arXiv subjects

Rebekah Hermsmeier

Publications and source records attributed to Rebekah Hermsmeier.

7 recordsLinked to original sources

arXiv2024

Highly spin-polarized molecules via collisional microwave pumping

We propose a general technique to produce cold spin-polarized molecules, in which rotationally excited states are first populated by coherent microwave excitation, and then allowed to spin-flip and relax via collisional quenching, which populates a single final spin state. We illustrate the high selectivity of the technique for $^{13}$C$^{16}$O molecules immersed in a cold buffer gas of helium atoms, achieving a high degree ($\geq$95\%) of nuclear spin polarization at 1 K.

physics.chem-ph↗

arXiv2024

Universality and threshold laws for collision lifetimes at ultralow temperatures

While collision lifetimes are a fundamental property of few-body scattering events, their behavior at ultralow temperatures is not completely understood. We derive a general expression for the Smith lifetime Q-matrix using multichannel quantum defect theory, which allows us to obtain the average time delay in the incident $s$-wave collision channel in the limit of zero collision energy $E$, $Q_{11}=A/\sqrt{E} + B$, where $A$ and $B$ are constants. We show that the time delay is dominated by elastic scattering, and contains an additional multichannel contribution independent of the two-body scattering length. We also obtain the expressions for the collision lifetime using the universal model of Idziaszek and Julienne [Phys. Rev. Lett. 104, 113202 (2010)]. The lifetime acquires an imaginary part in the presence of inelastic loss due to the lack of unitarity of the S-matrix, and shortens with increasing the short-range loss parameter $y$.

physics.chem-ph↗

arXiv2024

Magnetically tunable electric dipolar interactions of ultracold polar molecules in the quantum ergodic regime

By leveraging the hyperfine interaction between the rotational and nuclear spin degrees of freedom, we demonstrate extensive magnetic control over the electric dipole moments, electric dipolar interactions, and ac Stark shifts of ground-state alkali-dimer molecules such as KRb$(X^1Σ)$. The control is enabled by narrow avoided crossings and the highly ergodic character of molecular eigenstates at low magnetic fields, offering a general and robust way of continuously tuning the intermolecular electric dipolar interaction for applications in quantum simulation and sensing.

physics.atom-ph↗

arXiv2023

Nuclear spin relaxation in cold atom-molecule collisions

We explore the quantum dynamics of nuclear spin relaxation in cold collisions of $^1Σ^+$ molecules with structureless atoms in an external magnetic field. To this end, we develop a rigorous coupled-channel methodology, which accounts for rotational and nuclear spin degrees of freedom of $^1Σ^+$ molecules, their interaction with an external magnetic field, as well as for anisotropic atom-molecule interactions. We apply the methodology to study collisional relaxation of the nuclear spin sublevels of $^{13}$CO molecules immersed in a cold buffer gas of $^4$He atoms. We find that nuclear spin relaxation in the ground rotational manifold of CO occurs extremely slowly due to the absence of direct couplings between the nuclear spin sublevels. The rates of collisional transitions between the $N=1$ nuclear spin states of CO are generally much higher due to the direct nuclear spin-rotation coupling between the states. These transitions obey selection rules, which depend on the values of space-fixed projections of rotational and nuclear spin angular momenta for the initial and final molecular states. For some initial states, we also observe a strong magnetic field dependence, which can be understood using the first Born approximation. We use our calculated nuclear spin relaxation rates to investigate the thermalization of a single nuclear spin state of CO$(N=0)$ immersed in a cold buffer gas of He. The calculated nuclear spin relaxation times ($T_1\simeq 0.5$ s at $T=1$ K) display a steep temperature dependence decreasing rapidly at elevated temperatures due to the increased population of rotationally excited states, which undergo nuclear spin relaxation at a much faster rate. Thus, long relaxation times of $N=0$ nuclear spin states in cold collisions with buffer gas atoms can only be maintained at sufficiently low temperatures ($kT\ll 2B_e$), where $B_e$ is the rotational constant.

physics.chem-ph↗

arXiv2021

Total angular momentum representation for state-to-state quantum scattering of cold molecules in a magnetic field

We show that the integral cross sections for state-to-state quantum scattering of cold molecules in a magnetic field can be efficiently computed using the total angular momentum representation despite the presence of unphysical Zeeman states in the eigenspectrum of the asymptotic Hamiltonian. We demonstrate that the unphysical states arise due to the incompleteness of the space-fixed total angular momentum basis caused by using a fixed cutoff value $J_\text{max}$ for the total angular momentum of the collision complex $J$. As a result, certain orbital angular momentum ($l$) basis states lack the full range of $J$ values required by the angular momentum addition rules, resulting in the appearance of unphysical states. We find that by augmenting the basis with a full range of $J$-states for every $l$, it is possible to completely eliminate the unphysical states from quantum scattering calculations on molecular collisions in external fields. To illustrate the procedure, we use the augmented basis sets to calculate the state-to-state cross sections for rotational and spin relaxation in cold collisions of $^{40}$CaH$(X^2Σ^+,v=0,N=1, M_N=1, M_S=1/2)$ molecules with $^4$He atoms in a magnetic field. We find excellent agreement with benchmark calculations, validating our proposed procedure. We find that $N$-conserving spin relaxation to the lowest-energy Zeeman state of the $N=1$ manifold, $|11\frac{1}{2}\rangle\to |1-1-\frac{1}{2}\rangle$ is nearly completely suppressed due to the lack of spin-rotation coupling between the fully spin-stretched Zeeman states. Our results demonstrate the possibility of rigorous, computationally efficient, and unphysical state-free quantum scattering calculations on cold molecular collisions in an external magnetic field.

physics.chem-ph↗

arXiv2021

Quantum spin state selectivity and magnetic tuning of ultracold chemical reactions of triplet alkali-metal dimers with alkali-metal atoms

We demonstrate that it is possible to efficiently control ultracold chemical reactions of alkali-metal atoms colliding with open-shell alkali-metal dimers in their metastable triplet states by choosing the internal hyperfine and rovibrational states of the reactants as well as by inducing magnetic Feshbach resonances with an external magnetic field. We base these conclusions on coupled-channel statistical calculations that include the effects of hyperfine contact and magnetic-field-induced Zeeman interactions on ultracold chemical reactions of hyperfine-resolved ground-state Na and the triplet NaLi(a$^3Σ^+$) producing singlet Na$_2$($^1Σ^+_g$) and a Li atom. We find that the reaction rates are sensitive to the initial hyperfine states of the reactants. The chemical reaction of fully spin-polarized, high-spin states of rotationless NaLi(a$^3Σ^+, v = 0, N = 0$) molecules with fully spin-polarized Na is suppressed by a factor of 10-100 compared to that of unpolarized reactants. We interpret these findings within the adiabatic state model, which treats the reaction as a sequence of nonadiabatic transitions between the initial non-reactive high-spin state and the final low-spin states of the reaction complex. In addition, we show that magnetic Feshbach resonances can similarly change reaction rate coefficients by several orders of magnitude. Some of these resonances are due to resonant trimer bound states dissociating to the $N=2$ rotational state of NaLi(a$^3Σ^+, v = 0$) and would thus exist in systems without hyperfine interactions.

physics.chem-ph↗

arXiv2021

Near-threshold scaling of resonant inelastic collisions at ultralow temperatures

We show that the cross sections for a broad range of resonant {\it inelastic} processes accompanied by excitation exchange (such as spin-exchange, Förster resonant, or angular momentum exchange) exhibit an unconventional near-threshold scaling $E^{Δm_{12}}$, where $E$ is the collision energy, $Δm_{12}=m_1'+m_2'-m_1-m_2$, and $m_i$ and $m_i'$ are the initial and final angular momentum projections of the colliding species ($i=1,\,2$). In particular, the inelastic cross sections for $Δm_{12}=0$ transitions display an unconventional $E^0$ scaling similar to that of elastic cross sections, and their rates vanish as $T^{Δm_{12}+1/2}$. For collisions dominated by even partial waves (such as those of identical bosons in the same internal state) the scaling is modified to $σ_\text{inel}\propto E^{Δm_{12} +1} $ if $Δm_{12}$ is odd. We present accurate quantum scattering calculations that illustrate these modified threshold laws for resonant spin exchange in ultracold Rb+Rb and O$_2$+O$_2$ collisions. Our results illustrate that the threshold scaling of collision cross sections is determined only by the energetics of the underlying process (resonant vs. exothermic) rather than by whether the internal states of colliding particles is changed in the collision.

physics.atom-ph↗