Transcript Laas - OSU MSS 68 FINAL.pptx

A mm/submm Wave Spectrometer
to Quantify Astrochemical
Reaction Rates
Jacob C. Laas, Brian M. Hays, & Susanna L. Widicus Weaver
Department of Chemistry, Emory University, Atlanta, GA 30322
Motivation
Observational astronomy is becoming “data enabled”,
and the molecular inventory of ISM is diverse
SOFIA (NASA/DLR)
Herschel (ESA)
ALMA (ESO/NAOJ/NRAO), photo credit: J. Guarda (ALMA)
Chemical models rely on quantitative rate information
 laboratory support plays central role
Methanol as Case Study
hν
Ice mantle
CO H
2
H2 O H2
H2
H2
H
HCO+
H2
N2H+
H2
H2 O
NH3
CH3OH
H2
H2
H2
CH3CH2OH
H2
Dust grain
HCO+ H
H2CO
H2
H2
H2CO
CH3CN
H2
H2
H2
CO
CH3NH2
CH3COCH3
NH2CHO
CH3OCHO
HCOOH
H2 O
H2O, CH3OH, CO,
NH3 , H2CO, etc…
CH3OH
hν
CH2OH + H
CH3O + H
CH3 + OH
H2CO + H2
CH2OH
CH3O
CH3
HCO
HCOCH2OH
HCOOCH3
-H
CH3CHO
+OH
CH3COOH
Methanol as Case Study
Branching ratios have
yet to be quantitatively
measured
CH3OH
hν
CH2OH + H
CH3O + H
CH3 + OH
H2CO + H2
CH3O & CH2OH
need more laboratory
spectral information
CH2OH
CH3O
CH3
HCO
HCOCH2OH
HCOOCH3
-H
CH3CHO
+OH
CH3COOH
Scientific Approach
1.
Build mm/submm spectrometer
2.
Benchmark spectrometer
–
–
radical prep/detection
sensitivity
3.
Spectral support of photodissociation products
4.
Observe and quantify methanol photodissociation products
Experimental Design
• mm/submm wave source is analog signal
generator + frequency multiplier kit to access
50-1000 GHz
• Spectral beam is coupled to multipass optical
cell (Perry cell) to give ~7 passes
• Spectroscopy is performed 2-2.5 cm
downstream from valve
Experimental Design
• No other differences
– both scans show <20 K rot. temp.
– no line broadening
Normalized Intensities (arb.)
• Multiple passes enables 3- to 5-fold increase in SNR
30
Single Pass
20
10
0
-10
-20
Multiple Passes
-30
145.100
145.101
145.102
Methanol spectral comparison
Single pass
Laas, Hays, & Widicus Weaver 2013, JPC A, online only
7 passes
Spectral Analysis
• Direct absorption enables quantification of sample (Beer’s Law)
• Boltzmann analysis is performed via rotation diagram
– Uncalibrated integrated intensities preclude absolute, but not relative concentrations
– Linearity of graph ensures LTE is reasonable approximation
(𝜈ℎ2 𝐵𝑔𝑢
+
3000
Intensity (arb. unit)
3-1,3 - 2-1,2
2000
30,3 - 20,2
31,3 - 21,2
blended:
32,3 - 22,2 &
3 - 2,3-2-2,2
1000
-
32,3 - 22,2
+
32,3 - 22,2
-
0
-1000
145.094
145.100
145.124 145.128 145.132
Frequency (GHz)
+
Trot = 15.1 ± 0.8 K
ln 𝐼𝑛𝑡. 𝑘
+
30,3 - 20,2
𝐸𝑢𝑝𝑝𝑒𝑟 (𝐾)
High-Voltage Discharge Study
• A high-voltage plasma discharge was performed on methanol to test ability of
spectrometer to prepare, detect, and quantify highly-reactive dissociation
products
– H2CO and CH3O as previously reported products (Melnik et al., JCP, 2011)
• Discharge source is based on design by McCarthy et al. (2000-present)
High-Voltage Discharge Study
• Both H2CO and CH3O have been routinely detected under this scheme
–
–
+600 HV is applied to front electrode
~1% CH3OH in Ar
CH3O
• Detected via (2x) LIAs
Intensity (arb.)
Intensity (arb.)
H2CO
Freq. (GHz)
Freq. (GHz)
High-Voltage Discharge Study
• Line intensities exhibit cold rotational temperature
– negligible change to methanol with discharge running
• Dissociation products detected products @ ~0.03% wrt CH3OH
CH3O
ln 𝐼𝑛𝑡. 𝑘
(𝜈ℎ2 𝐵𝑔𝑢
H2CO
Trot = 14 ± 5 K
Trot = 5.4 ± 2.7 K
𝐸𝑢𝑝𝑝𝑒𝑟 (𝐾)
Ongoing and Future Work:
Photodissociation
• Astrochemical modeling predicts
important role in molecular clouds
• No concrete detection of CH3O via UV
discharge lamp or excimer laser
0.6
?
0.4
0.2
0.0
-0.2
-0.4
137.4500
137.4505
Frequency (GHz)
137.4510
Ongoing and Future Work:
CH2OH spectral study
1500
• No rotational spectrum yet
10 K
100 K
1000
• Spectroscopic constants via
high-resolution IR study
(~0.0005 cm-1 accuracy)
(Roberts et al., JPC A, 2013)
Intensity (arb.)
500
0
-500
-1000
-1500
100
200
300
Frequency (GHz)
400
500
Ongoing and Future Work
• Experimental setup has been benchmarked and promises
to be a powerful tool for lab-astro support
– spectrometer design is general and versatile for other applications
• Characterize CH2OH rotational spectrum
• Search for CH2OH in astronomical data
• Photodissociation via stronger UV source and more spectral data
Acknowledgements
Widicus Weaver group (Emory)
T. Orlando (GA Tech)
M. Heaven (Emory)
E. Herbst (UVA)
J. Bowman (Emory)
$$$:
NASA HSO OT1 Analysis
Program (No. 1428755)
NASA APRA (NNX11AI07G)
NSF Career (CHE-1150492)