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Spin-dipole-induced lifetime of the least-bound (5)Sigma(+)(g) state of He(2 S-3(1))+He(2 S-3(1))

Timothy J. Beams, G. Peach, I. B. Whittingham

Published Jul 1, 2006• 0 citations
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Abstract

The properties of the least-bound vibrational level (v=14) of the (5)Sigma(+)(g) state formed during the ultracold collision of two spin-polarized metastable 2 S-3(1) helium atoms are crucial to studies of photoassociation spectroscopy of metastable helium. We report a calculation of the autoionization lifetime tau(g) of this state induced by spin-dipole coupling of the (5)Sigma(+)(g) state to the (1)Sigma(+)(g) state from which Penning and associative ionization processes are highly probable. We find tau(g)approximate to 150 mu s, significantly larger than the recent experimental estimates of (0.5-3) mu s.

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The properties of the least-bound vibrational level (v=14) of the (5)Sigma(+)(g) state formed during the ultracold collision of two spin-polarized metastable 2 S-3(1) helium atoms are crucial to studies of photoassociation spectroscopy of metastable helium. We report a calculation of the autoionization lifetime tau(g) of this state induced by spin-dipole coupling of the (5)Sigma(+)(g) state to the (1)Sigma(+)(g) state from which Penning and associative ionization processes are highly probable. We find tau(g)approximate to 150 mu s, significantly larger than the recent experimental estimates of (0.5-3) mu s.
The properties of the least-bound vibrational level (v=14)(v=14)(v=14) of the $^{5}\ensuremath{\Sigma}_{g}^{+}$ state formed during the ultracold collision of two spin-polarized metastable 23S12\phantom{\rule{0.2em}{0ex}}^{3}S_{1}23S1​ helium atoms are crucial to studies of photoassociation spectroscopy of metastable helium. We report a calculation of the autoionization lifetime ${\ensuremath{\tau}}_{g}$ of this state induced by spin-dipole coupling of the $^{5}\ensuremath{\Sigma}_{g}^{+}$ state to the $^{1}\ensuremath{\Sigma}_{g}^{+}$ state from which Penning and associative ionization processes are highly probable. We find ${\ensuremath{\tau}}_{g}\ensuremath{\approx}150\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{s}$, significantly larger than the recent experimental estimates of $(0.5\ensuremath{-}3)\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{s}$.
We describe two-photon photoassociation (PA) experiments operated in an ultracold gas of metastable 4He∗^{4}\mathrm{He}^{*}4He∗ atoms in the 23S12\phantom{\rule{0.2em}{0ex}}^{3}S_{1}23S1​ state. Atom-molecule dark resonances as well as Raman signals provide information on the exotic molecule in the least bound vibrational J=2J=2J=2, v=14v=14v=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.
A quantum-mechanical close-coupled calculation for collisions of spin-polarized triplet helium atoms in a magnetostatic trap is presented in which the Penning and associative ionization processes are represented by a complex optical potential. The rate coefficients for the spin relaxation and relaxation-induced Penning ionization processes for a range of magnetic fields and temperatures, spanning the cold and ultracold temperature regimes, have been calculated and the sensitivity of these results to uncertainties in the molecular potentials and autoionization width investigated. The rate constants for both elastic and inelastic processes were found to be particularly sensitive to the short-range ${}^{5}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential and, to a lesser extent, the rate constants for relaxation-induced Penning ionization displayed sensitivity to the short-range form of the ${}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential and the autoionization width.
Spin-dipole mediated interactions between tightly confined metastable helium atoms couple the spin-polarized quintet ${^{5}\ensuremath{\sum}}_{g}^{+}$ state to the singlet ${^{1}\ensuremath{\sum}}_{g}^{+}$ state from which autoionization is highly probable, resulting in finite lifetimes for the trap eigenstates. We extend our earlier study on spherically symmetric harmonic traps to the interesting cases of axially symmetric anisotropic harmonic traps and report results for the lowest 10 states in ``cigarlike'' and ``pancakelike'' traps with average frequencies of 100 kHz and 1 MHz. We find that there is a significant suppression of ionization, and subsequent increase in lifetimes, at trap aspect ratios A=p/qA=p/qA=p/q, where ppp and qqq are integers, for those states that are degenerate in the absence of collisions or spin-dipole interactions.
A theoretical study is performed for the photoassociation of metastable helium. Combining short-range, high-level, ab initio calculations for 5 Σ g / u + and 5 Π g / u symmetries and long-range behaviour, better described in Hund's case (c), potential-energy curves for attractive, 5 Σ u + and 5 Π g , as well as repulsive, 5 Π u and 5 Σ g + , states of quintet metastable helium molecules dissociating to 2 3 S + 2 3 P are obtained. Taking into account the atomic spin-orbit couplings, vibrational levels are derived for near-dissociation levels at the 3 P 2 asymptote. Comparison with results from three recent photoassociation experiments obtained under different conditions (μK temperatures with spin alignment and mK temperatures without) allowed for an assignation of about three quarters of the observed levels. The robustness of the vibrational numbering is confirmed by testing a modification of the short-range potentials.
We observe light-induced frequency shifts in one-color photoassociative spectra of magnetically trapped 4 He* atoms in the metastable 2 3 S 1 state. A pair of ultracold spin-polarized 2 3 S 1 helium atoms is excited into a molecular bound state in the purely long-range 0 u + potential connected to the 2 3 S 1 -2 3 P 0 asymptote. The shift arises from the optical coupling of the molecular excited bound state with the scattering states and the bound states of two colliding 2 3 S 1 atoms. We measure the frequency-shifts for several ro-vibrational levels in the 0 u + potential and find a linear dependence on the photoassociation laser intensity. Comparison with a theoretical analysis provides a good indication for the s -wave scattering length a of the quintet ( 5 Σ g + ) potential, a = 7.2 ± 0.6 nm, which is significantly lower than most previous results obtained by non-spectroscopic methods.