Laser-induced, blackbody-radiation-assisted rovibrational cooling of symmetric-top molecular ions: NH3+ and ND3+

Quantum-state preparation of molecular ions is a prerequisite for precision spectroscopy and controlled studies of cold ion-molecule dynamics. While such control has been extensively developed for diatomic ions and proposed for linear polyatomic ions, corresponding strategies for symmetric-top molecular ions remain largely unexplored. We present a theoretical investigation of blackbody-radiation (BBR)-assisted rovibrational dynamics and laser cooling in the symmetric-top ions NH3+ and ND3+, prepared in specific rovibrational states by resonance enhanced multiphoton ionization of the neutral precursor. State-resolved radiative lifetimes and equilibration times are computed, revealing that vibrationally excited states decay rapidly, while the ground-state redistribution is dominated by slow BBR-driven rovibrational transitions, as pure rotational transitions are forbidden in the nonpolar NH3+ and ND3+ ions. BBR-assisted laser pumping via the ν2 umbrella-bending mode efficiently cools rotational levels within fixed K manifolds; however, ΔK = 0 selection rules induce a bottleneck, limiting access to the absolute rovibrational ground state for some initially prepared states. Isotopic substitution to ND3+ slows the redistribution dynamics due to the lower transition dipole moment. At room temperature, these cooling schemes yield > 90% and >85% of the population in selected rovibrational states of the NH3+ and ND3+ ions, respectively. In contrast, at temperatures below 100 K, BBR-induced redistribution is strongly suppressed for ions initially produced in the rovibrational ground state, effectively freezing the population for extended storage times. The schemes presented here provide a practical pathway for preparing state-selected molecular ions for precision spectroscopy and controlled ion-molecule reaction studies.