Ghost features in Doppler-broadened spectra of rovibrational transitions in trapped HD$^+$ ions

TL;DR

This paper investigates ghost features in Doppler-broadened spectra of rovibrational transitions in trapped HD$^+$ ions, revealing their origin and providing guidelines for their assessment using advanced lineshape models.

Contribution

It introduces a detailed theoretical analysis of ghost features in Doppler-broadened spectra, emphasizing the importance of comprehensive lineshape models for accurate spectral interpretation.

Findings

Ghost features depend on Doppler width, transition rates, and saturation levels.

The study explains the origin of ghost features in saturated spectra.

Guidelines are provided to assess the appearance of ghost features.

Abstract

Doppler broadening plays an important role in laser rovibrational spectroscopy of trapped deuterated molecular hydrogen ions (HD$^+$), even at the millikelvin temperatures achieved through sympathetic cooling by laser-cooled beryllium ions. Recently, Biesheuvel \textit{et al.} \cite{Biesheuvel2016} presented a theoretical lineshape model for such transitions which not only considers linestrengths and Doppler broadening, but also the finite sample size and population redistribution by blackbody radiation, which are important in view of the long storage and probe times achievable in ion traps. Here, we employ the rate equation model developed by Biesheuvel \textit{et al.} to theoretically study the Doppler-broadened hyperfine structure of the $(v,L):(0,3)\rightarrow(4,2)$ rovibrational transition in HD$^+$ at 1442~nm. We observe prominent yet hitherto unrecognized ghost features in the simulated spectrum, whose positions depend on the Doppler width, transition rates, and saturation levels of the hyperfine components addressed by the laser. We explain the origin and behavior of such features, and we provide a simple quantitative guideline to assess whether ghost features may appear. As such ghost features may be common to saturated Doppler-broadened spectra of rotational and vibrational transitions in trapped ions composed of partly overlapping lines, our work illustrates the necessity to use lineshape models that take into account all the relevant physics.