1. science
2. chemistry

What Is Known and What are the
Questions?
Eric Herbst, University of Virginia
INTRODUCTION “Errors using inadequate data are much less than those using no data at all.” Charles Babbage (1792-­‐1871) INTRODUCTION AND MODELLING Semenov, Walsh, Vasyunin, Garrod, Taquet, GraOer BRIEF HISTORY OF SURFACE
TECHNIQUES
1.  Rate Equations (Pickles & Williams 1977; Hasegawa,
Herbst, Leung 1992-3; Caselli et al. 1998; Garrod
2008, Garrod et al. 2009;2010; 2013; Taquet 2012)
2.  Macroscopic Stochastic Approaches (Charnley 2001;
Biham et al. 2001; Green et al. 2001; Stantcheva et al.
2002; Du & Parise 2011; Bron et al. 2014)
3.  Microscopic Stochastic Approaches; kMC or CTRW
a) H2 formation (Chang et al. 2005; Cuppen & Herbst
2005; Cuppen & Garrod 2011; Iqbal et al. 2014)
b) Grain/Mantle Growth/simulations of lab (Cuppen &
Herbst 2007; Cuppen et al. 2009; Garrod 2013;
Lamberts et al. s 2014)
c) Gas-Grain Models (Vasyunin et al. 2009;Vasyunin &
Herbst 2013; Chang & Herbst 2014)
4.  Molecular Dynamics/Quantum Chemistry (Kroes &
Andersson 2005; Bromley et al. 2014)
ReacOve Mechanisms accretion
desorption
Eley-Rideal
hopping
ED tunnelling
Langmuir-Hinshelwood
“flat” corrugated surface in 1D Eb Model of Competition
Diffusion
barrier
Chemical activation barrier
Ea < Eb means that tunneling under the
activation energy is more likely than
tunneling under the diffusion barrier.
Bulk Motion ??
•  1. Connected pores ???? •  2. subsOtuOonal (Cuppen, Fayolle, Garrod) •  3. intersOOal: sites of weak binding (Chang, Cuppen+) •  4. non-­‐thermal diffusion (Cuppen) •  5. vacancy mediaOon (Cuppen) Problems with Rate EquaOons •  a) inaccurate treatment of random walk; problem known as “back diffusion” (Krug) •  b) overesOmate of rate in “accreOon limit” where average number of reacOve parOcles less than unity and discreteness and fluctuaOons important (Tielens) •  c) cannot completely take into account microscopic structure of surface (e.g. roughness) and internal ice layers Two Phase vs. Three Phase vs. Multi-Phase
All react
independently of
position
Reactions on surface and
on pore walls
CTRW (MICROSCOPIC) APPROACH Chang, Cuppen & Herbst (2005)
•  A Monte Carlo approach in which the actual posiOons of individual species on (or off) a laece are followed with Ome. Can use to follow reacOons (LH,ER) and mantle build-­‐up. (on-­‐laece shown) A B C
Development of ice mantle in cold interstellar core Cuppen & Herbst, ApJ, 2007 Surface chemistry Grown on rough surface of amorphous carbon Internal chemistry? Chang and Herbst 2014: an internal layer with
interstitial sites (yellow) and processes; leads
to much higher radical abundances.
What we know of astronomical relevance
•  How to construct networks:
gas-phase, gas-grain
•  Many important gas-phase
reactions at some range of
temperatures
•  The gas-phase chemistry of
cold cores (to a significant
extent)
•  The need for surface and ice
chemistry
•  The formation of ices in cold
cores (CO2, CH3OH possible
exceptions)
•  Thermal and Non-thermal
desorption mechanisms
•  The strengths and
weaknesses of rate equation
models for bare grains and
ice mantles
•  The formation of COMs via
non-thermal excitation
•  How to improve rate equation
models with modifications
and stochastic treatments
(esp. H2 formation.)
•  How to construct
macroscopic and microscopic
stochastic models
•  Reasonable estimates for
most processes.
SOME GENERAL QUESTIONS
1.  Will we be able to construct detailed microscopic models under all relevant condiOons with complete sets of surface reacOons? A: For microscopic-­‐macroscopic models, low temperatures only for short period of Ome (young cold cores). For mixed (less detailed) approaches, can study warm-­‐up in hot cores (Vasyunin & Herbst 2013). Modified rate method can be used. 2.  Will the horizontal/verOcal/pore chemistry occurring on and in amorphous ices be amenable to soluOon? For what quesOons would such a soluOon be needed? A: VerOcal diffusion via intersOOal sites included in cold core model (Chang & Herbst 2014). Pore chemistry first included in models of Kalvans (2013) and Taquet (2012). Discussed at recent Faraday meeOng. Clearly important for ices. MORE GENERAL QUESTIONS
3.  How do we couple relaxaOon/excess energy into gas-­‐grain models? A: The simplest approach to relaxaOon is to use a 1-­‐D Langevin chain (Dzegilenko et al. 1996). Perhaps molecular dynamics treatments can also be used. Excess energy given off in one reacOon may affect another reacOon. 4. To what extent does grain/ice chemistry occur for other species than H2 in diffuse clouds? NH? NH3? A: ? 10.  Can we develop one gas-­‐grain network between 10 K and 800-­‐1000 K? A: we and others (Walsh; Furuya, Aikawa et al., KIDA) are trying. Building a new 10-800 K gas-grain network
Gas-phase
network of
Harada (up
to 800 K)**
Gas-grain network
of Garrod/Laas et
al. (cold/hot cores)
Remove duplications in gasphase rx. Add high T bare
surface rx (e.g. H+H)
Add 3-body reactions
with correct T
dependence; add
deuterium fractionation
if desired; check high T
radiative association
** See kida.uva2011 & more recent compilaOon. Check with
UMIST 2012
II. Rate Processes
Desorption Mechanisms
•  Thermal desorpOon (Brown): How is interstellar desorpOon related to complexiOes of TPD for mixed ices? Can one use analyOcal formulae for rates or is ViO method bener? Pre-­‐
exponenOal factor? Related to order. Surprise value for desorpOon energy of atomic oxygen? •  Nonthermal desorpOon (Dulieu) chemical/reacOve: breakthrough! Previous treatments empirical or simple RRK approach.. Work by Dulieu et al. •  CD = exp(-­‐εEdesorb/ΔHr/N) •  Other sources of energy such as heat of reacOon (Cuppen) Desorption Mechanisms II. PhotodesorpOon (Fayolle): VUV synchrotron beamline allows monochromaOc radiaOon, roles of photolysis and photodesorpOon. A number of mechanisms. DIET (desorpOon induced by electronic transiOons); proceeds via mainly discrete bound-­‐bound transiOons (CO, N2) Depth by isotopic studies of CO. Indirect mechanisms can occur in which excitaOon can be transferred betweeen different species (Dzegilenko and Herbst 1995). CO2: photodissociaOon, DIET, indirect DIET Methanol, water: linle direct photodesorpOon, perhaps correlated with conOnuous VUV spectrum. Molecules in a mixed ice cannot be modeled as individual species. HELP! Desorption Mechanisms III.
Cosmic ray desorpOon: consensus that it is either not needed in models or should be treated in more detail, if possible. Confusion as to whether rate equaOon method can be used for such a stochasOc process. But stochasOc method is not easily used either. Diffusion & Rate Coefficient Measurements (Theule, Karssenmeijer, Fedoseev, Nyman) Currently, “absolute” rate coefficients are finally being measured in the laboratory, but under these condiOons, the diffusive and chemical barriers cannot yet be separated, which appears to be necessary in the ISM. Possible approaches: measure diffusion rates separately from reacOon rate coefficients. Or esOmate acOvaOon energy barriers from gas-­‐phase values. Role of tunneling. What about Eley-­‐Rideal reacOons? How do we even know that this mechanism occurs? Yates claims that zero acOvaOon energy is inconsistent with the LH mechanism. Francois is able to disOnguish them, but how? Important Reactions/
Parameters
1.  Methanol network; abstraction vs
association
2.  OH abstraction to form new radicals
(“chain propagation”)
3.  Bulk reactions without much motion.
4.  X and Y factors
5.  A standard ice.
6.  The role of non-thermal electron
bombardment??
7.  High temperature surface chemistry on
bare grains via chemisorption??
Sticking, Relaxation, and Applications
Sticking of Ions: ???? Ionic reactions on grains, in
mantles?
Relaxation: the simplest approach is to use a 1-­‐D Langevin chain (Aikawa et al. 1999). Perhaps molecular dynamics treatments can also be used. Excess energy given off in one reacOon may affect another reacOon.
Applications: how important do rate changes/
uncertainties affect model results? How much detail
is needed? Depends upon question asked.
TWO SPECIFIC PROBLEMS
1. H2 FORMATION IN DIFFUSE
CLOUDS
2. ROLE OF HYDRODYNAMICS
Grain Formation of H2 in Diffuse Clouds
physisorption
flat
? rough
Tunneling?
Temperature
fluctuations?
Radiation?
chemisorption
Large
barriers
Small
barriers
PAH’s?
Amorphous C?
Problems with Hydrodynamics
1.  Cannot use stochastic methods for chemistry.
2.  Mass points can traverse widely varying physical
conditions. For example, at low temperatures
and high densities, with normal assumptions,
gas-grain chemical simulations deposit much H2
on grains, strongly changing the chemistry.
3.  Difficult to run for astronomical ages.
4.  Need a gas-grain code that works from 10 K to
1000 K since what can happen is unexpected
(currently using code of K. Furuya)
Encounter Desorption
Adsorption energy for H2: 440 K on water ice; 23 K on H2
Hincelin, Chang, Herbst, in prep.
Steady-­‐state abundances of H2 on grains at 10 K A young protoplanetary disk Hincelin et al. 2013 What are the Questions?
As we know, There are known knowns. There are things we know we know. We also know There are known unknowns. That is to say, We know there are some things We do not know. But there are also unknown unknowns, The ones we don’t know We don’t know. Acknowledgments
•  Sources of Funding:
•  NASA, NSF •  Recent group members:
Paul Rimmer, Nanase
Harada, Donghui Quan, Yezhe Pei, George Hassel,
Anton Vasyunin, Tatiana Vasyunina, Qiang Chang,
Dawn Graninger, Tobias Albertsson, Chris
Shingledecker, Jenny Bergner, Ugo Hincelin, Angela
Occhiogrosso, Chad Bernier, Kinsuk Acharyya