Observation and calculation of the quasi-bound rovibrational levels of the electronic ground state of H2+_2^+

Although the existence of quasi-bound rotational levels of the $X^+ \ ^2\Sigma_g^+$ground state of H$_2^+$has been predicted a long time ago, these states have never been observed. Calculated positions and widths of quasi-bound rotational levels located close to the top of the centrifugal barriers have not been reported either. Given the role that such states play in the recombination of H(1s) and H$^+$to form H$_2^+$, this lack of data may be regarded as one of the largest unknown aspects of this otherwise accurately known fundamental molecular cation. We present measurements of the positions and widths of the lowest-lying quasi-bound rotational levels of H$_2^+$and compare the experimental results with the positions and widths we calculate using a potential model for the$X^+$state of H$_2^+$ which includes adiabatic, nonadiabatic, relativistic and radiative corrections to the Born-Oppenheimer approximation.Comment: 5 pages, 3 figure

Structure and dynamics of H2+_2^+ near the dissociation threshold: a combined experimental and computational investigation

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of H$_2$ has been recorded in the vicinity of the dissociative-ionization threshold following three-photon excitation via selected rotational levels of the B $^1\Sigma_u^+$ ($v=19$) and $\bar{\rm H}$ $^1\Sigma_g^+$ ($v=11$) intermediate states. The spectra consist of transitions to bound levels of the X$^+$ $^2\Sigma_g^+$ state of H$_2^+$ with $v^+$ in the range 14-19 and $N^+$ in the range 0-9, of the A$^+$ $^2\Sigma_u^+$ state with $v^+=0$ and $N^+=0-2$, and of shape resonances corresponding to the X$^+\ (v^+=17,N^+=7)$ and X$^+\ (v^+=18,N^+=4)$quasibound levels. Calculations of the level structure of H$_2^+$have been carried out and the influence of adiabatic, nonadiabatic, relativistic and radiative corrections on the positions of these levels, and in the case of the shape resonances also on their widths, has been investigated. Different methods of calculating the widths and profiles of the shape resonances have been tested for comparison with the experimental observations. Slow oscillations of the dissociative-ionization yield have been observed and reflect, in first approximation, the Franck-Condon factors of the$\bar{\text H}$$\rightarrow$X$^+$, A$^+$ bound - free transitions

Pulsed excitation of Rydberg-atom-pair states in an ultracold Cs gas

Pulsed laser excitation of a dense ultracold Cs vapor has been used to study the pairwise interactions between Cs atoms excited to $n$p$_{3/2}$ Rydberg states of principal quantum numbers in the range $n=22-36$. Molecular resonances were observed that correspond to excitation of Rydberg-atom-pair states correlated not only to the $n$p$_{3/2}+n$p$_{3/2}$ dissociation asymptotes, but also to $n$s$_{1/2}+(n+1)$s$_{1/2}$, $n$s$_{1/2}+n'$f$_{j}$, and $(n-4)$f$_{j}+(n-3)$f$_{j}$ $(j=5/2,7/2)$ dissociation asymptotes. These pair resonances are interpreted as arising from dipole-dipole, and higher-order long-range-interaction terms between the Rydberg atoms on the basis of i) their spectral positions, ii) their response to static and pulsed electric fields, and iii) millimeter-wave spectra between pair states correlated to different pair-dissociation asymptotes. The Rydberg-atom--pair states were found to spontaneously decay by Penning ionization and the dynamics of the ionization process were investigated during the first 10 $\mu$s following initial photoexcitation. To interpret the experimental observations, a potential model was derived that is based on the numerical determination of the eigenvalues and eigenfunctions of the long-range interaction Hamiltonian. With this potential model, which does not include adjustable parameters, all experimental observations could be accounted for, and the results demonstrate that long-range-interaction models provide a global and accurate description of interactions in ultracold Rydberg gases and that they correctly account for, and enable the analysis of, phenomena as diverse as the formation of Rydberg macrodimers, Penning ionization in dense Rydberg gases, and Rydberg-excitation blockade effects.Comment: 17 pages, 12 figure

Long-range Rydberg molecules, Rydberg macrodimers and Rydberg aggregates in an ultracold Cs gas

We present an overview of our recent investigations of long-range interactions in an ultracold Cs Rydberg gas. These interactions are studied by high-resolution photoassociation spectroscopy, using excitation close to one-photon transitions into $n$p$_{3/2}$ Rydberg states with pulsed and continuous-wave ultraviolet laser radiation, and lead to the formation of long-range Cs$_2$ molecules. We observe two types of molecular resonances. The first type originates from the correlated excitation of two atoms into Rydberg-atom-pair states interacting at long range via multipole-multipole interactions. The second type results from the interaction of one atom excited to a Rydberg state with one atom in the electronic ground state. Which type of resonances is observed in the experiments depends on the laser intensity and frequency and on the pulse sequences used to prepare the Rydberg states. We obtain insights into both types of molecular resonances by modelling the interaction potentials, using a multipole expansion of the long-range interaction for the first type of resonances and a Fermi-contact pseudo-potential for the second type of resonances. We analyse the relation of these long-range molecular resonances to molecular Rydberg states and ion-pair states, and discuss their decay channels into atomic and molecular ions. In experiments carried out with a two-colour two-photon excitation scheme, we observe a large enhancement of Rydberg-excitation probability, which we interpret as a saturable autocatalytic antiblockade phenomenon.Comment: 28 pages, 11 figures, submitted to EPJ S

Experimental characterization of singlet scattering channels in long-range Rydberg molecules

We observe the formation of long-range Cs$_2$ Rydberg molecules consisting of a Rydberg and a ground-state atom by photoassociation spectroscopy in an ultracold Cs gas near 6s$_{1/2}$($F$=3,4)$\rightarrow$np$_{3/2}$ resonances (n=26-34). The spectra reveal two types of molecular states recently predicted by D. A. Anderson, S. A. Miller, and G. Raithel [Phys. Rev. A 90, 062518 (2014)]: states bound purely by triplet s-wave scattering with binding energies ranging from 400 MHz at n=26 to 80 MHz at n=34, and states bound by mixed singlet-triplet s-wave scattering with smaller and F-dependent binding energies. The experimental observations are accounted for by an effective Hamiltonian including s-wave scattering pseudopotentials, the hyperfine interaction of the ground-state atom, and the spin-orbit interaction of the Rydberg atom. The analysis enabled the characterization of the role of singlet scattering in the formation of long-range Rydberg molecules and the determination of an effective singlet s-wave scattering length for low-energy electron-Cs collisions.Comment: v2 with corrections and modifications - to appear in Phys. Rev. Let

High-resolution photoelectron-spectroscopic investigation of the H2_2O+^+ cation in its A~+{\mathrm {\tilde A^+}} electronic state

The photoelectron spectrum of water has been recorded in the vicinity of the ${\mathrm {\tilde A^+}}$ $\leftarrow$ $\tilde{\mathrm{X}}$ transition between 112 000 and 116 000 cm$^{-1}$ (13.89-14.38 eV). The high-resolution allowed the observation of the rotational structure of several bands. Rotational assignments of the transitions involving the $\Pi(080)$, $\Sigma(070)$ and $\Pi(060)$ vibronic states of the $\tilde{\mathrm{A}}^+$ electronic state are deduced from previous studies of the $\tilde{\mathrm{A}}^+ - \tilde{\mathrm{X}}^+$ band system of H$_2$O$^+$ (Lew, Can. J. Phys. 54, 2028 (1976) and Huet et al., J. Chem. Phys. 107, 5645 (1997)) and photoionization selection rules. The transition to the $\Sigma(030)$ vibronic state is tentatively assigned.Comment: 10 pages, 4 figure

High-resolution spectroscopy of He2+_2^+ using Rydberg-series extrapolation and Zeeman-decelerated supersonic beams of metastable He2_2

Recently, high-resolution spectroscopy of slow beams of metastable helium molecules (He$_2^*$) generated by multistage Zeeman deceleration was used in combination with Rydberg-series extrapolation techniques to obtain the lowest rotational interval in the molecular helium ion at a precision of 18 MHz [Jansen et al. Phys. Rev. Lett. 115 (13) (2015) 133202], limited by the temporal width of the Fourier-transform-limited laser pulses used to record the spectra. We present here an extension of these measurements in which we have (1) measured higher rotational intervals of He${_2}^+$, (2) replaced the pulsed UV laser by a cw UV laser and improved the resolution of the spectra by a factor of more than five, and (3) studied $M_J$ redistribution processes in regions of low magnetic fields of the Zeeman decelerator and shown how these processes can be exploited to assign transitions originating from specific spin-rotational levels ($N^{\prime\prime},J^{\prime\prime}$) of He$_2^*$.Comment: 28 pages, 8 figure

High-Resolution Spectroscopy of High Rydberg States: Chemical and Technological Applications

High Rydberg states of atoms and molecules possess unusual properties that can be exploited in chemistry and technology. The extreme sensitivity of these states to external influences makes them ideal probes of their environment and they can be used to measure electric fields and ion concentrations in the gas phase with high accuracy. The highest Rydberg states with principal quantum number n ≥ 200 lie energetically so close to successive ionization thresholds in atoms and molecules that they can be used to determine ionization potentials precisely and to extract detailed information on the energy level structure of molecular cations. To investigate and better exploit the properties of high Rydberg states, we have developed new high-resolution vacuum ultraviolet laser sources and combined these with millimetre waves in double-resonance experiments. In these experiments a spectral resolution of up to 60 kHz can be achieved. High-resolution spectroscopy is ideally suited to study the fascinating behaviour of high Rydberg states and opens the way to promising applications in the gas-phase chemistry of unstable and charged particles and in measurement technology

Precision measurement of the rotational energy-level structure of the three-electron molecule He2+_2^+

The term values of all rotational levels of the $^4$He{_2}^+\,X^+\,^2\Sigma_u^+\,(\nu^+=0) ground vibronic state with rotational quantum number $N^+\le 19$ have been determined with an accuracy of 8 x 10$^{-4}$ cm$^{-1}$ ($\sim{25}$ MHz) by MQDT-assisted Rydberg spectroscopy of metastable He$_2^*$. Comparison of these term values with term values recently calculated ab initio by Tung et al. [J. Chem. Phys. 136, 104309 (2012)] reveal discrepancies that rapidly increase with increasing rotational quantum number and reach values of 0.07 cm$^{-1}$ ($\sim{2.1}$ GHz) at $N^+=19$.Comment: 11 pages, 6 figure