Low-Energy Electrons in Astrochemistry

Transcription

1 Low-Energy Electrons in Astrochemistry electron stimulated desorption Chris Arumainayagam Wellesley College Low-Energy (2 20 ev) electrons D(C O) 3.5 ev ~ Å CH 3 OH,NH 3, H 2 O Ta(110) postirradiation analysis P = Torr T = 90 K

2 Phys. Chem. Chem. Phys., 2010,12, DOI: /B917005G

3 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

4 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 ev bare silicaceous or carbonaceous interstellar dust grain

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6 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.

7 Electron-induced dissociation mechanisms AB + e Electron Impact Excitation > 3 ev Electron Attachment < 15 ev AB * + e AB * Dipolar Dissociation Autodetachment Dissociative Attachment Associative Attachment A * + B A + + B AB + energy A + B AB + energy Electron Impact Ionization AB +* + 2e Fragmentation A + B +* ~ 10 ev

8 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)

9 Another Key Difference between Photons and Electrons LUMO π* π* HOMO n hν n π π LUMO π* π* HOMO n e n π π

10 Low-energy electron-induced radiolysis in cosmic ices radical-radical reactions H, CH 2 OH, CH 3 O HOCH 2 CH 2 OH radical formation CH 3 OH* excitation CH 3 OH low-energy electron

11 Mass Spec Signal Mass Spec Signal Intensity Isothermal Electron Stimulated Desorption Experimental Procedures IRAS Irradiated 0.02 Time (sec) Liquid N 2 Port 0.00 Nonirradiated Wavenumbers (cm 1 ) e - Filament Mass Spec Ta(110) Products Power Supply Post-Irradiation Temperature Programmed Desorption Methanol Temperature (K)

12 PART I RADIOLYSIS OF METHANOL

13 Post-Irradiation Temperature-Programmed Desorption 12 CH 3 OH on Mo(110) C.R. Arumainayagam, et. al. J. Phys. Chem., 99 (1995) 9530

14 Conclusion 1 Post-irradiation temperature programmed desorption is useful for identifying labile radiolysis products (e.g., CH 3 OCH 2 OH)

15 Methoxymethanol Yield (Arb. Units) Radiolysis Yield vs. Incident Electron Energy Methoxymethanol: CH 3 OCH 2 OH (m/e = 61) 1.80E E E E E E E E E E E+007 CH 3 O + CH₂OH CH 3 OCH₂OH Electron Energy (ev) Threshold: 6 7 ev Boyer, Boamah, Sullivan, Arumainayagam, Bass, Sanche, (J. Phys. Chem. C, 2014, 118, )

16 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).

17 Methanol Radicals Radical 1 Radical 2 CH 2 OH HCO CH 3 O CH 3 OH H CH 2 OH HOCH 2 CH 2 OH Ethylene Glycol CH 2 OHCHO Glycolaldehyde CH 3 OCH 2 OH Methoxymethanol CH 3 CH 2 OH Ethanol HOCH 2 OH Methylene Glycol CH 3 OH Methanol HCO CH 2 OHCHO Glycolaldehyde CHOCHO Glyoxal CH 3 OCHO Methyl Formate CH 3 CHO Acetaldehyde HCOOH Formic Acid H 2 CO Formaldehyde CH 3 O CH 3 OCH 2 OH Methoxymethanol CH 3 OCHO Methyl Formate CH 3 OOCH 3 Dimethyl Peroxide CH 3 OCH 3 Dimethyl Ether CH 3 OOH Methyl Hydroperoxide CH 3 OH Methanol CH 3 CH 3 CH 2 OH Ethanol CH 3 CHO Acetaldehyde CH 3 OCH 3 Dimethyl Ether CH 3 CH 3 Ethane CH 3 OH Methanol CH 4 Methane OH HOCH 2 OH Methylene Glycol HCOOH Formic Acid CH 3 OOH Methyl Hydroperoxide CH 3 OH Methanol HOOH Hydrogen Peroxide H 2 O Water H CH 3 OH Methanol H 2 CO Formaldehyde CH 3 OH Methanol CH 4 Methane H 2 O Water H 2 Dihydrogen

18 What is the difference between a photon and an electron? Electrons ( 20 ev) UV Study 1 CH 2 OH H 2 CO HCO CO CO 2 CH 4 HCOOH CH 3 OCHO CH 3 OCH 3 HOCH 2 CH 2 OH CH 3 OCH 2 OH HCOCH 2 OH CH 3 CH 2 OH C 2 H 6 CH 3 CHO CH 3 COOH 1 Oberg et. al. (A&A 504, (2009))

19 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

20 Irradiated Methanol: IRAS Product Analysis 100 monolayers CH 3 OH 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: /mnras/stw593)

21 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

22 PART II RADIOLYSIS OF AMMONIA

23 Why study ammonia? Öberg, K., et al. The Spitzer Ice Legacy: Ice Evolution from cores to protostars. The Astrophysical Journal 740(2011): 16 pp.

24 Possible Radiolysis Products of Ammonia Hydrazine N-3 Species Diazene N-2 Species

25 Mass Spec Spec Signal Katie Shulenberger `14 (Wellesely College) Detection of Hydrazine and Diazene at High Incident Electron Energies N 2 D 4 Film Thickness: 100 ML Electron Energy: 1000 ev Electron Dose: 8.5 C ND 3 N 2 D 2 m/z 36 (N2 D 4 + ) m/z 36 (N 2 D + 4 ) m/z 34 (N 2 D + 3 ) m/z 32 (N 2 D + 2 ) Temperature (K)

26 Radiolysis Products of Ammonia Hydrazine (N 2 H 4 ) + Diazene (N 2 H 2 ) NH + * NH + H 2 Zheng, W. et al. The Astrophysical Journal. 674: , 2008 February 20

27 Model: Bimolecular Intermediate Step R k 1 I + I k 2 I P dr dt = k 1R di dt = k 1R k 2 I 2 dp dt = k 2 2 I2 Katherine Tran 15 (Wellesley College) R(t) = R 0 e k 1t I t =? P t =?

28 Hydrazine Yield Results: Yield vs Fluence Film Thickness: 500 ML Electron Energy: 1000 ev Transmitted Current: 3.3 A -- k 1 = 0.15 s -1 ; k 2 = 40 cm 2 s -1 molecule Fluence ( C/cm 2 )

29 Hydrazine Yield Results: Yield vs Film Thickness Electron Energy: 1000 ev Dose Rate: 0.33 C/s Electron Dose: 33 C Film Thickness (ML)

30 Results: Low Energy Experiments Leon Sanche, Andrew Bass & Sasan Esmaili

31 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.

32 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 National Science Foundation (CHE , CHE , and CHE ) Current Lab Members 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