Preparation and detection of states with simultaneous spin alignment and selectable molecular orientation in PbO

Abstract

We are pursuing an experiment to measure the electric dipole moment of the electron using the molecule PbO. This measurement requires the ability to prepare quantum states with orientation of the molecular axis and, simultaneously, alignment of the electron spin perpendicular to this axis.It also requires efficient detection of the evolution of the spin alignment direction within such a state. We describe a series of experiments that have achieved these goals, and the features and limitations of the techniques. We also discuss possible new approaches for improved efficiency in this and similar systems.A permanent electric dipole moment (EDM) of the electron, d e , would violate both parity and time-reversal invariance [1], since d e = 2d e S (where S is the electron spin).There is substantial interest in measurements of d e with sensitivity beyond the current limit d e < 1.6 × 10 −27 e·cm [2]. A non-zero value of d e within the next few orders of magnitude would be a clear indication of physics beyond the Standard Model (SM). Moreover, a nonzero EDM within this range is predicted to occur in a wide range of theories that extend the SM [3]. Heavy polar molecules have a significant advantage over atoms in the search for d e [4]. As a result of the strong hybridization of the atomic orbitals in such molecules, there is a large internal electric field E int = E intn , wheren is the direction of the molecule's internuclear axis. If d e = 0, unpaired electrons interact with this internal field giving rise to a linear Stark shift described by the Hamiltonian H EDM = −d e · E int . This leads to an observable energy shift in molecules that can be 2-3 orders of magnitude larger than what is achievable in atoms under typical laboratory conditions [5, 6].Despite this advantage, molecules pose significant experimental challenges. For example, the thermal Boltzmann distribution limits the population in any particular rovibrational state. This reduces the counting rate in the experiment, and hence the overall sensitivity to d e . In addition, molecules suitable for measuring d e must have unpaired electron spins; such species are chemical free radicals and generally are not thermodynamically stable. This leads to substantial experimental difficulties and typically reduces counting rates even further.Measurements using the molecule PbO hold considerable promise for improved sensitivity to d e , as described in Refs. [7,8,9] and summarized here. PbO is not a free radical; rather, its ground state X(0) 1 Σ + has closed shells [10]. The EDM measurement is conducted in the metastable a(1) [ 3 Σ + ] state, which has two unpaired electron spins and hence is sensitive to d e . The a(1) state can be populated by laser excitation and has a lifetime of τ a ∼ = 82 µs. The Ω-doublet substructure of this |Ω| = 1 state allows it to be easily polarized with a modest external electric field E 15 V/cm. Together these properties allow an EDM measurement using PbO contained in a closed cell, operating at substantial vapor density. These propertie...

The Buffer Gas Beam: An Intense, Cold, and Slow Source for Atoms and Molecules

“…For this reason, effusive beam sources of these molecules are typically not feasible; the Boltzmann distribution of rotational states at temperatures where the species have appreciable vapor pressure put only a tiny fraction in any single rotational state (however, performing an electron EDM experiment with molecules in a hot vapor cell is a possibility [112]). A molecular beam for an EDM search should therefore have low temperature and, in light of Eq.…”

Magnetic and electric dipole moments of theH3Δ1state in ThO

“…The -doublets enable the molecule to be completely polarized in small electric fields. The fully polarized molecule accesses the full eEDM sensitivity of the molecule, while suppressing E-field-induced systematic errors such as those due to leakage currents and geometric phases [7,11,12]. The extremely small magnetic moment of the H 3 1 state makes the molecule less sensitive to effects arising from fluctuating B fields and motional ( v × E/c 2 ) magnetic fields [7].…”

Microwave state transfer and adiabatic dynamics of magnetically trapped polar molecules

“…In general, if one wishes to explore polar radicals in mixed fields in any states except the doubly-stretched ones, either optical trapping or an IoffePritchard trap with a sufficiently large offset B-field as to be above all the crossings is necessary; this implies that polar radicals in their lower parity doublet are extremely challenging candidates for slowing with a magnetic coilgun [27,28]. More broadly, in precision measurement applications it is imperative to take a close look [29] at the exact Stark-Zeeman spectrum of the system of interest, as the spectral distortions from the avoided crossings can persist over very large ranges in magnetic field.…”