CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
MOLECULAR DYNAMICS MODELING OF IMPACT-INDUCED
SHOCK WAVES IN HYDROCARBONS
Mark L. Elert1, Sergey Zybin2, and C. T. White3
Chemistry Department, U. S. Naval Academy, Annapolis, MD 21402
Department of Chemistry, The George Washington University, Washington, D.C. 20052
3
Code 6189, Naval Research Laboratory, Washington, D.C. 20375
2
Abstract. We use nonequilibrium molecular dynamics (MD) simulations to study the behavior of
hydrocarbons under shock compression and spallation processes in shock and rarefaction waves
generated by the high-velocity impact of a flyer plate into a target material. The interatomic forces
were introduced using a recently modified reactive empirical bond order (REBO) potential with
intermolecular interactions, termed the adaptive intermolecular REBO potential (AIREBO). This
potential allows us to simulate as many as ten thousand molecules on a single processor, providing a
relatively large cross-section at the shock front in hydrocarbon solids. We performed plane-wave
impact experiments with different flyer velocities and observed the chemical dissociation of methane
and acetylene molecules in the shock layer, followed by polymerization into carbon chains for certain
flyer velocities. The hydrocarbon oligomers survive into the rarefaction region, indicating that stable
molecular products have been formed. These results may be significant for the understanding of
shock-induced chemical reactions resulting from meteorite impact in planetary atmospheres and
methane ice surfaces.
several thousand atoms for at least several
picoseconds. A large cross section is necessary to
allow the formation of oligomers without spurious
effects from periodic boundary conditions, and a
sufficiently long time scale is required to determine
whether reaction products survive into the
rarefaction region behind the advancing shock
front. Although workers at Los Alamos National
Laboratory have recently demonstrated the
feasibility of reactive shock simulations employing
up to 576 benzene molecules [3] and 1728 methane
molecules [4] for times on the order of one
picosecond using a semiempirical tight-binding
method, larger simulations must rely on empirical
potentials. In the present study we use the reactive
empirical bond order (REBO) potential for
hydrocarbons developed by Brenner [5] and
modified by Stuart, Tutein, and Harrison [6] to
include intermolecular and torsional effects. This
INTRODUCTION
The chemistry of hydrocarbons subjected to
shock impact may be relevant to our understanding
of the composition and evolution of planetary
atmospheres and to the formation of life. Comets
and carbonaceous asteroids are known to be rich in
organic compounds, but these materials may have
undergone pyrolysis, or alternatively may have
formed more complex compounds via shock
synthesis in the atmosphere, during impact onto the
surface of the early earth [1,2]. In addition to its
possible cosmological interest, shock wave impact
on hydrocabons may provide a unique environment
for the synthesis of organic compounds under novel
kinetically-controlled conditions.
To effectively study the shock-induced chemistry
of condensed-phase hydrocarbons using molecular
dynamics, it is necessary to simulate the behavior of
1406
potential can accurately model bond-breaking and
bond-forming processes in hydrocarbons, including
changes in the hybridization of carbon atoms. The
potential has been successfully applied to such
diverse problems as the study of phase transitions in
carbon [7,8], the use of carbon nanotubes as STM
tips [9], and the compression and friction of
anchored hydrocarbon chains on diamond surfaces
[10]. Preliminary two-dimensional simulations of
shock-induced chemistry in acetylene using this
potential have been reported previously [11].
longer than about fifteen carbons were found. Most
of the reaction products were straight chain
polymers, although some ring structures were also
formed. Four-carbon chains were the dominant
product at an impact speed of 12 km/s, but at higher
impact speeds the distribution favored three-carbon
chains, as shown in Fig. 3. In fact, the data suggest
that product hydrocarbons of maximum complexity
CALCULATIONS
MD simulations of shock impact on solid
methane and acetylene were carried out by
impacting crystals of the subject hydrocarbon with
a flyer plate of the same material. Periodic
boundary conditions were employed in the two
directions perpendicular to that of shock wave
propagation. Flyer plates were between four and
eight unit cells in thickness, and the cross section of
the periodic supercell was eight unit cells in each of
the transverse dimensions. The time step for
integration of the equations of motion was between
0.05 and 0.10 fs, depending on flyer plate impact
speed.
For acetylene, shock wave chemistry was
investigated as a function of flyer plate impact
speed and thickness. Some polymerization was
observed for impact speeds as low as ten km/s and
flyer plates four unit cells in width. At higher
impact speeds, the fraction of carbon atoms
incorporated into chains at least three atoms in
length increased significantly, as shown in Fig. 1.
The fraction of reacting carbon atoms depended
also on the width of the impacting flyer plate,
which determines the total energy delivered to the
target crystal. Figure 2 shows the fraction of
carbon atoms in oligomers of three or more carbons
at comparable times in simulations with different
flyer plate widths.
Most of the hydrocarbon oligomers formed in the
acetylene simulations were quite short, and were of
course hydrogen-deficient as a result of the 1:1
carbon-hydrogen stoichiometry of the reactant
material. Typically about ten percent of the carbons
participating in chains of three or more atoms were
found in chains of eight or more, and no oligomers
20
30
40
Ryer Plate Speed, km/s
FIGURE 1. The fraction of carbon atoms in the target acetylene
crystal which are involved in chains of at least three carbons in
length, as a function of flyer plate speed. All measurements
were made at a point approximately 1.5 ps after the start of the
simulation, when most of the energy of the flyer plate had been
dissipated in the target crystal. Flyer plates were five unit cells
thick.
0.00
2
4
6
8
10
Flyer Plate Width (Unit Cells)
FIGURE 2. The fraction of carbon atoms in the acetylene
crystal participating in chains of three or more carbons after
passage of the shock wave, as a function of flyer plate thickness,
for an impact speed of 20 km/s.
1407
20
30
40
Ryer Plate Speed, km/s
FIGURE 4. Carbon-carbon bonds resulting from shock impact
of methane at a flyer plate impact speed of 30 km/s are shown as
heavy lines. Carbon-hydrogen bonds are shown as lighter lines.
FIGURE 3. The ratio of the number of carbon atoms found in
four-carbon chains to the number in three-carbon chains, as a
function of flyer plate impact speed, for the same four simulation
snapshots described in Fig. 1.
of methane using a semiempirical tight-binding
method.
This lends further support to the
conclusion of those authors that the initial products
of shock-induced chemistry in methane are
molecular hydrogen and hydrocarbon oligomers,
although experimental studies [12, 13] indicate that
solid amorphous carbon is formed at much longer
time scales.
are formed preferentially when the incident energy
density is only moderately greater than the
threshold value needed to induce polymerization.
The results for simulations of shock impact in
methane are qualitatively different from those in
acetylene. With no pi bonding and a high H:C
ratio, methane is much less likely than unsaturated
hydrocarbons to undergo polymerization reactions.
Simulations with flyer plate impact speeds of
twenty km/s produced essentially no carbon-carbon
bonding, and even impacts of 25 km/s produced
only C2 products. More significant chemistry was
found with flyer impacts of 30 km/s. A sample
frame from such a simulation is shown in Fig. 4. In
this figure, the shock wave is proceeding from left
to right through the methane crystal. A few unit
cells of the unshocked crystal are visible at right.
(In this view, looking through a thickness of eight
unit cells, the free rotation of methane molecules in
their lattice positions is apparent.) Carbon-carbon
bonds, shown as thick lines in the figure, have
formed behind the shock front, and a few examples
of C3 and C4 chains are evident.
The products resulting from the 30 km/s impact
simulations are strikingly similar to those predicted
by Kress et al. [4] in their double-shock simulations
CONCLUSION
MD simulations of shock-induced chemistry in
hydrocarbons using the AIREBO potential [6] can
produce information relevant to the study of organic
material processing in meteor and comet impacts
with planetary atmospheres. Acetylene is found to
readily form hydrocarbon oligomers at impact
speeds which are modest on a planetary scale,
whereas methane is unreactive up to much higher
flyer plate velocities.
Although acetylene
polymerization is thought to lead eventually to the
formation of polycyclic aromatic hydrocarbons
(PAHs) in interplanetary dust particles and large
solar system objects [14], the present study
indicates that PAHs are not the initial products of
shock impact on acetylene.
1408
8. Glosli, J. N. and Ree, F. H., /. Chem. Phys. 110, 441446 (1999).
9. Harrison, J. A., Stuart, S. J., Robertson, D. H., and
White, C. T., /. Phys. Chem. B 101, 9682-9685
(1997).
10. Tutein, A. B., Stuart, S. J., and Harrison, J. A., /.
Phys. Chem. B 103, 11357-11365 (1999); Tutein, A.
B., Stuart, S. J., and Harrison, J. A., Langmuir 16,
291-296 (2000).
11.Elert, M. L., Swanson, D. R., and White, C. T,
"Molecular Dynamics Simulation of Shock-Induced
Chemistry in Acetylene," in Shock Compression of
Condensed Matter - 1999, edited by M. D. Furnish,
L. C. Chhabildas, and R. S. Hixson, AIP Conference
Proceedings 505, New York, 2000, pp. 283-286.
12. Nellis, W. J., Ree, F. H., van Thiel, M., and Mitchell,
A. C., /. Chem. Phys. 75, 3055-3063 (1981).
13. Nellis, W. J., Hamilton, D. C., and Mitchell, A. C., /.
Chem. Phys. 115, 1015-1019 (2001).
14. Sagan, C., Khare, B. N., Thompson, W. R.,
McDonald, G. D., Wing, M. R., Bada, J. L., Vo-Dinh,
T., and Arakawa, E. T., Astrophys. J. 414, 399-405
(1993).
ACKNOWLEDGMENTS
This work was supported by the Office of Naval
Research. MLE received additional support from
the Naval Academy Research Council.
REFERENCES
1. Chyba, C., and Sagan, C., Nature 355, 125-132
(1992).
2. Chyba, C., Thomas, P. J., Brookshaw, L., and Sagan,
C., Science 249, 366-373 (1990).
3. Bickham, S. R., Kress, J. D., and Collins, L. A., /.
Chem. Phys. 112, 9695-9698 (2000).
4. Kress, J. D., Bickham, S. R., Collins, L. A., and
Holian, B. L., Phys. Rev. Lett. 83, 3896-3899 (1999).
5. Brenner, D. W., Phys. Rev. B 42, 9458-9471 (1990).
6. Stuart, S. J., Tutein, A. B., and Harrison, J. A., /.
Chem. Phys. 112, 6472-6486 (2000).
7. Glosli, J. N. and Ree, F. H., Phys. Rev. Lett. 82,
4659-4662 (1999).
1409