Session KW1: Positron Collisions

Positron Scattering from Large Molecules.

We report on the experimental progress in positron scattering with biologically relevant molecules. In particular, large molecules, many of which are solid at room temperature requiring the use of a vacuum oven to produce a vapor suitable for beam experiments. Our positron source is comprised of a sealed sodium-22 ($^{\mathrm{22}}$Na) source, rare gas (neon) moderator (RGM) coupled to a Surko-style buffer gas trap (BGT). This system can produce an energy tunable positron beam which is pulsed and has a thermal energy spread. A room temperature scattering cell is used for gaseous and liquid targets, and a vacuum oven is used for targets which are solid at room temperature. The energy loss of the positron is used to discriminate between scattering channels allowing a range of scattering channels to be investigated. Of particular importance is the formation of the positronium (Ps) atom in positron-molecules collisions. Empirical models of the Ps formation cross section provides a reasonable guide, but no \textit{ab initio} theory is available to describe the Ps formation cross section in positron scattering from large molecules. Our most recent work considered positron scattering from Pyridine, the simplest azine, has shown remarkable agreement between the independent atom model (IAM) with the screening additivity rule (SCAR) including interference effects. We will discuss the current state-of-the-art in positron-molecule scattering at energies above \textasciitilde 1eV and comment on future work involving DNA and RNA basis. Additionally, we will discuss future work in positron molecules scattering below 1 eV.   

Quantum suppression of antihydrogen formation in positronium-antiproton collisions

Antihydrogen ($\bar{\rm H}$) can be created in collisions of antiprotons ($\bar{p}$) with positronium (Ps) through charge-transfer reaction $\bar{p} + {\rm Ps}(n_i l_i) \rightarrow \bar{{\rm H}}(n' l') +e^-,$ where the Ps state is characterised by the principal and orbital angular momentum quantum numbers $n_i$ and $l_i$ respectively, with $n'$ and $l'$ being the corresponding values for $\bar{\rm H}$. The AEgIS and GBAR Collaborations at CERN's Antiproton Decelerator plan to use this reaction to form beams of $\bar{\rm H}$ for studies of antimatter gravity. Both collaborations intend to employ laser-excited Ps as the scattering target. Theoretical simulations required for these experimental programs are based on classical trajectory methods. We used the quantum-mechanical convergent close-coupling (CCC) approach to test the validity of the classical methods. The two-centre CCC approach is a general-purpose formalism applicable to a wide range of atomic collision processes. It was originally developed for positron scattering on atomic hydrogen including Ps formation. The CCC approach is based on discretisation of the continuum and combines the expansion of the total wave function over the atomic states with that over the Ps states. Such a double expansion allows for explicit Ps formation, and allows for the distinction within the ionisation processes between Ps-formation and breakup channels. The CCC method was applied to study $\bar{\rm H}$ formation at low energies relevant to the aforementioned experimental activities. We found increases of several orders of magnitude in $\bar{\rm H}$-formation cross sections $\sigma_{\bar{{\rm H}}}$ when $n_i$ was raised from 1 to 2 and 3, with the cross sections for the excited states displaying a characteristic $1/E$ dependence at low Ps kinetic energies $E$. Recently we extended the previous studies by considering Ps principal quantum numbers up to $n_i=5$. We established that the dramatic increase in $\sigma_{\bar{{\rm H}}}$, when $n_i$ is increased from 2 to 3 and from 1 to 2, was absent for the higher values of $n_i$. In the Ps kinetic energy region where the data for all $n_i$ behave as $1/E$, there is only a factor of around 3 between the $\sigma_{\bar{{\rm H}}}$ for $n_i = 5$ when compared to $n_i =3$. This is to be contrasted with the approximately factor of 30 increase between $n_i = 2$ and 3, and a several orders of magnitude enhancement in the formation of $\bar{\rm H}$ via $\bar{ p}$ scattering with Ps in an $n_i = 2$ excited state over the ground state. It was concluded that quantum effects dramatically suppress the increase of $\sigma_{\bar{{\rm H}}}$, in sharp contrast to expectations from Bohr-like and classical theories. If the trend in $\sigma_{\bar{{\rm H}}}$ persists at high $n_i$, then the implications for the current experimental efforts, which aim to exploit efficient charge transfer from excited-state Ps to produce $\bar{\rm H}$, could be severe.   

Positron mass stopping powers in atomic and molecular hydrogen

In quantifying the collision processes, the particular quantity of interest is the stopping power (SP), because of its use in modeling projectile transport through matter. Many applications require accurate SP values at low and medium energies to model the projectile's entire path, however it is significantly inhibited by the lack of reliable experimental and theoretical data. The SP calculations used to date are mostly based on high-energy approximations combined with the Bragg's additivity rule for molecules. Both approaches need careful checking at low and intermediate energies where an accurate account of channel coupling effects is important. In this report, we present results for positron impact electronic excitations and the SP of atomic and molecular hydrogen calculated using the convergent close-coupling (CCC) method. Our results cover low to high energy range and explicitly include Ps formation and a large number of target excitation and ionization channels. We check Bragg's additivity rule both for positron and electron projectiles by comparing results for atomic and molecular hydrogen.   

Measurement of New Features in Positron-molecule Annihilation Spectrum Using a High-Resolution Positron Beam.

Experiments have shown that the low-energy (sub-eV) annihilation spectra of positrons on molecules is dominated by relatively sharp features that have been identified as vibrational Feshbach resonances.\footnote{Gribakin, et al., Rev. Mod. Phys. {\bf 82}, 2577 (2010).} The ability to resolve these features is limited by difficulties encountered in creating beams with sufficiently narrow energy spreads. Recently, we have demonstrated the operation of a cryogenic, buffer-gas trap, with total beam energy spreads as low as 7 meV FWHM and temporal spreads of sub-microsecond duration.\footnote{Natisin, et al., Appl. Phys. Lett. {\bf 108}, 024102 (2016).} Here, experiments are presented using the narrow energy spread beam to measure positron annihilation energy spectra with enhanced energy resolution. New features are observed, including the identification of resonances due to IR-inactive vibrational modes.\footnote{Natisin, et al, Phys. Rev. Lett., {\bf 119}, 113402 (2017).} Attempts to directly measure the effect of overtone and combination modes,\footnote{Gribakin, et al., Phys. Rev. A, {\bf 96}, 062709 (2017).} will also be reported, as will a discussion of the current limits to the measurements and prospects for the future.