Colloquium

Positronium trapping at material surfaces

Bernardo Barbiellini
Northeastern University
Wednesday, January 30, 2008
Higgins 310, 4 pm

When an electron meets its positively charged antimatter counterpart, the positron, they can form an exotic atom known as positronium (Ps) before annihilating each other within hundreds of picoseconds in a burst of gamma rays. The possibility of having a Ps atom trapped at a material surface is of great fundamental interest, since it can lead to new insight regarding quantum sticking and is a necessary first step to try to obtain the dipositronium (Ps2) molecule on a material host. Some experiments in the past have produced evidence for Ps bound states on a quartz surface, but firm theoretical support for such a conclusion was lacking. We present first-principles density-functional calculations of the key parameters determining the interaction potential between Ps and an alpha-quartz surface [1]. We show that there is indeed a bound state with an energy value which agrees very well with the experimental estimate of ~ 0.15 eV.
Further, a brief energy analysis invoking the Langmuir-Hinshelwood mechanism shows that once two Ps atoms are trapped on a surface, they can easily combine to form a Ps2 molecule [2].

[1] R. Saniz, B. Barbiellini, P. M. Platzman, and A. J. Freeman. Phys. Rev. Lett. 99, 096101, (2007).

[2] D.B. Cassidy, D.B., and A.P. Mills Jr., Nature 449, 195 (2007).