Interaction of Small Hydrocarbons with Fusion Relevant Beryllium Thin Films

Ion-surface collision studies are carried out with small deuterated hydrocarbon cations i.e . CD x + with x = 2-4 colliding with fusion relevant Beryllium (Be) thin films with ions incident energy as low as 0 eV and as high as E in = 100 eV. Be films are coated on stainless steel surface by the technique of Thermionic Vacuum Arc (TVA); a novel thin film deposition method with primary as well distinguished characteristic of control of ion flux and respective dose towards the substrate. Prior to scattering, methane-d 4 99 atom % D is ionized by electron impact and ions are mass and energy analyzed. Ionization and collisions are performed in ultra high vacuum conditions. In these kinds of collision experiments, we have recorded secondary ion mass spectra and plotted respective incident energy resolved abundances of secondary product ions. Relative abundances in percentage of total secondary ions are plotted and it is observed that such beryllium films can accumulate charged hydrocarbon layers as surface adsorbates. These self assembled layers play a primary role in surface-scattering of primary ions. Moreover, it is seen that bond dissociation energy in lighter hydrocarbons is higher than that for heavier species and shows primarily that the deuterium atoms are loosely bounded to carbon atoms in heavier hydrocarbons than in lighter ones.


INTRODUCTION
tudy of reactive collisions of ions with surfaces is the area accelerated in last few years towards characterizing gaseous molecular ions and also nature of the surfaces. These studies are evidenced by most recent research work on properties of multiplycharged ions, on their spectroscopy and on their gasphase reactivity [1][2][3][4][5]. Considerable interests are taken to study physical sputtering as well chemical sputtering caused by colliding slow ions with hyper thermal energy range (up to100 eV) and the processes like Surface Induced Dissociation (SID), Charge Exchange Reactions (CER) and reflection properties have been identified and investigated [6][7][8]. In addition to being of fundamental importance, reactions of polyatomic ions with surfaces are relevant in applications like modifying surfaces to prepare 1 Department of Physics, University of Sargodha, Sargodha, Pakistan.
Email: a bilal.rasul@uos.edu.pk (Corresponding Author), b aroogaroog04@gmail.com advanced electronic materials and plasma-wall interactions in fusion plasma [9][10][11][12]. A transfer of translational kinetic energy of projectiles into internal modes of energy has been observed during surface collisions of polyatomic ions, causing their dissociation. This energy transfer may help in characterizing the structures of projectile ions as well nature of surfaces [13,14].
In our earlier papers, we have reported SID and CER of hydrocarbon cations and NH3 + with a variety of materials including stainless steel and fusion relevant materials like tungsten and others [15][16][17][18][19]. Reflections of hydrocarbons with fusion relevant tungsten (plasma sprayed tungsten) and Carbon Fiber Composite (CFC) surfaces have been studied experimentally [20][21][22]. An urgent demand for data on collisions of low-energy (0-100 eV) small hydrocarbon ions C1-C3 group, with

Thermionic Vacuum Arc (TVA) setup
Adhesive and dense layers of metals vapors like tungsten (W), beryllium (Be), nickel (Ni) or chromium (Cr) are deposited in this technique, in high vacuum conditions (<10 -3 Pa). An externally heated tungsten filament with 100 mA of electron current accelerates electrons towards an anode biased at 1-6 kV i.e Be inside a W holder. A Wehnelt cylinder focuses the said beam and it ignites an electrical discharge at the anode and creates a strong local heating resulting in metal evaporation. These metal vapors are ionized and may cause the expansion of the plasma in the whole available volume. The parameters like filament current, anode voltage, cathode-anode distance and orientation of the anode are the keys to control the discharge conditions, ultimately whole deposition process. Using the optimized parameters like in [23], a deposition rate of about 5±0.5 nm s -1 is achieved.

Double focusing mass spectrometer, BESTOF
Ion-surface collisions experiments were carried out at double focusing sector field mass spectrometer, designed in reverse geometry (i.e. BE) and combined with collision chamber containing Surface (S) and linear Time-Of-Flight (TOF) mass spectrometer, namely BESTOF [24][25]  Normally, background pressure of the collision chamber is kept less than 10 -9 Torr if the valve between the collision chamber and the mass spectrometer remains closed. However, during our experiments, opening this beam-line valve increases the pressure in this chamber to about 2.0×10 -8 Torr. Under these conditions, the number of collisions between background molecules and the surface is of the order of 10 12 mm -2 s -1 . When the ion current was kept constant of the order of 10 11 particles mm -2 s -1 , it is worth noticing that the surface would always be covered with a few layers of hydrocarbons.
After reflection in front of the charged surface or collision with adsorbed layers, majority of the product ions resulting from a variety of reaction channels leave surface. A secondary lens stack guides them towards a pulsed deflection and acceleration field, a starting point for the TOF. These secondary ions are detected by a double stage Multi-Channel Plate (MCP), connected to a multi-channel scaler with time resolution of 2 ns per channel.

Beryllium film structure
A variety of surface diagnostic techniques are applied to study the structure and composition of Be films at the National Institute of Lasers, Plasma and Radiation Physics, Bucharest, Romania. Roughness of about 300±50 nm is observed in Atomic Force Microscopy (AFM) images (Fig. 1) while the Environmental Scanning Electron Microscopy (ESEM) showed a smooth and pin-hole free Be coating (Fig. 2). The X-Ray Diffraction (XRD) analysis proved the crystalline structure of the deposited film while some contaminations in the outermost layer (3 nm) are found when studied by Auger Electron Spectroscopy (AES).

Surface induced dissociation
Secondary ion mass spectra are taken by TOF MS explained in experimental section of this paper, after the collisions of CD2 + , CD3 + and CD4 + ions with beryllium film deposited on stainless steel by TVA.
The incident energy Ein range chosen for the projectile ions is from about 0 eV to a maximum of 100 eV which includes the energy range of hydrocarbons expected in next step fusion devices [26]. In Fig. 3, we show some of the mass spectra recorded at an incident energy of Ein = 30 eV. A very few ionic species appear at this low energy, as mentioned there in.
We plot incident Energy Resolved Mass Spectra (ERMS) for surface induced dissociation products after the impact of CD2 + , CD3 + and CD4 + ion beams with the beryllium film in the incident energy range from about Ein = 0 eV-100 eV. These are the plots of normalized abundance of the product ions ∑ versus incident energy. Fig. 4 summarizes these graphs. Here and onwards, by Appearance Threshold Energy (ATE) of the product ions, we mean the incident energy of the primary ions corresponding to relative abundance of 1 % of a specific product ion. We see that the overall range of the ATE, to separate one D atom from the projectile ion, is lower than to separate two and respectively three deuterium atoms. Energy range of appearance threshold at the loss of one D atom from CDx + lies between 10-20 eV where as this range for the loss of two D atoms is around 18-68 eV and for three D atoms this energy range is about 32-80 eV. Moreover, we see that the value of ATE for one D atom loss, is lower for heavier projectile ions than in the case of lighter projectiles e.g. ATE for one D-atom loss from CD4 + (product ion CD3 + m/z 18) lies at 10 eV whereas this value is 15 eV for the projectile ion CD3 + (product ion CD2 + m/z 16) and in the case of CD2 + , this energy threshold lies at 20 eV. Such sequential variation in threshold energy is also seen for the separation of two D-atoms i.e. ATE for two Datoms loss from CD4 + (product ion CD2 + m/z 16) lies at 20 eV and for projectile ion CD3 + (product ion CD + m/z 14), this value is 35 eV whereas in the case of CD2 + (product ion C + m/z 12), this energy threshold lies at 70 eV. Similar behavior was observed for the dissociative reaction channels which result in the formation of products at the loss of three D atoms from the projectile ions. We see that the product ion CD + m/z 14 at the loss of three D atoms from CD4 + , appears with 1 % of relative abundance at 32 eV where this energy threshold lies at about 80 eV for the projectile ion CD3 + (product ion C + m/z 12

CONCLUSIONS
Small deuterated hydrocarbon cations CD2 + , CD3 + and CD4 + are collided with fusion relevant beryllium thin films in the incident energy range from about 0 eV to 100 eV. Be is coated on polished stainless steel surfaces by TVA and a variety of experimental techniques reveal that smooth and crack free crystalline coatings are obtained. Secondary ion mass spectra are recorded by linear TOF mass analyzer and respective incident energy resolved mass spectra are presented in this article. We observe that bond dissociation energies are higher for lighter hydrocarbons than for heavier species respectively, C-D bonds seem stronger in lighter hydrocarbons than in heavier ones.