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Abstract. We give a snapshot of the rapidly developing field of ultracold polar molecules and walk the reader through the papers appearing in this Topical Issue.PACS. 33. Molecular properties and interactions with photons -33.80.Ps Optical cooling of molecules; trapping -34. Atomic and molecular collision processes and interactions -39. Instrumentation and techniques for atomic and molecular physics

Previous investigations have shown that the instantaneous eigenstates of a molecule interacting via its polarizability with a strong electric field of a nonresonant laser pulse are pendular hybrids of field-free rotational states, aligned along the field direction. However, nonadiabatic effects during the time evolution of the initial field-free rotational state could cause the molecule to end up in a state described by a linear combination of pendular states ͑a rotational wavepacket͒ whose alignment properties are not a priori known. We report a computational study of the time evolution of these states. We solve the reduced time-dependent Schrödinger equation for an effective Hamiltonian acting within the vibronic ground state. Our numerical results show that the time evolution and the achievement of adiabatic behavior depend critically on the detailed characteristics of the laser pulse and the rotational constant of the molecule.

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Recent experiments have demonstrated the efficacy of orienting low rotational states of a linear polar molecule in a static electric field, εS, or aligning a molecule (polar or not) in an intense nonresonant laser field, εL. We present theoretical results showing that the combined action of εS and εL can markedly sharpen orientation, particularly by introducing a pseudo-first-order Stark effect for tunneling doublets created by the polarizability interaction. Also, if εS and εL are not collinear, the molecular axis can be localized with respect to φ as well as θ, since M states as well as J states undergo hybridization. Another benefit is a means to eliminate “wrong way orientation” which otherwise occurs for “low-field seeking” states.

Interaction of the strong electric field of an intense laser beam with the anisotropic polarizability of a linear molecule creates pendular states, superpositions of the field-free rotational states, in which the molecular axis librates about the field direction. Angular motion in the low-lying pendular states is thereby restricted by a double-well potential, governed by the laser intensity. The pendular energy levels occur as pairs of opposite parity, with separations corresponding to the frequency for tunneling between the wells. If the molecule is polar or paramagnetic, introducing a static electric or magnetic field connects the nearly degenerate pendular levels and thus induces strong pseudo-first-order Stark or Zeeman effects. This can be exploited in many schemes to control and manipulate molecular trajectories.