1.  DETERMINATION OF METHANOL PHOTOLYSIS BRANCHING RATIOS VIA ROTATIONAL SPECTROSCOPY
Carson Powers, Morgan N. McCabe, Susanna L. Widicus Weaver
Department of Chemistry, Emory University
Wednesday, June 21, 2017

2. Methanol Photolysis in the Interstellar Medium (ISM)
H2O, CO, CH3OH,
NH3 , H2CO
Ice mantle
Radicals are mobile on grain surfaces at T > 20 K and can combine with other radicals.
glycolaldehyde
methyl formate
CH3CO
HCO
CH2OH + H
CH3O + H
CH3 + OH hν
CH3OH →
The most complex chemistry observed in the interstellar medium (ISM) has been observed in so-called “hot cores”, which are dense and warm regions of gas and dust within which the process of star formation is occurring.
Methanol is ubiquitous in the ISM! If we look at the space around a young star, the space close to the star is typically very diffuse and very hot.
NEW TALK (JUNE 2017): “Let me start off by telling you all why we should even care about methanol. The first point I should mention is that methanol is ubiquitous in the interstellar medium (or ISM). Depending on the distance from a star, the molecules in a source in the ISM may be at different densities, temperatures, and even phases. In the dense clouds, far from the stellar object, methanol frozen out on dust grains can make up between fifteen and twenty percent of the total composition of the ice, and methanol in the condensed phase is of interest to the astrochemical community at large, which brings me to the next slide…”
NEW TALK, DRAFT 3, CONTINUATION: “… and the formation of complex organic molecules (or COMs). Implicit in the study of astrochemistry are attempts at explaining how larger complex organic molecules can form in the ISM. It is difficult for these molecules to form in warmer regions of the ISM (thermal instability of molecules, number density of molecules per cm^3, etc.). Therefore, it is believed that these radical-radical reactions in both the gas and condensed phase are a large contribution to the formation of these COM’s, and these can occur at temperatures as low as 20K in the condensed phase. Photolysis of methanol is therefore believed to be an extremely important driver of this chemistry, and the radicals produced from this reaction can then react with radicals from other reactions to produce these COM’s. Seen below are the reactions of methoxy and hydroxy methyl with formyl radical produced from the photolysis of formaldehyde, which lead to production of glycolaldehyde and methyl formate, which are structural isomers of one another. Methyl radical, after reacting with formyl radical, can then further react with another methyl radical to produce acetone.” → → → → →
CH3 →
acetone
Garrod R. T., Widicus Weaver S. L., & Herbst E. Astrophys. J., 682, , 2008.

3. Previous Studies of Methanol Photolysis Branching Ratios
Hagege et al. (1968) Öberg et al. (2009) (gas phase, mass spec) (condensed phase, IR)
~73%
C 𝐇 𝟑 OH → C 𝐇 𝟐 OH + H → C 𝐇 𝟑 O + H → C 𝐇 𝟐 + 𝐇 𝟐 O → C 𝐇 𝟑 + OH → HCOH + 𝐇 𝟐 → CO + 2 𝐇 𝟐 → 𝐇 𝟐 CO + 𝐇 𝟐
~75%
~15%
~5%
~12%
“Previous work has been carried out on the photolysis of methanol in both the gas phase, and the condensed phase. Hagege et al. conducted a gas phase study using mass spec, and determined the branching ratios for methanol near the peak of the absorption cross-section to be the percentages seen here. Of particular interest is the percentage for the main branch of methanol, seen here to be shared by the radicals methoxy and hydroxymethyl. Because both radicals possess the same mass, the branching ratio cannot be split, and for the reasons mentioned in the last slide, having the ratios between the radicals possess great implications for the diversity of the structural isomers previously mentioned in the ISM. Later studies, performed by Oberg et al. in 2009, observed the gas phase products of several complex organic evaporated from grain surfaces, using IR spectroscopy. However, they chose to back-calculate the branching ratios for these channels using a model which only considers the first generation dissociation and recombination of radicals for methanol, and no further. This, in turn, means that the branching ratios determined were not obtained directly, and because of the multiple dissociation and recombination events occurring on the surface of the ice, the true branching ratio from only the primary dissociation of methanol is unknown. This is where our study enters: Through the use of gas phase, direct absorption rotational spectroscopy, we aim to determine the exact branching ratios for methanol photodissociation, with no secondary dissociation or recombination events occurring. In order to accomplish this, we used the spectrometer design…”
~20%
~0%
Öberg et al. A&A, 504, , 2009.
Hagege J., Roberge P.C., Vermeil C. Trans. Faraday Soc., 64, , 1968.

4. Experimental Setup Pulsed Valve with Fused Silica Capillary Tube
Beam Block
Excimer Laser
Cylindrical Focusing Lens
Oscilloscope
Microwave Synthesizer (250 kHz-50GHz)
“Seen here is the experimental setup my group used in the production of methanol photolysis products. Argon was used as a carrier gas and was bubbled through liquid methanol down the sample line. The sample passed through a fused silica tube fixed on the pulsed valve, and a UV excimer laser at 193 nm was shone across the tube. Photolysis occurs in the tube, and the products are rotationally and vibrationally cooled via supersonic expansion. The expansion is probed with light in the millimeter/submillimeter regime, and the pure rotational spectra of the molecules is obtained via direct absorption spectroscopy.
Multipass Optical System
Detector
Millimeter/Submillimeter Frequency Multiplier (x3-x27)

5. Multipass Optical Path

6. Parent Methanol Reference Lines
3 + 1, ,1
3 + 0, ,2
3 − 1, − 1,1
3 + 0, ,2
1 + 1, ,1
4 + 0, ,3

7. Rotation Diagram for Parent Methanol
T = 12 ± 5 K
Nmethanol =
(1.19 ± 0.03)×1017 cm-2

8. Laser Photolysis + Methanol Depletion

9. Formaldehyde Photolysis Product
3 1, ,2
2 1, ,1
2 0, ,1
2 1, ,0
3 0, ,2
T = 19 K
N/Nmethanol = (3.7 ± 0.4)×10-5

10. Methanol Dissociation
Energy (kcal/mol)
140
120
100 80 60 40 20
COLLISION INDUCED DISSOCIATION/collisional quenching
Lyman alpha =
kcal/mol
Hays B. M., Wehres N., Alligood DePrince B. A., Roy A. L. M., Laas J. C., & Widicus Weaver S. L.  Chem. Phys. Lett., 630, 18-26, 2015. 

11. Methoxy Photolysis Product
Scaling x100
Scaling x10
3 0, ,0
3 2, ,0
3 −1, ,0
T = 3.7 K
N/Nmethanol = (6 ± 2)×10-4

12. Hydroxymethyl Photolysis Product
T = 0.6 K
N/Nmethanol = (8.1 ± 3)×10-4
Based on frequencies reported by Bermudez, Bailleux and Cernicharo, 2016, A&A, 598, A9.
Caveats:
-- No line strength information included
-- Our fit of the reported lines does not converge

13. Methanol Photolysis Branching Ratios
Hagege et al. Öberg et al. This Work
(1968) (2009)
CH3OH + hν CH3 + OH < 5% % 5%* %*
CH3O + H ~75% % % %
CH2OH + H % % %
H2CO + H % % 2% 2%
*assumed
Hagege et al. Trans. Faraday Soc., 64, 1968
Öberg et al. A&A 504, 2009

14. Conclusions and Future Work
Determine optimal laser position on tube
Try different Ar/CH3OH ratios
Collect more hydroxymethyl lines, refine fit
Measure other branching ratios for COMs
Try other laser wavelengths, compare branching ratio changes

15. Acknowledgements Luyao Zou, AJ Mesko, Kevin Roenitz
Undergraduates: (on project) Samuel Zinga; (off project) Elena Jordanov, Lindsay Rhoades, Houston Smith
Past Group Members: Brian Hays and Jake Laas
NASA Emerging Worlds Award NNX15AH74G

17. Boltzmann Diagram Analysis
Variables
Values h
Planck constant c
Speed of light A
Einstein A coefficient
g 𝑢
Upper State degeneracy k
Boltzmann constant ν
Frequency (MHz)
𝑁 𝑇
Number density
𝑄( 𝑇 𝑟𝑜𝑡 )
Rotational Partition Function
𝐸 𝑢
Upper State Energy
𝑇 𝑟𝑜𝑡
Rotational Temperature
Formula for integrated line intensities: −∞ ∞ 𝐼 𝑏 𝑑 ν = ℎ 𝑐 3 𝐴 𝑔 𝑢 8𝜋𝑘 ν 2 𝑁 𝑇 𝑄( 𝑇 𝑟𝑜𝑡 ) 𝑒 − 𝐸 𝑢 /𝑘 𝑇 𝑟𝑜𝑡
Conversion of Einstein A to B coefficient: 𝐴 1→0 = 𝐵 1←0 8𝜋ℎ ν 3 𝑐 3
Y versus X: ln[( −∞ ∞ 𝐼 𝑏 𝑑 ν)(k/( ℎ 2 νB 𝑔 𝑢 ))] versus 𝐸 𝑢 = 𝐸 1 +ℎν
Inverse of slope is proportional absolute 𝑇 𝑟𝑜𝑡 of molecules in supersonic expansion
The relationship 𝑒 𝑦−𝑖𝑛𝑡𝑒𝑟𝑐𝑒𝑝𝑡 𝑄( 𝑇 𝑟𝑜𝑡 ) allows for the determination of relative abundance ratio
degenerate if it corresponds to two or more different measurable states of a quantum system.