# Infrared spectroscopy of molecular ions in selected rotational and spin-orbit states

## TL;DR

This paper introduces a novel experimental setup for recording infrared spectra of molecular ions in specific rotational and spin-orbit states, enabling detailed spectroscopic analysis of selected ion states.

## Contribution

The work presents a new state-selective IR spectroscopy method for molecular ions using mass-analyzed-threshold-ionization and reaction detection, improving state specificity over previous techniques.

## Key findings

- IR spectra of C₂H₂⁺ in specific rotational levels obtained
- Method demonstrates selective excitation and detection of ion states
- Spectra reveal details of spin-orbit components in the ground state

## Abstract

First results are presented obtained with an experimental setup developed to record IR spectra of rotationally state-selected ions. The method we use is a state-selective version of a method developed by S. Schlemmer, D. Gerlich and coworkers (Int. J. Mass. Spec. 185, 589 (1999); J. Chem. Phys. 117, 2068 (2002)) to record IR spectra of ions. Ions are produced in specific rotational levels using mass-analyzed-threshold-ionization spectroscopy. The state-selected ions generated by pulsed-field ionization of Rydberg states of high principal quantum number ($n\approx200$) are extracted toward an octupole ion guide containing a neutral target gas. Prior to entering the octupole, the ions are excited by an IR laser. The target gas is chosen so that only excited ions react to form product ions. These product ions are detected mass selectively as a function of the IR laser wavenumber. To illustrate this method, we present IR spectra of C$_2$H$_2^+$ in selected rotational levels of the $^2\Pi_{\textrm{u,}3/2}$ and $^2\Pi_{\textrm{u,}1/2}$ spin-orbit components of the vibronic ground state.