Session SF3: Nonequilibrium Kinetics of Low-temperature Plasmas

A reliable Collisional Radiative Model for Xenon Plasma

Xenon is used nearly in all modern HETs as propellant [1] and it is also added as a trace gas to characterize the low temperature plasmas. Our aim is to obtain highly needed detailed fine-structure resolved cross sections for the electron impact excitation of xenon using relativistic distorted wave (RDW) approach and demonstrate the application of the calculated data in modelling of low temperature xenon plasma by developing a C-R model following our earlier work on Kr [2]. Various transitions from the ground 5$p^{\mathrm{6}}$ state to the excited 5$p^{\mathrm{5}}$6$s$, 5$p^{\mathrm{5}}$6$p$, \textit{5p}$^{5}$\textit{5d}, 5$p^{\mathrm{5}}$7$s$ and 5$p^{\mathrm{5}}$7$p$ states as well as among these excited states are considered and their cross sections are calculated. The cross sections are incorporated in a C-R model which is coupled with OES measurements of Czerwiec \textit{et al}.[3] for diagnostics of low temperature inductively coupled Xe plasma (ICP). [1] Y. H. Chiu \textit{et al}., \textit{J. Appl. Phys.} \textbf{99}, 113304, 2006 [2] R.K. Gangwar \textit{et al.}, \textit{Plasma Sources Sci. Technol.} \textbf{25}, 35025, 2016 [3]T. Czerwiec and D.B. Graves, \textit{J. Phys. D. Appl. Phys.} \textbf{37}, 2827, 2004.   

Strong Vibrational Non-Equilibrium in N$_2$ and CO$_2$ by Microsecond Pulsed Microwave Plasma

Novel means for electrification of chemical processes are essential in order to keep up with the modernizing energy supply. In this context, we investigate microwave plasma to utilize electrical power for activation and conversion of stable molecules, in particular CO$_2$ and N$_2$. Preferential vibrational excitation is considered as pathway to their fixation reactions with ultimate efficiency. In this study, the microwave power was pulsed at low duty cycle to limit gas heating and to reveal vibrational excitation dynamics. The typical pulsing scheme was 100 microsec ON-time followed by 25 ms OFF-time. Raman scattering yielded the vibrational and rotational temperature evolution during the pulse. Strong non-equilibrium was observed both in N$_2$ and CO$_2$. Specifically, in N$_2$ the rotational temperature, taken as proxy for the gas temperature, slowly increased to 1200K, after the vibrational temperature already peaked at 3500K. In CO$_2$, also non-equilibrium between different vibrational degrees of freedom was observed. In fact, the asymmetric stretch immediately heated to over 1000K before equilibrating with the symmetric stretch and bending mode at 1800K. Finally, the vibrational excitation dynamics were correlated to CO yield and efficiency in scans of ON-time.   

O atom kinetics in RF CCP oxygen plasma at increased pressures

In this study, the 81 MHz symmetric CCP discharge in quartz tube at increased pressures (10 to 100 Torr) was used to study O$_{2}$ dissociation mechanism. The use of both spatially resolved and time-resolved actinometry technique on Kr in the modulated rf discharge allowed measuring atom loss frequency $\nu$$_{loss}$$^{O}$ and dissociation degree and thereby determining dissociation rate constant k$_{diss}$$^{O2}$. The O atom loss is connected with surface recombination at lower pressure and volume reactions at the higher pressure. The variation of plasma parameters allowed studying the O$_{2}$ dissociation and O atom loss mechanisms in a wide range of gas temperature from $\sim$ 500 K up to 1800 K. The possible mechanism of increasing k$_{diss}$$^{O2}$ at low E/N as well as the role of ozone generation in $\nu$$_{loss}$$^{O}$ is discussed in detail.   

EUV emission from Sn plasmas

We report on our continuing efforts to understand the EUV emission from tin plasmas of interest to nanolithography. Recent investigations have examined both the strong in-band emission at 13.5~nm, as well as the out-of-band emission at shorter wavelengths, from measurements of molten tin droplets illuminated by a high-intensity laser. The line features in the out-of-band region have been assigned to various transitions within the ions of most relevance (Sn$^{8+}$ to Sn$^{15+}$). Recent theoretical work has been extended to examine the emission from tin at various temperatures and densities. Good agreement is found between the theoretical predictions and experiment for the in-band emission from plasmas generated at various laser energies. The agreement between theory and experiment for the out-of-band features is also quite reasonable. We describe the experimental effort to produce such tin plasmas and the associated diagnostics. We will discuss the atomic structure calculations made using the Los Alamos suite of atomic physics codes and emission calculations that have been made using the ATOMIC code to predict the emission at various plasma conditions. The role of radiation transport effects and possible gradients will also be discussed.