Singh, Maisenbacher, Lin, Axelrod, Panda, and Müller (Phys. Rev. Research 5, 033008 (2023)) proposed trapping small, chemically stable molecules (N2_2, CO, O2_2, HCl) by buffer-gas loading a roughly 10 K deep, very far-off-resonant quasielectrostatic dipole trap produced by 1064-nm continuous-wave light at about 300 GW/cm2^2 in a near-concentric build-up cavity. The feasibility of this trap rests on three unmeasured molecule-limited quantities that the source explicitly leaves open: (1) molecule-specific multiphoton ionization and electronic photodissociation rates under such continuous-wave intensities, including possible resonance-enhanced (REMPI) enhancements that Popruzhenko's Perelomov-Popov-Terent'ev-type formula cannot capture; (2) the magnitude and mechanism of two-body loss for trapped molecules, i.e. whether 'universal loss' with a coefficient of order 10−1010^{-10} cm3^3/s applies or the coefficient is orders of magnitude smaller, which fixes the achievable density, evaporative-cooling prospects, and sample lifetime; and (3) whether infrared multiphoton dissociation occurs at cryogenic (1.5 K) temperature for hydrogen-containing or vibrationally dense molecules with more than three atoms, which decides whether the trap extends beyond triatomics. These three molecule-limited quantities remain unmeasured: no molecular ionization or dissociation rate has been measured at continuous-wave intensities of hundreds of GW/cm2^2, no closed-shell stable molecule has been trapped in this regime, and no cold-temperature infrared-multiphoton-dissociation data exist.

Background

Optical dipole traps confine neutral particles through the interaction of an induced dipole moment with a far-detuned laser field; for 1064-nm light the photon energy is tiny compared with electronic excitation energies of most small molecules, so the trapping potential is 'quasielectrostatic' and nearly independent of the molecule's internal state, energy-level structure, and dipole moments. Deep traps (here of order 10 K, requiring continuous-wave intensities of hundreds of GW/cm2^2) are technologically demanding: a Fabry-Pérot build-up cavity — two highly reflective mirrors in a near-concentric (almost hemispherical) geometry — enhances a modest input laser into a circulating standing wave whose antinodes provide the trap. Buffer-gas cooling is the technique of thermalizing molecules against cold helium gas inside a cryogenic cell (here at 1.5 K); the proposal is to load molecules directly from such a cell into the deep trap and then pump out the helium, predicting 10410^4-10610^6 trapped molecules of species such as N2_2, CO, O2_2, and HCl that cannot be laser cooled. This would extend ultracold physics, cold-chemistry studies, and precision measurement beyond the laser-coolable radical and bialkali molecules used today.

Three molecule-limited processes decide whether the scheme works. First, multiphoton ionization (MPI): at these intensities a molecule can absorb many 1064-nm photons and ionize. The source estimates rates using Popruzhenko's formula, a Perelomov-Popov-Terent'ev (PPT)-type strong-field ionization rate for hydrogen-like systems (S. V. Popruzhenko, V. D. Mur, V. S. Popov, and D. Bauer, Phys. Rev. Lett. 101, 193003 (2008)), which matches noble-gas data only within about two orders of magnitude and ignores resonance-enhanced multiphoton ionization (REMPI), where an intermediate electronic resonance can boost the ionization rate by several orders. The source therefore restricts candidate molecules to ionization potentials I0≥12I_0 \geq 12 eV with an estimated rate Wi≲10−9W_i \lesssim 10^{-9} s−1^{-1}, a deliberately conservative criterion awaiting measurement. Second, photodissociation, in particular infrared multiphoton dissociation (IRMPD): a polyatomic molecule can absorb many photons resonant with rovibrational transitions, climb through a dense 'quasicontinuum' of vibrational states, and dissociate. The mechanism requires closely spaced vibrational levels, so it is essentially excluded for diatomics and unlikely for triatomics, but for molecules with more than three atoms — especially hydrogen-containing ones such as CH4_4 or CH3_3OH, where near-infrared frequencies match high vibrational overtones — the source finds only room-temperature absorption data and cannot say whether IRMPD will occur at cryogenic temperature. Third, two-body loss: collisions between trapped molecules can eject both from the trap, characterized by a loss coefficient β\beta. In optical-trap experiments on bialkali molecules a near-unity fraction of collisions is lossy ('universal loss', with β∼10−10\beta \sim 10^{-10} cm3^3/s at the relevant temperatures); Christianen, Zwierlein, Groenenboom, and Karman (Phys. Rev. Lett. 123, 123402 (2019)) attributed this to photoexcitation of the transient two-body collision complex by the trap light. The source argues universal loss should not apply to small, chemically stable (closed-shell) molecules, whose mutual interactions (of order 20 meV) are far too weak to shift excited states by the several eV needed for single-photon excitation of the complex and whose complex lifetimes are of order picoseconds, but adopts universal loss as a worst-case bound. This theoretical picture is contested: Bause et al. (Phys. Rev. Research 3, 033013 (2021)) found universal loss for NaK even in a dark box trap at intensities about 1000 times lower, disagreeing with the photoexcitation prediction by at least two orders of magnitude. Empirically, near-universal loss keeps appearing for other molecule classes in optical traps — two CaF molecules colliding in optical tweezers show β=7(4)×10−11\beta = 7(4) \times 10^{-11} cm3^3/s (Cheuk et al., Phys. Rev. Lett. 125, 043401 (2020)), and SrOH in an ordinary optical dipole trap is lost with β≈4×10−10\beta \approx 4 \times 10^{-10} cm3^3/s (Nasir et al., arXiv:2607.16913 (2026)) — but no closed-shell stable molecule has been tested in the deep quasielectrostatic regime, where the very high photon flux makes any photoinduced channel more likely, not less. The loading model itself also relies on low-temperature molecule-helium collision cross sections at 1.5 K that are estimated rather than measured, and the source notes that the cavity must hold its mode against mirror thermoelastic deformation and alignment drift — intensities the same group has since approached experimentally, achieving continuous-wave intensities above 500 GW/cm2^2 (arXiv:2602.23476) and modeling a thermoelastic ceiling near 2.9 TW/cm2^2 (arXiv:2604.07676) in room-temperature cavities.

Problem Statement

Determine the molecule-limited feasibility of the buffer-gas-loaded deep optical trap proposed by Singh et al. (Phys. Rev. Research 5, 033008 (2023)), by establishing, for representative small chemically stable molecules (N2_2, CO, O2_2, HCl, and the more-than-three-atom candidates CH4_4 and SF6_6) under continuous-wave 1064-nm light at intensities of order 300 GW/cm2^2:

  1. The molecule-specific multiphoton ionization and electronic photodissociation rates, including any resonance-enhanced (REMPI) contributions, sufficient to validate or refute the source's conservative selection criterion Wi≲10−9W_i \lesssim 10^{-9} s−1^{-1} for ionization (derived from Popruzhenko's PPT-type formula) and the source's assessment that electronic photodissociation is unlikely for small, chemically stable molecules. The source notes that rate limits can be set in the high-intensity cavity even without buffer-gas cooling, so room-temperature cavity measurements are admissible.

  2. The two-body loss coefficient βQ-Q\beta_{Q\text{-}Q} between trapped molecules and its mechanism, establishing whether universal loss (a near-unity loss probability per collision, with β\beta of order 10−1010^{-10} cm3^3/s) applies, as in bialkali optical-trap experiments, or whether the coefficient is orders of magnitude smaller as the source's short-complex-lifetime argument suggests; this coefficient fixes the achievable trapped density, the prospects for evaporative cooling toward the mK regime, and the long-time sample lifetime.

  3. Whether infrared multiphoton dissociation (IRMPD), the rovibrational dissociation channel, occurs at cryogenic (about 1.5 K) temperature for hydrogen-containing or vibrationally dense molecules with more than three atoms — an independent determination, distinct from and required in addition to the electronic photodissociation rates of item 1.

These three determinations jointly decide whether chemically stable molecules survive the trap light and can be held at useful density in such a trap — the prerequisite for its proposed use in molecular spectroscopy, cold chemical reactions, and precision measurement.

References

  1. A. Singh, L. Maisenbacher, Z. Lin, J. J. Axelrod, C. D. Panda, H. Müller, "Dynamics of a buffer-gas-loaded, deep optical trap for molecules", Phys. Rev. Research 5, 033008 (2023). DOI: 10.1103/PhysRevResearch.5.033008, arXiv:2301.12620, https://doi.org/10.1103/PhysRevResearch.5.033008
  2. J. J. Axelrod, L. Maisenbacher, A. Singh, I. M. Pope, P. N. Petrov, J. T. Zhang, H. Müller, "A thermoelastic limit on the focal intensity in Fabry-Pérot cavities", arXiv:2604.07676 (2026), https://arxiv.org/abs/2604.07676
  3. L. Maisenbacher, A. Singh, I. M. Pope, H. Müller, "Ultrahigh continuous-wave intensities in high-NA optical cavities through suppression of the parametric oscillatory instability", arXiv:2602.23476 (2026), https://arxiv.org/abs/2602.23476
  4. S. V. Popruzhenko, V. D. Mur, V. S. Popov, D. Bauer, "Strong field ionization rate for arbitrary laser frequencies", Phys. Rev. Lett. 101, 193003 (2008). DOI: 10.1103/PhysRevLett.101.193003, https://doi.org/10.1103/PhysRevLett.101.193003
  5. A. Christianen, M. W. Zwierlein, G. C. Groenenboom, T. Karman, "Photoinduced two-body loss of ultracold molecules", Phys. Rev. Lett. 123, 123402 (2019). DOI: 10.1103/PhysRevLett.123.123402, https://doi.org/10.1103/PhysRevLett.123.123402
  6. R. Bause, A. Schindewolf, R. Tao, M. Duda, X.-Y. Chen, G. Quéméner, T. Karman, A. Christianen, I. Bloch, X.-Y. Luo, "Collisions of ultracold molecules in bright and dark optical dipole traps", Phys. Rev. Research 3, 033013 (2021). DOI: 10.1103/PhysRevResearch.3.033013, arXiv:2103.00889, https://doi.org/10.1103/PhysRevResearch.3.033013
  7. L. W. Cheuk, L. Anderegg, Y. Bao, S. Burchesky, S. Yu, W. Ketterle, K.-K. Ni, J. M. Doyle, "Observation of collisions between two ultracold ground-state CaF molecules", Phys. Rev. Lett. 125, 043401 (2020). DOI: 10.1103/PhysRevLett.125.043401, arXiv:2002.00048, https://doi.org/10.1103/PhysRevLett.125.043401
  8. A. Nasir, A. Lunstad, M. Li, S. Salim, S. Liu, C. Hallas, Z. Lasner, J. M. Doyle, "Order of magnitude improved optical trapping of molecules through transverse cooling", arXiv:2607.16913 (2026), https://arxiv.org/abs/2607.16913
  9. B. J. Schellenberg, E. H. Prinsen, J. Nauta, L. F. Pašteka, A. Borschevsky, S. Hoekstra, "Single-photon loading of polar molecules into an optical trap", Phys. Rev. A 113, 013113 (2026). DOI: 10.1103/mybg-d2ym, arXiv:2507.17521, https://doi.org/10.1103/mybg-d2ym
  10. O. Amit, D. Taray, V. Wirthl, V. Weis, M. W. Syed, A. Ozawa, J. Weitenberg, S. G. Karshenboim, J. T. M. Walraven, L. Maisenbacher, R. Pohl, Z. Burkley, F. Schmid, T. W. Hänsch, D. C. Yost, Th. Udem, "Towards trapping of hydrogen atoms for computable optical clock applications", Phys. Rev. A 112, 033101 (2025). DOI: 10.1103/3bnr-q23f, https://doi.org/10.1103/3bnr-q23f
  11. M. H. Rayment, S. D. Hogan, "Electrostatic trapping of N2_2 molecules in high Rydberg states", Phys. Rev. Lett. 132, 113201 (2024). DOI: 10.1103/PhysRevLett.132.113201, arXiv:2403.12315, https://doi.org/10.1103/PhysRevLett.132.113201
  12. E. H. Prinsen, A. Borschevsky, S. Hoekstra, A. K. Dutta, S. Chakraborty, B. J. Schellenberg, L. F. Pašteka, I. A. Aucar, "Ab initio calculations of the static and dynamic polarizability of BaOH", Phys. Rev. A 113, 022809 (2026). DOI: 10.1103/r71x-bnst, arXiv:2510.19564, https://doi.org/10.1103/r71x-bnst
  13. H. Jóźwiak, T. V. Tscherbul, P. Wcisło, "Hyperfine and Zeeman interactions in ultracold collisions of molecular hydrogen with atomic lithium", J. Chem. Phys. 160, 094304 (2024). DOI: 10.1063/5.0193148, arXiv:2312.13652, https://doi.org/10.1063/5.0193148
  14. B. M. Reible, S. Ahmadkhani, L. Delle Site, "Open quantum systems beyond equilibrium: Lindblad equation and path integral molecular dynamics", Phys. Rev. A 113, 042205 (2026). DOI: 10.1103/rrhf-h7xp, arXiv:2603.10839, https://doi.org/10.1103/rrhf-h7xp