10 pulses ;  20  s t =0 t  250  s Real - time phase scoring and correction >1 M co - averages routine Linux DAQ software GHz 200 W GHz TWTA Data 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: C 2 H 2 + CS 2 discharge 150 k gas pulses, 1.5 M chirps : ~ Hz 1% C 2 H 2, 1% C 2 S, 98% Ne 1 kV discharge

Example: C 2 H 2 + CS 2 discharge  495 features with SNR > 3  Of the strongest 34 features ( SNR > 50):  13 artifacts ( LO ; harmonics ; scope spurs )  10 assigned ( C 3 S, c - C 3 H 2, SO 2, HC 3 S, C 5 S, H 2 C 3 S, C 2 S )  11 unidentified 150 k gas pulses, 1.5 M chirps : ~ Hz 1% C 2 H 2, 1% C 2 S, 98% Ne 1 kV discharge

Chirped-pulse vs. cavity Chirped - pulse 8.5 h

Chirped-pulse vs. cavity Chirped - pulse Cavity 8.5 h 11 h

Chirped-pulse vs. cavity Chirped - pulse Cavity 8.5 h 11 h 8.5 h 9 s ( h )

Chirped-pulse vs. cavity Chirped - pulse Cavity 8.5 h 11 h 8.5 h 9 s ( h )  Bandwidth  Intensity accuracy  Resolution  Frequency accuracy  Sensitivity  Bandwidth  Intensity accuracy  Resolution  Frequency accuracy  Sensitivity

Chirped-pulse + cavity Chirped - pulse Cavity

Chirped-pulse + cavity Chirped - pulse Cavity

Chirped-pulse + cavity Chirped - pulse Cavity

Automated analysis with cavity QtFTM : Cavity control software designed for programmatic, automated acquisition

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 Magnetic field Gas 2 present Gas 1 present Buffer gas impurities Gas A cmplx or impurity Gas B cmplx or impurity Closed - shell intermed / prod Open - shell I / P

Example acquisition: Discharge test Discharge enabled Discharge disabled

Example acquisition: Discharge test

DC

Taxonomy flowchart Yes No YesNoYesNo YNNYYNYN YNYNYNYNYNYNYNYN Detected in cavity ? Discharge ? Magnetic ? CS 2 only ? C 2 H 2 only ?

Taxonomy flowchart Yes No YesNoYesNo YNNYYNYN YNYNYNYNYNYNYNYN Detected in cavity ? Discharge ? Magnetic ? CS 2 only ? C 2 H 2 only ? Non - detections : 330/495 ( past expt : 20%)

Taxonometric classification Discharge Magnetic field C 2 H 2 present CS 2 present

Taxonometric classification Discharge Magnetic field C 2 H 2 present CS 2 present Next step : cross - correlation within groups using double resonance

MW-MW double resonance f FTM J = 0 J = 2 J = 1 f FTM

MW-MW double resonance f DR f FTM J = 0 J = 2 J = 1 f FTM f DR

MW-MW double resonance f DR f FTM J = 0 J = 2 J = 1 f FTM f DR

MW-MW double resonance f DR f FTM J = 0 J = 2 J = 1 f FTM f DR

DR cross correlation Discharge Magnetic field C 2 H 2 present CS 2 present For each line, look for DR link at every other ( unique ) frequency # scans  N lines 2 /2

DR cross correlation – C 2 H 2 magnetic

No DR

DR cross correlation – C 2 H 2 magnetic Link !

DR cross correlation – C 2 H 2 magnetic 8/17 lines : c - C 3 H 2 Remainder : unassigned. No obvious linkages

DR cross correlation – C 2 H 2 magnetic 8/17 lines : c - C 3 H 2 Remainder : unassigned. No obvious linkages

DR cross correlation Discharge Magnetic field C 2 H 2 present CS 2 present Only the 60 strongest lines are shown in this example

DR cross correlation – CS 2 + C 2 H 2 discharge 4: C 5 S 4: C 4 S 2: C 3 S 5 =1 2: C 13 CCS Remainder : unassigned. Some linkages are likely unassigned vibrational states of molecules above

DR cross correlation – CS 2 + C 2 H 2 discharge 4: C 5 S 4: C 4 S 2: C 3 S 5 =1 2: C 13 CCS Remainder : unassigned. Some linkages are likely unassigned vibrational states of molecules above

Spectral taxonomy Future : 2-26 GHz CP coverage