Molecular scattering behavior has generally proven difficult to study at low collision energies. We formed a molecular beam of OH radicals with a narrow velocity distribution and a tunable absolute velocity by passing the beam through a Stark decelerator. The transition probabilities for inelastic scattering of the OH radicals with Xe atoms were measured as a function of the collision energy in range of 50 to 400 wavenumber, with an overall energy resolution of about 13 wavenumbers. The behavior of the cross sections for inelastic scattering near the energetic thresholds was accurately measured, and excellent agreement was obtained with cross-sections derived from coupled-channel calculations on ab initio computed potential energy surfaces.The study of collisions between gas-phase atoms and molecules is a well-established method of gathering detailed information about their individual structures and mutual interaction (1 ). The level of detail obtained by these studies depends on the quality of preparation of the collision partners before the collision (2-4 ) and on how accurately the products are analyzed afterward (5-7 ). In recent years, it has become increasingly possible to control the internal and external degrees of freedom of the scattering partners, allowing the potential energy surfaces that govern the molecular collisions to be probed in ever greater detail. The most detailed information is obtained when crossed molecular beams are used to produce intense jets of molecules with a well-defined velocity, confined to only a few internal quantum states. Further state selection can be achieved by optical preparation of a single quantum state or by purification of the beam with the use of electrostatic or magnetic multipole fields (2, 3 ). These methods allow the orientation of the molecules to be controlled before the collision (8, 9 ) and the orientation of the scattered products can be measured (10 ).One of the most important parameters describing a scattering event is the collision energy of the scatterers. However, control over the collision energy has been a difficult experimental task. Since the 1980's, ingenious crossed beam machines have been engineered to vary the crossing angle of the intersecting beams, allowing variation of the collision energy while maintaining particle densities high enough for scattering (11 ). It was thereby possible to measure threshold behavior of rotational energy transfer (12, 13 ), or to tune the collision energy over the reaction barrier for reactive scattering (14, 15 ). These methods led to considerable improvement in the control over collision energy at high energiesfor example to probe short-range interactions. However, a similar level of control over collisions at low energies, which are sensitive probes for long-range interactions, is generally lacking. The angle of the 1

The parameter W a , which characterizes nuclear-spin-dependent parity violation (PV) in the molecular spinrotational Hamiltonian, was computed with a quasirelativistic Hartree-Fock approach for radium fluoride (RaF) and found to be one of the largest absolute values predicted so far. The peculiar electronic structure of RaF leads to highly diagonal Franck-Condon matrices between the energetically lowest two electronic states, which qualifies RaF for direct laser cooling. A subset of diatomic molecules with a wide range of internal structures suitable for this cooling technique is also indicated. As trapped cold molecules offer superior coherence times, RaF can be considered promising for high-precision experiments aimed at molecular PV.

We present a combined experimental and theoretical study on the radiative lifetime of CO in the a (3)Pi(1,2), v=0 state. CO molecules in a beam are prepared in selected rotational levels of this metastable state, Stark-decelerated, and electrostatically trapped. From the phosphorescence decay in the trap, the radiative lifetime is measured to be 2.63+/-0.03 ms for the a (3)Pi(1), v=0, J=1 level. From the spin-orbit coupling between the a (3)Pi and the A (1)Pi states a 20% longer radiative lifetime of 3.16 ms is calculated for this level. It is concluded that coupling to other (1)Pi states contributes to the observed phosphorescence rate of metastable CO.

Optical pumping by blackbody radiation is a feature shared by all polar molecules and fundamentally limits the time that these molecules can be kept in a single quantum state in a trap. To demonstrate and quantify this, we have monitored the optical pumping of electrostatically trapped OH and OD radicals by room-temperature blackbody radiation. Transfer of these molecules to rotationally excited states by blackbody radiation at 295 K limits the 1=e trapping time for OH and OD in the X 2 3=2 , v 00 0, J 00 3=2f state to 2.8 and 7.1 s, respectively. DOI: 10.1103/PhysRevLett.98.133001 PACS numbers: 33.80.Ps, 33.55.Be, 44.40.+a In his 1917 paper Einstein showed [1] that even in the absence of collisions the velocity distribution of a molecular gas takes on a Maxwellian distribution due to the momentum transfer that takes place in the absorption and emission of blackbody radiation. The absorbed and emitted photons optically pump the rotational and vibrational transitions, resulting in thermal distributions over the available states. The rotational temperature of the CN molecule in interstellar space [2], for example, is the result of optical pumping by the cosmic microwave background-radiation [3].The influence of blackbody radiation on atoms and molecules is in general small and it is rare that it can be observed directly in laboratory experiments. However, in a number of cases the interaction with blackbody radiation is experimentally observable and important. The first dynamical effects of blackbody radiation on the population of atomic levels were noticed when studying the lifetime of highly excited Rydberg states in atoms [4]. Atoms in these states can have dipole moments of thousands of Debye, and have sufficient spectral overlap with the spectrum of roomtemperature blackbody radiation. The excitation (and ionization) rates can be on the order of 1000 s ÿ1 , implying that the effect can already be observed on a s time scale.The excitation rates in ground state atoms and molecules are generally much lower, and therefore require a longer interaction time to be observed. Only with the development of ion traps, together with a sufficient reduction of collisional energy exchange (i.e., a good vacuum at room temperature), could the photodissociation of molecular ions and clusters by blackbody radiation be directly observed [5,6]. Ions in storage rings are also trapped long enough for the interaction with blackbody radiation to be noticeable [7].The effect of blackbody radiation on neutral molecules in a trap has until now been left experimentally unexplored, partly because the conditions to observe the effect were not met, and partly because the importance of this effect was not always realized. Polar molecules generally have strong vibrational and/or rotational transitions in the infrared region of the spectrum. As a result they can relatively easily be optically pumped by room-temperature blackbody radiation, and this fundamentally limits the time that these molecules can be kept in a single quantum state in ro...