Ion Neutralization at Metal Surface vac When a ionized atom is - - PowerPoint PPT Presentation

Ion Neutralization at Metal Surface

When a ionized atom is projected onto a solid surface, an excited solid-atom system is formed. The Ion-Neutralization process is a process by which the excited solid-atom system de-excite itself. The metal surface is represented by the Fermi level (εF, the electrochemical potential of electrons in the metal) and the vacuum level (εvac, the energy of an electron at rest, in vacuum, φ = work function). The Ion is represented by the its scheme of energy levels (and their separation to the same Vacuum level). The attractive ionic potential is in the form of a well which, as the metal–ion separation decreases, lowers the potential barrier that an electron at the metal surface must surmount to be captured in a stationary ionic state.

Valence band Conduction band

εF εvac φ ~ 4.5 eV

~ 4 eV

Different neutralization (de-excitation) mechanisms can take place:

• “one-electron process”: an electron in the metal band tunnels out from the surface to an excited state of the ion that is

energetically degenerate with the surface state - This is called Resonance Neutralization [Fig.(a)].

• “two-electron process”: one electron in the metal band tunnels out from the surface to a more tightly bound state of the Ion.

Energy is conserved by the emission of a second (Auger) electron [Fig.(b) - called Auger Neutralization - AN] or a photon [Fig.(c)]. The Auger electron is in the metal surface and can be excited above the vacuum level if the energy balance is positive.

For singly charged ions the Auger electron emission is more probable than photon emission because the Auger transition lifetime is about 106 times shorter than the radiative lifetime of approximately 10–8 s.

Vacant ionic states are represented by open circles, occupied surface states by filled circles.

Being two-electron processes, Auger processes are generally less efficient than resonant charge transfer - however Auger processes may be the dominating process where the Resonant Process are energetically forbidden.

Journal of Microscopy, Vol. 205, Pt 1 January 2002, pp. 86–95

• Resonance Neutralization
• Auger Neutralization - AN

MicroBooNE Case

with a c.r. flux at surface of ~9kHz and an

• avg. track length of 2.1 m,

≈1.5x109 e-Ion pairs/m3 s are generated Ions drift to the cathode, and the Ion current j+ impinging upon the metal surface (x=d) is j+ = n+(x=d) * vd+ ≈ 4x109 I+/m2 s The Tot. Ion flow rate at the Cathode Surface (Sc=25 m2) is j+ * Sc ≈ 1011 I+/s

The (measured) average number of AN electrons emitted per Ar+ ion is 𝜹i (Ar+) = 10-1 e/I+ ⟹ AN Electron emission rate: (Ion Flow Rate) * 𝜹i = 1010 e/s

= 𝒫(few nA) [and negligible Photon emission rate]

arXiv:1703.10491 [physics.ins-det]

(rare) Cosmic events may produce a blast of ionization in the TPC Volume - factor 103 or more wrt avg. cosmic ionization ⟹ AN mechanism may potentially induce outburst current of ~few μA delayed 𝒫(min) of time - when the Ions reach the Cathode

Cosmic Evt.s form ICARUS Run at Surface

• Jun. 2001

Caveat 1: the AN mechanism is well known in the case of atomic Ions (e.g. Ar+). In Liquid Argon Ar+ ions quickly convert into Ar2+ ionic dimers. The AN mechanism is known to hold as well for ionic dimers but is less efficient than for Ions. To be noted: no experimental data were found available for AN processes with Ar2+

Caveat 2: in case of insulating surface (resistive Cathode) - instead of metal surface - the situation is more complex as additional energy dissipation channels are possible. Whether AN remains the dominant neutralization channel, … we don’t know (no data are reported in literature).