Excited-state Collisions Research Articles

Nearly isotropic Lyman-α 1 radiation following nonradiative electron capture in Xe54+ + Kr collisions at 197 MeV u−1

We measured the angular distribution of the subsequently emitted Lyman-α 1 transition of Xe53+* ions following nonradiative electron capture in collisions of 197 MeV u−1 Xe54+ projectiles with gaseous krypton target. The alignment of the projectile 2p 3/2 state and the relative population of its magnetic substates were further deduced. In contrast to the available lower energy results (Yang et al 2020 Phys. Rev. A 102 042803), it is found that at 197 MeV u−1 the Lyman-α 1 radiation becomes much less anisotropic and the magnetic substates are nearly statistically populated. Moreover, the experimental findings are compared with the results of the corresponding radiative electron capture mechanism, obviously different energy dependence of the magnetic-substate population is obtained and discussed using semiclassical arguments, which help reveal the population mechanism of excited states in collisions of highly charged high-Z ions with atoms.

State-Dependent Optical Lattices for the Strontium Optical Qubit.

We demonstrate state-dependent optical lattices for the Sr optical qubit at the tune-out wavelength for its ground state. We tightly trap excited state atoms while suppressing the effect of the lattice on ground state atoms by more than 4 orders of magnitude. This highly independent control over the qubit states removes inelastic excited state collisions as the main obstacle for quantum simulation and computation schemes based on the Sr optical qubit. Our results also reveal large discrepancies in the atomic data used to calibrate the largest systematic effect of Sr optical lattice clocks.

Engineering Excited-State Interactions at Ultracold Temperatures.

Using a recently developed method for precisely controlling collision energy, we observe a dramatic suppression of inelastic collisions between an atom and ion (Ca+Yb^{+}) at low collision energy. This suppression, which is expected to be a universal phenomenon, arises when the spontaneous emission lifetime of the excited state is comparable to or shorter than the collision complex lifetime. We develop a technique to remove this suppression and engineer excited-state interactions. By dressing the system with a strong catalyst laser, a significant fraction of the collision complexes can be excited at a specified atom-ion separation. This technique allows excited-state collisions to be studied, even at ultracold temperature, and provides a general method for engineering ultracold excited-state interactions.

Singlet and triplet states dissociation of doubly charged HDO, H2O and D2O as a function of internal energy

SynopsisThe fragmentation of HDO2+, H2O2+ and D2O2+ is reported as a function of the excitation energy. The doubly charged ions were prepared selectively to triplet or singlet excited states in collisions with F+ and H+ at 3 keV, respectively. For HDO2+ the isotopic ratio, ratio of probabilities for the two dissociation channels OD+ — H+ over OH+ — D+, was estimated to be 4.5 ± 0.8 using H+ and 3.8 ± 0.8 using F+ projectiles. Results are compared with recent calculations.

Fragmentation of doubly charged HDO, H2O, and D2O molecules induced by proton and monocharged fluorine beam impact at 3 keV.

Doubly charged ions HDO(2+), H2O(2+), and D2O(2+) were prepared selectively to triplet or singlet excited states in collisions with F(+) or H(+) projectiles at 3 keV. Excitation energies of dications following two-body or three-body dissociation channels were measured and compared with recent calculations using ab initio multi-reference configuration interaction method [Gervais et al., J. Chem. Phys. 131, 024302 (2009)]. For HDO(2+), preferential cleavage of O-H rather than O-D bond has been observed and the ratio between the populations of the fragmentation channels OD(+)_H(+) and OH(+)_D(+) were measured. The kinetic energy release has been measured and compared with previous experiments.

Efficient demagnetization cooling of atoms and its limits

Demagnetization cooling relies on spin-orbit coupling that brings motional and spin degrees of freedom into thermal equilibrium. In the case of a gas, one has the advantage that the spin degree of freedom can be cooled very efficiently using optical pumping. We investigate demagnetization cooling of a chromium gas in a deep optical dipole trap over a large temperature range and reach high densities up to $5\times 10^{19} m^{-3}$. We study the loss mechanism under such extreme conditions and identify excited-state collisions as the main limiting process. We discuss that in some systems demagnetization cooling has a realistic potential of reaching degeneracy by optical cooling only.

Comparison of rotationally inelastic rate constants for [formula omitted] + Ne measured using pulsed and cw lasers

We have measured rotationally inelastic rate constants for the processLi2∗A1Σu+(vi=5,ji=30)+Ne→Li2∗A1Σu+(vf=5,jf)+Ne using pulsed laser excitation and compared them to values previously measured using cw laser excitation, applying uniform analysis techniques to both sets of data. We compare the experimental results with rate constants calculated from quasiclassical trajectories on an ab initio potential energy surface, finding that the two experimental methods give consistent results when the different distributions of excited-state collision speeds are taken into account.

Communication: Direct angle-resolved measurements of collision dynamics with electronically excited molecules: NO(A2Σ+) + Ar

We report direct doubly differential (quantum state and angle-resolved) scattering measurements involving short-lived electronically excited molecules using crossed molecular beams. In our experiment, supersonic beams of nitric oxide and argon atoms collide at 90°. In the crossing region, NO molecules are excited to the A(2)Σ(+)state by a pulsed nanosecond laser, undergo rotationally inelastic collisions with Ar atoms, and are then detected 400 ns later (approximately twice the radiative lifetime of the A(2)Σ(+)state) by 1 + 1(') multiphoton ionization via the E(2)Σ(+) state. The velocity distributions of the scattered molecules are recorded using velocity-mapped ion imaging. The resulting images provide a direct measurement of the state-to-state differential scattering cross sections. These results demonstrate that sufficient scattering events occur during the short lifetimes typical of molecular excited states (∼200 ns, in this case) to allow spectroscopically detected quantum-state-resolved measurements of products of excited-state collisions.

Numerical calculations of energy, nucleus size and coulomb decay rate for ddμ* resonance states in the variational approach using new wavefunctions

This paper provides a theoretical complement to the experimental measurement of the population of excited dμ(2s) and dμ(1s) atoms in a deuterium. The population of these atoms plays an important role in a muon catalyzed fusion cycle. Symmetric and non-symmetric muonic molecular ions have been predicted to form in excited states in collisions between excited muonic atoms and hydrogen molecules. One example is the ddμ*, which is a muonic deuterium–deuterium symmetric ion in excited state and is initially produced in the interaction of dμ(2s) atoms with deuterium nuclei. Our calculations interpret the experimental findings in terms of the so-called side-path model. This model essentially deals with the interaction mentioned above in which the ddμ* ion undergoes Coulomb de-excitation where the excitation energy is shared between a dμ(1s) atom and one deuterium. The structure of ddμ* is studied here using the numerical, variational method and the given wavefunctions. Few resonance energies for ddμ* molecular states are calculated below the 2s threshold. For more precise assessment of the reliability of the given wavefunctions, the nucleus sizes and Coulomb decay rates for the zeroth, first and second vibrational meta-stable states of the mentioned ion are also calculated. The obtained results are close to those previously reported. The advantage of the given method over previous methods is that the used wavefunction has only two terms, which simplifies the calculations with the same results as those from the complicated coupled rearrangement channel method with a Gaussian basis set. These energies are the base data required for size, formation and decay rate calculations of the ddμ* ion.

Collisional deexcitation of exotic hydrogen atoms in highly excited states

The deexcitation of exotic hydrogen atoms in highly excited states in collisions with hydrogen molecules has been studied using the classical-trajectory Monte Carlo method. The Coulomb transitions with large change of principal quantum number n have been found to be the dominant collisional deexcitation mechanism at high n. The molecular structure of the hydrogen target is shown to be essential for the dominance of transitions with large \Delta n. The external Auger effect has been studied in the eikonal approximation. The resulting partial wave cross-sections are consistent with unitarity and provide a more reliable input for cascade calculations than the previously used Born approximation.

Loading an optical dipole trap

We present a detailed experimental study of the physics involved in transferring atoms from a magneto-optical trap (MOT) to an optical dipole trap. The loading is a dynamical process determined by a loading rate and a density dependent loss rate. The loading rate depends on cooling and the flux of atoms into the trapping volume, and the loss rate is due to excited state collisions induced by the MOT light fields. From this study we found ways to optimize the loading of the optical dipole trap. Key ingredients for maximum loading are found to be a reduction of the hyperfine repump intensity, increased detuning of the MOT light, and a displacement of the optical dipole trap center with respect to the MOT. A factor of 2 increase in the number of loaded atoms is demonstrated by using a hyperfine repump beam with a shadow in it. In this way we load $8\ifmmode\times\else\texttimes\fi{}{10}^{6}{}^{85}\mathrm{Rb}$ atoms into a 1 mK deep optical dipole trap with a waist of $58 \ensuremath{\mu}\mathrm{m},$ which is 40% of the atoms initially trapped in the MOT.

Low Saturation Intensities in Two-Photon Ultracold Collisions

Interactions between ultracold atoms are of considerable interest because of the appearance of novel lightinduced dynamics at low temperature and the onset of quantum behavior [1]. Ultracold collisions proved to be crucial in attaining Bose-Einstein condensation, and optical control of ultracold collisions appears to provide a promising technique for producing large ensembles of ultracold molecules [2]. In this Letter, we report on a study of light-induced ultracold atomic collisions which, among other striking observations, display saturation intensities for two-photon processes that are much lower than observed for single-photon processes. Any realistic theoretical treatment of excited state collisions must include spontaneous emission as collision times are comparable to excited state lifetimes [3]. Furthermore, at ultracold temperatures— unlike thermal collisions —the hyperfine interaction plays an important role in the collision dynamics [4], though the analysis of virtually all experiments to date has ignored hyperfine interactions. For the two-photon experiment reported here, we analyzed the measurements using a semiclassical model incorporating all hyperfine energy levels. This allows us to elucidate the interplay between the various processes of flux depletion, spontaneous emission, optical shielding, and Franck-Condon overlap. Furthermore, we can quantitatively explain the striking observation of very low saturation intensities in the two-photon collision process.

Magneto-optical trapping and ultracold collisions of potassium atoms

We present measurements of loading and loss rates for a vapor-cell optical trap of the two naturally occurring potassium isotopes 39K and 41K. The unresolved excited-state hyperfine structure makes trapping of K fundamentally different from trapping of the other alkalis and leads to an enhanced loading rate. We measure the loading rate as a function of laser intensity, beam size, and detuning and find that the results are in reasonable agreement with a simple rate-equation model for the loading process. The dependence of the loss rate on trapped-atom density determines the contribution to the loss rates from excited-state collisions. We find a substantial difference between the collisional loss rates for the two isotopes.

Laser-induced ultracold Rb(5S1/2)+Rb(5P1/2) collisions.

Measurements of collisional loss of optically trapped Rb atoms are presented for excitation of the colliding atoms by light tuned near the 5[ital S][sub 1/2][r arrow]5[ital P][sub 1/2] transitions of [sup 85]Rb and [sup 87]Rb. Using the [ital P][sub 1/2] state allows the effects of spontaneous emission on excited-state collisions to be studied with minimal complications arising from hyperfine structure. The shapes of the collision spectra are nearly identical for the two isotopes and are consistent with a simple model for the role of spontaneous emission during the collision.

High densities of cold atoms in a dark spontaneous-force optical trap.

A new magneto-optical trap is demonstrated which confines atoms predominantly in a ``dark'' hyperfine level, that does not interact with the trapping light. This leads to much higher atomic densities as repulsive forces between atoms due to rescattered radiation are reduced and trap loss due to excited-state collisions is diminished. In such a trap, more than ${10}^{10}$ sodium atoms have been confined to densities approaching ${10}^{12}$ atoms ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$.

Calculations of inelastic collisions of excited states of Xe with He and Ar

Coupled channel calculations of thermal collisions of Xe(5p5nl) (nl=6s,6p,5d) with He and Ar have been performed using potential curves and coupling matrix elements that we previously calculated. Coupling among all of the closely spaced 6s′, 6p, and 5d fine structure levels of Xe is fully included. These excited levels lie in the range 76 000–84 000 cm−1 above the Xe ground state, and the spacing of adjacent levels ranges from 84 to 1460 cm−1. Rate constants for level-to-level transitions are calculated at T=300 and T=800 and compared with recent experiments; agreement is generally good. The rate constants are very sensitive to the specific initial and final levels and to the collision partner. This sensitivity can be explained by the pattern of crossings and avoided crossings of potential curves corresponding to individual fine structure levels. The calculations thereby illustrate the importance of spin–orbit effects in heavy atom systems and provide insight into a large amount of data.

Excited-state collisions of trappedRb85atoms

We descrbe a new method for measuring excited-state collisions between optically trapped atoms. With this method, trap-loss collision rates are deduced from the loading behavior of clouds of trapped atoms in the regime where radiation trapping limits the atom density. Our measurements indicate that $^{85}\mathrm{Rb}$ trap-loss collisions occur at significantly smaller rates than expected both from previous work on Cs and from recent models. In addition, the dependence of the trap-loss collisions on the frequency of the light used to excite the atom pairs is also different from that of Cs, suggesting that assumptions about the dynamics in these models need modification.

Associative ionization in collisions of slowed and trapped sodium.

Collisions of trapped atoms occur over time intervals that are often longer than radiative lifetimes. This leads to radiative interruption of excited-state collision dynamics and constrains collisional interactions to atom pairs that are in close proximity at the time of excitation. Using classical dynamics I show that this causes Na associative ionization to be dominated by a four-step process in which the longer-range ${\mathrm{Na}}^{\mathrm{*}}$-Na interaction initiates inward motion that is completed as ${\mathrm{Na}}^{\mathrm{*}}$+${\mathrm{Na}}^{\mathrm{*}}$\ensuremath{\rightarrow}${\mathrm{Na}}_{2}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$+e. Thus low-temperature associative ionization does not result from ${\mathrm{Na}}^{\mathrm{*}}$+${\mathrm{Na}}^{\mathrm{*}}$ collisions. I predict strong single- and two-wavelength dependences, which have a molecular character due to the close proximity at the time of photoabsorption. These are amenable to experimental observation. I also calculate saturation that results from the depletion of interacting pairs. This produces very-low-power saturation and explains the associative ionization measurements of Gould et al. [Phys. Rev. Lett. 60, 788 (1988)].

Exoergic collisions of cold Na-Na.

We calculate rates for two exothermic excited-state collision processes involving sodium at ultracold temperatures. We predict that the rate for fine-structure changing collisions exceeds that of radiative redistribution with release of enough energy to cause loss from current optical traps. A semiclassical treatment is used which accounts for the frequency dependence of absorption and spontaneous emission in midcollision.

Singly and doubly charged ion collisions with neutrals into excited states

Recent experimental results on charge transfer into excited states in collisions between singly and doubly charged ions with He, Ne, Ar, H 2 and O 2 are presented, respectively. The energy range of the ion beams of He q+, Ar q+, Ne + ( q = 1,2) ions is (70–170)× q keV. The optical measurement was performed by an Optical Multichannel Analysis System (OMA). The wavelength range covered was 200–800 nm. Different processes of excitation have been observed in single and double charged ion collisions with neutrals. The dependence of emission cross sections on the potential energy defect is discussed.