Measuring Atomic & Molecular Polarizabilities
Atom interferometric measurements of atomic polarizability are an order
of magnitude more accurate than previous methods.
C.R. Ekstrom, J. Schmiedmayer, M.S. Chapman, T.D. Hammond, and D.E.
Pritchard, Phys. Rev. A 51, 3883 (1995). Measurement
of the Electric Polarizability of Sodium with an Atom Interferometer.
We propose to make significant advances in the accuracy of atomic and
molecular measurements with atom interferometry. Accuracy in our previous
experiments was primarily limited by uncertainties in the velocity distribution
of our beam and by nonuniformities in our interaction region. Thin-film
microfabrication techniques will enable the construction of an improved
ultra-thin interaction region whose dimensions will be determined accurately
using optical interferometry. The incorporation of vibration isolation
to ensure zero phase stability and to permit smaller period gratings will
further improve precision measurements of polarizabilities.
We have also recently conceived a variant of our previously proposed
technique for velocity multiplexing [HPC95] which will increase the relative
accuracy of our phase shift measurements. Our new proposal continues to
remove uncertainties in the source velocity distribution as a systematic
error in our experiments, while at the same time retaining a higher fraction
of the original beam intensity. Our present variation employs a Ramsey
separated oscillatory field setup which entangles an atom’s velocity with
its internal state. The normally incoherent interference patterns produced
by each velocity sub-class will then be phase shifted via a Stern-Gerlach
magnet, allowing 100% of the atoms to be both velocity multiplexed and
to contribute in-phase to the resulting interference.
Improvements of this type should permit polarizability measurements
with uncertainties in the E-4 range and will permit a direct measurement
of the anisotropic polarizability of a molecule as described below.
Polarizability of Multiple Alkalis
We propose to make precision measurements of the polarizabilities of all
the alkalis. With the improvements mentioned above, we expect to obtain
better than 0.1% accuracy for lithium through potassium in a separated
beam configuration. These measurements will be intercompared and extended
to atomic rubidium and cesium (which are too heavy to separate in our interferometer)
by using the same gradient field to deflect all the different species.
A cesium measurement is especially desirable in order to check atomic structure
theories used in measurements of parity violation parameters [MIB78,
NMW88, WBC97]. Additional applications of these precise polarizability
measurements include improved determinations of Van der Waals interactions,
electric dipole transition rates, and calculations of long-range interatomic
potentials relevant to cold collision theory and Bose-Einstein condensation.
Anisotropic Polarizability of Sodium Molecules
We propose to make the first measurement of both the parallel and the perpendicular
components of the
polarizability of the dimer molecule Na2 in the ground state manifold
using our techniques of molecular [CEH95b] and contrast [SEC94] interferometry.
Simultaneously, we expect to measure the average ground state polarizability
with an accuracy below 0.1%, a factor of 100 improvement over earlier measurements
for alkali dimers [MMS74]. This crucial level of improvement will permit
tests of the various approximations used in molecular structure calculations
[MIB88, BOK94] We have performed calculations of the phase shift and contrast
expected for Na2 molecules when an electric field is applied to one path
in an atom interferometer. The results show considerable structure in the
phase and contrast as a function of applied field; this arises from beating
of interference patterns for molecules with different quantum numbers associated
with molecular orientation, and will permit accurate determination of both
polarizability components. By using our velocity multiplexing scheme we
may be able to separate the various states by taking advantage of
the sharp rephasing properties of a multi-peaked velocity distribution
[HPC95].