Observation of deeply bound ${}^{85}\mathrm{Rb}_2$ vibrational levels using Feshbach optimized photoassociation

We demonstrate Feshbach optimized photoassociation (FOPA) into the $0_g^{-}(5S_{1/2}+5P_{1/2})$ state in ${}^{85}\mathrm{Rb}_2$. FOPA uses the enhancement of the amplitude of the initial atomic scattering wave function due to a Feshbach resonance to increase the molecular formation rate from photoassociation. We observe three vibrational levels, $v=127$, 140, and 150, with previously unmeasured binding energies of 256, 154, and $96{\text{cm}}^{-1}$. We measure the frequency, central magnetic field position, and magnetic field width of each Feshbach resonance. Our findings experimentally confirm that this technique can measure vibrational levels lower than those accessible to traditional photoassociative spectroscopy.

Figure 1

A diagram of the photoassociation process. Two rubidium atoms Rb+Rb in a trapped, ultracold gas collide with relative kinetic energy $E$ above a ground molecular potential $V_X$. During the collision, a photon of energy $E_{PA}$ promotes the pair into a bound vibrational state of energy $E_{ex}$ in the excited molecular potential $V_{ex}$ that dissociates to a ground-state atom and singly excited atom $(\mathrm{Rb}+{\mathrm{Rb}}^{*})$.

Figure 2

Two atoms initially in a particular separated-atom state collide with relative kinetic energy $E$. A magnetic field shifts the open-channel potential ${\text{V}}_X$, such that the collision energy is equal to a bound-state energy $E_a$ of the closed channel ${\text{V}}_a$. This leads to coupling of the two channels and enhancement of the initial scattering wave-function amplitude at small internuclear separation.

Figure 3

The experimental setup for the far-off resonance trap (FORT). Six orthogonal beams (red arrows) overlap at the zero of a magnetic quadrupole field to form a magneto-optical trap (MOT). A focused Ti:sapphire laser (red cones) creates the FORT. Helmholtz coils (HC) produce the magnetic field necessary for the Feshbach resonance. The probe laser passes through the trapped atoms, and the atom density is measured by the absorption of the probe laser measured by the CCD camera.

Figure 4

The timing sequence to induce Feshbach optimized photoassociation. The magneto-optical trap is loaded, and the atoms are transferred to a far-off-resonance trap tuned to a photoassociation resonance. The photoassociation rate is enhanced by a Feshbach resonance, the far-of-resonance trap is turned off, and the shadow of the atoms is imaged on a CCD camera. The vertical axis indicates the quantity being off or on and, in the case of the MOT repump laser, being at a lower intensity. The vertical axis is not to scale.

Figure 5

The circles represent the fractional number of atoms remaining in the trap for each value of the magnetic field at a trapping laser frequency of 12 323 ${\text{cm}}^{-1}$, which excites the colliding atoms into the $\nu =127$ state of the $0_g^{-}$ excited molecular potential. The curve is a Lorentzian fit to the data.

Figure 6

The same as Fig. 5, with a trapping laser frequency of 12 425 ${\text{cm}}^{-1}$, which excites the colliding atoms into the $\nu =140$ state of the $0_g^{-}$ excited molecular potential.

Figure 7

The same as Fig. 5, with a trapping laser frequency of 12 483 ${\text{cm}}^{-1}$, which excites the colliding atoms into the $\nu =150$ state of the $0_g^{-}$ excited molecular potential.