Low-Energy (220 eV) electrons or photons

P = 1109 Torr

20 200 ÅNH3

T = 90 K

D(NH) ≈ 4.1 eV

Radiolysis of Cosmic Ice Analogs of Ammonia, an Interstellar Hydride

Leslie Gates and Chris ArumainayagamWellesley College, Massachusetts, USA

electron stimulated desorption

post-irradiation analysis

High-Energy (1000 eV) electrons

Phys. Chem. Chem. Phys., 2010,12, 5947-5969DOI: 10.1039/B917005G

Interstellar synthesis of prebiotics: Widely Accepted Hypothesis

Dark dense molecular clouds

thin ice layers (10 K)

bare silicaceous or carbonaceous interstellar dust grain

H2 cloud

0.1 μm

Origins of UV light in dark, dense molecular clouds

UV light from stars cannot penetrate dark, dense molecular clouds

icy interstellar dust grain

nearby star

UV light formation within dark, dense molecular clouds

UV light3-12 eV

Cosmic Ray107-1020 eV

icy interstellar dust grain

Low-energy (< 20 eV) electrons

Proposed HypothesisUV photolysis in

cosmic ices

NH3

NH3*

H, NH2

UV

excitation

radical formation

radical-radical reactions

N2H4

HHN

H

HHN

H

Our Hypothesis

Low-energy electrons (< 20 eV)could play a significant role in thesynthesis of “complex” organicmolecules previously thought toform exclusively via photons

Cosmic ray107-1020 eV

secondary electron cascade (0-20 eV)

Formation of secondary electrons in cosmic ices and dust grains

bare silicaceous or carbonaceous

interstellar dust grain

thin (~100 ML) ice layers (10 K)

Secondary Electron “Tail”

Dissociation Cross-Section

Dissociation Yield

DEA Resonances Ionization

and

Excitation

Importance of Low-Energy Electrons

C. Arumainayagam et al., Surface Science Reports 65 (2010) 1‒44.

1 MeV particle → 100,000 low-energy electrons

Electron-induced dissociation mechanisms

AB + e–

AB* + e–

AB–*

AB+* + 2e–

A* + B

A+ + B–

AB + energy

A● + B+*

A●+ B–

AB– + energy

Electron Impact Excitation

Electron Attachment

Electron Impact

Ionization

AutodetachmentDissociative

Electron Attachment

Associative Attachment

Fragmentation

Dipolar Dissociation

< 15 eV

> 3 eV

> 10 eV

(B ) (A B) (B)H D EA

“Thermodynamic Threshold”

*

C. Arumainayagam et al., Surface Science Reports 65 (2010) 1‒44.

How to break a 5 eV bond with a 3 eV electron?

Energetics of Photochemistry

EE

∆E

Another Key Difference between Photons and Electrons

hν

e–

π*

n

π

π*

n

π

π*

n

π

HOMO

LUMO

π*

n

π

LUMO

HOMO

Low-energy electron-induced radiolysis in

cosmic ices

NH3

NH3*

H, NH2

excitation

radical formation

radical-radical reactions

N2H4

HHN

H

HHN

H

low-energy electrons

cosmic ray

Ammonia

Liquid N2 Port

Ta(110)

e-

Mass Spec

Filament

PowerSupply

Products

Isothermal Electron Stimulated Desorption

Time (sec)

Mas

s Sp

ec S

ign

al

Post-Irradiation Temperature

Programmed Desorption

Temperature (K)

Mas

s Sp

ec S

ign

al

Experimental Procedures

4000 3500 3000 2500 2000 1500 1000

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Inte

nsi

ty

Wavenumbers (cm1

)

Nonirradiated

Irradiated

IRAS

Why study Ammonia?

Öberg, K., et al. “The Spitzer Ice Legacy: Ice Evolution from cores to protostars.” The Astrophysical Journal 740(2011): 16 pp.

Hydrazine

Diazene

N-3 Species

N-2 Species

Possible Radiolysis Products of Ammonia

100 150 200 250 300 350 400 450 500

m/z 36 (N2D

4

+)

m/z 34 (N2D

3

+)

m/z 32 (N2D

2

+)

Mas

s S

pec

Sig

nal

Temperature (K)

Film Thickness: 100 ML

Electron Energy: 1000 eV

Electron Dose: 8.5 C

ND3

Detection of Hydrazine and Diazene at High Incident Electron Energies

100 150 200 250 300 350 400 450 500

m/z 36 (N2D

4

+)

m/z 34 (N2D

3

+)

m/z 32 (N2D

2

+)

Mas

s S

pec

Sig

nal

Temperature (K)

N2D

4 Film Thickness: 100 ML

Electron Energy: 1000 eV

Electron Dose: 8.5 C

ND3

Katie Shulenberger `14 (Wellesely College)

100 150 200 250 300 350 400 450 500

N2D

2 m/z 36 (N2D

4

+)

m/z 34 (N2D

3

+)

m/z 32 (N2D

2

+)

Mas

s S

pec

Sig

nal

Temperature (K)

N2D

4 Film Thickness: 100 ML

Electron Energy: 1000 eV

Electron Dose: 8.5 C

ND3

Proposed Mechanisms of Hydrazine and Diazene from Ammonia

Zheng, W. et al. The Astrophysical Journal. 674:1242-1250, 2008 February 20

Hydrazine (N2H4)

Diazene (N2H2)

+●NH●

●NH●

+ H2

*

+

𝒅𝑰

𝒅𝒕= 𝒌𝟏𝑹 − 𝒌𝟐𝑰𝟐

𝒅𝑹

𝒅𝒕= −𝒌𝟏𝑹 𝑹(𝒕) = 𝑹𝟎𝒆−𝒌𝟏𝒕

𝑰 𝒕 =?

𝑷 𝒕 =?

𝑅𝑘1

𝐼

𝐼 + 𝐼𝑘2

𝑃

𝒅𝑷

𝒅𝒕=

𝒌𝟐

𝟐𝑰𝟐

Model: Bimolecular Intermediate Step

Katherine Tran ’15 (Wellesley College)

Results: Hydrazine Yield vs Electron Fluence

0 100 200 300 400 500

Hy

dra

zin

e Y

ield

-- k1 = 0.15 s-1; k2 = 40 cm

2s

-1molecule

-1

Film Thickness: 500 ML

Electron Energy: 1000 eV

Transmitted Current: 3.3A

Fluence (C/cm2 )

Numeric Fit

Results: Yield vs Film Thickness

0 50 100 150 200

Hyd

razin

e Y

ield

Film Thickness (ML)

Electron Energy: 1000 eV

Dose Rate: 0.33 C/s

Electron Dose: 33 CQuadratic fit

Production of Hydrazine and Diazene at Incident Electron Energy of 10 eV at 90 K

N2H2

N2H4

Production of Hydrazine at Incident Electron Energy of 7 eV at 25 K

Leon Sanche, Andrew Bass & Sasan Esmaili

1000 eV7 eV

Importance of Surface Temperature

Hydrazine

Desorption of ●NH2

• Electron–induced reaction of ammoniayields hydrazine (N2H4) and diazene(N2H2) with high-(1000 eV) or low-energy(7-20 eV) electrons.

• The results are consistent with ourhypothesis that high energy radiolysis ismediated by low-energy electrons

Final Conclusions

Acknowledgements

Collaborators

Dr. Andrew Bass, Université de Sherbrooke

Sasan Esmaili, Université de Sherbrooke

Leon Sanche, Université de Sherbrooke

Dr. Petra Swiderek, University of Bremen

Esther Bohler, , University of Bremen

Former lab membersKatie Shulenberger ‘14Katherine D Tran ’15Sebiha Abdullahi ‘15Jane L. Zhu ’16Carina Belvin ‘16Kathleen Regovish ‘16Lauren Heller ‘17Milica Markovic ‘17

Helen Cumberbatch Julia LukensZoe PeelerAlice ZhouJyoti CampbellKendra CuiJean HuangHope SchneiderAnna Caldwell-OverdierJeniffer Perea

Ally BaoChristina BuffoEliana MarosicaMayla ThompsonRhoda Tano-MenkaStephanie VillafaneSubha BaniyaElla MullikinAlly BaoJustine HuangAury Hay

Current Lab Members

Funding SourceNational Science Foundation(CHE-1465161, CHE-1012674, CHE-1005032)

• Radical-radical reactions: no energy barrier• PE falls monotonically as distance ↓• Radical energy ↑ ⇒ Reaction probability ↓• Temperature ↑ ⇒ Rate constant ↓

Assume reaction is not diffusion limited

0 10 20 30 40 50 60

Irradiation Time (s)

Dia

zen

e Y

ield

Film Thickness: 500 ML ND3

Electron Energy: 1000 eV

Transmitted Current: 3.3 A

Results: Yield vs Irradiation Time

Bunsen-Roscoe Law of Photochemistry

A photochemical effect is directly proportional to the total energy dose, irrespective of the time required to deliver that dose

Despite constant electron dose across experiments, we observe varying ammonia radiolysis product yields

Dose Rate Effect in Radiation Chemistry

U.S. Department of Health & Human Services

Hydrazine Data

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

0.00E+000

5.00E+008

1.00E+009

1.50E+009

2.00E+009

2.50E+009

3.00E+009

Hyd

razin

e Y

ield

Incident Current (A)

Film Thickness: 500 ML ND3

Electron Energy: 1000 eV

Fluence: 132 C

Diazene Data

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2

0.00E+000

5.00E+008

1.00E+009

1.50E+009

2.00E+009

2.50E+009

3.00E+009

Dia

zen

e Y

ield

Incident Current (A)

Film Thickness: 500 ML ND3

Electron Energy: 1000 eV

Fluence: 132 C