Modeling state-selective photodetachment in cold ion traps: Rotational state “crowding” in small anions

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.

Rovibrational quenching of C2− anions in collisions with He, Ne, and Ar atoms

“…In a recent paper we calculated cross sections and rate coefficients for C 2 − -He rotationally inelastic collisions, treating the anion as a rigid rotor [49]. The rates for rotational excitation and quenching were found to be in line with those for similar ionic molecules interacting with helium [46]. Simulations of cooling rotational motion at typical helium pressures in ion traps showed thermalization to Boltzmann populations occurred within tenths of seconds.…”

“…− would ideally start with ions initially cooled to around 10 K, for example by helium buffer gas cooling in a cryogenic ion trap [45,46]. Processes used to generate C 2 − involve applying an electric discharge to a mixture of C 2 H 2 and CO 2 in a carrier gas [25,47] which may form the anion in excited vibrational states.…”

Beyond the helium buffer: 12C−2 rotational cooling in cold traps with H2 as a partner gas: interaction forces and quantum dynamics

“…Equations (12) were solved using the fourth order Runge-Kutta method. The rates due to spontaneous radiative processes are usually negligible in such simulations due to the high buffer gas pressure resulting in collisional rates being the dominant process in changing the molecule's rotational state [68]. For C − 2 , this is an even better approximation since the anion has no dipole moment and thus dipole transitions are forbidden.…”

Rotational state-changing collisions of C2H− and C2N− anions with He under interstellar and cold ion trap conditions: A computational comparison

“…Inelastic cross sections were computed for all transitions between N = 0 to N = 15, which was deemed to be sufficient to model buffer gas dynamics in a cold trap up to about 50 K (see below), while the same range of levels is expected to be the one most significantly populated during low-energy collisional exchanges with He atoms within ISM environments dynamical conditions. Scattering calculations for C 2 H − were carried out by us in a former study 48 using the PES of Dumouchel et al as discussed above 27 . Another quantity that we shall compute from the cross sections is the state-to-state inelastic rotational rates over a range of temperatures from thresholds up to about 100 K, to cover the range of T values expected to be significant for processes in the ISM environments we are discussing here.…”

“…Following the prescription indicated in equation 5, we have calculated the relative values of the photo-detachment cross sections for the C 2 N − anion. The data for the C 2 H − anion we use here for comparison were presented before 48,66 and therefore we shall only discuss them during the analysis of the compared data below.…”