Toward Ultracold Organic Chemistry: Prospects of Laser Cooling Large Organic Molecules

Abstract

Ultracold organic chemistry enables studies of reaction dynamics and mechanisms in quantum regime. Access to ultracold molecules hinges on the ability to efficiently scatter multiple photons via quasi-closed cycling transitions and, in practice, is complicated by the complex electronic structure of polyatomic molecules. Using equation-of-motion coupled-cluster (EOM-CC) calculations, we demonstrate that an alkaline earth metal attached to various aromatic ligands (such as benzene, phenol, cyclopentadienyl and pyrrolide) feature nearly-closed cycling transitions with only a few additional repump lasers. We also show that aromatic ligands such as benzene can accommodate multiple cycling centers in various geometrical configurations and may open new avenues in quantum information science, precision measurements, and ultracold chemistry. File list (2) download file view on ChemRxiv cooling_large.pdf (10.42 MiB) download file view on ChemRxiv si.pdf (1.48 MiB)

Laser cooled molecules

“…More extensive exploration of these optical cycling centers, as done by Li et al (2019), and the identification of other bonds or structures that behave similarly, will allow for laser cooling to be applied to an increasingly diverse set of complex molecules. Of particular interest are large organic and chiral molecules for studying fundamental symmetries of nature and ultracold quantum chemistry (Ivanov et al, 2020). Experimentally, the CH 3 ligand has been explored in preliminary work on laser cooling of YbOCH 3 (Augenbraun et al, 2020b).…”

Franck-Condon tuning of optical cycling centers by organic functionalization

“…Using multireference wave functions and ground state and timedependent density functional theory (TD-DFT) calculations [27], we investigate the Franck-Condon factors (FCFs, q v ,v ≡ | v |v | 2 , which in many cases approximate the vibrational branching ratio) of alkaline earth phenoxides. We show that (i) electronic transitions in Ca and Sr phenoxides are promising for optical cycling (see also [22]) and (ii) electron-withdrawing substituents on the phenyl ring make the M-O bond more ionic via induction and resonance effects. This substitution suppresses the FCF-induced vibrational branching of spontaneous emission roughly in proportion to the total electron withdrawing strength of the substituents.…”

“…However, recently, a few molecules have been exper-imentally shown to have sufficiently closed optical cycling transitions to allow laser cooling [17][18][19][20][21][22]. These molecules are characterized by vibrational branching ratios that strongly favor decay to a small number of ground-state vibrational levels, meaning only a handful of lasers are required to achieve optical cycling.…”

“…Building upon the M-O-R motif [22,25], here we investigate functionalized phenyl rings for R and introduce a guiding principle by which the vibrational wavefunction overlap can be enhanced by straightforward chemical substitution within the molecular ligand. Using multireference wave functions and ground state and timedependent density functional theory (TD-DFT) calculations [27], we investigate the Franck-Condon factors (FCFs, q v ,v ≡ | v |v | 2 , which in many cases approximate the vibrational branching ratio) of alkaline earth phenoxides.…”

“…Fundamentally, the large values of η 0,0 attained by these species can be traced to the highly ionic nature of the M-O bond; the bonding electron of neutral M is pulled sufficiently far from the M + ion core that excitations of the remaining electron on the core do not perturb the bond. This suggests (see also [22]) that if the electron withdrawing strength of the ligand can be increased, the FCF limit on the number of emitted photons would likewise increase [48]. However, if electron withdrawing chemical groups are located too close to the metal atom, they pull on it and bend the bond, significantly degrading the diagonal FCFs.…”