Dual-comb spectroscopy of ammonia formation in non-thermal plasmas

Abstract

Plasma-activated chemical transformations promise the efficient synthesis of salient chemical products. However, the reaction pathways that lead to desirable products are often unknown, and key quantum-state-resolved information regarding the involved molecular species is lacking. Here we use quantum cascade laser dual-comb spectroscopy (QCL-DCS) to probe plasma-activated NH3 generation with rotational and vibrational state resolution, quantifying state-specific number densities via broadband spectral analysis. The measurements reveal unique translational, rotational and vibrational temperatures for NH3 products, indicative of a highly reactive, non-thermal environment. Ultimately, we postulate on the energy transfer mechanisms that explain trends in temperatures and number densities observed for NH3 generated in low-pressure nitrogen-hydrogen (N2–H2) plasmas.

Precision spectroscopy of non-thermal molecular plasmas using mid-infrared optical frequency comb Fourier transform spectroscopy

“…via V-T/R processes), resulting in higher T trans values compared to T rot . We note that a similar behavior has been observed for plasma-generated ammonia [27]. Since both NH 3 and HCN are polar molecules with relatively strong dipole moments and a high tendence for strong hydrogen bond formation, the observation of similar behavior may indicate the strong involvement of surface processes in their formation, leading to formation of molecules initially in vibrationally excited states.…”

“…As shown in this figure, the populations in the vibrational states 01 1 0, 02 • 0, and 02 2 0 can be characterized by a vibrational temperature of 336 ± 4 K determined from the slope of the linear fit (red line). We acknowledge that a more comprehensive evaluation of the vibrational temperatures can be also be achieved through broadband fitting routines, as demonstrated elsewhere [27,43].…”

“…spectroscopy of reactive PNC processes-is very limited and mostly lacking quantum-state knowledge on the plasma-generated species. Very recently we have applied quantum cascade laser dual comb spectroscopy to investigate ammonia formation built in non-thermal N 2 -H 2 plasmas [27]. Dual comb spectroscopy has also been applied to detect small traces of atomic Rb and K near 780 nm [28] as well as Fe near 533 nm [29] generated by laser-induced breakdown spectroscopy.…”