Cavity Ring-down UV spectroscopy of the C$^2\Sigma^+$-X$^2\Pi$ electronic transition of CH

TL;DR

This study used cavity ring-down UV spectroscopy to measure and analyze the rotational spectra of the CH radical's electronic transition, providing more accurate molecular constants and insights into its excited states.

Contribution

The paper presents the first high-resolution, rotationally resolved spectra of the CH radical's C$^2\Sigma^+$-X$^2\Pi$ transition using cavity ring-down spectroscopy, improving molecular constants and understanding of predissociation.

Findings

Over 200 new or more accurate transition measurements

Determined a precise equilibrium bond length of 1.115798 Å

Placed constraints on predissociation lifetimes

Abstract

Rotationally resolved spectra of the C$^2\Sigma^+$-X$^2\Pi$ electronic system of the CH radical were measured using cavity ring-down spectroscopy in supersonically expanding, planar hydrocarbon plasma. The experimental conditions allowed the study of highly excited rotational levels starting from vibrationally excited states. Here we present some 200+ new or more accurately recorded transitions in the 0-0, 1-1 and 2-2 vibronic bands in the ultraviolet between 30900-32400 cm$^{-1}$ (324-309 nm). The resulting data, compared to earlier measurements, allows for the determination of more precise molecular constants for each vibrational state and therefore more precise equilibrium values. From this an equilibrium bond length of 1.115798(17) \r{A} for the C$^2{\Sigma}^+$ state is determined. A comprehensive list with observed transitions for each band has been compiled from all available experimental studies and constraints are placed on the predissociation lifetimes.