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...

With a Stark decelerator, beams of neutral polar molecules can be accelerated, guided at a constant velocity, or decelerated. The effectiveness of this process is determined by the six-dimensional ͑6D͒ volume in phase space from which molecules are accepted by the Stark decelerator. Couplings between the longitudinal and transverse motion of the molecules in the decelerator can reduce this acceptance. These couplings are nearly absent when the decelerator operates such that only every third electric-field stage is used for deceleration, while extra transverse focusing is provided by the intermediate stages. For many applications, the acceptance of a Stark decelerator in this so-called s = 3 mode significantly exceeds that of a decelerator in the conventionally used ͑s =1͒ mode. This has been experimentally verified by passing a beam of OH radicals through a 2.6-m-long Stark decelerator. The experiments are in quantitative agreement with the results of trajectory calculations, and can qualitatively be explained with a simple model for the 6D acceptance. These results imply that the 6D acceptance of a Stark decelerator in the s = 3 mode of operation approaches the optimum value, i.e., the value that is obtained when any couplings are neglected.

The Stark deceleration method exploits the concepts of charged particle accelerator physics to produce molecular beams with a tunable velocity. These tamed molecular beams offer interesting perspectives for precise crossed beam scattering studies as a function of the collision energy. The method has advanced sufficiently to compete with state-of-the-art beam methods that are used for scattering studies throughout. This is demonstrated here for the scattering of OH radicals (X 2 P 3/2 , J = 3/2, f) with Ar atoms, a benchmark system for the scattering of open-shell molecules with atoms. Parity-resolved integral state-to-state inelastic scattering cross sections are measured at collision energies between 80 and 800 cm À1 . The threshold behavior and collision energy dependence of 13 inelastic scattering channels is accurately determined. Excellent agreement is obtained with the cross sections predicted by close-coupling scattering calculations based on the most accurate ab initio OH + Ar potential energy surfaces to date.

We report on the Stark deceleration and electrostatic trapping of ¹⁴NH (a¹Δ) radicals. In the trap, the molecules are excited on the spin-forbidden A³∏ <- a¹Δ transition and detected via their subsequent fluorescence to the X³∑⁻ ground state. The 1/e trapping time is 1.4 ± 0.1 s, from which a lower limit of 2.7 s for the radiative lifetime of the a¹Δ, v=0, ,J=2 state is deduced. The spectral profile of the molecules in the trapping field is measured to probe their spatial distribution. Electrostatic trapping of metastable NH followed by optical pumping of the trapped molecules to the electronic ground state is an important step towards accumulation of these radicals in a magnetic trap

Fluorescence resonance energy transfer (FRET) is a distance-sensitive method that correlates changes in fluorescence intensity with conformational changes, for example, of biomolecules in the cellular environment. Applied to the gas phase in combination with Fourier transform ion cyclotron resonance mass spectrometry, it opens up possibilities to define structural/ conformational properties of molecular ions, in the absence of solvent, and without the need for purification of the sample. For successfully observing FRET in the gas phase it is important to find suitable fluorophores. In this study several fluorescent dyes were examined, and the correlation between solution-phase and gas-phase fluorescence data were studied. he study of biomolecular conformation in the gas phase has attracted great attention because it opens possibilities to compare gas-phase and solution-phase structures and to understand the effect of the solvent on a molecule. Because matrix-assisted laser desorption/ionization (MALDI) [1] and electrospray ionization (ESI) [2] typically generate unsolvated ions without any adducts, their structure also provides an ideal model system for theoretical calculations of conformations. A major question today is whether singly or multiply charged biomolecular ions produced by soft ionization methods retain their native (or at least a "native-like"), active conformation in the gas phase. This is often assumed but needs to be proven rigorously.Mass spectrometry has many attractive features for studying biomolecules in the gas phase, including the capability of the isolation of ions and elimination of unwanted species before detection and a positive identification of the molecule from the exact mass or from tandem mass spectrometry (MS/MS) data. There are mass spectrometric methods available for obtaining conformational information of molecules in the gas phase: blackbody infrared radiative dissociation where ions undergo unimolecular dissociation by exchanging their energy with the surroundings by absorption and emission of infrared photons [3], collision-induced dissociation where ions are dissociated as a result of interaction with a target neutral species [4], hydrogendeuterium exchange where hydrogen atoms of the protein are exchanged with the deuterium atoms from a solution [5], covalent or noncovalent tagging of biomolecules based on the surface accessibility of specific moieties [6,7], and ion mobility spectrometry where the measured cross section of the molecule is compared with a theoretically calculated cross section [8]. However, generally speaking, these methods yield only indirect information about the gas-phase conformation, which is often derived from either the overall cross section of the molecule, from a fragmentation pattern, or from a dissociation rate.Recently, the efficient trapping capabilities of mass spectrometry have been coupled with the high sensitivity of fluorescence detection to provide structural and spectroscopic information of molecules in the gas phase. The general idea...