Potential Energy Retention of Slow Highly Charged Ar-Ions in Chemical Clean Silicon Surfaces


Potential Energy Retention of Slow Highly Charged Ar-Ions in Chemical Clean Silicon Surfaces

Kost, D.; Facsko, S.; Tyrroff, H.; Zschornack, G.; Möller, W.

Highly charged ions (HCI) carry a large amount of potential energy, which is defined as the sum of the binding energies of all electrons that are removed from the atom. In the case of low velocities of the ions this energy is equal or can exceed the kinetic energy of the ions. Retaining such a large energy in a very small surface area of typically about 1-10nm^2 and in a very short interaction time of typically 5-10fs an enormous power flux of 10^13 W/cm^2 is given. In respect of this, high yields of photons and electrons have been observed, which carry away a fraction of the potential energy. Some experiments give a quantitative estimation of an energy reemission coefficent in the order of only 10%. In our experiment we determine the fraction of the retained potential energy of the ions by a calorimetric measurement.

For improved calorimetric measurements of the retention of the potential energy of highly charged ions a UHV device with a base pressure of p<10^-9mbar was constructed and connected to the 7.5 GHz ECR ion source at the Forschungszentrum Rossendorf. The ECR ion source is equipped with a sector field magnetic separation and a beam deceleration system. The ions are accelerated with a voltage of 5 kV and after beam transport decelerated to final kinetic energies down to 60eVxq.

By using a calorimetric setup the increase of the target temperature is measured during the irradiation with argon ions.
The retained energy of Ar q+ (q= 1 up to 9) ions into the Si(100) surface is determined at kinetic energies between 60 eVxq and 200 eVxq. By extrapolation to zero kinetic energy the retained fraction of the potential energy is obtained.

For absolute calorimetric measurements a small resistive heater is used for the calibration of the calorimetric setup. Using the resistive heater a function between the temperature increase and the deposited electrical power of the heater is obtained. Assuming that the electrical power is equivalent to the retained power of the incident ions the retained power of the ions can obtained from the temperature increase.

By sputtering with Ar+ the silicon surface is cleaned before the measurement. The chemical state of the target surface is controlled by AES using LEED
optics. Once being clean there are no other Auger electron signals in spite of silicon signals detectable over a time of 7 hours.

The obtained retained potential energy of the different charge states of the argon ions is related to the total potential energy. The potential energy retention coefficient results as 0.7 +/- 0.2 and decreases weakly with increasing charge state. This values are compared with earlier measurements [1] on a copper surface. The potential energy retention coefficient for silicon is about three times larger than the results with the copper surface.

[1] U. Kentsch, H. Tyrroff, G. Zschornack, and W. Möller, Retention of the potential energy of multiply charged argon ions incident on copper, Phys. Rev. Lett. 87(10), 4273 (2001).

  • Poster
    International Conference on Photonic, Electronic and Atomic Collisions, 23.-29.07.2003, Stockholm, Schweden

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