Metastable Atoms and Molecules

T. J. Gay

C. H. Dugan

Matthias Raunhardt

John W. Keto, F. J. Soley, M. Stockton +1 more

We have studied the dynamic properties of the $\mathrm{He}(2^{3}S)$ and the ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ atomic and molecular metastable states of helium that are produced in electron-bombarded superfluid helium. The steady-state concentration of ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecules is shown to be limited by collisions between pairs of these molecules. Measurements of the rate coefficient for this bimolecular reaction as a function of temperature and pressure show that the reaction is diffusion limited and that the diffusion coefficient for the metastable molecules in the superfluid is determined by ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$-roton collisions. The bilinear destruction of metastable molecules has been monitored for times as long as 0.1 sec after the initial population of the state, and this establishes a lower bound of 0.1 sec on the radiative lifetime of the $(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ state in superfluid helium. The lifetime and decay mode of the $\mathrm{He}(2^{3}S)$ atoms are distinctly different from that of the corresponding ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecule. In particular, the characteristic lifetime of the $\mathrm{He}(2^{3}S)$ state in superfluid helium is essentially independent of the temperature between 2.08 and 1.4 \ifmmode^\circ\else\textdegree\fi{}K and is measured to be about 15 \ensuremath{\mu}sec. Our experimental observations indicate that the destruction of metastable atoms results in the population of high vibrational levels of the $a^{3}\ensuremath{\Sigma}_{u}^{+}$ molecular state. However, it is shown that the production of $(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecules responds immediately to the electron beam (response time 3 \ensuremath{\mu}sec) and thus the formation of this state in the liquid does not depend upon the destruction of metastable atoms. Despite the fact that the lifetimes and decay modes of the metastable $\mathrm{He}(2^{3}S)$ atoms and $(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecules are not related, the production rates for these states are nearly equal and correspond to about 450 and 500 per incident 160-keV electron, respectively.

Steven Moal, M. Portier, Nassim Zahzam +1 more

We describe two-photon photoassociation (PA) experiments operated in an ultracold gas of metastable $^{4}\mathrm{He}^{*}$ atoms in the $2\phantom{\rule{0.2em}{0ex}}^{3}S_{1}$ state. Atom-molecule dark resonances as well as Raman signals provide information on the exotic molecule in the least bound vibrational $J=2$, $v=14$ state of the $^{5}\ensuremath{\Sigma}_{g}^{+}$ interaction potential of two spin-polarized metastable atoms. The physical origin of the various two-photon PA signals is first discussed. Their linewidths are interpreted in terms of processes limiting the lifetime $\ensuremath{\tau}$ of the exotic molecule. A value of $\ensuremath{\tau}=1.4\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{s}$ is found, which significantly differs from two recent calculations of the Penning ionization rate induced by spin-dipole coupling [T. J. Beams et al., Phys. Rev. A 74, 014702 (2006); M. Portier and G. V. Shlyapnikov (unpublished)]. The limitation of $\ensuremath{\tau}$ by atom-molecule inelastic collisions is investigated and ruled out.

Anand Bahl, Jared K. Wahlstrand, Howard M. Milchberg +1 more

We present first of a kind, quantitative comparison between SSSI measurements of transient nonlinear polarization and plasma generation, and the metastable electronic state description of the optical response in atoms and molecules.