Low-Energy Electrons in Astrochemistry Chris Arumainayagam Wellesley College electron stimulated desorption Low-Energy (220 eV) electrons D(C‒O) ≈ 3.5 eV postirradiation analysis ~20-200 Å CH3OH,NH3,H2O Ta(110) P = 1109 Torr T = 90 K Phys. Chem. Chem. Phys., 2010,12, 5947-5969 DOI: 10.1039/B917005G Our Hypothesis Low-energy electrons (< 20 ev) could play a significant role in the synthesis of “complex” organic molecules previously thought to form exclusively via photons Formation of secondary electrons in cosmic ices and dust grains secondary electron cascade (0-20 eV) thin (~ 100 ML) ice layers (10 K) Cosmic ray 107-1020 eV bare silicaceous or carbonaceous interstellar dust grain Importance of Low-Energy Electrons Secondary Electron “Tail” Dissociation Cross-Section Dissociation Yield 1 MeV particle → 100,000 low-energy electrons DEA Resonances Ionization and Excitation C. Arumainayagam et al., Surface Science Reports 65 (2010) 1‒44. Electron-induced dissociation mechanisms Electron Impact Excitation > 3 eV AB + e– Electron Attachment AB* + e– Autodetachment Dissociative Attachment AB–* Associative Attachment < 15 eV Electron Impact Ionization ~ 10 eV Dipolar Dissociation AB+* + 2e– Fragmentation A* + B A+ + B– AB + energy A●+ B– AB– + energy A● + B+* How to break a 5 eV bond with a 3 eV electron? “Thermodynamic Threshold” AB + e- A + * B H (B ) D(A B) EA(B) Another Key Difference between Photons and Electrons LUMO hν HOMO LUMO π* π* n n π π π* π* e– HOMO n n π π Low-energy electron-induced radiolysis in cosmic ices radical formation CH3OH* excitation CH3OH low-energy electron radical-radical reactions H, CH2OH, CH3O HOCH2CH2OH 0.14 Experimental Procedures 0.10 0.08 0.06 Irradiated 0.04 0.02 0.00 Liquid N2 Port Time (sec) e- Mass Spec Ta(110) Post-Irradiation Temperature Programmed Desorption Temperature (K) Nonirradiated -0.02 4000 3500 3000 Methanol 2500 2000 1500 1 Wavenumbers (cm ) Filament Products Mass Spec Signal IRAS 0.12 Intensity Mass Spec Signal Isothermal Electron Stimulated Desorption Power Supply 1000 PART I RADIOLYSIS OF METHANOL Post-Irradiation Temperature-Programmed Desorption 12CH OH on Mo(110) 3 C.R. Arumainayagam, et. al. J. Phys. Chem., 99 (1995) 9530 Conclusion 1 Post-irradiation temperature programmed desorption is useful for identifying labile radiolysis products (e.g., CH3OCH2OH) Methoxymethanol Yield (Arb. Units) Radiolysis Yield vs. Incident Electron Energy Methoxymethanol: CH3OCH2OH (m/e = 61) CH3O● + 1.80E+008 ●CH₂OH → CH3OCH₂OH 1.60E+008 1.40E+008 1.20E+008 1.00E+008 8.00E+007 Threshold: 6 – 7 eV 6.00E+007 4.00E+007 2.00E+007 0.00E+000 -2.00E+007 2 4 6 8 10 12 14 16 18 20 22 Electron Energy (eV) Boyer, Boamah, Sullivan, Arumainayagam, Bass, Sanche, (J. Phys. Chem. C, 2014, 118, 22592-22600) Conclusion 2 • Dissociative electron attachment may not play an importance role in radiation-induced chemical synthesis reactions of methanol • Radical-radical reactions are the likely mechanism for the formation of ethylene glycol and methoxymethanol • Barrier-less radical-radical reactions may be rapid in interstellar ices because of the low temperatures (10 to 100 K). Methanol Radicals Radical 1 •CH2OH •HCO CH3O• •CH3 •OH •H •CH2OH HOCH2CH2OH Ethylene Glycol CH2OHCHO Glycolaldehyde CH3OCH2OH Methoxymethanol CH3CH2OH Ethanol HOCH2OH Methylene Glycol CH3OH Methanol •HCO CH2OHCHO Glycolaldehyde CHOCHO Glyoxal CH3OCHO Methyl Formate CH3CHO Acetaldehyde HCOOH Formic Acid H2CO Formaldehyde CH3O• CH3OCH2OH Methoxymethanol CH3OCHO Methyl Formate CH3OOCH3 Dimethyl Peroxide CH3OCH3 Dimethyl Ether CH3OOH Methyl Hydroperoxide CH3OH Methanol •CH3 CH3CH2OH Ethanol CH3CHO Acetaldehyde CH3OCH3 Dimethyl Ether CH3CH3 Ethane CH3OH Methanol CH4 Methane •OH HOCH2OH Methylene Glycol HCOOH Formic Acid CH3OOH Methyl Hydroperoxide CH3OH Methanol HOOH Hydrogen Peroxide H2O Water •H CH3OH Methanol H2CO Formaldehyde CH3OH Methanol CH4 Methane H2O Water H2 Dihydrogen Radical 2 What is the difference between a photon and an electron? •CH2OH H2CO HCO CO CO2 CH4 HCOOH CH3OCHO CH3OCH3 HOCH2CH2OH CH3OCH2OH HCOCH2OH CH3CH2OH C2H6 CH3CHO CH3COOH Electrons (≤20 eV) UV Study1 1Oberg et. al. (A&A 504, 891–913 (2009)) Conclusion 3 • Post-irradiation temperature programmed desorption can be used to identify components in a complex mixture of radiolysis products • The identified electron-induced methanol radiolysis products include many that have been previously identified as being formed by methanol UV photolysis in the interstellar medium • Post-irradiation temperature programmed desorption results cannot be used to conclude if identified products are nascent radiolysis products Irradiated Methanol: IRAS Product Analysis 100 monolayers CH3OH at 90 K irradiated with 14 eV electrons 2400 µC Sullivan, Boamah, Shulenberger, Chapman, Atkinson, Boyer, Arumainayagam; Monthly Notices of the Royal Astronomical Society (doi: 10.1093/mnras/stw593) Conclusion 4 • Thermal processing above 90 K not necessary for product formation • IRAS not as effective as TPD for identifying species in complex product mixture PART II RADIOLYSIS OF AMMONIA Why study ammonia? Öberg, K., et al. “The Spitzer Ice Legacy: Ice Evolution from cores to protostars.” The Astrophysical Journal 740(2011): 16 pp. Possible Radiolysis Products of Ammonia Hydrazine N-3 Species Diazene N-2 Species Detection of Hydrazine and Diazene at High Incident Electron Energies Mass Signal Signal Spec SpecSignal Mass MassSpec N2D4 Film Thickness: 100 ML Electron Energy: 1000 eV Electron Dose: 8.5 C ND3 3 N2D2 + m/z 36 (N22D44+) 2 4 + m/z 34 (N22D33++) 2 3 + m/z 32 (N22D22++) 2 2 100 100 150 150 200 200 250 250 300 300 350 350 400 400 450 450 500 500 Temperature (K) Katie Shulenberger `14 (Wellesely College) Radiolysis Products of Ammonia Hydrazine (N2H4) + Diazene (N2H2) * ●NH + ●NH ● ● + H2 Zheng, W. et al. The Astrophysical Journal. 674:1242-1250, 2008 February 20 Model: Bimolecular Intermediate Step 𝑅 𝐼+𝐼 𝒅𝑹 = −𝒌𝟏 𝑹 𝒅𝒕 𝒅𝑰 = 𝒌𝟏 𝑹 − 𝒌𝟐 𝑰𝟐 𝒅𝒕 𝒅𝑷 𝒌𝟐 𝟐 = 𝑰 𝒅𝒕 𝟐 𝑘1 𝐼 𝑘2 𝑃 𝑹(𝒕) = 𝑹𝟎 𝒆−𝒌𝟏𝒕 𝑰 𝒕 =? Katherine Tran ’15 (Wellesley College) 𝑷 𝒕 =? Results: Yield vs Fluence Hydrazine Yield Film Thickness: 500 ML Electron Energy: 1000 eV Transmitted Current: 3.3A -- k1 = 0.15 s-1; k2 = 40 cm2s-1molecule-1 0 100 200 300 Fluence (C/cm2 ) 400 500 Hydrazine Yield Results: Yield vs Film Thickness Electron Energy: 1000 eV Dose Rate: 0.33 C/s Electron Dose: 33 C 0 50 100 150 Film Thickness (ML) 200 Results: Low Energy Experiments Leon Sanche, Andrew Bass & Sasan Esmaili Final Conclusions • Low-energy (< 20 eV) electroninduced condensed phase reactions may contribute to the interstellar synthesis of “complex” molecules previously thought to form exclusively via UV photons • Molecules such as methoxymethanol may serve as tracer molecules for the differences between photon- and electron-induced reactions. Acknowledgements Former Lab Members Collaborators Dr. Léon Sanche, Université de Sherbrooke Dr. Andrew Bass, Université de Sherbrooke Dr. Petra Swiderek, University of Bremen Dr. Michael Boyer, Clark University Funding Source Current Lab Members National Science Foundation (CHE-1465161, CHE-1012674, and CHE 1005032) Jane Zhu Clarissa Verish Helen Cumberbatch Julia Lukens Zoe Peeler Alice Zhou Jyoti Campbell Kendra Cui Mebatsion Gebre Jean Huang Hope Schneider Aida Abou-Zamzam Ally Bao Christina Buffo Eliana Marosica Leslie Gates Mayla Thompson Rachel Kim Rhoda Tano-Menka Stephanie Villafane Subha Baniya
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