[Paper Review] Laser-probing the rotational cooling of molecular ions by electron collisions

Ábel Kálosi, M. Grieser+10 more|arXiv (Cornell University)|Mar 31, 2022

Atomic and Molecular Physics57 references19 citations

TL;DR

This study experimentally measures rotational cooling rates of CH+ molecular ions via inelastic electron collisions using laser-probing in a cryogenic storage ring. By achieving electron-induced cooling to the J = 0 ground state faster than radiative relaxation, the work provides the first state-resolved experimental benchmark for low-temperature electron collision rates, validating recent theoretical predictions and enabling future studies of molecular ions in astrophysical and plasma environments.

ABSTRACT

We present state-selected measurements of rotational cooling and excitation rates of CH$^+$ molecular ions by inelastic electron collisions. The experiments are carried out at the Cryogenic Storage Ring, making use of a monoenergetic electron beam at matched velocity in combination with state-sensitive laser-dissociation of the CH$^+$ ions for simultaneous monitoring of the rotational level populations. Employing storage times of up to 600 s, we create conditions where electron-induced cooling to the $J = 0$ ground state dominates over radiative relaxation, allowing for the experimental determination of inelastic electron collision rates to benchmark state-of-the-art theoretical calculations. On a broader scale, our experiments pave the way to probe inelastic electron collisions for a variety of molecular ions relevant in various plasma environments.

Motivation & Objective

• To measure state-resolved rotational cooling and excitation rates of CH+ molecular ions by inelastic electron collisions at low temperatures.
• To experimentally benchmark theoretical calculations of electron collision cross sections for rotational transitions in molecular ions.
• To demonstrate that electron collisions can dominate over radiative cooling in cold molecular ion systems.
• To establish a platform for future measurements of inelastic electron collision rates in a broad range of molecular ions relevant to astrophysical and plasma environments.
• To develop and apply laser-dissociation diagnostics for real-time monitoring of rotational level populations in stored ions.

Proposed method

• Utilization of the Cryogenic Storage Ring (CSR) at Max-Planck-Institut für Kernphysik to store CH+ ions at ~20 K, enabling controlled electron collisions.
• Employment of a velocity-matched, monoenergetic electron beam (20 eV, T⊥ ≈ 26 K) to ensure resonant interaction with co-moving CH+ ions.
• Application of tunable nanosecond laser pulses (300 nm) for resonant photodissociation to probe rotational state populations via H-atom detection on a microchannel plate (MCP) detector.
• Simultaneous measurement of dissociation rates from electron-capture and radiative decay processes to isolate inelastic collision effects.
• Use of phase-space cooling via electron beam to achieve ion beam diameter < 6 mm, ensuring high overlap and sensitivity.
• Analysis of time-dependent changes in J-level populations to extract inelastic collision rate coefficients, corrected for dissociative recombination (DR) and radiative decay.

Experimental results

Research questions

• RQ1 What are the state-resolved inelastic electron collision rate coefficients for rotational transitions in CH+ at temperatures relevant to the interstellar medium?
• RQ2 Can electron collisions dominate over radiative cooling in cooling CH+ ions to the J = 0 ground state in a cryogenic environment?
• RQ3 To what extent do experimental measurements of rotational population evolution agree with state-of-the-art theoretical calculations for electron-impact rotational excitation and de-excitation?
• RQ4 How can laser-dissociation diagnostics be used to resolve individual rotational levels in stored molecular ions under controlled electron collision conditions?
• RQ5 What is the role of J-specific dissociative recombination in distorting rotational population measurements, and how can it be corrected?

Key findings

• Electron-induced rotational cooling to the J = 0 state outperforms radiative cooling, reducing the time to reach J = 0 dominance from ~600 s (radiative only) to significantly shorter times under electron collision conditions.
• The measured electron collision rate coefficient for J = 1 → 0 de-excitation is (3.2 ± 1.5) × 10⁻³ s⁻¹, consistent with theoretical predictions and confirming the dominance of electron collisions at low temperatures.
• Dissociative recombination (DR) rates were estimated to be about an order of magnitude smaller than inelastic collision rates, indicating that DR does not significantly distort the rotational cooling dynamics in this experiment.
• The effective electron density of 7.0(6) × 10⁵ cm⁻³ was sufficient to make collisional transition rates exceed radiative rates, confirming the dominance of electron collisions under these conditions.
• Laser probing enabled state-selective detection of rotational populations with high sensitivity, allowing resolution of individual J levels (J ≤ 3) and time-dependent evolution over up to 600 s of storage.
• The results validate recent theoretical calculations of low-temperature inelastic electron collision rates for CH+, providing the first experimental benchmark for this class of processes.