Archisman Sinha | Searcharxiv

arXiv2026

Anomalous Peak Formation in the Second Stability Zone of Quadrupole Mass Filters: Role of Non-linear Resonances Induced by Single Rod Defects

Operating a quadrupole mass filter (QMF) within its second stability zone offers superior mass resolving power but introduces extreme sensitivity to structural imperfections. While symmetric misalignments are well-documented, this work combines analytical modeling with SIMION trajectory simulations to investigate the unaddressed electrodynamic impact of a completely localized, asymmetric defect on a single rod. Unlike diagonally symmetric perturbations, which reduce the four-fold rotational symmetry to two-fold symmetry while maintaining a smooth stability landscape, a single-rod defect breaks the remaining symmetry, giving rise to pronounced transmission ridges within the second stability zone. Spatial multipole expansion reveals that this structural breakdown injects odd-parity harmonics -- dominated by the hexapole A3 field -- which couple the orthogonal transverse equations of motion. By tracking secular indices through explicit zone-II Floquet projection mappings, we demonstrate that these asymmetric fields drive destructive, higher-order nonlinear secular resonances. Ions traversing these precise parametric coordinates undergo rapid amplitude growth and collide with adjacent electrodes, establishing critical geometric tolerance frameworks and operating conditions for high-performance mass spectrometry.

physics.chem-ph↗

arXiv2026

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 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.

physics.chem-ph↗