We have investigated the photoassociation (PA) spectra of ultracold
88Sr atoms near the $5s^2 ^1S_0+5s5p ^1P_1$ atomic asymptote. The intensity modulation of the PA lines was used to reconstruct the ground-state scattering wave function, whose nodal point inferred the s-wave scattering length of
a=1.1(2) nm. The observed PA resonances also determined a precise lifetime of the
1P1 state to be 5.263(4) ns.
We present results from two-photon photoassociative spectroscopy of the least-bound vibrational level of the $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+}$ state of the
88Sr2 dimer. Measurement of the binding energy allows us to determine the
s-wave scattering length ${a}_{88}=\ensuremath{-}1.4(6){a}_{0}$. For the intermediate state, we use a bound level on the metastable $^{1}S_{0}\text{\ensuremath{-}}^{3}P_{1}$ potential, which provides large Franck-Condon transition factors and narrow one-photon photoassociative lines that are advantageous for observing quantum-optical effects such as Autler-Townes resonance splittings.
We have investigated photoassociation (PA) spectra of ultracold
88Sr atoms near the
5s21S0+5s5p1P1 atomic asymptote. The intensity modulation of the PA lines was used to reconstruct the ground-state scattering wave function, whose last nodal point was determined to be
r0=3.78(18)nm. The PA lines also determined a precise lifetime of the
1P1 state to be
5.263(4)ns.
Using new experimental measurements of photoassociation resonances near the
1S0→3P1 intercombination transition in
84Sr and
86Sr, we present an updated study into the mass-scaling behavior of bosonic strontium dimers. A previous mass-scaling model [Borkowski et al., Phys. Rev. A 90, 032713 (2014)] was able to incorporate a large number of photoassociation resonances for
88Sr, but at the time only a handful of resonances close to the dissociation limit were known for
84Sr and
86Sr. In this work, we perform a more thorough measurement of
84Sr and
86Sr bound states, identifying multiple new resonances at deeper binding energies out to
E/h=−5 GHz. We also identify several previously measured resonances that cannot be experimentally reproduced and provide alternative binding energies instead. With this improved spectrum, we develop a mass-scaled model that reproduces the observed binding energies of
86Sr and
88Sr to within 1 MHz. In order to accurately reproduce the deeper bound states, our model includes a second
1u channel and more faithfully reproduces the depth of the potential. As determined by the previous mass-scaling study,
84Sr
0u+ levels are strongly perturbed by the avoided crossing between the
1S0+3P1 0u+ (3Πu) and
1S0+1D2 0u+ (1Σu+) potential curves and therefore are not included in this mass-scaled model, but are accurately reproduced using an isotope-specific model with slightly different quantum defect parameters. In addition, the optical lengths of the
84Sr
0u+, ν=−2 to
ν=−5 states are measured and compared to numerical estimates to characterize their use as optical Feshbach resonances.
We report photoassociative spectroscopy of
88Sr2 in a magneto-optical trap operating on the $^{1}S_{0}\ensuremath{\rightarrow}^{3}P_{1}$ intercombination line at 689 nm. Photoassociative transitions are driven with a laser red detuned by 600--2400 MHz from the $^{1}S_{0}\ensuremath{\rightarrow}^{1}P_{1}$ atomic resonance at 461 nm. Photoassociation takes place at extremely large internuclear separation, and the photoassociative spectrum is strongly affected by relativistic retardation. A fit of the transition frequencies determines the
1P1 atomic lifetime ($\ensuremath{\tau}=5.22\ifmmode\pm\else\textpm\fi{}0.03\text{ }\text{ }\mathrm{n}\mathrm{s}$) and resolves a discrepancy between experiment and recent theoretical calculations.
This dissertation describes two-photon photoassociation to the least-bound vibrational level of the X
1Σg+ electronic ground state of the
86Sr
2 dimer, which represents the first observation of this naturally occurring halo molecule. We measure the binding energy of this state to be
Eb=83.00(7)(20)\,kHz. Using the precise determination of the halo state binding energy and the universal theory for a very weakly-bound state on a potential that asymptotes to a van der Waals form, we determine
s-wave scattering lengths for all strontium isotopes. Our results are consistent with, but substantially more accurate than the previously determined values found from spectroscopic determination of long-range coefficients using other strontium isotopes. With a radial expectation value of
⟨r⟩≈21\,nm the halo state extends well into the classically forbidden region, which results in a large sensitivity of the dimer binding energy to light near-resonant with a bound-bound transition on the metastable
1S0−3P1 potential. This suggests that
86Sr may be a promising candidate for manipulating atomic interactions via optical coupling of the molecular bound states and for probing naturally occurring Efimov states. Furthermore, we observe novel multi-photon spectral loss features due to strong coupling and small detuning of the photoassociation lasers. Numerical simulations of a simple three-level model undergoing a simultaneous two-frequency drive, shows that solutions of the time-dependent Schr\"{o}dinger equation accurately reproduce these additional loss features. A Floquet analysis of this model, yields analytic formulas for predicting the AC Stark shift of the halo molecule. Additionally, we report on the current state and recent modifications of our ultracold neutral strontium apparatus with a particular emphasis on the installation of a
532\,nm optical lattice. Characterization of this laser system and applications are discussed for further investigation of the strontium-
86 halo molecule and additional photoassociation experiments.