Measurement of hydrogen and nitrogen via collision-induced infrared absorption

Abstract

A novel spectroscopic approach has been developed for laser absorption sensing of homonuclear diatomic molecules at high pressures relevant to modern energy devices. The spectroscopic approach exploits the collision-induced absorption (CIA) effect, wherein collisions between pairs or clusters of molecules generate transient electric dipole moments. This effect causes ‘infrared inactive’ molecules, including hydrogen (H<inf>2</inf>) and nitrogen (N<inf>2</inf>), to exhibit infrared absorption spectra at sufficiently high gas densities. The CIA spectra of the H<inf>2</inf> [sbnd]H<inf>2</inf> and N<inf>2</inf> [sbnd]N<inf>2</inf> pairs in the fundamental bands were experimentally studied up to 70 atm and found to be in close agreement with the HITRAN CIA database. Notably, the CIA spectra scale quadratically with gas density and are found to be 3 to 5 orders of magnitude stronger than their molecular spectra at elevated pressures. Additional measurements were made to accurately model CIA spectra with varying mixture compositions. These spectra enable a two-color fixed-wavelength direct-absorption strategy to simultaneously measure H<inf>2</inf> and N<inf>2</inf> in binary mixtures, targeting their fundamental CIA bands near 2.1μm and 4.3μm, respectively. A multi-pressure sensing strategy was further introduced to effectively separate CIA spectra from molecular interference by exploiting their distinct density dependencies. The measurements presented demonstrate a new infrared absorption spectroscopy approach that will enable direct and sensitive measurement of homonuclear diatomic molecules under conditions relevant to practical energy systems.