A MOLECULAR DYNAMICS STUDY OF ELECTRONIC SPUTTERING OF LARGE ORGANIC MOLECULES

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Submitted on 1 Jan 1989

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A MOLECULAR DYNAMICS STUDY OF

ELECTRONIC SPUTTERING OF LARGE ORGANIC MOLECULES

D. Fenyö, B. Sundqvist, B. Karlsson, R. Johnson

To cite this version:

D. Fenyö, B. Sundqvist, B. Karlsson, R. Johnson. A MOLECULAR DYNAMICS STUDY OF ELEC-

TRONIC SPUTTERING OF LARGE ORGANIC MOLECULES. Journal de Physique Colloques,

1989, 50 (C2), pp.C2-33-C2-35. �10.1051/jphyscol:1989206�. �jpa-00229402�

JOURNAL DE PHYSIQUE

Colloque C2, suppl6ment au n02, Tome 50, f6vrier 1989

A MOLECULAR DYNAMICS STUDY OF ELECTRONIC SPUTTERING OF LARGE ORGANIC MOLECULES

D. FENYO, B.U.R. SUNDQVIST, B. KARLSSON and R.E. JOHNSON*

Division of Ions Physics, Department of Radiation Sciences, Box 535, S-751 21 Uppsala, Sweden

' ~ e p a r t m e n t of Nuclear Engineering and Engineering Physics, University of Virginia, Charlottesville, VA 22901, U.S.A.

D a n s c e t r a v a i l u n m o d s l e d e " s p u t t e r i n g " e l e c t r o n l q u e d e g r o s s e s m o l e c u l e s o r g a n i q u e s e s t e s s a y 6 e n u t i t i s a n t d e s simulations d y n a m l q u e s . L e s o b s e r v a b l e s c a l c u l e e s s o n t c o m p a r e e s a u x r e s u l t a t s e x p e r i m e n t a u x .

Abstract

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In this study a model for electronic sputtering of large organic molecules is tested by using molecular dynamics simulations. Calculated observables are compared with experimental data.

It has been suggested 111 that a fast heavy ion passing through an organic solid will cause an expansion of the lattice around the ion track. Different mechanisms like coulomb explosion 121, repulsive decays I31 and vibrational excitations of the molecules /I/ may contribute to this expansion. These mechanisms have recently been discussed by Johnson 141. In order to test to what extent sputtering of large organic molecules can be initiated by molecular expansion, molecular dynamics (MD) calculations were canied out. One of the advantages of MD is that the motion of each molecule can be followed.

2

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TEIE SIMULATIONS

The organic solid was assumed to consist of spherical molecules interacting through a Lennard-Jones potential. The parameters of the potential were set to give the correct density, cohesive energy and velocity of sound. Newton's second law for the molecules in the solid was made discreet according to the method of Verlet 151 by introducing a time step of l o i 4 s. The only influence on the solid of an incident ion is that molecules near the ion track expand by a small amount on a short time scale. Fragmentation and ionization of molecules are not considered, The expanded molecules shift the potential, the force becomes initially repulsive and a compressional wave is emitted. As a consequence of the expanding ion track region both expanded and non expanded molecules are ejected. In Fig. 1 the cylindrical sample, containing 1668 molecules with mass 10000 u, is shown at different times after the expansion.

3

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RESULTS AND DISCUSSION

The total molecular yield has been measured to scale as [(d~ldx),]~ /6/. Assuming the increase in potential energy due to the expanding molecules to be proportional to the electronic stopping power, the simulations are consistent with this experimental result at least for low stopping powers. Only -1 % of (dE/dx), needs to be converted into intermolecular potential energy, giving an radial expansion of the order of 10 %, to explain the measured yields.

In Fig. 2 angular distributions of non expanded molecules are shown for 45" and 0" angle of incidence. For 45' angle of incidence the angular distribution is peeked at an angle which is not normal to the surface and for normal incidence it has a minimum for normal ejection. The results are consistent with recent experimental observations for molecular ions of insulin /7/ if it is assumed that non expanded molecules in the calculations correspond to intact molecular ions. As the experimental results are for secondary ions, measurements on angular distributions of the total molecular yield are needed for a direct comparison.

Recent experiments on Langmuir-Blodgett films of fatty acids /8,9/ irradiated with fast heavy ions indicate that craters are formed. In these simulations it is also possible to study the size and shape of craters in the sample. Such calculations are under way.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989206

C2-34 JOURNAL DE PHYSIQUE

Fig. 1. The sample at three different times (0, 30,60 and 90 ps). Expanded molecules are shown as filled circles and the non expanded molecules as open circles.

Ejection angle

Fig. 2. Angular distributions of non expanded molecules for a)45Oand b) .OO ' angle of incidence.

4 - CONCLUSIONS

With these simulations we have shown that the simple model of molecular expansion can qualitatively reproduce some experimental data on electronic sputtering like total yield of intact neutral molecules as a function of electronic stopping power, angular distribution of ejecta and crater formation. An improved model may also make it possible to make computer experiments with the MD technique on electronic sputtering of molecules adsorbed on a polymer surface.

REFERENCES

/I/ WILLIAMS, P. and B.U.R. SUNDQVIST, Phys. Rev. Lett. 58 (1987) 1031 121 HAFF, P., Appl. Phys. Lett., 29 (1976).473

/3/ JOHNSON, R.E. and B. SUNDQVIST, Int. J. Mass Spectrom. Ion Phys. 53 (1983) 337 /4/ JOHNSON, R.E., Int. J. Mass Spec. Ion Proc. 79 (1987) 357

151 VERLET, L., Phys. Rev. 159 (1967) 98

/6/ HEDIN, A., P.

HAKANSSON,

M. SALEHPOUR and B.U.R. SUNDQVIST, Phys. Rev. B 35 (1987) 7377 171 ENS, W., B.U.R SUNDQVIST, P. HAKANSSON, A. HEDIN and G. JONSSON, submitted to Phys. Rev. Lett.

in Jan. 1988

/8/ SAVE, G., P HAKANSSON, B.U.R SUNDQVIST, R.E. JOHNSON, E. SODERSTROM, S.E. LINDQVIST and J. BERG, Appl. Phys. Lett., 51 (1987) 1379

/9/ BOLBACH, G., S. DELLA-NEGRA, C. DEPRUN, Y. LeBEYEC and K.G. STANDING, Rap. Comm. Mass.

Spectrom. 1 (1987) 22