DETERMINATION OF METHANOL PHOTOLYSIS BRANCHING RATIOS VIA ROTATIONAL SPECTROSCOPY - PowerPoint Presentation

Slide1

 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, 2017Slide2

Garrod R. T., Widicus Weaver S. L., & Herbst E.

Astrophys

. J., 682, 283-304, 2008.

CH

3

acetone

glycolaldehyde

methyl formate

CH

3

CO

HCO

CH

2

OH + H

CH

3O + HCH3 + OH

hν

CH3OH

H

2

O, CO, CH

3

OH,

NH

3

, H

2

CO

Ice mantle

hν

Radicals are mobile on grain surfaces at T > 20 K and can combine with other radicals.

Methanol Photolysis in the Interstellar Medium (ISM)Slide3

C

OH

C

OH + H

C

O + H

C

+ O

C + OH

HCOH +

CO + 2

CO +

Ö

berg et al.

A&A,

504, 891-913, 2009.Hagege J.,

Roberge P.C., Vermeil C. Trans. Faraday Soc., 64, 3288-3299, 1968.

~75%

~5%

~20%

Hagege et al. (1968) Öberg et al. (2009)

(gas phase, mass spec) (condensed phase, IR)

~73%

~15%

~12%

~0%

Previous Studies of Methanol Photolysis Branching RatiosSlide4

Experimental Setup

Excimer Laser

Cylindrical Focusing Lens

Pulsed Valve with Fused Silica Capillary Tube

Beam Block

Millimeter/Submillimeter Frequency Multiplier

(x3-x27)

Multipass

Optical System

Detector

Oscilloscope

Microwave Synthesizer (250 kHz-50GHz)Slide5

Multipass

Optical PathSlide6

-

-

-

-

-

-

Parent Methanol Reference LinesSlide7

T = 12 ± 5 K

N

methanol =

(1.19 ± 0.03)×1017

cm-2

Rotation Diagram for Parent MethanolSlide8

Laser Photolysis + Methanol DepletionSlide9

T = 19 KN/

N

methanol = (3.7 ± 0.4)×10-5

-

-

-

-

-

Formaldehyde Photolysis ProductSlide10

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. 

Energy (kcal/

mol

)

140

120

100

80

60

40

20

0

Lyman alpha =

235.19 kcal/

mol

Methanol Dissociation Slide11

-

-

Scaling x100

Scaling x10

-

T = 3.7 K

N/

N

methanol

= (6 ± 2)×10

-4

Methoxy Photolysis ProductSlide12

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 convergeT = 0.6 KN/

Nmethanol = (8.1 ± 3)×10-4

Hydroxymethyl Photolysis ProductSlide13

Hagege et al. Öberg et al. This Work

(1968) (2009)

CH3OH + h CH

3 + OH < 5% 12% 5%* 12%* CH3O + H ~75% 15% 39% 36%

CH2OH + H 73% 53% 49% H2CO + H 20% 0% 2% 2%

*assumed

Hagege

et al. Trans. Faraday Soc., 64, 1968

Ö

berg et al. A&A 504, 2009

Methanol Photolysis Branching RatiosSlide14

Determine optimal laser position on tubeTry different Ar/CH3OH ratiosCollect more hydroxymethyl lines, refine fitMeasure other branching ratios for COMs

Try other laser wavelengths, compare branching ratio changes

Conclusions and Future WorkSlide15

Luyao Zou, AJ Mesko, Kevin RoenitzUndergraduates: (on project) Samuel Zinga; (off project) Elena Jordanov, Lindsay Rhoades, Houston SmithPast Group Members: Brian Hays and Jake Laas

NASA Emerging Worlds Award NNX15AH74G

AcknowledgementsSlide16
Slide17

Formula for integrated line intensities:

=

Conversion of Einstein A to B coefficient:

=

Y versus X: ln[(

)(k/(

B

))] versus

=

Inverse of slope is proportional absolute

of molecules in supersonic expansion

The relationship

allows for the determination of relative abundance ratio

Variables

Values

h

Planck constant

c

Speed

of light

A

Einstein A coefficient

Upper State degeneracy

k

Boltzmann

constant

Frequency

(MHz)Number density)Rotational Partition FunctionUpper State EnergyRotational TemperatureVariablesValueshPlanck constant

cSpeed of lightAEinstein A coefficientUpper State degeneracykBoltzmann constantFrequency (MHz)Number densityRotational Partition FunctionUpper State EnergyRotational TemperatureBoltzmann Diagram Analysis