State Selective Preparation and Nondestructive Detection of Trapped ${\rm O}_2^+$

TL;DR

This paper demonstrates a method for state-selective preparation and nondestructive detection of trapped O$_2^+$ ions using resonance-enhanced multiphoton ionization, enabling precise control and analysis of molecular quantum states for advanced scientific applications.

Contribution

It introduces a novel approach combining (2+1) REMPI with nondestructive detection techniques for specific rovibrational states of O$_2^+$ ions in traps.

Findings

Resolved the O$_2$ $d^1\Pi_g$ state spectrum and clarified its band origin.

Identified optimal transitions for different rotational temperatures.

Demonstrated nondestructive detection of trapped molecular ions.

Abstract

The ability to prepare molecular ions in selected quantum states enables studies in areas such as chemistry, metrology, spectroscopy, quantum information, and precision measurements. Here, we demonstrate $(2+1)$ resonance-enhanced multiphoton ionization (REMPI) of oxygen, both in a molecular beam and in an ion trap. The two-photon transition in the REMPI spectrum is rotationally resolved, allowing ionization from a selected rovibrational state of O$_2$. Fits to this spectrum determine spectroscopic parameters of the O$_2$ $d\,^1\Pi_g$ state and resolve a discrepancy in the literature regarding its band origin. The trapped molecular ions are cooled by co-trapped atomic ions. Fluorescence mass spectrometry nondestructively demonstrates the presence of the photoionized O$_2^+$. We discuss strategies for maximizing the fraction of ions produced in the ground rovibrational state. For $(2+1)$ REMPI through the $d\,^1\Pi_g$ state, we show that the Q(1) transition is preferred for neutral O$_2$ at rotational temperatures below 50~K, while the O(3) transition is more suitable at higher temperatures. The combination of state-selective loading and nondestructive detection of trapped molecular ions has applications in optical clocks, tests of fundamental physics, and control of chemical reactions.