Collisional relaxation kinetics fororthoandparaNH₂⁻under photodetachment in cold ion traps

Abstract

The collisional cooling of the internal rotational states of the nonlinear anion NH2- (1A1), occurring at the low temperature of a cold ion trap under helium buffer gas cooling, is examined via quantum dynamics calculations and ion decay rate measurements. The calculations employ a novel ab initio potential energy surface that describes the interaction anisotropy and range of action between the molecular anions and the neutral He atoms. The state changing integral cross sections are employed to obtain the state-to-state rate coefficients, separately for the ortho- and the para-NH2- ions. These rates are in turn used to compute the state population evolution in the trap for both species, once photodetachment by a laser is initiated in the trap. The present work shows results for the combined losses of both species after the photodetachment laser is switched on and analyzes the differences of loss kinetics between the two hyperfine isomers.

Collision-driven state-changing efficiency of different buffer gases in cold traps: He(1S), Ar(1S) and p-H2(1Σ) on trapped CN−(1Σ)

“…It is therefore of direct interest when modelling possible operating conditions in the traps to know the size and temperature dependence of the collisioninduced state-changing processes for the molecular anion under study as the buffer gas selected is replaced by another, to test a broader range of operating conditions. In our previous work [11][12][13][14] we have examined a variety of small molecular anions and discussed the relative importance of their inelastic collision rate constants involving rotational states at different trap temperatures. Recent experimental work in our group has also been directed to photo-detachment studies of the CN À anion with He as the buffer gas of choice.…”

Collisional Quantum Dynamics for MgH− (1Σ+) With He as a Buffer Gas: Ionic State-Changing Reactions in Cold Traps

“…We have further studied the time evolution of the fractional populations of the lower rotational states in order to model the collisional preparation of the anionic molecule in the trap to further perform state-selected photo-detachment experiments as already done in our group for OH − /He (Hauser et al, 2015) and for NH2-/He (Gianturco et al, 2018; Hernández Vera et al, 2018; Lakhmanskaya et al, 2018). The time evolution of the fractional populations at different temperatures for the trap is shown by our present calculations to be less efficient than in the case of MgH + /He and that lower temperatures would be needed to reach a steady-state population dominated by the lowest two rotational states.…”

“…The data shown by Figure 10 report the computed rates for excitation processes between the lowest five rotational states, of MgH − (X 1 Σ + ) that we expect to be involved in the cold ion traps of the planned experiments (Hauser et al, 2015; Gianturco et al, 2018). Those given by Figure 11 describe the corresponding de-excitation transition rates between the set of rotational states of the rotationally cooling anion in the trap.…”

“…In that work, in fact, both the experiments and the calculations of OH − and NH2- anionic systems (trapped down to 10–20 K temperature regimes) have shown clearly that it is possible for such trapped molecules to be kept in specific rotational states of their ground electronic state and | v > = 0 vibrational state. Such selected initial conditions can then be used to study photo-detachment processes where only one or two rotational states are depleted from the trap by laser detachment of their access electron (Gianturco et al, 2018).…”

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

“…To follow the above steps in modelling the PD process, the state-changing collisional rates for the molecular anion of interest need to be known at different operating temperatures. Furthermore, we need to know as best as possible the PD rates (obtained from the PD cross sections as functions of the initial rotational state, as we have previously discussed 48,65 ). As a comment about the calculated cross sections and rates for state-to-state inelastic processes, it suffices to say that the dominant inelastic processes we found for the present systems are those for which the state-changing values of ∆N ± 1, 2 and for which the rates at a temperature of, say 20 K, are of the order of 10 −10 in units of cm 3 s −1 .…”