Engineered Molecules

We engineer polyatomic molecule to combine features advantageous to precision measurements and quantum science such as laser-coolability, high polarizability, sensitivity to fundamental symmetry violations, exotic nuclei, control over electromagnetic interactions, and tailoring to specific measurement methods.

One of the broader questions which drives our work is: How can we design molecules with features tailored to some scientific goal?  An example of this is our work with studying fundamental symmetry violations with polyatomic molecules, where we proposed molecules which combined some set of useful features which did not exist simultanously in "known" molecules, then created them, studied them spectroscopically, and are now focusing on performing measurements of nuclear and electronic symmetry violations.

This approach relies on knowledge about different optically-active metals for laser control, different ligands to confer different properties, and the structure and properties of the resulting molecule formed by bonding them.  To extend our fundamental symmetry studies further, as well as enable access to molecular enhancement for other applications in nuclear physics, physical chemistry, and quantum sensing, it is critical to continue building the toolbox through basic studies of molecular design and engineering, first via theoretical predictions and then by experimental spectroscopic study.  Some of the more specific questions  we are trying to answer are:

New ligands and symmetries

As discussed on the page describing our research into using polyatomic molecules to study fundamental symmetries, the fact that polyatomic molecules always have states of lowered symmetry makes them very useful to realize high polarizability and implement quantum control.  Our main experimental work has focused on metal hydroxide (MOH) molecules, where M=Sr, Yb, Ra is an alkaline earth (or similar) atom, which can cycle photons for optical control, and have bent states enabling high polarization in ~100 V/cm fields and the ability to robustly reject systematic errors and tune electromagnetic properties.  However, these states have a lifetime limited to a few hundred milliseconds due to they are not in the vibrational ground state.  On the other hand, states with lower symmetry in their vibrational ground state, such as permanently bent molecules or those having a (non-linear) symmetric top structure, can have all of these benefits but in states with considerably longer lifetimes of minutes or more.  We are therefore interesting in studying alkaline earth atoms bonded to other ligands which result in non-linear molecules with fundamentally different symmetries and properties, such as -NH2, -CH3, -OCH3, -SH, and more. 

Figure: states with angular momentum about a symmetry axis, such as rigid body rotation of a CH3 group or bending motion, results in "parity doubling" structures which are very useful and interesting.

 We want to answer questions such as:

Funding and Support: We gratefully acknowledge the Heising Simons Foundation and an NSF CAREER award for supporting this work

Relevant Publications:

Molecules with multiple valence electrons

The best characterized approach to constructing polyatomic molecules with cycling centers uses species with a single valence electron and a single bond.  This approach works very well, but those molecules all have the same electronic structure of a single, metal-centered s-electron, similar to an alkali atom.  While the polyatomic structure can be used to tune certain properties of the molecule, others can only be tuned by changing the electronic structure itself.  For example, alkaline earth atoms have very different structure than alkali atoms due to the fact that they have two valence electrons instead of one, giving rise to novel features like non-magnetic states, optical clock transitions, and much more.  We can then ask the fudamental question - can we find approaches to constructing polyatomic molecules with more than one valence electron which cycle photons?

Figure from one of our papers showing the rough idea - can we find optical cycling centers with multiple valence electrons, and recipes for constructing polyatomic species out of them?

We can take inspiration from diatomics - the aluminum-containing molecules AlF and AlCl, for example, cycle photons extremely well.  Analogous to the alkaline earth-based species, we might be tempted to see if species "which bond like F," such as -OH, preserve the optical cycling center.  Sadly, we find that it does not; the Al-OH bond is too covalent, and the lone pairs on the O deflect the Al-O-H bond angle in a way which strongly depends on the electronic state.  The result is a "floppy" molecule whose bond angle changes significantly when the metal electrons are excited, resulting in a strong coupling between the electronic and vibrational degrees of freedom.  Substituting more electronegative ligands does not solve the problem.

An alternative approach which does preserve the optical cycling characteristics is to instead use a more covalent ligand, such as -SH.  Intuitively, the larger S atom results in the Al and H atoms being further apart, repelling each other less, and therefore locking in to the perpendicular bonding orbitals around the S atom.  The geometry is therefore stabilized by this unique bonding pattern, resulting in a decoupling of the electronic and vibrational degrees of freedom.  This is related to the reason why the hydrogen sulfide molecule H-S-H has a bond angle of around 92 degrees, while the water molecule H-O-H has a bond angle of around 104 degrees.

Figure from one of our papers showing the intuitive idea - the larger Al-S and S-H distance reduce repulsion between the bonding partners and lock the bonds into the perpendicular S orbitals.

This approach gives rise to a number of different electronic structures with decoupling of electronic and vibrational degrees of freedom, including species with two, three, and four valence electrons, across many p-block elements.

Relavent publication: Multivalent optical cycling centers: towards control of polyatomics with multi-electron degrees of freedom. P. Yu, A. Lopez, W. A. Goddard III, N. R. Hutzler. Phys. Chem. Chem. Phys. 25, 154 (2023)

Molecules with multiple optical cycling centers: "Hypermetallic Molecules"

Much of the discussion above is framed as taking an optically-active metal center and bonding some ligand to it to achieve desirable properties.  For certain molecules, the properties of the electronic structure, particularly the ability to achieve optical control via photon cycling, is largely unaffected by choice of ligand with similar bonding patters.  If the optical cycling properties of a metal only depend on the bond to that metal, then large molecules should be able to support multiple, quasi-independent optical cycling centers as long as there is a sufficiently decoupling "linker" between them.  This could have many interesting applications for quantum information processing, quantum sensing, and precision measurements.  For example, one atom could be the primary qubit/sensor, and the other could be used for laser-cooling, environmental monitoring, or state preparation/readout/heralding/etc. via couplings between the two atoms.  This also relaxes requirements on the photon cycling properties of the qubit/sensor atom, which enables advanced quantum control methods for atoms that don’t make bonds amenable to photon cycling.

Figure: Ground and excited state orbitals for the molecules YbCCCa, from one of our papers.  The ground state corresponds to an electron localized on each of the Ca (a) and Yb (b) metals.  There are also excited, metal-centered electronic states (c, d, and e) that can be used to cycle photons.  Thus the -CC- acts as a linker between the optically-active metals.

Funding and Support: This was was previously supported by the DOE Basic Energy Sciences program.

Relevant Publication: Hypermetallic Polar Molecules for Precision Measurements, M. J. O'Rourke, N. R. Hutzler, Phys. Rev. A 100, 022502 (2019)