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 ro-vibrational states by resonance enhanced multiphoton ionization (REMPI) 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 ro-vibrational transitions as pure rotational transitions are forbidden in the non-polar NH3+ and ND3+ ions. BBR-assisted laser pumping via the nu2 umbrella-bending mode efficiently cools rotational levels within fixed K manifolds; however, Delta K = 0 selection rules induce a bottleneck, limiting access to the absolute rovibrational ground state for some initially prepared states. Isotopic substitution to ND3+ further slows dynamics due to reduced transition dipoles. At room temperature, these cooling schemes yield more than 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 (v=0, J, K), effectively freezing the population for extended storage times. In comparison, vibrationally excited states exhibit lifetimes on the order of a few milliseconds, independent of the temperature of the BBR field.

Figures (9)

• Figure 1: Blackbody radiation (BBR) spectral intensity calculated using Planck’s law for temperatures of 300, 250, 200, 150, 100 and 77 K. The vibrational modes of NH$_3^+$ are indicated by vertical dashed lines in blue and ND$_3^+$ in vertical dotted lines in red indicating their effective spectral overlap with the BBR field.
• Figure 2: Energy-level structure illustrating the allowed rovibrational transitions for (a) NH$_3^+$ and (b) ND$_3^+$. For excitation of the $\nu_2$ vibrational mode, only parallel transitions with $\Delta K = 0$ are allowed. Owing to nuclear-spin statistics, the lowest rovibrational level of fermionic NH$_3^+$ is $\ket{0,0,0}$, whereas that of bosonic ND$_3^+$ is $\ket{0,1,0}$. In the $K=0$ manifold of NH$_3^+$, alternate $J$ levels are absent, while in ND$_3^+$ all $J$ levels are present but with modified statistical weights. (The states with lower statistical weight are denoted in dashed -- lines). In the $K=1$ manifold, all $J$ levels occur for both isotopologues. For the $K=0$ manifold, only $P$- and $R$-branch transitions are allowed, whereas in the $K=1$ manifold $P$-, $Q$-, and $R$-branch transitions are permitted.
• Figure 3: Spectra of NH$_3^+$ involving the $\nu_2$ mode of vibration. The stick spectrum of ro-vibrational transitions as calculated in this work is shown in blue at 300 K. Inverted below this simulation is the computational spectrum (in red) reported by Yurchenko et alyurchenko2008ab.
• Figure 4: BBR temperature dependent decay of ground state $|0,0,0\rangle$ NH$_3^+$ (a) and $|0,1,0\rangle$ ND$_3^+$ (b) molecular ions. The decay is slower in the case of ND$_3^+$ because of the reduced value of the transition dipole moment. Nonetheless, the decay freezes at a temperature below 100 K, for both species. The state index is denoted as $\ket{\nu_2,J,K}$
• Figure 5: Proposed laser-driven transitions applied for cooling the NH$_3^+$ molecular ion in the $K=0$ (left) and $K=1$ (right) manifolds. The $P$ branch transitions are denoted with upward arrows. The downward cooling transitions are denoted using red dashed lines. While only $P$- and $R$-type decay transitions are possible in the $K=0$ manifold, $P$- and $Q$-type decay transitions are accessible in the $K=1$ manifold. $J$ level designations are given beside each state. Laser driven transition with the same color scheme can be pumped using a common laser source e.g. $P(2)$, $P(4)$ and $P(6)$.