The preparation of cold molecules is of great importance in many contexts, such as fundamental physics investigations, high-resolution spectroscopy of complex molecules, cold chemistry and astrochemistry. One versatile and widely applied method to cool molecules is helium buffer-gas cooling in either a supersonic beam expansion or a cryogenic trap environment. Another more recent method applicable to trapped molecular ions relies on sympathetic translational cooling, through collisional interactions with co-trapped, laser-cooled atomic ions, into spatially ordered structures called Coulomb crystals, combined with laser-controlled internal-state preparation. Here we present experimental results on helium buffer-gas cooling of the rotational degrees of freedom of MgH(+) molecular ions, which have been trapped and sympathetically cooled in a cryogenic linear radio-frequency quadrupole trap. With helium collision rates of only about ten per second--that is, four to five orders of magnitude lower than in typical buffer-gas cooling settings--we have cooled a single molecular ion to a rotational temperature of 7.5(+0.9)(-0.7) kelvin, the lowest such temperature so far measured. In addition, by varying the shape of, or the number of atomic and molecular ions in, larger Coulomb crystals, or both, we have tuned the effective rotational temperature from about 7 kelvin to about 60 kelvin by changing the translational micromotion energy of the ions. The extremely low helium collision rate may allow for sympathetic sideband cooling of single molecular ions, and eventually make quantum-logic spectroscopy of buffer-gas-cooled molecular ions feasible. Furthermore, application of the present cooling scheme to complex molecular ions should enable single- or few-state manipulations of individual molecules of biological interest.

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Absolute total cross sections (TCSs) for electron scattering from boron trifluoride (BF(3)) and phosphorus trifluoride (PF(3)) molecules have been measured using a linear transmission method. The electron energy ranges from 0.6 to 370 eV for BF(3) and from 0.5 to 370 eV for PF(3). The TCS energy dependence for BF(3) exhibits two very pronounced enhancements: resonantlike narrow feature located near 3.6 eV with the maximum value of 19.2 x 10(-20) m(2), and intermediate energy very broad enhancement with two humps, one centered around 21 eV (18.8 x 10(-20) m(2) in the maximum) and the other near 45 eV (19.5 x 10(-20) m(2)). For PF(3) the TCS has quite different low-energy dependence: at 0.5 eV it has a high value of 70 x 10(-20) m(2) and decreases steeply towards higher energies. Beyond the minimum near 5.5 eV, the TCS reveals two distinct humps: the resonant one centered near 11 eV with the peak value of 32.9 x 10(-20) m(2) and the second one much broader around 35 eV (27.9 x 10(-20) m(2)). The present TCSs for trifluorides are compared to each other as well as to previous TCS data for selected perfluorides and to results for their perhydrided counterparts. The differences and similarities in the shape and magnitude of TCSs are pointed out.

We present experimental values of the electron impact coherence parameters (EICP) and reduced Stokes parameters for excitation of 51P1 state of cadmium atoms. The results have been obtained using electron–photon coincidence technique for incident electron energies 80 eV and 60 eV and electron scattering angles in the range of 5° to 50°. We also present an additional set of data for electron energy 100 eV and scattering angle 50° which complements our previous results. All the experimental values are compared with theoretical relativistic distorted-wave approximation (RDWA) calculations. The first Born approximation (FBA) predictions of the alignment angle are also presented. The theoretical results are in good qualitative agreement with the experimental data.

The electron–photon coincidence technique has been used as the most sensitive tool for investigating inelastic collisions of atoms with electrons and heavy particles and led to new insights into collision dynamics theory. However, for purely geometrical reasons, due to finite sizes of the electron energy analysers and electron beam sources, the measurements have not been carried out for the largest scattering angles. We present a novel system for electron–photon coincidence experiments on electron inelastic scattering with the magnetic angle changer allowing determination of excitation parameters at arbitrarily large scattering angles. As shown by experimental tests, our device can be successfully used in the electron–photon coincidence experiment and it has unprecedented efficacy—deflection angle up to 60° for 100 eV electrons, without a ferromagnetic core.

Abstract-In the experiments involving trapped ions, the application of a cooling procedure is required. The optical Doppler scheme is one of the most commonly used methods for slowing down ions motion. In this paper we present an optical system sufficient for cooling calcium ions in such a scheme. The system allows also for optical detection of trapped ions.Experiments with trapped ions require an efficient method for cooling and detecting the ions inside the trap [1]. In the presented experimental set-up [2] it is intended to investigate various collisional effects involving ions of different species, including atomic and molecular ones, for example: further ionization, dissociation, chargetransfer collisions, chemical reactions, elastic scattering energy transfer, etc.The trapped ions will be slowed down using a sympathetic cooling scheme, which involves direct optical quenching of at least one species of ions forming the ensemble. For numerous reasons (atomic mass 40 close to many other ion species, easy evaporating, easy optical access) calcium ions were chosen as the cooling medium. To ensure efficient calcium cooling, two laser systems are necessary [3].The application of two lasers is explained in Fig. 1, where a simplified scheme of energy levels of calcium ions is shown. Doppler cooling is ensured by the UV laser driving 4 2 S 1/2 -4 2 P 1/2 transition, while the NIR laser is necessary to avoid trapping ions in the dark 3 2 D 3/2 state.The apparatus consists of a linear segmented Paul trap placed in a vacuum chamber. The trap is equipped with an electron gun, oven providing a calcium beam, and source of a molecular beam (not used for calcium cooling). The optical part of the apparatus contains two laser systems (397 and 866nm) used for Doppler cooling and detection of calcium ions.Both systems are external cavity diode lasers. As the lasers wavelengths tend to drift in time, stabilization schemes must be applied to provide efficient cooling.As the UV photon momentum is higher than NIR and the S-P transition is much stronger than that of P-D, the fine tuning of a 397nm beam is more important than a 866nm one. This way, for a UV beam a more precise * E-mail: lklos@izyka.umk.pl method of Pound-Drever-Hall [4] locking was used while for NIR, a simpler FM lock [5] scheme was applied. Both stabilization schemes use vacuum optical cavities made of ultra-low expansion glass (ULE), 150mm long each, providing a free spectral range of 2GHz and finesse of 1000.The Doppler cooling efficiency depends on the UV laser detuning as presented in Fig. 2. The details of derivation of the curve can be found in ref. [6]. The efficiency can be understood as the rate of energy loss by the ion ensemble during the interaction with the laser. Positive values mean cooling the system (red-detuned laser) and negative stand for heating up the ions (bluedetuned laser). As the ions gain energy from the RF electric field, collisions with the residual gas, etc., the equilibrium energy of the ions depends on this rate. The efficiency also ...

We present a method to measure the decay rate of the first excited vibrational state of polar molecular ions that are part of a Coulomb crystal in a cryogenic linear Paul trap. Specifically, we have monitored the decay of the |ν = 1, J = 1)(X) towards the |ν = 0, J = 0)(X) level in MgH+ by saturated laser excitation of the |ν = 0, J = 2)(X)-|ν = 1, J = 1)(X) transition followed by state selective resonance enhanced two-photon dissociation out of the |ν = 0, J=2)(X) level. The experimentally observed rate of 6.32(0.69) s(-1) is in excellent agreement with the theory value of 6.13(0.03) s(-1) (this Letter). The technique enables the determination of decay rates, and thus absorption strengths, with an accuracy at the few percent level.

An absolute total cross section (TCS) for electron scattering from phosphine (PH 3 ) molecules was obtained in a linear transmission experiment at energies ranging from low (0.5 eV) to intermediate (370 eV). The dominant behaviour of the TCS energy function is a very pronounced low-energy enhancement with two distinct resonant-like humps peaked at around 2.4 and 6 eV. Above 10 eV the TCS is a rather featureless, monotonically decreasing function of energy. Our experimental results are compared with the theoretical predictions and intermediate-energy measurements. The similarities and differences of experimental TCS data for isoelectronic hydrides containing third-period atoms (SiH 4 , PH 3 , H 2 S and HCl) are also pointed out and discussed.