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451. HOTATOM: Stochastic Program for Simulated Collision
Events
by Trina Valencich, Department of Chemistry, California
State University of Los Angeles, Los Angeles,
California 90032
This program determines product yields and formation
energy distributions for competing reactions of
energetic atoms or molecules. For this purpose, it is
an inexpensive and reliable alternative to programs
that solve integral equations; and it contains more
realistic corrections for subsequent unimolecular
effects. It is a fully stochastic program that deals
with a pre-selected number of simulated collision
events.
The process at each collision is considered to be
A + S %Æ X + Y
where A is an energetic species (usually, but not
always, a translationally hot atom) and S is a
substrate molecule.Four kinds of channels are
permitted in the calculations corresponding to the
following processes:
Type 0: X = A and Y = S, at different energies than
before (elastic and/or inelastic scattering). There
will be one of these channels for each species present
in a sufficient amount to contribute to moderation
effects. The user supplies collision cross-sections
s(E) and (at several E) moderation functions, i.e.,
probabilities for the subsequent energy of A. Hard-
sphere moderation functions or the more realistic
"Porter-Kunt Type C" functions (cf. J. Chem. Phys., 52,
3240 (1970)) may be inserted if desired. The energy E
is maintained in the laboratory reference frame. If it
is desired to supply moderation functions in terms of
relative energy, this may be specified; and the user
may also provide scattering angle distribution
functions or select internal isotropic random
scattering. The user may, of course, transform the
data externally if some other kind of scattering is
preferred.
Type 1: X is a diatomic molecule containing A (an
atomic abstraction reaction). Only s(E) need be
supplied for this case.
Type 2: Y is an atom (conceivably, a small molecule);
A is part of X; and X has 3 or more atoms (most
typically, a hot-atom substitution process). Besides
s(E), the user supplies needed parameters for
unimolecular rate calculations, tabular probabilities
at several E for the amount of internal energy (E*)
deposited in X by the substitution process, and
scattering angle distributions (or isotropic random
scattering may again be specified). An automatic
default of a uniform E* probability is also available.
Subsequent unimolecular calculations are carried out by
the rotationally-controlled modification of RRKM theory
(J. Chem. Phys., 57, 332 (1972)) appropriate to the
substitution considerations.In the event the
substitution producing X fails to survive and A is
released, A continues to undergo collisions (an
important feature of the program). Relative to
laboratory energy transformation is automatic in this
case.
Type 3: Y is A; and S has enough internal energy to
decompose (a radiation-damage type of reaction). In a
manner similar to reaction channel Type 2, the user
supplies s(E), tabular probabilities at several E for
the amount of internal energy deposited in S,
unimolecular decomposition parameters, and scattering
angle distributions (or isotropic random scattering is
specified). The unimolecular calculation determines
whether the target is stabilized or decomposes and
records the energy at which the event occurred and
determines the laboratory reference frame energy for
Y's next collision.
The program prints numbers of stabilized products from
channels of Types 1, 2, 3; the number of A which are
thermalized, the numbers of decomposed products for
each possible decomposition path from channels of Types
2 and 3, and the total number of collisions. The user
may also request sequential listing of every collision
outcome other than nonreactive scattering along with
the energy of its occurence. He may also have
histograms of these prepared automatically and have
every A energy during the calculation listed in detail.
The program is designed for either regular or
minicomputer use. Output does not exceed 80 columns in
width and may be directed either to lineprinter or
teletype. The memory requirements depend on the array
sizes.
FORTRAN IV (CDC)
Lines of Code: 1500
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