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Physical Review A

Investigation of nonadiabatic effects in molecular-hydrogen Rydberg states by electric field ionization

T. J. Morgan, C. F. Barnett, J. A. Ray, Arnold Russek

Published Sep 1, 1979• 14 citations
Publisher

Abstract

The technique of electric field ionization has been used to study molecular Rydberg states of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ formed by electron capture of ${\mathrm{H}}_{2}^{+}$ or ${\mathrm{D}}_{2}^{+}$ beams in ${\mathrm{H}}_{2}$. The neutral molecules were passed through two intense electric field ionization regions in tandem, separated by an \ensuremath{\sim} ${10}^{\ensuremath{-}7}$-sec drift region. Up to 15% of the Rydberg population removed by the first ionization region was repopulated during transit through the drift region. This result has been interpreted in terms of nonadiabatic state mixing of nearly degenerate adiabatic molecular states. A difference in the state repopulation between Rydberg states of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ was observed, indicating the importance of the rovibrational spectrum on the repopulation mechanism. By comparison, similar studies on atomic Rydberg states showed no difference between H and D. Moreover, the atoms exhibited only a small repopulation effect, consistent with the contribution due to $\stackrel{\ensuremath{\rightarrow}}{1}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{s}}$ coupling.

DOI
10.1103/physreva.20.1062

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The technique of electric field ionization has been used to study molecular Rydberg states of H2{\mathrm{H}}_{2}H2​ and D2{\mathrm{D}}_{2}D2​ formed by electron capture of H2+{\mathrm{H}}_{2}^{+}H2+​ or D2+{\mathrm{D}}_{2}^{+}D2+​ beams in H2{\mathrm{H}}_{2}H2​. The neutral molecules were passed through two intense electric field ionization regions in tandem, separated by an \ensuremath{\sim} 10\ensuremath−7{10}^{\ensuremath{-}7}10\ensuremath−7-sec drift region. Up to 15% of the Rydberg population removed by the first ionization region was repopulated during transit through the drift region. This result has been interpreted in terms of nonadiabatic state mixing of nearly degenerate adiabatic molecular states. A difference in the state repopulation between Rydberg states of H2{\mathrm{H}}_{2}H2​ and D2{\mathrm{D}}_{2}D2​ was observed, indicating the importance of the rovibrational spectrum on the repopulation mechanism. By comparison, similar studies on atomic Rydberg states showed no difference between H and D. Moreover, the atoms exhibited only a small repopulation effect, consistent with the contribution due to 1\ensuremath→\ifmmode⋅\else\textperiodcentered\fis\ensuremath→\stackrel{\ensuremath{\rightarrow}}{1}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{s}}1\ensuremath→​\ifmmode⋅\else\textperiodcentered\fis\ensuremath→ coupling.
The lll-mixing effect and state-transfer induced by an external electric field pulse in ultracold Rydberg gases are observed with cesium atoms excited to nS\mathit{nS}nS states, and measured with a state selective field ionization technique. High-lll states are populated from the initially excited nS\mathit{nS}nS states due to non-adiabatic transitions through avoided crossings that are formed between the nS\mathit{nS}nS state and the ($n\ensuremath{-}4$) manifold. The population of the product state is investigated as a function of the electric field pulse parameters. The coherent property of the transfer is demonstrated using a 2-pulse sequence and the results are consistent with the theoretical calculations. This lll-mixing phenomenon in the electric field may be used to help forming a molecular potential and create Rydberg molecules.
Hydrogen molecules are excited in a molecular beam to Rydberg states around n=17-18 and are exposed to the inhomogeneous electric field of an electric dipole. The large dipole moment produced in the selected Stark eigenstates leads to strong forces on the H2 molecules in the inhomogeneous electric field. The trajectories of the molecules are monitored using ion-imaging and time of flight measurements. With the dipole rods mounted parallel to the beam direction, the high-field-seeking and low-field-seeking Stark states are deflected towards and away from the dipole, respectively. The magnitude of the deflection is measured as a function of the parabolic quantum number k and of the duration of the applied field. It is also shown that a large deflection is observed when populating the (17d2)1 state at zero field and switching the dipole field on after a delay. With the dipole mounted perpendicular to the beam direction, the molecules are either accelerated or decelerated as they move towards the dipole. The Rydberg states are found to survive for over 100 micros after the dipole field is switched off before being ionized at the detector and the time of flight is measured. A greater percentage change in kinetic energy is achieved by initial seeding of the beam in helium or neon followed by inhomogeneous field deceleration/acceleration. Molecular dynamics trajectory simulations are presented highlighting the extent to which the trajectories can be predicted based on the known Stark map. The spectroscopy of the populated states is discussed in detail and it is established that the N+=2, J=1, MJ=0 states populated here have a special stability with respect to decay by predissociation.
The outer electron of a Rydberg atom is so weakly bound to the atomic core that a relatively small electric field may cause the electron to be stripped from the atom. The field strength at which this ionization happens is strongly dependent on the state of the Rydberg atom. Pulsed electric field ionization serves, therefore, not only as a means by which a Rydberg atom may be efficiently detected, but also a means by which the state of the Rydberg atom may be analyzed.\nThe first observation of predominantly diabatic passage to field ionization of a non-hydrogenic Rydberg atom has resulted in an improved understanding of the adiabatic and diabatic ionization processes. The magnitudes of the various field ionization features have been measured and found to agree with theory, and by detailed analysis, it is now possible to label the various field ionization signal features with the appropriate (VBAR)m(,l)(VBAR) quantum numbers. A much better understanding of the laser excitation process, the Rydberg state production, and the response of the excited state to an increasing electric field has enabled measurement of some Rydberg fine-structure intervals, through observation of field ionization quantum beats.
Rydberg-state excitation of atoms in strong infrared laser fields provides a new complementary aspect of the perspective of atom--strong field interactions. In this article, we perform an experimental and theoretical study on the corresponding process of diatomic molecules, N2{\mathrm{N}}_{2}N2​ and O2{\mathrm{O}}_{2}O2​. We show that neutral molecules can also survive strong 800-nm laser fields in high Rydberg states, while their behavior is remarkably different in comparison with their companion atoms, Ar and Xe. The Rydberg excitation of N2{\mathrm{N}}_{2}N2​ generally behaves similarly to Ar, while that of O2{\mathrm{O}}_{2}O2​ is more significantly suppressed than the ionization compared to Xe in a high intensity region, which can be understood in the frame of a semiclassical picture, together with their different structures of molecular orbitals. However, distinct quantum features in the Rydberg excitation processes that are apparently beyond the semiclassical picture have been identified, i.e., the less suppressed probability of O2{\mathrm{O}}_{2}O2​ at low intensity and the oscillation behavior of the ratio between N2{\mathrm{N}}_{2}N2​ and Ar, indicating that our understanding of the relevant physics is still far from complete.
The detection of electrons from the surface interaction of Rydberg hydrogen atoms and molecules at a metal surface is investigated experimentally for the first time. The experimental detection of an electron signal in a time gate corresponding to when the Rydberg atoms or molecules are expected to collide with the surface provides evidence for the previously proposed backscattered electron loss (So et al 2009 Phys. Rev. A 79 012901) as a route of surface ionization under electron-extraction fields. Comparison of the ion- and electron-extraction results shows that the dynamics as the Rydberg states approach the surface are very different in the two cases, leading to the possible loss of control of the electronic polarization in the initially prepared Rydberg state from the traversal of level crossings during the Rydberg trajectory. The difference in the ionization dynamics observed for the Rydberg H and H2 cases also illustrates some of the fundamental differences between atomic and molecular, and hydrogenic and non-hydrogenic systems.
The theory of collisions of Rydberg atoms with rare-gas atoms is generalized to include the effects of an applied static electric field. Two effects occur. First, the field causes the states to split, and the increased energy separations lead to a decrease in the inelastic cross sections. Second, there is a change in the form of the wave functions, because the various Stark states in the field have electron charge distributions that may be localized in very different regions of space. The present work focuses on this second effect. With the description of the initial and final Stark states of the Rydberg atom by the proper electronic wave functions obtained in parabolic coordinates, cross sections are obtained for a variety of specific transitions of Rydberg atoms (n=10,15,and20n=10, 15, \mathrm{and} 20n=10,15,and20) caused by collisions. The calculations are strictly valid for hydrogen, but exhibit general features that are expected to apply to other Rydberg atoms as well. The results show a dramatic effect on the cross section for particular transitions depending on the degree of overlap of the initial and final charge distributions. In addition, it is observed that transitions are favored that involve small changes in the azimuthal quantum number mmm. The origin of this tendency is discussed.