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arXiv:2105.11995 · Physical Review A

Near-threshold scaling of resonant inelastic collisions at ultralow temperatures

Rebekah Hermsmeier, Adrien Devolder, Paul W. Brumer, Timur V. Tscherbul

Published Jan 14, 2022• 3 citations
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Abstract

We show that the cross sections for a broad range of resonant inelastic processes accompanied by excitation exchange (such as spin exchange, F\"orster resonance, or angular momentum exchange) exhibit a near-threshold scaling ${E}^{\mathrm{\ensuremath{\Delta}}{m}_{12}}$, where $E$ is the collision energy, $\mathrm{\ensuremath{\Delta}}{m}_{12}={m}_{1}^{\ensuremath{'}}+{m}_{2}^{\ensuremath{'}}\ensuremath{-}{m}_{1}\ensuremath{-}{m}_{2}$, and ${m}_{i}$ and ${m}_{i}^{\ensuremath{'}}$ are the initial and final angular momentum projections of the colliding species ($i=1,2$). In particular, the inelastic cross sections for $\mathrm{\ensuremath{\Delta}}{m}_{12}=0$ transitions display an unconventional ${E}^{0}$ scaling similar to that of elastic cross sections, and their rates vanish as ${T}^{\mathrm{\ensuremath{\Delta}}{m}_{12}+1/2}$. For collisions dominated by even partial waves (such as those of identical bosons in the same internal state) the scaling is modified to ${\ensuremath{\sigma}}_{\text{inel}}\ensuremath{\propto}{E}^{\mathrm{\ensuremath{\Delta}}{m}_{12}+1}$ if $\mathrm{\ensuremath{\Delta}}{m}_{12}$ is odd. We present accurate quantum scattering calculations that illustrate these modified threshold laws for resonant spin exchange in ultracold $\text{Rb}+\text{Rb}$ and ${\mathrm{O}}_{2}+{\mathrm{O}}_{2}$ collisions. Our results illustrate that the well-known ${k}^{\ensuremath{-}1}$ threshold scaling of inelastic cross sections only applies to exothermic, rather than resonant, inelastic processes.

DOI
10.1103/physreva.105.l011302

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When experiment challenges theory: Scattering of vibrationally excited molecules in the cold collision energy regime

F. Javier Aoiz

The last decade has seen substantial advances in the exploration of inelastic and reactive collisions at collision energies in the cold ( < $<$ 1 K) and ultracold ( < 1 $<1$ mK) regime.1-13 These advances ushered in an understanding of cold and ultracold collisions that is impacting research areas ranging from molecular spectroscopy to quantum simulation and computation to quantum control of chemical reaction dynamics. Cold collisions are dominated by resonances, as amply manifest in most of the systems studied.1, 14 Resonances are also present in the thermal ( > $>$ 100 K) and hyperthermal ( > $>$ 1000 K) collision energy regimes,15 but obscured by background (non-resonant) scattering and the presence of many partial waves pertaining to higher angular momenta that blur the partial waves' individual contributions. Moreover, the outcome of thermal and hyperthermal collisions is somewhat insensitive to the details of the interaction potential – the potential energy surface (PES) – especially to its attractive long-range region. Only in a handful of cases, such as the F+HD exchange reaction, resonances in the thermal regime have been experimentally detected and theoretically accounted for by quantum mechanical scattering calculations.16, 17 In contrast, in the cold regime, collisions are dominated by only a few partial waves whose effects on the integral and differential scattering cross-sections can be singled out and linked to particular resonances. The cold collision energy regime has been attained by making use of crossed molecular beams that were either Stark- or Zeeman-decelerated or whose intersection angle could be reduced all the way down to the merging-beams limit. A beam emerging from a Stark or Zeeman decelerator has a variable, well-defined laboratory velocity. When crossed with a secondary beam at small angles (5 ∘ $^{\circ }$ –10 ∘ $^{\circ }$ ), collision energies in the 0.3–15 K could be readily achieved, as demonstrated by the group of van de Meerakker.9, 11 The technique of merged beams makes use of the ability to bend one of the beams by a magnetic or electric field such that the crossing angle becomes zero, thereby affording collision energies in the range 0.1 K–10 K. Merging beams have been used in studies of Penning ionization by the groups of Narevicius5, 12 and Osterwalder18 and made it possible to unravel shape and Feshbach resonances arising in this process. The techniques mentioned so far require the reactants to be polar or paramagnetic. For reactants that are neither, collision energies as low as 5 K could be achieved by taking advantage of a favorable combination of low intersection angles and low reduced mass of the reactants. This approach has been demonstrated by Bergeat, Costes, Naulin, and coworkers in a series of reactive3, 19 and inelastic collision experiments8, 20 of H 2 $_2$ . Yet, another approach to achieving low relative collision velocities relies on the coexpansion of the collision partners in a single molecular beam.21 If their masses are similar, both partners will end up with nearly the same velocities, within a few ms − 1 $^{-1}$ . Using this technique, Zare, Mukherjee, and coworkers carried out a series of experiments in which rotational quenching of HD by H 2 $_2$ , D 2 $_2$ , and He was investigated. Moreover, in these experiments, the HD molecule was prepared in v $v$ =1 and j $j$ =2 using Stark-field-induced adiabatic Raman passage that allowed, in addition, to align the rotational angular momentum of the HD reactant molecule with respect to the initial relative velocity of the HD + He reactants.22-24 Collisional energy transfer between rotation and translation, in which a molecule is excited or relaxed via scattering by an atom or another molecule, has been of particular significance for developing our understanding of inelastic collisions. Most notably, the scattering of NO molecules by rare gas (Rg) atoms has served as a benchmark system for both experimentalists and theorists for decades. In its ground electronic state, NO is a 2 Π $^2\Pi$ molecule with a single unpaired electron in an antibonding π ∗ $\pi ^*$ orbital. In this state, the rotational levels are split into two spin–orbit manifolds, defined by the projection | Ω | $|\Omega |$ ( = 1 2 $=\frac{1}{2}$ or 3 2 $\frac{3}{2}$ ) of the electronic angular momentum (orbital and spin) on the internuclear axis. The rotational levels with the same j $j$ in the two manifolds are separated by approximately 123 cm − 1 $^{-1}$ for v $v$ =0 (117 cm − 1 $^{-1}$ for v $v$ =10). In addition, each rotational level within each manifold is further split into two Λ $\Lambda$ -doublet states ( ε = ± 1 $\epsilon = \pm 1$ , e/f), which are nearly degenerate, but differ in parity, p = ( − 1 ) j − ε / 2 $p = (-1)^{j-\epsilon /2}$ . As the Rg atom approaches the NO molecule, the degeneracy of the Λ $\Lambda$ doublets is lifted, giving rise to two PESs, A ′ $A^{\prime }$ and A ′ ′ $A^{\prime \prime }$ , in which the orbital of the unpaired electron lies, respectively, in the plane of the three atoms or perpendicular to it. In computational practice, it is more convenient to use linear combinations (sums and differences) of the A ′ ′ $A^{\prime \prime }$ and A ′ $A^{\prime }$ PESs. Using hexapole focusing or Stark deceleration combined with detection via velocity ion imaging, fully state-to-state-resolved integral and differential cross sections for rotationally elastic and inelastic collisions could be measured with high resolution.9, 11, 25, 26 More than that, the orientation and alignment of the scattered (outgoing) NO molecules could be measured as a function of the scattering angle27-30 and, quite recently, the effects of orientation of the reactant (incoming) NO molecules on the differential cross section in collisions with Rg, mostly Ar, could be established.31-35 On the theory side, quantum mechanical close coupling (QMCC) scattering calculations on accurate two-dimensional ab initio PESs using the benchmark coupled-cluster, CCSD(T), method with the NO internuclear distance fixed at its equilibrium value36 were performed and found to account for all the details revealed by the experiments. Although state of the art, most of these experiments except for those described in Refs. 9, 11 were carried out at thermal energies for NO molecules in their lowest level, | v = 0 , j = 1 2 , Ω = 1 2 , f ⟩ $|v=0, j=\frac{1}{2}, \Omega =\frac{1}{2}, f\rangle$ . Scattering of highly vibrationally excited molecules in the cold energy regime remained an unexplored frontier until the recent work by the groups of Arthur Suits, Ad van der Avoird, Gerrit Groenenboom, and Hua Guo who added spin–orbit and rotational state selection and looked for spin- and rotational-state changing collisions to boot. Published in the current issue of Natural Sciences,37 this work is a culmination of a 3-year collaboration that included both implementation of new experimental techniques and boosting of theoretical methods. Working with the NO+Ar system, the authors made use of stimulated emission pumping via the electronically excited A 2 Σ + $A^2\Sigma ^+$ state to promote NO( 2 Π 1 2 $^2\Pi _{\frac{1}{2}}$ ) molecules to the vibrationally excited state with v = 10 $v=10$ and a selected spin–orbit state | Ω | $|\Omega |$ and rotational state j $j$ , | v = 10 , j , Ω , f ⟩ $|v=10, j,\Omega , f\rangle$ .38, 39 Low (cold) collision energies were achieved in a novel experimental setup of two beams (dual-beam arrangement) crossing at 4 ∘ $^{\circ }$ and intersected by the pump and dump laser beams of the stimulated-emission pumping scheme. Downstream, the collision products were probed using 1+1 ′ $^{\prime }$ REMPI with the ionization laser tuned just above the ionization threshold. Fully state-resolved angular distributions of the scattered NO molecules were measured by velocity ion slice imaging and detected by a charge-coupled device (CCD). In order to be able to detect the product (outgoing) molecules resulting from the cold NO + Ar collisions, some excess energy had to be made available to the products. This came in the form of the initial spin–orbit and/or rotational state energy. In the study highlighted herein,37 angular distributions leading to the spin–orbit changing transition | v = 10 , Ω = 3 2 , j = 3 2 f ⟩ → | v ′ = 10 , Ω ′ = 1 2 , j ′ = 9 2 , e ⟩ $|v=10, \Omega =\frac{3}{2}, j=\frac{3}{2} f\rangle \rightarrow |v^{\prime }=10, \Omega ^{\prime }=\frac{1}{2}, j^{\prime }=\frac{9}{2}, e \rangle$ were measured at energies in the 2 K–5 K range. Although intrabeam scattering could have been used as an alternative as it had been in previous work,21 the authors demonstrated that a dual-beam setup provides a superior angular and energy resolution. The highlighted experiment challenged the existing theoretical treatments of the NO+ Rg systems: first, the approximation of fixing NO at its equilibrium distance was bound to fail for NO in v = 10 $v=10$ due to the concomitant elongation of the NO bond. Second, for such a highly excited vibrational state, both the long- and short-range interactions are affected, triggering nonadiabatic effects involving rotational and spin–orbit degrees of freedom. The above rendered the single-reference coupled-cluster method used in previous treatments of NO( 2 Π 1 2 , v = 0 $^2\Pi _{\frac{1}{2}},v=0$ ) inadequate. Therefore, the authors of the highlighted paper37 constructed a new coupled-cluster PES as well as a multireference configuration-interaction (MRCI) PES and carried out full-fledged (3D) QMCC calculations on both PESs. These tour-de-force calculations reproduced quite well the experimental differential cross sections and defined a new state of the art in scattering calculations. However, some discrepancies remain as at low collision energies, and the accuracy of the long-range potential needed is a challenge for any ab initio method. Moreover, treating spin-changing collisions that are dominated by the essentially attractive difference potential, V A ′ ′ − V A ′ $V_{A^{\prime \prime }}-V_{A^{\prime }}$ , puts additional demands on the accuracy of the A ′ ′ $A^{\prime \prime }$ and A ′ $A^{\prime }$ PESs. The calculated integral cross section in the 3–4 cm − 1 $^{-1}$ energy interval exhibits a bump that may indicate the presence of resonances that show up as backscattering in the measured differential cross section. A partial-wave analysis points to overlapping resonances associated with contributions from several of the highest partial waves involved. In any case, tracing the resonances may require additional improvements of the existing PESs. All in all, the highlighted combined experimental and theoretical study sets a new standard for our understanding of cold collisions. There is no conflict of interest. The peer review history for this article is available at https://publons.com/publon/10.1002/ntls.20210088. This Highlight has been internally reviewed and edited by Bretislav Friedrich and Gerard Meijer. No data available.

Natural Sciences•Nov 11, 2021•0 citations
We show that the cross sections for a broad range of resonant inelastic processes accompanied by excitation exchange (such as spin exchange, F"orster resonance, or angular momentum exchange) exhibit a near-threshold scaling E\ensuremathΔm12{E}^{\mathrm{\ensuremath{\Delta}}{m}_{12}}E\ensuremathΔm12​, where EEE is the collision energy, \ensuremathΔm12=m1\ensuremath′+m2\ensuremath′\ensuremath−m1\ensuremath−m2\mathrm{\ensuremath{\Delta}}{m}_{12}={m}_{1}^{\ensuremath{'}}+{m}_{2}^{\ensuremath{'}}\ensuremath{-}{m}_{1}\ensuremath{-}{m}_{2}\ensuremathΔm12​=m1\ensuremath′​+m2\ensuremath′​\ensuremath−m1​\ensuremath−m2​, and mi{m}_{i}mi​ and mi\ensuremath′{m}_{i}^{\ensuremath{'}}mi\ensuremath′​ are the initial and final angular momentum projections of the colliding species (i=1,2i=1,2i=1,2). In particular, the inelastic cross sections for \ensuremathΔm12=0\mathrm{\ensuremath{\Delta}}{m}_{12}=0\ensuremathΔm12​=0 transitions display an unconventional E0{E}^{0}E0 scaling similar to that of elastic cross sections, and their rates vanish as T\ensuremathΔm12+1/2{T}^{\mathrm{\ensuremath{\Delta}}{m}_{12}+1/2}T\ensuremathΔm12​+1/2. For collisions dominated by even partial waves (such as those of identical bosons in the same internal state) the scaling is modified to \ensuremathσinel\ensuremath∝E\ensuremathΔm12+1{\ensuremath{\sigma}}_{\text{inel}}\ensuremath{\propto}{E}^{\mathrm{\ensuremath{\Delta}}{m}_{12}+1}\ensuremathσinel​\ensuremath∝E\ensuremathΔm12​+1 if \ensuremathΔm12\mathrm{\ensuremath{\Delta}}{m}_{12}\ensuremathΔm12​ is odd. We present accurate quantum scattering calculations that illustrate these modified threshold laws for resonant spin exchange in ultracold Rb+Rb\text{Rb}+\text{Rb}Rb+Rb and O2+O2{\mathrm{O}}_{2}+{\mathrm{O}}_{2}O2​+O2​ collisions. Our results illustrate that the well-known k\ensuremath−1{k}^{\ensuremath{-}1}k\ensuremath−1 threshold scaling of inelastic cross sections only applies to exothermic, rather than resonant, inelastic processes.
Accurate calculations of the near resonant charge exchange cross-sections in HD + , HT + and DT + at very low energies are presented. The charge exchange process between an ion and its parent atom is a near resonant process that becomes inelastic when two different isotopes are involved. We find that, at very low energies, the charge exchange cross-section follows Wigner's law for inelastic processes and becomes much larger than the cross-section for elastic collisions which tends to a finite limit. The efficiency of inelastic charge exchange increases as the mass difference between the two isotopes decreases.
Abstract Taking the ultracold 85 Rb– 87 Rb collision system as an example, we investigated the Feshbach resonances of nonzero partial waves above the threshold. The self-energy at the threshold, which represents the coupling strength between open and closed channels, is considered a critical parameter to quantitatively describe the properties of Feshbach resonances. The total elastic and inelastic cross sections are calculated as functions of the magnetic field B and collision energy E col , ranging from 0.1 to 600 μ K. For a large absolute value of the self-energy at the threshold, the resonance decays rapidly with increasing collision energy, and narrow resonances of nonzero partial waves can be clearly resolved in the contour plot of the inelastic cross section versus the collision energy and magnetic field. It was found that the resonance tail appeared at the given magnetic field when the cross section decreased from the maximal value of the resonance peak to the minimum value, where a long resonance tail indicates an appreciable resonance in a relatively large region of collision energy. This relationship between the self-energy and the properties of Feshbach resonances still exists in the thermally averaged inelastic rate coefficient. The bound-state energies for nonzero partial waves split owing to the spin–spin interaction, which results in multiple nearly-overlapping resonances. Both the spin–spin and second-order spin–orbit effects are included. However, multiple nearly-overlapping resonances for nonzero partial waves are difficult to resolve in thermally averaged rate coefficients.
Nonadiabatic inelastic collision cross sections have been computed at low and ultralow energies, focusing in the present work on the Ca+{}^{+}+(4s2S)(4s{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}S)(4s2S) + Rb(5s2S)(5s{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}S)(5s2S) $\ensuremath{\rightarrow}$ Ca(4s4p3P)(4s4p{\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}P)(4s4p3P) + Rb+(1S){}^{+}{(}^{1}\phantom{\rule{-0.16em}{0ex}}S)+(1S) charge-exchange process at ultralow collision energies (EEE $\ensuremath{\leqslant}$ 10${}^{\ensuremath{-}6}$ eV) induced by spin-orbit nonadiabatic coupling terms. The calculations reveal a wealth of sharp resonances which can substantially enhance charge-exchange probabilities over a specific energy window of around 10${}^{\ensuremath{-}6}$ to 10${}^{\ensuremath{-}8}$ eV. Such resonances are analyzed in detail, and the corresponding molecular mechanism is discussed for a specific example, where the nonadiabatic transition probabilities are seen to increase by nearly 2 orders of magnitude.
Using the convergent close-coupling theory we study the threshold behavior of cross sections for positronium (Ps) of energy EEE scattering on antiprotons. In the case of Ps(1s\mathrm{Ps}(1sPs(1s) elastic scattering, simple power laws are observed for all partial waves studied. The partial-wave summed cross section is nearly constant, and dominates the antihydrogen formation cross section at all considered energies, even though the latter is exothermic and behaves as 1/E1/21/{E}^{1/2}1/E1/2. For Ps(2s\mathrm{Ps}(2sPs(2s), oscillations spanning orders of magnitude on top of the 1/E1/E1/E behavior are found in the elastic and quasielastic cross sections. The antihydrogen formation is influenced by dipole-supported resonances below the threshold of inelastic processes. Resonance energies form a geometric progression relative to the threshold. The exothermic antihydrogen formation cross sections behave as 1/E1/E1/E at low energies, but are oscillation free. We demonstrate that all these rich features are reproduced by the threshold theory developed by Gailitis [J. Phys. B: At. Mol. Phys. 15, 3423 (1982)].
Relaxation of O(3Pj=0)\mathrm{O}{(}^{3}{P}_{j=0})O(3Pj=0​) by collisions with He at ultracold temperatures is investigated, and it is illustrated that cross sections and rate constants for the $j=\stackrel{\ensuremath{\rightarrow}}{0}j=2$ and $j=\stackrel{\ensuremath{\rightarrow}}{0}j=1$ transitions exhibit qualitatively different threshold behaviors stemming from the different mechanisms of these nonadiabatic transitions. The results allow us to make the conclusion that, in general, rate constants for the nonadiabatic transitions governed by electrostatic couplings should obey Wigner's threshold law for s-wave scattering approaching a finite value at $\stackrel{\ensuremath{\rightarrow}}{T}0\mathrm{K},$ while cross sections and rate constants for the transitions determined by Coriolis couplings should vanish in the zero-temperature limit.
The last decade has seen substantial advances in the exploration of inelastic and reactive collisions at collision energies in the cold ( < <<< 1 K) and ultracold ( < 1 <1<1<1 mK) regime.1-13 These advances ushered in an understanding of cold and ultracold collisions that is impacting research areas ranging from molecular spectroscopy to quantum simulation and computation to quantum control of chemical reaction dynamics. Cold collisions are dominated by resonances, as amply manifest in most of the systems studied.1, 14 Resonances are also present in the thermal ( > >>> 100 K) and hyperthermal ( > >>> 1000 K) collision energy regimes,15 but obscured by background (non-resonant) scattering and the presence of many partial waves pertaining to higher angular momenta that blur the partial waves' individual contributions. Moreover, the outcome of thermal and hyperthermal collisions is somewhat insensitive to the details of the interaction potential – the potential energy surface (PES) – especially to its attractive long-range region. Only in a handful of cases, such as the F+HD exchange reaction, resonances in the thermal regime have been experimentally detected and theoretically accounted for by quantum mechanical scattering calculations.16, 17 In contrast, in the cold regime, collisions are dominated by only a few partial waves whose effects on the integral and differential scattering cross-sections can be singled out and linked to particular resonances. The cold collision energy regime has been attained by making use of crossed molecular beams that were either Stark- or Zeeman-decelerated or whose intersection angle could be reduced all the way down to the merging-beams limit. A beam emerging from a Stark or Zeeman decelerator has a variable, well-defined laboratory velocity. When crossed with a secondary beam at small angles (5 ∘ ∘^{\circ }∘ –10 ∘ ∘^{\circ }∘ ), collision energies in the 0.3–15 K could be readily achieved, as demonstrated by the group of van de Meerakker.9, 11 The technique of merged beams makes use of the ability to bend one of the beams by a magnetic or electric field such that the crossing angle becomes zero, thereby affording collision energies in the range 0.1 K–10 K. Merging beams have been used in studies of Penning ionization by the groups of Narevicius5, 12 and Osterwalder18 and made it possible to unravel shape and Feshbach resonances arising in this process. The techniques mentioned so far require the reactants to be polar or paramagnetic. For reactants that are neither, collision energies as low as 5 K could be achieved by taking advantage of a favorable combination of low intersection angles and low reduced mass of the reactants. This approach has been demonstrated by Bergeat, Costes, Naulin, and coworkers in a series of reactive3, 19 and inelastic collision experiments8, 20 of H 2 2_22​ . Yet, another approach to achieving low relative collision velocities relies on the coexpansion of the collision partners in a single molecular beam.21 If their masses are similar, both partners will end up with nearly the same velocities, within a few ms − 1 −1^{-1}−1 . Using this technique, Zare, Mukherjee, and coworkers carried out a series of experiments in which rotational quenching of HD by H 2 2_22​ , D 2 2_22​ , and He was investigated. Moreover, in these experiments, the HD molecule was prepared in v vvv =1 and j jjj =2 using Stark-field-induced adiabatic Raman passage that allowed, in addition, to align the rotational angular momentum of the HD reactant molecule with respect to the initial relative velocity of the HD + He reactants.22-24 Collisional energy transfer between rotation and translation, in which a molecule is excited or relaxed via scattering by an atom or another molecule, has been of particular significance for developing our understanding of inelastic collisions. Most notably, the scattering of NO molecules by rare gas (Rg) atoms has served as a benchmark system for both experimentalists and theorists for decades. In its ground electronic state, NO is a 2 Π 2Π^2\Pi2Π molecule with a single unpaired electron in an antibonding π ∗ π∗\pi ^*π∗ orbital. In this state, the rotational levels are split into two spin–orbit manifolds, defined by the projection | Ω | ∣Ω∣|\Omega |∣Ω∣ ( = 1 2 =12=\frac{1}{2}=21​ or 3 2 32\frac{3}{2}23​ ) of the electronic angular momentum (orbital and spin) on the internuclear axis. The rotational levels with the same j jjj in the two manifolds are separated by approximately 123 cm − 1 −1^{-1}−1 for v vvv =0 (117 cm − 1 −1^{-1}−1 for v vvv =10). In addition, each rotational level within each manifold is further split into two Λ Λ\LambdaΛ -doublet states ( ε = ± 1 ϵ=±1\epsilon = \pm 1ϵ=±1 , e/f), which are nearly degenerate, but differ in parity, p = ( − 1 ) j − ε / 2 p=(−1)j−ϵ/2p = (-1)^{j-\epsilon /2}p=(−1)j−ϵ/2 . As the Rg atom approaches the NO molecule, the degeneracy of the Λ Λ\LambdaΛ doublets is lifted, giving rise to two PESs, A ′ A′A^{\prime }A′ and A ′ ′ A′′A^{\prime \prime }A′′ , in which the orbital of the unpaired electron lies, respectively, in the plane of the three atoms or perpendicular to it. In computational practice, it is more convenient to use linear combinations (sums and differences) of the A ′ ′ A′′A^{\prime \prime }A′′ and A ′ A′A^{\prime }A′ PESs. Using hexapole focusing or Stark deceleration combined with detection via velocity ion imaging, fully state-to-state-resolved integral and differential cross sections for rotationally elastic and inelastic collisions could be measured with high resolution.9, 11, 25, 26 More than that, the orientation and alignment of the scattered (outgoing) NO molecules could be measured as a function of the scattering angle27-30 and, quite recently, the effects of orientation of the reactant (incoming) NO molecules on the differential cross section in collisions with Rg, mostly Ar, could be established.31-35 On the theory side, quantum mechanical close coupling (QMCC) scattering calculations on accurate two-dimensional ab initio PESs using the benchmark coupled-cluster, CCSD(T), method with the NO internuclear distance fixed at its equilibrium value36 were performed and found to account for all the details revealed by the experiments. Although state of the art, most of these experiments except for those described in Refs. 9, 11 were carried out at thermal energies for NO molecules in their lowest level, | v = 0 , j = 1 2 , Ω = 1 2 , f ⟩ ∣v=0,j=12,Ω=12,f⟩|v=0, j=\frac{1}{2}, \Omega =\frac{1}{2}, f\rangle∣v=0,j=21​,Ω=21​,f⟩ . Scattering of highly vibrationally excited molecules in the cold energy regime remained an unexplored frontier until the recent work by the groups of Arthur Suits, Ad van der Avoird, Gerrit Groenenboom, and Hua Guo who added spin–orbit and rotational state selection and looked for spin- and rotational-state changing collisions to boot. Published in the current issue of Natural Sciences,37 this work is a culmination of a 3-year collaboration that included both implementation of new experimental techniques and boosting of theoretical methods. Working with the NO+Ar system, the authors made use of stimulated emission pumping via the electronically excited A 2 Σ + A2Σ+A^2\Sigma ^+A2Σ+ state to promote NO( 2 Π 1 2 2Π12^2\Pi _{\frac{1}{2}}2Π21​​ ) molecules to the vibrationally excited state with v = 10 v=10v=10v=10 and a selected spin–orbit state | Ω | ∣Ω∣|\Omega |∣Ω∣ and rotational state j jjj , | v = 10 , j , Ω , f ⟩ ∣v=10,j,Ω,f⟩|v=10, j,\Omega , f\rangle∣v=10,j,Ω,f⟩ .38, 39 Low (cold) collision energies were achieved in a novel experimental setup of two beams (dual-beam arrangement) crossing at 4 ∘ ∘^{\circ }∘ and intersected by the pump and dump laser beams of the stimulated-emission pumping scheme. Downstream, the collision products were probed using 1+1 ′ ′^{\prime }′ REMPI with the ionization laser tuned just above the ionization threshold. Fully state-resolved angular distributions of the scattered NO molecules were measured by velocity ion slice imaging and detected by a charge-coupled device (CCD). In order to be able to detect the product (outgoing) molecules resulting from the cold NO + Ar collisions, some excess energy had to be made available to the products. This came in the form of the initial spin–orbit and/or rotational state energy. In the study highlighted herein,37 angular distributions leading to the spin–orbit changing transition | v = 10 , Ω = 3 2 , j = 3 2 f ⟩ → | v ′ = 10 , Ω ′ = 1 2 , j ′ = 9 2 , e ⟩ ∣v=10,Ω=32,j=32f⟩→∣v′=10,Ω′=12,j′=92,e⟩|v=10, \Omega =\frac{3}{2}, j=\frac{3}{2} f\rangle \rightarrow |v^{\prime }=10, \Omega ^{\prime }=\frac{1}{2}, j^{\prime }=\frac{9}{2}, e \rangle∣v=10,Ω=23​,j=23​f⟩→∣v′=10,Ω′=21​,j′=29​,e⟩ were measured at energies in the 2 K–5 K range. Although intrabeam scattering could have been used as an alternative as it had been in previous work,21 the authors demonstrated that a dual-beam setup provides a superior angular and energy resolution. The highlighted experiment challenged the existing theoretical treatments of the NO+ Rg systems: first, the approximation of fixing NO at its equilibrium distance was bound to fail for NO in v = 10 v=10v=10v=10 due to the concomitant elongation of the NO bond. Second, for such a highly excited vibrational state, both the long- and short-range interactions are affected, triggering nonadiabatic effects involving rotational and spin–orbit degrees of freedom. The above rendered the single-reference coupled-cluster method used in previous treatments of NO( 2 Π 1 2 , v = 0 2Π12,v=0^2\Pi _{\frac{1}{2}},v=02Π21​​,v=0 ) inadequate. Therefore, the authors of the highlighted paper37 constructed a new coupled-cluster PES as well as a multireference configuration-interaction (MRCI) PES and carried out full-fledged (3D) QMCC calculations on both PESs. These tour-de-force calculations reproduced quite well the experimental differential cross sections and defined a new state of the art in scattering calculations. However, some discrepancies remain as at low collision energies, and the accuracy of the long-range potential needed is a challenge for any ab initio method. Moreover, treating spin-changing collisions that are dominated by the essentially attractive difference potential, V A ′ ′ − V A ′ VA′′−VA′V_{A^{\prime \prime }}-V_{A^{\prime }}VA′′​−VA′​ , puts additional demands on the accuracy of the A ′ ′ A′′A^{\prime \prime }A′′ and A ′ A′A^{\prime }A′ PESs. The calculated integral cross section in the 3–4 cm − 1 −1^{-1}−1 energy interval exhibits a bump that may indicate the presence of resonances that show up as backscattering in the measured differential cross section. A partial-wave analysis points to overlapping resonances associated with contributions from several of the highest partial waves involved. In any case, tracing the resonances may require additional improvements of the existing PESs. All in all, the highlighted combined experimental and theoretical study sets a new standard for our understanding of cold collisions. There is no conflict of interest. The peer review history for this article is available at https://publons.com/publon/10.1002/ntls.20210088. This Highlight has been internally reviewed and edited by Bretislav Friedrich and Gerard Meijer. No data available.