Abstract
Using accurate ab initio calculations of the interaction forces, we employ a quantum mechanical description of the collisional state-changing processes that occur in a cold ion trap with He as a buffer gas. We generate the corresponding inelastic rates for rotational transitions involving three simple molecular anions OH-(1Σ), MgH-(1Σ), and C2H-(1Σ) colliding with the helium atoms of the trap. We show that the rotational constants of these molecular anions are such that within the low-temperature regimes of a cold ion trap (up to about 50 K), a different proportion of molecular states are significantly populated when loading helium as a buffer gas in the trap. By varying the trap operating conditions, population equilibrium at the relevant range of temperatures is reached within different time scales. In the modeling of the photodetachment experiments, we analyze the effects of varying the chosen values for photodetachment rates as well as the laser photon fluxes. Additionally, the changing of the collision dynamics under different buffer gas densities is examined and the best operating conditions, for the different anions, for yielding higher populations of specific rotational states within the ion traps are extracted. The present modeling thus illustrates possible preparation of the trap conditions for carrying out more efficiently state-selected experiments with the trapped anions.
Highlights
Cooling and controlling molecular ions has been an important topic for many years, owing to the importance of cold molecules for many applications such as precision spectroscopy and metrology,1–3 quantum-state controlled chemistry,4,5 or laboratory astophysics.6–8 Available techniques for the preparation of molecular ions in specific quantum states of vibrational or rotational motion and in selected hyperfine states are photoionization of suitable precursors9 or buffer gas cooling.10 Optical pumping11,12 or depletion of excited states13 have been demonstrated
Solving the problem of rotational quenching dynamics corresponds to solving the time-independent Schrödinger equation (TISE) for the nuclei that move on the potential energy surfaces discussed in Sec
We have analyzed the computational modeling of molecular photodetachment experiments involving small molecular anions confined in cold ion traps, with helium as the buffer gas of choice for the translational and internal cooling of molecules
Summary
Cooling and controlling molecular ions has been an important topic for many years, owing to the importance of cold molecules for many applications such as precision spectroscopy and metrology, quantum-state controlled chemistry, or laboratory astophysics. Available techniques for the preparation of molecular ions in specific quantum states of vibrational or rotational motion and in selected hyperfine states are photoionization of suitable precursors or buffer gas cooling. Optical pumping or depletion of excited states have been demonstrated. The photodetachment processes of interest here will be interpreted as causing transitions between the molecular anion, M−, in its ground electronic state and the neutral molecule, M, in its ground electronic state Near threshold, both the neutral molecule produced and the initial anion are considered in their vibrational ground state but in different rotational states depending on the photon energy.. The population losses in the trap will depend on the structure of the anion under consideration, the physical nature of their interaction with the He buffer gas, and the interplay between the collisional density of the uploaded gas and the photon flux of the neutralizing laser.
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