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

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...

We present femtosecond midinfrared pump-probe measurements of the molecular motion and energy-transfer dynamics of a water molecule that is enclosed by acetone molecules. These confined water molecules show hydrogen-bond and orientational dynamics that are much slower than in bulk liquid water. This behavior is surprising because the hydrogen bonds to the CAO groups of the acetone molecules are weaker than the hydrogen bonds in bulk water. The energy transfer between the OOH groups of the confined water molecules has a time constant of 1.3 ؎ 0.2 ps, which is >20 times slower than in bulk water. We find that this energy transfer is governed completely by the rate at which hydrogen bonds are broken and reformed, and we identify the short-lived molecular complex that forms the transition state of this process.hydrogen bonding ͉ infrared pump-probe spectroscopy ͉ energy transfer W ater plays an essential role in many chemical and biological processes. Over the last decades, this notion has motivated a lot of work on the dynamical properties of bulk liquid water (1-7). However, the role of water in (bio)chemical processes is often played by a limited number of water molecules in a strongly restricted molecular environment. For example, the stability, structure, and biological function of proteins are largely determined by only a few surrounding layers of water molecules (8). When the water molecules participate directly in a reaction, the number of involved water molecules is even smaller. For example, the proton-pumping function of bacteriorhodopsin involves changes of the hydrogen network that is formed by particular amino acids of the protein and only a few confined water molecules (9-12).Recently, the dynamics of water in restricted environments was studied by comparing the spectral dynamics of an optically excited probe molecule embedded in a hydrated (bio)molecule with the spectral dynamics of the same probe molecule in bulk water (13). The spectral dynamics reflect the collective rearrangement of the solvating water, and were found to be much slower within the hydrated (bio)molecule than in bulk water. In this article, we present a study of the hydrogen-bond and energy-transfer dynamics of individual H 2 O and 1 H 2 HO molecules in a confined environment. In this study, we probed the dynamics of the water molecules directly with femtosecond midinfrared laser pulses that are resonant with the OOH stretch vibrations. Experimental MethodsThe system of confined water molecules is prepared by dissolving water (0.4 mol͞liter) in a mixture of acetone (4.0 mol͞liter) and CCl 4 . The structures that are formed in this mixture have a polar internal part consisting of an enclosed water molecule forming hydrogen bonds to the CAO groups of a few surrounding acetone molecules, and an apolar external part formed by the methyl groups of the acetone molecules. The favorable interaction between the methyl groups and the CCl 4 molecules allows these structures to enter the apolar CCl 4 matrix. The molecular ratio of H 2 O͞aceton...