Surface Chemistry: New
Methods, New Results
ERIC HERBST
DEPARTMENTS OF PHYSICS
AND ASTRONOMY
THE OHIO STATE UNIVERSITY
106 sites
TYPES OF SURFACE REACTIONS
REACTANTS: MAINLY MOBILE
ATOMS AND RADICALS
A +
H +
B 
H  H2
AB
association
X  XH (X = O, C, N, CO,
etc.)
WHICH CONVERTS
H +
O  OH  H2O
C  CH  CH2  CH3  CH4
N  NH  NH2  NH3
CO  HCO  H2CO  H3CO  CH3OH
X + Y  XY
??????????
MODELLING DIFFUSIVE
SURFACE CHEMISTRY
Rate Equations
dNH/dt = kaccnH - kevapNH - KH-HNHNH
Advantages gas-phase and grain
chemistry are coupled in
time-dependent
calculations
Problems averages obtained only
Accurate if large numbers
of reactive species on
grains; reality is that
small numbers may exist
especially for H
Rates of Diffusion
 Standard
astrochemical (e.g. Hasegawa
et al. 1991)
 Slow H (P1): H slowed down to olivine
(carbon) value of Pirronello et al. (1997)
 Slow (P2): all other species slowed
proportionally
 With P1/2, H2 formation efficient only in
narrow T range dependent on surface
MODIFIED RATE
APPROACH
Slows diffusive rate coefficients so that
rate of reaction does not exceed rate of accretion
See Caselli et al. (1998)
Advantages modifies practical rate
method to approach Monte Carlo
Problems
semi-empirical, no way to
prove correct in all situations
New models: Caselli, Ruffle, Roberts, Herbst
GRAIN MANTLE GROWTH
(COLD CLOUDS; silicate
grains)
% Agreement in TMC-1
Gas-phase species
Roberts & Herbst 2002
TMC-1: COMPARISON WITH SWAS
H2O
CO
O2
Roberts and Herbst 2002
STOCHASTIC METHODS
Based on solution of master equation,
which is a kinetic-type equation in
which one calculates not
concentrations but probabilities that
certain numbers of species are
present. Can solve directly (Hartquist,
Biham) or via Monte Carlo realization
(Charnley).
MONTE CARLO APPROACH
Call two random numbers – which lead to
increment of time and which process occurs
during that time. Repeat procedure.
Advantages
takes discrete nature of
grains into account; formally exact
Problems so far limited to simple
systems with fixed gas-phase
abundances
PROBABILISTIC MASTER
EQUATION
dNH/dt = kaccnH - kevapNH - KH-HNHNH
replaced when NH << 1 by a series of
coupled equations for Pn(H):
<NH> =
 n Pn(H)
dP0(H)/dt = ……….
PROTOSTELLAR CORES
Current Picture: surface chemistry during late previous
stage.
H
D
O
CO
H2CO
+ isotopomers
CO2
CH3OH
H2O ice
Shock, heat disrupt mantle:gas-phase chemistry
ensues!
RE: rate equation
MR: modified rate
MC: Monte Carlo
ME: master eq.
“High density”
O, CO > H
Dear David,
The very best of luck
in your new state of
active “retirement.”
With best
wishes, Eric
dense (giant) molecular clouds
organic molecules
H
core
4 -3
n = 10 cm
T = 10 K
2
PDR’s
embedded
stars
hot
ionized
gas
HII region
protoplanetary disk
studied in millimeter-wave and IR