It has been roughly three decades since laser cooling techniques produced ultracold atoms [1][2][3], leading to rapid advances in a vast array of fields. Unfortunately laser cooling has not yet been extended to molecules because of their complex internal structure. However, this complexity makes molecules potentially useful for a wide range of applications [4]. For example, heteronuclear molecules possess permanent electric dipole moments which lead to long-range, tunable, anisotropic dipole-dipole interactions. The combination of the dipole-dipole interaction and the precise control over molecular degrees of freedom possible at ultracold temperatures make ultracold molecules attractive candidates for use in quantum simulation of condensed matter systems [5] and quantum computation [6]. Also ultracold molecules could provide unique opportunities for studying chemical dynamics [7,8] and for tests of fundamental symmetries [9][10][11]. Here we experimentally demonstrate laser cooling of the polar molecule strontium monofluoride (SrF). Using an optical cycling scheme requiring only three lasers [12], we have observed both Sisyphus and Doppler cooling forces which have substantially reduced the transverse temperature of a SrF molecular beam. Currently the only technique for producing ultracold molecules is by binding together ultracold alkali atoms through Feshbach resonance [13] or photoassociation [14]. By contrast, different proposed applications for ultracold molecules require a variety of molecular energy-level structures (e.g. unpaired electronic spin [5,9,11,15], Omega doublets[16], etc). Our method provides a new route to ultracold temperatures for molecules. In particular it bridges the gap between ultracold temperatures and the ∼ 1 K temperatures attainable with directly cooled molecules (e.g. with cryogenic buffer gas cooling [17] or decelerated supersonic beams [18]). Ultimately our technique should enable the production of large samples of molecules at ultracold temperatures for species that are chemically distinct from bialkalis.

We demonstrate a scheme for optical cycling in the polar, diatomic molecule strontium monofluoride (SrF) using the X 2 Σ + → A 2 Π 1/2 electronic transition. SrF's highly diagonal Franck-Condon factors suppress vibrational branching. We eliminate rotational branching by employing a quasicycling N = 1 → N ′ = 0 type transition in conjunction with magnetic field remixing of dark Zeeman sublevels. We observe cycling fluorescence and deflection through radiative force of an SrF molecular beam using this scheme. With straightforward improvements our scheme promises to allow more than 10 5 photon scatters, possibly enabling the direct laser cooling of SrF. [5,6,7], as qubits in a quantum computer [8], and for studying ultracold chemistry [9,10]. Currently the best technique for creating ultracold molecules relies on their assembly from preexisting ultracold atoms [11,12], and recent experiments have produced ground state polar molecules near quantum degeneracy [12]. Unfortunately only bialkali molecules have been produced this way and the number of molecules created is fairly small (∼ 10 4 ) [12]. There is substantial interest in developing techniques for direct cooling of molecules, enabling use of species with different structures (e.g. unpaired electron spins, which are important for several proposed applications [2, 4, 6, 13]), and possibly also much larger samples. Molecules have been directly cooled using a cryogenic buffer gas [14] or via supersonic expansion followed by beam deceleration [15], but so far the temperature reached with these techniques is limited to ∼ 10 mK.The prospect of direct laser cooling of molecules is attractive because large numbers could in principle be obtained for new types of species at ultracold temperatures. Unfortunately, molecules possess rotational and vibrational degrees of freedom resulting in decays to unwanted sublevels, so finding the closed cycling transitions required for laser cooling is a challenge. However, recent proposals have shown that certain molecules may be amenable to laser cooling because their Franck-Condon factors (FCFs) suppress decays to excited vibrational states [16,17]. Furthermore Stuhl et al. pointed out that a F = 1 → F ′ = 0 type transition eliminates decays to all but the initial F = 1 state [17]. (We use N , v, I, S, F , M , and P as the rotational, vibrational, nuclear spin, electronic spin, total angular momentum, Zeeman, and parity quantum numbers respectively.) In this scheme dark Zeeman levels in the ground state must be remixed to obtain optical cycling [17,18]. Here we propose and demonstrate a scheme for optical cycling that uses the X 2 Σ + 1/2 → A 2 Π 1/2 electronic transition in the molecule SrF. This transition has the required highly diagonal FCFs. Use of a P branch N = 1 → N ′ = 0 type rotational transition eliminates rotational branching, while a magnetic field remixes the dark Zeeman sublevels. The X(N = 1) state of SrF has four hyperfine structure (HFS) levels, so we outline a technique for simultaneously addressing all of these ...

We demonstrate deceleration of a beam of neutral strontium monofluoride molecules using radiative forces. Under certain conditions, the deceleration results in a substantial flux of detected molecules with velocities ≲50  m/s. Simulations and other data indicate that the detection of molecules below this velocity is greatly diminished by transverse divergence from the beam. The observed slowing, from ∼140  m/s, corresponds to scattering ≳10(4) photons. We also observe longitudinal velocity compression under different conditions. Combined with molecular laser cooling techniques, this lays the groundwork to create slow and cold molecular beams suitable for trap loading.

We demonstrate and characterize a cryogenic buffer gas-cooled molecular beam source capable of producing bright beams of free radicals and refractory species. Details of the beam properties (brightness, forward velocity distribution, transverse velocity spread, rotational and vibrational temperatures) are measured under varying conditions for the molecular species SrF. Under typical conditions we produce a beam of brightness 1.2×10 11 molecules/sr/pulse in the X 2 Σ + (v = 0, Nrot = 0) state, with 140 m s forward velocity and a rotational temperature of ≈ 1 K. This source compares favorably to other methods for producing beams of free radicals and refractory species for many types of experiments. We provide details of construction that may be helpful for others attempting to use this method.

In a dense gas of 300 microK 85Rb atoms of n approximately 50 ionization occurs on a 100 ns time scale, far too fast to be explained by the motion of the atoms or photoionization by 300 K blackbody radiation. Rapid ionization is accompanied by spectral broadening, with the spectrum becoming continuous at n=88 at a density of 5x10(10)cm(-3). The atomic transitions broaden both smoothly and by the emergence of new features, which we attribute to multiple atom absorptions. We attribute the rapid ionization to a sequence of near resonant dipole-dipole transitions through virtual states in this intrinsically many-body system, culminating in the ionization of some of the atoms.