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Radiation Chemistry vs. Photochemistry Cosmic Synthesis of Prebiotic Molecules
Chris ArumainayagamWellesley College, Massachusetts, USA
First Evidence of molecules in space:Annie Jump Cannon(Wellesley College Class of 1884)
Molecular Physics, 2015,113, 2159-2168
Five Mechanisms for Processing Interstellar Ices
Shock Waves
Arumainayagam et. al., Chemical Society Reviews 2019, 48, 8, 2267–2496.
What is the difference between radiation chemistry and photochemistry?
Radiation Chemistry
Radiation chemistry is the “study of the chemical changes produced by the absorption of radiation of sufficiently high energy to produce ionization.”
RADIATION CHEMISTRY INVOLVES IONIZATION.
R.J. Woods, Basics of Radiation Chemistry, in: R.D.C. William J. Cooper, Kevin E. O'Shea
Flux of Cosmic Rays Reaching Earth
cosmic ray107‐1020 eV
secondary electron cascade (0‐20 eV)
Formation of Secondary Electrons in Cosmic Ices and Dust Grains
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 par cle → 100,000 low‐energy electrons
Electron‐induced Dissociation Mechanisms
AB + e–
AB* + e–
AB–*
AB+* + 2e–
A* + BA+ + 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
not possible for photons
Photochemistry involves “chemical processes which occur from the electronically excited state formed by photon absorption.”
– visible (1.8 eV – 3.1 eV)– near‐UV (3.1 – 4.1 eV)– far (deep)‐UV (4.1 – 6.2 eV)
– vacuum‐UV (6.2 –12.4 eV)
Photochemistry
B. Wardle, Principles and Applications of Photochemistry, Wiley, Chichester, UK, 2009.
no ionization;only excitation
ionization ANDexcitation
Condensed Phase Ionization Energy
Mater. Horiz., 2016, 3, 7‐‐10
Most previous “photochemistry” studies relevant to astrochemistry involve radiation
chemistry in addition to photochemistry.
VUV‐emission spectrum of Microwave‐Discharge Hydrogen‐Flow Lamps (MDHL)
CREDIT: Gustavo Adolfo Cruz Diaz
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
How do you create a little bit of heaven on earth?
UV Light
Low‐Energy (210 eV) electrons or photons
P = 1109 torr
20200 Å NH3
T = 90 K
D(NH) ≈ 4.1 eV
Electron/photon stimulated desorption
post‐irradiation analysis
High‐Energy (1000 eV) electrons
Experimental Procedures
Methanol
Liquid N2 Port
Ta(110)
e‐
Mass Spec
Filament
PowerSupply
Products
Post‐Irradiation TemperatureProgrammed Desorption
Temperature (K)
Mass Sp
ec Signa
l
Experimental ProceduresIsothermal Electron Stimulated Desorption
Time (sec)
Mass Sp
ec Signa
l
Wavenumber (cm‐1)
Mass Sp
ec Signa
l
Photon Source
N‐3 Species
NHN
H2Ntriazene
Hydrazine
DiazeneN‐2 Species
Possible Radiolysis/Photolysis Products of Ammonia
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
Products of High Energy Electrons From Condensed Ammonia
RADIATION CHEMISTRY
Arumainayagam, ACS Earth and Space Chemistry 2019, 3, 800−810
Products of Low‐Energy Photons From Condensed Ammonia
PHOTOCHEMISTRY
Arumainayagam, ACS Earth and Space Chemistry 2018, 2 (9), 863-868.
Energetics of Ammonia Photolysis
• Ionization energy for gas phase ammonia: 10.1 eV• Threshold for low‐energy secondary electrons: 8.5 eV • Absorption threshold photon energy: 6.2 eV
Most previous “photochemistry” studies relevant to astrochemistry involve radiation chemistry in addition to photochemistry.
Our experiments were done at < 7.4 eV.
Our experiments involve only photochemistry.
Conclusions
• Low‐energy (< 20 eV) electrons may be the dominant species responsible for the synthesis of prebiotic molecules in the interstellar medium
AcknowledgementsCollaboratorsDr. Andrew Bass, Université de SherbrookeDr. Leon Sanche, Université de SherbrookeDr. Petra Swiderek, University of BremenDr. Michael Boyer (Clark University)Undergraduate Lab MembersKatie Shulenberger ‘14Katherine D. Tran ’15Sebiha Abdullahi ‘15Jane L. Zhu ’16Carina Belvin ‘16Kathleen Regovish ‘16Lauren Heller ‘17Milica Markovic ’17Julia Lukens ‘17Stephanie Villafane ’17Helen Cumberbatch ‘ 17Zoe Peeler ’17Alice Zhou’ 17
Jyoti Campbell’18Kendra Cui’18Leslie Gates’18Hope Schneider’18Jean Huang’18Subha Baniya’19Jeniffer Perea’19Christina Buffo’19Mayla Thompson’19Rhoda Tano‐Menka’19Ally Bao’19Aury Hay’20Ella Mullikin’20
Mileva Van Tuyl ‘21Iffy Nwolah’21Lan Dau’21Nina Sachdev’21Christine Zhang’21Sophie Coppieters ‘t Wallant’21Sophia Bussey’21Hannah Anderson ’22Natalie O’Hearn ‘22
Funding SourceNational Science Foundation(CHE‐1465161, CHE‐1012674, CHE‐1005032)