We have constructed an apparatus combining the experimental techniques of cold target recoil ion momentum spectroscopy and a laser cooled target. We measure angle differential cross sections in Li(+)+Na-->Li+Na(+) electron transfer collisions in the keV energy regime with a momentum resolution of 0.12 a.u. yielding an order of magnitude better angular resolution than previous measurements. We resolve Fraunhofer-type diffraction patterns in the differential cross sections. Good agreement with predictions of the semiclassical impact parameter method is obtained.

Using the inherent timing stability of pulses from a mode-locked laser, we have precisely measured the cesium 6P 3/2 excited state lifetime. An initial pump pulse excites cesium atoms in two counterpropagating atomic beams to the 6P 3/2 level. A subsequent synchronized probe pulse ionizes atoms which remain in the excited state, and the photo-ions are collected and counted. By selecting pump pulses which vary in time with respect to the probe pulses, we obtain a sampling of the excited state population in time, resulting in a lifetime value of 30.462(46) ns. The measurement uncertainty (0.15%) is slightly larger than our previous report of 0.12% [Phys. Rev. A 84, 010501(R) (2011)] due to the inclusion of additional data and systematic errors. In this follow-up paper we present details of the primary systematic errors encountered in the measurement, which include atomic motion within the intensity profiles of the laser beams, quantum beating in the photo-ion signal, and radiation trapping. Improvements to further reduce the experimental uncertainty are also discussed.

Utilizing two-photon excitation in hot Rb vapor we demonstrate the generation of collimated optical fields at 420 and 1324 nm. Input laser beams at 780 and 776 nm enter a heated Rb vapor cell collinear and circularly polarized, driving Rb atoms to the 5D 5∕2 state. Under phase-matching conditions coherence among the 5S 1∕2 → 5P 3∕2 → 5D 5∕2 → 6P 3∕2 transitions produces a blue (420 nm) beam by four-wave mixing. We also observe a forward and backward propagating IR (1324 nm) beam, due to cascading decays through the 6S 1∕2 → 5P 1∕2 states. Power saturation of the generated beams is investigated by scaling the input powers to greater than 200 mW, resulting in a coherent blue beam of 9.1 mW power, almost an order of magnitude larger than previously achieved. We measure the dependences of both beams in relation to the Rb density, the frequency detuning between Rb ground-state hyperfine levels, and the input laser intensities. A wide range of phenomena can be created by exploiting nonlinear optical processes in a dense atomic vapor. Large enhancements of these processes are possible through the generation of quantum coherences among atomic states and include effects, such as electromagnetically induced transparency, fast and slow light propagation, four-wave mixing (FWM), and lasing without inversion. FWM in particular has been shown to produce both efficient frequency upconversion [1-3] and downconversion [4] using low-power continuous-wave (cw) lasers. The newly created optical fields are narrowband tunable coherent light sources [5], with wavelengths from the IR to approaching the UV depending upon the atomic states involved. Frequency upconversion by FWM has most often been studied in Rb vapor, first demonstrated using low-power cw lasers by Zibrov et al. in 2002 [6] where 15 μW of coherent radiation at 420 nm was achieved. The method relies upon input lasers at 780 and 776 nm [see Fig. 1(a)] driving Rb atoms from the 5S 1∕2 ground state to the 5D 5∕2 state by two-photon excitation, with the 5P 3∕2 level as an intermediate state. A third optical field between the 5D 5∕2 → 6P 3∕2 levels at 5.23 μm is initiated through spontaneous emission. Strong atomic coherences are thus formed in a diamond-type energy level structure, creating coherent blue light at 420 nm (6P 3∕2 → 5S 1∕2 ) by FWM. Recent experiments achieved first 40 μW of 420 nm light through the additional coupling of both 5S 1∕2 hyperfine ground-state levels [1], and subsequently 1.1 mW by further optimization of input laser polarizations and frequencies [3]. The generated blue beam exhibits a high degree of spatial coherence [2], with a spectral linewidth typically limited by the linewidths of the applied laser fields [7]. The absolute frequency of the blue light has been found to be centered on the 6P 3∕2 → 5S 1∕2 transition, with tunability of ≥100 MHz possible by adjustment of the input laser frequencies [5]. Incorporating an additional laser at 795 nm has been shown to both enhance and suppress the FWM process through control of optical pumping [8]. ...

The fine-structure intervals in the nϭ29 state of Si 2ϩ separating levels from Lϭ8 to Lϭ14 have been measured by microwave spectroscopy. A beam of Si 3ϩ ͑Na-like silicon͒ captures a single electron from an n ϭ10 Rydberg target, forming highly excited Rydberg states of Si 2ϩ near nϭ29. Specific L levels within n ϭ29 are selectively detected by excitation with a Doppler-tuned CO 2 laser, followed by Stark ionization. This allows the detection of microwave induced transitions between different L levels in the nϭ29 state, determining the fine-structure intervals. The fine-structure pattern can be used to deduce the dipole polarizability of the Si 3ϩ ion, which forms the core of the Rydberg system. The result ␣ d ϭ7.404(11) is in good agreement with calculations that are comparable in precision.

Fine structure intervals connecting n = 19 Rydberg levels of Si+ with L between 9 and 16 were measured precisely using the RESIS/microwave technique. The fine structure pattern conforms closely with that predicted by an effective potential model, and indicates a value of 11.666(4)a30 for the adiabatic dipole polarizability of the Mg-like ion, Si2+.

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The fine structure of high-angular-momentum n = 9 and 10 Rydberg states of barium has been measured precisely, using the resonant excitation Stark ionization spectroscopy method. Optical transitions corresponding to ͑n , nЈ͒ = ͑10, 30͒, ͑9,17͒, and ͑9,20͒ were induced with a Doppler-tuned CO 2 laser, determining the finestructure energies corresponding to all n = 9 and 10 levels with L ജ 6. The pattern of these fine-structure energies conforms closely with an effective potential model, by comparison with which the dipole and quadrupole polarizabilities of Ba + can be determined. Combining our data with earlier measurements made it possible to deduce, in addition, the portion of ␣ 2 due to the lowest excited D state of Ba + . Our best estimates of these three properties are ␣ 1 = 124.30͑16͒a 0 3 , ␣ 2 = 2462͑361͒a 0 5 , and ␣ 2 0 = 1828͑88͒a 0 5 .

Measurements of the mixing rates and cross sections for collisional excitation transfer between the 5P1/2 and 5P3/2 states of rubidium (Rb) in the presence of 4He buffer gas are presented. Selected pulses from a high repetition rate, mode-locked femtosecond laser are used to excite either Rb state with the fluorescence due to collisional excitation transfer observed by time-correlated single-photon counting. The time dependence of this fluorescence is fitted to the solution of rate equations which include the mixing rate, atomic lifetimes and any quenching processes. The variation in the mixing rate over a large range of buffer gas densities allows the determination of both the binary collisional transfer cross section and a three-body collisional transfer rate. We do not observe any collisional quenching effects at 4He pressures up to 6 atm and discuss in detail other systematic effects considered in the experiment.