Atomic oxygen densities in the gas and liquid phase: utilizing ps- and fs-TALIF to interrogate the transport of O from the gas into the liquid phase

Recently, there has been an emerging realization of the importance of atomic oxygen in plasma-induced chemistry, particularly in the aqueous phase. Unlike many reactive species generated by plasmas, atomic oxygen is not found in biological systems in nature, so its effects remain largely unknown. However, several studies have alluded to its potential, documenting atomic oxygen’s central role in deactivating multiple cancer cell lines, cleaving DNA, and degrading a variety of organics. Quantifying solvated O atoms, critical for isolating its effects, has proven exceedingly difficult as chemical probes used for its detection suffer from a number of shortcomings. Here we show that aqueous O can be measured directly by employing two-photon absorption laser induced fluorescence (TALIF) with a femtosecond laser [1]. We demonstrate that given a sufficiently fast laser pulse, solvated O atoms can be excited without appreciable heating of the liquid and at a requisite efficiency for detection of a fluorescence signal despite the highly collisional aqueous environment. These measurements establish the proof of concept for an experimental technique to directly quantify atomic oxygen in liquid without the need for inherently problematic chemical probes. The measurements were conducted at the PCRF and combined with gas phase measurements conducted with a picosecond laser at PRF [2,3]. Gas phase measurements were performed in open atmosphere and with and without a liquid interface; to account for quenching in this heterogeneous and collisional environment, the effective lifetimes of excited atomic oxygen atoms were measured directly in situ.

[1] B. Myers, A. Dogariu, B. Beeler, K. Stapelmann, "Imaging solvated oxygen atoms with a femtosecond laser", Nature Comm. in revision

[2] B. Myers, E. Barnat, K. Stapelmann, J. Phys. D: Appl. Phys. 54 (2021) 455202

[3] K. Stapelmann, B. Myers, M. H. Quesada, E. Lenker, P. Ranieri, J. Phys. D: Appl. Phys. 54 (2021) 434003