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

Abstract

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.