Three methods of perturbing the ground state of the He4 dimer by electric fields are treated: the Stark shift by large static electric fields; dissociation by a pulsed electric field; and the coupling of a plane wave to the interaction-induced quadrupole moment of the dimer. The second process, with coupling to the interaction-induced polarizability anisotropy of the dimer, appears to offer the best prospect for observable perturbation of the dimer. The estimates are based on results of ab initio calculations of properties of the He–He system.
Three methods of perturbing the ground state of the He4 dimer by electric fields are treated: the Stark shift by large static electric fields; dissociation by a pulsed electric field; and the coupling of a plane wave to the interaction-induced quadrupole moment of the dimer. The second process, with coupling to the interaction-induced polarizability anisotropy of the dimer, appears to offer the best prospect for observable perturbation of the dimer. The estimates are based on results of ab initio calculations of properties of the He–He system.
Intense pulsed-laser fields have provided means to both induce spatial alignment of molecules and enhance strength of chemical bonds. The duration of the laser field typically ranges from hundreds of picoseconds to a few femtoseconds. Accordingly, the induced "laser-dressed" properties can be adiabatic, existing only during the pulse, or nonadiabatic, persisting into the subsequent field-free domain. We exemplify these aspects by treating the helium dimer, in its ground [Formula: see text] and first excited [Formula: see text] electronic states. The ground-state dimer when field-free is barely bound, so very responsive to electric fields. We examine two laser realms, designated (I) "intrusive" and (II) "impelling." I employs intense nonresonant laser fields, not strong enough to dislodge electrons, yet interact with the dimer polarizability to induce binding and pendular states in which the dimer axis librates about the electric field direction. II employs superintense high-frequency fields that impel the electrons to undergo quiver oscillations, which interact with the intrinsic Coulomb forces to form an effective binding potential. The dimer bond then becomes much stronger. For I, we map laser-induced pendular alignment within the X state, which is absent for the field-free dimer. For II, we evaluate vibronic transitions from the X to A states, governed by the amplitude of the quiver oscillations.
We study the effect of a strong static homogeneous electric field on the highly excited rovibrational levels of the 7 Li 133 Cs dimer in its electronic ground state. Our full rovibrational investigation of the system includes the interaction with the field due to the permanent electric dipole moment and the polarizability of the molecule. We explore the evolution of the states next to the dissociation threshold as the field strength is increased. The rotational and vibrational dynamics are influenced by the field; effects such as orientation, angular motion hybridization and squeezing of the vibrational motion are demonstrated and analyzed. The field also induces avoided crossings causing a strong mixing of the electrically dressed rovibrational states. Importantly, we show how some of these highly excited levels can be shifted to the continuum as the field strength is increased, and conversely how two atoms in the continuum can be brought into a bound state by lowering the electric field strength.
The tunability of the helium-helium interaction through an external electric field is investigated. For a static external field, electric-field-induced resonances and associated electric-field-induced bound states are calculated for the $^{4}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He},^{3}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$, and $^{3}\mathrm{He}\text{\ensuremath{-}}^{3}\mathrm{He}$ systems. Qualitative agreement is found with the literature for the $^{3}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$ and $^{3}\mathrm{He}\text{\ensuremath{-}}^{3}\mathrm{He}$ systems [E. Nielsen, D. V. Fedorov, and A. S. Jensen, Phys. Rev. Lett. 82, 2844 (1999)]. The implications of the predicted electric-field-induced resonances for $^{4}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$ on the wave packet dynamics, initiated by intense laser pulses, are investigated. Our results are expected to guide next generation experiments.
Dipole-dipole interactions between helium atoms in Rydberg-Stark states with principal quantum number n=53 and approximately linear Stark energy shifts, resulting from induced electric dipole moments of approximately 7900 D, have been investigated experimentally. The experiments were performed in pulsed supersonic metastable helium beams, with particle number densities of up to $\ensuremath{\sim}{10}^{9}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. In the presence of amplitude-modulated, radio-frequency electric fields, changes in the spectral intensity distributions associated with the transitions to these states that are attributed to dipole-dipole interactions within the ensembles of excited atoms have been observed. The experimental results are in excellent agreement with calculations of the Rydberg energy level structure carried out using Floquet methods, and excitations shared by up to four atoms. The use of these Rydberg-Stark states as sensors for nonresonant broadband radio-frequency electrical noise is also discussed.
The rapid development of experimental techniques to produce ultracold alkali molecules opens the ways to manipulate them and to control their dynamics using external electric fields. A prerequisite quantity for such studies is the knowledge of their static dipole polarizability. In this paper, we computed the variations with internuclear distance and with vibrational index of the static dipole polarizability components of all homonuclear alkali dimers including Fr(2), and of all heteronuclear alkali dimers involving Li to Cs, in their electronic ground state and in their lowest triplet state. We use the same quantum chemistry approach as in our work on dipole moments [Aymar and Dulieu, J. Chem. Phys. 122, 204302 (2005)], based on pseudopotentials for atomic core representation, Gaussian basis sets, and effective potentials for core polarization. Polarizabilities are extracted from electronic energies using the finite-field method. For the heaviest species Rb(2), Cs(2), and Fr(2) and for all heteronuclear alkali dimers, such results are presented for the first time. The accuracy of our results on atomic and molecular static dipole polarizabilities is discussed by comparing our values with the few available experimental data and elaborate calculations. We found that for all alkali pairs, the parallel and perpendicular components of the ground state polarizabilities at the equilibrium distance R(e) scale as (R(e))(3), which can be related to a simple electrostatic model of an ellipsoidal charge distribution. Prospects for possible alignment and orientation effects with these molecules in forthcoming experiments are discussed.
We study non-perturbative effects of a static electric field on two-color photoassociation of different atoms. A static electric field induces anisotropy in scattering between two different atoms and hybridizes field-free rotational states of heteronuclear dimers or polar molecules. In a previous paper [D. Chakraborty et al., J. Phys. B 44, 095201 (2011)], the effects of a static electric field on one-color photoassociation between different atoms has been described through field-modified ground-state scattering states, neglecting electric field effects on heteronuclear diatomic bound states. To study the effects of a static electric field on heteronuclear bound states, and the resulting influence on Raman-type two-color photoassociation between different atoms in the presence of a static electric field, we develop a non-perturbative numerical method to calculate static electric field-dressed heteronuclear bound states. We show that the static electric field induced scattering anisotropy as well as hybridization of rotational states strongly influence two-color photoassociation spectra, leading to significant enhancement in PA rate and large shift. In particular, for static electric field strengths of a few hundred kV/cm, two-color PA rate involving high-lying bound states in electronic ground-state increases by several orders of magnitude even in the weak photoassociative coupling regime.