---
title: Sr-SrOH Atom-Molecule Complex
url: https://www.emergentmind.com/topics/sr-sroh-system
type: topic
---

# Sr-SrOH Atom-Molecule Complex

The Sr-SrOH system denotes the atom-molecule complex formed by a ground-state strontium atom, $\mathrm{Sr}(^1S)$, and ground-state strontium monohydroxide, $\mathrm{SrOH}(X^2\Sigma^+)$). It is studied as a route to forming $\mathrm{Sr_2OH}$, described as a near prolate symmetric top / asymmetric top with rich rotational structure. Current theory characterizes the system as strongly anisotropic and effectively non-reactive under ultracold conditions, with a dense spectrum of near-threshold resonances and candidate optical pathways for coherent transfer from weakly bound atom-molecule states to the rovibrational ground state [2508.14543].

## 1. Physical definition and thermochemical character

In the ultracold-matter literature, the Sr-SrOH system consists of a Sr atom in its ${}^1S$ ground state colliding with an SrOH molecule in its $X^2\Sigma^+$ ground state. The target bound product is $\mathrm{Sr_2OH}$, which the paper describes as a near prolate symmetric top / asymmetric top. The proposed attraction of this platform is that it would extend ultracold assembly from diatomics and linear radicals to an asymmetric-top species while retaining an alkaline-earth-centered optical and electronic structure [2508.14543].

A central result is that the entrance channel is non-reactive in the thermochemical sense relevant to ultracold collisions. The reported reference energies are
$$
\Delta E(\mathrm{Sr}+\mathrm{SrOH}) = 0,
$$
$$
\Delta E(\mathrm{Sr_2}+\mathrm{OH}) = 54993.2\ \mathrm{cm^{-1}},
$$
$$
\Delta E(\mathrm{SrO}+\mathrm{SrH}) = 59864.7\ \mathrm{cm^{-1}}.
$$
Accordingly, spontaneous rearrangement into $\mathrm{Sr_2}+\mathrm{OH}$ or $\mathrm{SrO}+\mathrm{SrH}$ is not energetically accessible under ultracold conditions. The paper notes one important nuance: isotope exchange may still be possible in principle because its energetic cost is of order $0.003\ \mathrm{cm^{-1}}$ [2508.14543].

This non-reactive character is significant because strong anisotropy in atom-molecule systems is often associated with complex short-range dynamics. Here, however, the primary issue is not an open chemical-loss channel but the structure of the interaction potential and the near-threshold bound-state spectrum. A common misconception is therefore that strong anisotropy in Sr-SrOH implies barrierless chemistry; the published thermochemistry indicates instead that the dominant ultracold phenomena should be resonance-rich scattering and bound-state control rather than energetically allowed rearrangement [2508.14543].

## 2. SrOH and Sr as experimentally prepared constituents

The feasibility of Sr-SrOH studies depends on the unusually advanced state control already achieved for SrOH. SrOH is a triatomic, linear free radical in its vibronic ground state and a polyatomic analog of laser-cooled diatomics. In a cryogenic buffer-gas beam, SrOH was produced by laser ablation of a pressed $\mathrm{Sr(OH)_2}$ target inside a $\sim 2\ \mathrm{K}$ helium cryogenic buffer-gas cell, yielding roughly $\sim 10^9$ molecules per pulse with pulse duration $\sim 5\ \mathrm{ms}$, forward speed $130 \pm 20\ \mathrm{m/s}$, and transverse velocity spread $\pm 15\ \mathrm{m/s}$. Optical cycling on the rotationally closed $\tilde{X}^{2}\Sigma^{+}(000)\leftrightarrow\tilde{A}^{2}\Pi_{1/2}(000)$ transition at $688\ \mathrm{nm}$, together with a single $631\ \mathrm{nm}$ repumper for $\tilde{X}^{2}\Sigma^{+}(100)\leftrightarrow\tilde{B}^{2}\Sigma^{+}(000)$, produced a beam deflection of $0.2^\circ$ via scattering of $\sim 100$ photons per molecule [1603.04089].

The optical-cycling closure of SrOH was subsequently quantified much more deeply. Vibronic branching ratios from the first two electronically excited states were measured experimentally at the $\sim 10^{-5}$ level, and a Markov-chain analysis using the measured branching network predicted more than $1.5\times 10^4$ photon scatters on average before decay into an unaddressed vibrational state. The corresponding practical cooling scheme used 8–10 lasers, depending on rotational handling in manifolds such as $X(010)$ and $X(110)$, and supported the claim of $>10^4$ photon scatters per molecule [2205.11381].

That level of closure enabled trapping-based spectroscopy and optical trapping. MOT-assisted spectroscopy identified two new repumping transitions,
$X(12^00)\rightarrow \tilde{A}(020)\mu^2\Pi_{1/2}$ at $711.4\ \mathrm{nm}$ and
$X(12^20)\rightarrow \tilde{A}(020)\kappa^2\Pi_{1/2}$ at $697.7\ \mathrm{nm}$,
and their addition increased the trapped molecule number to $32400(4700)$, a 4.5-fold increase over the previous shallower cycle, with MOT lifetime reaching $210(35)\ \mathrm{ms}$ in the best reported conditions [2509.09786]. An optical dipole trap at $1064\ \mathrm{nm}$ then trapped $1400(300)$ SrOH molecules, with measured lifetimes of $320(30)\ \mathrm{ms}$ for $X(010)$, $135(17)\ \mathrm{ms}$ for $X(200)$, and $190(30)\ \mathrm{ms}$ for $X(03^10)$ [2509.01618].

Cold atomic Sr can also be generated in the same general cryogenic-source framework. Direct thermal emission from a pressed HfC/SrO target in a $4\ \mathrm{K}$ cryogenic buffer-gas beam source released $3.7(2)\times 10^{14}$ Sr atoms per pulse, and adding water vapor to the cell yielded $5.3(5)\times10^{10}$ SrOH molecules with peak SrOH density $3.0(3)\times10^{8}\ \mathrm{molecules/cm^3}$ [2407.09907]. Taken together, these results establish that both constituents of the Sr-SrOH system are not merely spectroscopic abstractions but experimentally producible, laser-addressable, and, for SrOH, trappable.

## 3. Ground-state interaction potential and structure of $\mathrm{Sr_2OH}$

The ground-state interaction in Sr-SrOH was computed with high-level electronic-structure theory. For Sr-SrOH, the potential was represented as
$$
V(R,\theta)=\sum_{\lambda=0}^{35} V_\lambda(R)P_\lambda(\cos\theta),
$$
with fitted long-range coefficients $C_{6,0}=3123$ a.u. and $C_{6,2}=50$ a.u. The same study reported the first computed static polarizability of SrOH, with permanent dipole moment $\mu = 1.75\ \mathrm{D}$ at RCCSD(T), compared with an experimental value of $1.90\ \mathrm{D}$, average polarizability $\bar{\alpha}=182.11$ a.u., and polarizability anisotropy $\Delta\alpha=55.65$ a.u. [2508.14543].

The fully relaxed bound complex $\mathrm{Sr_2OH}$ has the following reported equilibrium properties:

| Property | Reported value |
|---|---:|
| $E_{\min}$ | $-10852\ \mathrm{cm^{-1}}$ |
| $\mu$ | $1.77\ \mathrm{D}$ |
| $A$ | $0.76\ \mathrm{GHz}$ |
| $B$ | $0.80\ \mathrm{GHz}$ |
| $C$ | $15.86\ \mathrm{GHz}$ |
| $r_{\mathrm{Sr-Sr}}$ | $3.81\ \text{\AA}$ |
| $r_{\mathrm{Sr-O}}$ | $2.34\ \text{\AA}$ |
| $r_{\mathrm{O-H}}$ | $0.97\ \text{\AA}$ |
| $\angle\mathrm{Sr-O-Sr}$ | $\approx 109^\circ$ |
| $\angle\mathrm{Sr-O-H}$ | $\approx 125^\circ$ |

In the reduced Jacobi representation used for scattering, where the SrOH fragment is kept linear, the potential minimum is shallower and occurs at
$$
-5261\ \mathrm{cm^{-1}},\quad R=3.35\ \text{\AA},\quad \theta=130^\circ.
$$
The paper also reports a secondary minimum at
$$
-1974\ \mathrm{cm^{-1}} \quad (R=4.60\ \text{\AA},\ \theta=40^\circ),
$$
and saddle points in the linear geometries at $\theta=0^\circ,\ R=4.65\ \text{\AA}$ and $\theta=180^\circ,\ R=6.80\ \text{\AA}$ [2508.14543].

The anisotropy is unusually strong. The isotropic term $V_0(R)$ alone has a minimum
$$
V_0^{\min}=-1334\ \mathrm{cm^{-1}} \text{ at } R=4.50\ \text{\AA},
$$
whereas the full short-range interaction is dominated by a large, structureless $V_1(R)$ term, with higher $\lambda$ components still non-negligible. The paper attributes this to the directional ionic/metal-ligand bonding character of SrOH, the presence of two metal centers competing to interact with the OH ligand, and strong orientation dependence of the electron-density redistribution [2508.14543].

## 4. Ultracold scattering and the near-threshold resonance spectrum

Quantum scattering calculations for Sr-SrOH were carried out at collision energy $E_{\rm col}=1\ \mu\mathrm{K}$ using reduced mass $\mu = 47.83$ u, SrOH rotational constant $B = 0.249203\ \mathrm{cm^{-1}}$, rotational basis up to $j_{\max}=50$, and radial propagation from $R_{\min}=2.5\ \text{\AA}$ to $R_{\max}=300\ \text{\AA}$. Scattering lengths were extracted with **molscat** using the hybrid log-derivative Airy propagator. To assess sensitivity to short-range uncertainty, the full PES was scaled as
$$
V(R,\theta)\rightarrow \gamma V(R,\theta),
$$
with $\gamma$ varied over roughly $\pm 5\%$ [2508.14543].

The resulting scattering-length landscape is dominated by a dense forest of narrow resonances. Over the full $\pm 5\%$ scaling scan, the calculation identified
$$
159\ \text{distinct resonant features},
$$
equivalent to about
$$
\sim 16\ \text{resonances per 1\% change in }\gamma.
$$
The physical origin is the coupling of the entrance channel to a large number of near-threshold bound states involving rotationally excited SrOH states, end-over-end angular momentum $L$, and strong anisotropic couplings [2508.14543].

A particularly instructive comparison is between isotropic and anisotropic dynamics. If only the isotropic term $V_0$ is retained, the scattering length shows a single broad resonance. Restoring the full anisotropic potential splits that behavior into the dense resonance spectrum. This shows that the resonance proliferation is not a generic feature of a deep potential alone; it is specifically driven by the anisotropic structure of the Sr-SrOH interaction [2508.14543].

This distinction addresses another common misconception. Dense resonances do not imply that the system is chemically reactive. For Sr-SrOH, the calculations instead indicate a non-reactive but strongly coupled ultracold complex in which short-range sensitivity, rotational channel mixing, and near-threshold level density are the controlling features. The paper further suggests that magnetically tunable Feshbach resonances may exist, and it also highlights a mergoassociation route in which two optical tweezers are merged and a trap-induced avoided crossing is followed adiabatically to convert separated atom and molecule into a weakly bound molecule [2508.14543].

## 5. Excited states, transition dipoles, and coherent formation of ground-state $\mathrm{Sr_2OH}$

To examine optical assembly beyond scattering resonances, the Sr-SrOH study computed low-lying excited states in a reduced one-dimensional model. The targeted electronic manifolds were three ${}^{2}A'$ states and two ${}^{2}A''$ states, correlating asymptotically as follows:
- $1^2A' \to \mathrm{SrOH}(X^2\Sigma^+)+\mathrm{Sr}(^1S)$,
- $1^2A'', 2^2A', 3^2A' \to \mathrm{SrOH}(X^2\Sigma^+)+\mathrm{Sr}(^3P)$,
- $2^2A'', 4^2A' \to \mathrm{SrOH}(A^2\Pi)+\mathrm{Sr}(^1S)$ [2508.14543].

The excited-state curves are reported to be mostly smooth and nearly parallel to the ground-state curve. The same calculations found a conical intersection between $3^2A'$ and $4^2A'$ near $5.5$–$6.1\ \text{\AA}$, depending on geometry, and strong short-range state mixing in some channels. Transition dipole moments were evaluated through
$$
\mu(v_g, v_e) = \int_0^\infty \Psi_g^{v_g}(R)\,\mu(R)\,\Psi_e^{v_e}(R)\,dR.
$$
For states correlating to $\mathrm{SrOH}(X^2\Sigma^+)+\mathrm{Sr}(^3P)$, the transition dipole moment decays strongly at large $R$. By contrast, for states correlating to $\mathrm{SrOH}(A^2\Pi)+\mathrm{Sr}(^1S)$, the transition dipole moment remains above $6\ \mathrm{D}$ even at long range [2508.14543].

These properties motivate a STIRAP-based route to coherent molecule formation. The analysis used a Tang-Toennies form,
$$
V(R)=\left(A+BR+\frac{C}{R}+DR^2+ER^3\right)e^{-bR} -f_6(bR)\frac{C_6}{R^6},
$$
with fixed $b = 0.7$ and $C_6(\mathrm{Sr-SrOH}) \approx 2923.1$ a.u., estimated by scaling from $\mathrm{Sr}_2$. The basic idea is to connect an initial weakly bound atom-molecule state to the rovibrational ground state through an excited bound state. Because the ground and excited potentials are very similar, the Franck-Condon factors are nearly diagonal, which makes simple three-level two-photon STIRAP difficult. The paper therefore suggests multi-step transfer, for example
weakly bound state $\to v_g=20$ via $v_e=30$, and then $v_g=0$ via $v_e=6$.
Among the candidate intermediate states, $2^2A''$ is identified as the best because it maintains nonzero transition-dipole coupling across the full range [2508.14543].

The importance of this section is methodological as much as spectroscopic. The published result is not an experimental demonstration of coherent $\mathrm{Sr_2OH}$ formation. It is a one-dimensional STIRAP model supported by ab initio excited states and transition dipoles, and the paper explicitly treats the route as plausible but experimentally demanding [2508.14543].

## 6. Relation to the broader SrOH platform

The interest in the Sr-SrOH system is amplified by the fact that isolated SrOH is already an advanced ultracold and precision-measurement platform. SrOH was identified as the first and, so far, the only polyatomic molecule to be directly laser cooled to sub-millikelvin temperatures, and its nearly degenerate $\tilde{X}(200)\leftrightarrow\tilde{X}(03^{1}0)$ rovibrational transitions were analyzed as probes of ultralight bosonic dark matter. For the branches discussed in that work, the reported enhancement factors include $Q_\mu=-617$ at $\omega = 2\pi\times 1.1~\mathrm{GHz}$ and $\pm 23$ at $\omega = 2\pi\times 29$–$31~\mathrm{GHz}$, with estimates that $Q_\mu>10^3$ is achievable and that a one-day measurement could reach $\delta\mu/\mu\sim 10^{-17}$ [1805.08185].

Later MOT-assisted spectroscopy directly measured the $X(200)$–$X(03^10)$ structure, reporting that the observed $X(03^10;N=1)$ level lies about $2.275(4)\ \mathrm{cm^{-1}}$ above $X(200;N=1)$ and that many spacings in the $X(200)$–$X(03^10)$ band fall in the $1$–$100\ \mathrm{GHz}$ range [2509.09786]. Optical trapping then established that the relevant internal states are long-lived on the few-hundred-millisecond scale, with measured lifetimes $135(17)\ \mathrm{ms}$ for $X(200)$ and $190(30)\ \mathrm{ms}$ for $X(03^10)$, consistent with spontaneous radiative decay and black-body excitation limits [2509.01618].

A separate line of work on fully spin-polarized $\mathrm{Li}+\mathrm{SrOH}$ showed ratios of elastic to inelastic collision rates well in excess of $100$ over magnetic fields $1$–$1000\ \mathrm{G}$ and collision energies $10^{-5}$–$0.1\ \mathrm{K}$, with spin relaxation dominated by the direct magnetic dipole-dipole mechanism and the indirect spin-rotation mechanism strongly suppressed [1702.05856]. That study does not establish analogous cooling behavior for Sr-SrOH, but it does show that a heavy $^2\Sigma$ radical such as SrOH can participate in favorable ultracold collisions.

Within this wider context, the Sr-SrOH system occupies a specific niche. Isolated SrOH already supplies optical cycling, MOT loading, optical trapping, and precision-sensitive internal structure; Sr-SrOH adds a non-reactive atom-molecule entrance channel, a strongly anisotropic PES, an exceptionally dense near-threshold resonance spectrum, and a theoretically motivated route to coherent assembly of $\mathrm{Sr_2OH}$ [2508.14543]. A plausible implication is that successful control of Sr-SrOH would connect two strands of current research that are often treated separately: ultracold atom-molecule association and precision-ready polyatomic-state engineering.

Source: https://www.emergentmind.com/topics/sr-sroh-system