Probing Weakly Bound Molecules with Nonresonant Light

Abstract

We show that weakly bound molecules can be probed by ''shaking'' in a pulsed nonresonant laser field. The field introduces a centrifugal term which expels the highest vibrational level from the potential that binds it. Our numerical simulations applied to the Rb 2 and KRb Feshbach molecules indicate that shaking by feasible laser pulses can be used to accurately recover the square of the vibrational wave function and, by inversion, also the long-range part of the molecular potential. DOI: 10.1103/PhysRevLett.103.053003 PACS numbers: 33.80.Àb, 33.15.Àe Weakly bound species, such as halos [1], spend most of their vibrational period in the classically forbidden region of the electronic potential. The accuracy required to treat such a potential stretches current computational methods to their limits. Here we present a versatile technique for determining the interatomic potential from the dependence of the dissociation probability of the weakly bound or halo molecule on the intensity of a pulsed nonresonant laser field. We also show that a cw laser field can be used to control the molecules' size. The technique relies on the ability to impart a value of angular momentum to the weakly bound molecule such that the centrifugal term concomitant with it expels the molecule's vibrational level from the potential and thus causes the molecule to dissociate. Unlike in the case of rotational predissociation, the value of the imparted angular momentum is tunable, derived from the anisotropic polarizability interaction of the molecule with a nonresonant laser field. This interaction creates directional states in which the molecular axis librates (shakes) about the field vector [7]. The laser intensity needed to impart a preordained value of the angular momentum varies characteristically with the internuclear distance. It is this characteristic dependence that can be used to map out the probability density of the vibrational state from which the molecule was forced to dissociate. A highly accurate long-range molecular potential can then be obtained by inverting the vibrational probability density. This route to an accurate potential, independent of spectroscopy or scattering, complements what can be learned from either. We illustrate the technique's machinery by examining Feshbach molecules of acute interest, Rb 2 and KRb.Although the technique is applicable to weakly bound molecules in any rotational state, we limit our considerations here to the case of rotationless (diatomic) molecules, i.e., species in vibrational levels that can only support the ground rotational state. In addition, we consider the molecule to be in a 1 AE electronic state, with a Vðr ! 1Þ ¼ ÀC 6 r À6 asymptotic potential. In order for a level to be rotationless, its binding energy E b must fulfill the inequality, where m is the reduced mass and d 6 % 1:6 is a dimensionless parameter which depends solely on the exponent of the power law potential and can be evaluated analytically [8]. The centrifugal term in the effective potentialthen leads to dissociat...

CHAPTER 9. Molecular Impurities Interacting with a Many-particle Environment: From Ultracold Gases to Helium Nanodroplets

“…The effects are expected to be most pronounced for molecules which possess a small rotational constant B, such as in experiments involving molecules in highly-excited vibrational states. In the context of ultracold gases, the latter can be studied using weakly-bound molecules [140][141][142] created by photoassociation spectroscopy [137], or by measuring nonzero angular momentum Feshbach resonances [138], as schematically illustrated in Fig. 12.…”

“…Depending on the bath density, the energies of the bound molecular states will shift, and so will the positions of the continuumto-bound transitions. An alternative possibility is measuring the angulon self-energy as a shift of the microwave lines in the spectra of weakly bound molecules [140][141][142][143], prepared using one of these techniques.…”

Rotational structure of weakly bound molecular ions

“…This is because such states are nonclassical, often spending most of their lifetime in the farout, classically forbidden region of the potential [4]. Also, molecular species, whether ionic or neutral, in such states can be probed using nonresonant laser light, by "shaking" [19]. On the other hand, threshold states determine low-energy scattering behavior, subject to studies in traps [5].…”

Libration of Strongly‐Oriented Polar Molecules inside a Superfluid

“…Due to a more involved rotational structure of such species, the pendulon spectrum is expected to be even richer than that described in the present study. Finally, it would be of great interest to investigate the field effect on weaklybound halo-species (such as Feshbach molecules) immersed into a Bose-Einstein condensate [27][28][29]. There, novel effects are expected to take place due to a strong coupling of both molecular rotational and vibrational degrees of freedom to the condensate excitations.…”