Quantum-State-Specific Detection of Molecular Hydrogen by Three-Photon Ionization

Abstract

A sensitive method is demonstrated for determining (${v}^{\ensuremath{'}\ensuremath{'}}, {J}^{\ensuremath{'}\ensuremath{'}}$) populations of ${\mathrm{H}}_{2}$. Single rotational levels of the $E$, $F{^{1}\ensuremath{\Sigma}_{g}}^{+}$ state are prepared by two-photon excitation; absorption of a third quantum causes ionization. Detection of ${J}^{\ensuremath{'}\ensuremath{'}}$ levels of ${v}^{\ensuremath{'}\ensuremath{'}}=0, 1, \mathrm{and} 2$ is possible while scanning the dye laser fundamental over just 5 nm. Measured rotational populations at different temperatures (295-750 K) agree well with the corresponding Boltzmann distributions. Rotational line strengths show almost no ${J}^{\ensuremath{'}\ensuremath{'}}$ dependence for this process.