@article{zafar_martin_shannon_2019, title={Doppler-free, Stark broadened profiles at low plasma densities in helium}, volume={230}, ISSN={0022-4073}, url={http://dx.doi.org/10.1016/j.jqsrt.2019.03.020}, DOI={10.1016/j.jqsrt.2019.03.020}, abstractNote={This work utilizes Doppler-free saturation spectroscopy to measure Doppler-free, Stark broadened spectral profiles for the π-polarization of the HeI 21P → 61D transition in a low density (ne=7×1010−2×1012 cm−3) helium plasma. The measurements were performed in an electron-cyclotron resonance discharge at 23 mTorr with electron density being diagnosed using a combination of Langmuir probe and microwave interferometry techniques. The Doppler-free profiles were observed to be nearly symmetric at ne < 1011 cm−3 but markedly asymmetric above this transition point. Electron density is extracted from the spectral data via fitting to a spectral model based on quasi-static Stark broadening. The fit results are compared to ne measurements obtained using a combination of Langmuir probe and microwave interferometry techniques. The fit and measured ne are shown to agree within 20% on average. Finally, the quasi-static model is shown to be valid in the low-density regime for 21P → 61D helium transition.}, journal={Journal of Quantitative Spectroscopy and Radiative Transfer}, publisher={Elsevier BV}, author={Zafar, Abdullah and Martin, Elijah and Shannon, Steve}, year={2019}, month={Jun}, pages={48–55} }
 @article{zafar_martin_shannon_2018, title={High resolution magnetic field measurements in hydrogen and helium plasmas using active laser spectroscopy}, volume={89}, ISSN={0034-6748 1089-7623}, url={http://dx.doi.org/10.1063/1.5039334}, DOI={10.1063/1.5039334}, abstractNote={Passive spectroscopic measurements of Zeeman splitting have been used reliably to measure magnetic fields in plasmas for decades. However, a requirement is that the field magnitude must be sufficiently strong to be resolved over Doppler and instrument broadening (typically >10 000 G). A diagnostic for measuring magnetic fields spectroscopically well below this limit (>20 G) with high sensitivity has been developed at the Oak Ridge National Laboratory. The diagnostic relies on measuring a high resolution spectral profile using Doppler-free saturation spectroscopy (DFSS) and then fitting the spectrum to a quantum mechanical model. DFSS is an active, laser based technique that greatly reduces the influence of Doppler broadening and eliminates instrument broadening. To date, the diagnostic has been successfully employed to measure the magnetic field in magnetized (550-900 G), low-temperature (5-10 eV), low-density (1010–1012 cm−3), hydrogen and helium plasmas in the 5-200 mTorr pressure range using a low power (25 mW) diode laser. Implementing an approximate crossover resonance model, the measurements are shown to be accurate within 5 G for helium and 83 G for hydrogen. The accuracy in hydrogen can be improved to 39 G if the crossover resonances are neglected. A more robust crossover model can decrease this error to <1 G.}, number={10}, journal={Review of Scientific Instruments}, publisher={AIP Publishing}, author={Zafar, Abdullah and Martin, Elijah and Shannon, Steve}, year={2018}, month={Oct}, pages={10D126} }
 @article{zafar_martin_shannon_isler_caughman_2016, title={A temporally and spatially resolved electron density diagnostic method for the edge plasma based on Stark broadening}, volume={87}, ISSN={0034-6748 1089-7623}, url={http://dx.doi.org/10.1063/1.4955484}, DOI={10.1063/1.4955484}, abstractNote={An electron density diagnostic (≥1010 cm−3) capable of high temporal (ms) and spatial (mm) resolution is currently under development at Oak Ridge National Laboratory. The diagnostic is based on measuring the Stark broadened, Doppler-free spectral line profile of the n = 6–2 hydrogen Balmer series transition. The profile is then fit to a fully quantum mechanical model including the appropriate electric and magnetic field operators. The quasi-static approach used to calculate the Doppler-free spectral line profile is outlined here and the results from the model are presented for H-δ spectra for electron densities of 1010–1013 cm−3. The profile shows complex behavior due to the interaction between the magnetic substates of the atom.}, number={11}, journal={Review of Scientific Instruments}, publisher={AIP Publishing}, author={Zafar, A. and Martin, E. H. and Shannon, S. C. and Isler, R. C. and Caughman, J. B. O.}, year={2016}, month={Jul}, pages={11E505} }
 @article{li_zhang_qiao_yu_peterson_zafar_kumar_curtarolo_hunte_shannon_et al._2016, title={All The Catalytic Active Sites of MoS2 for Hydrogen Evolution}, volume={138}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.6b05940}, DOI={10.1021/jacs.6b05940}, abstractNote={MoS2 presents a promising low-cost catalyst for the hydrogen evolution reaction (HER), but the understanding about its active sites has remained limited. Here we present an unambiguous study of the catalytic activities of all possible reaction sites of MoS2, including edge sites, sulfur vacancies, and grain boundaries. We demonstrate that, in addition to the well-known catalytically active edge sites, sulfur vacancies provide another major active site for the HER, while the catalytic activity of grain boundaries is much weaker. The intrinsic turnover frequencies (Tafel slopes) of the edge sites, sulfur vacancies, and grain boundaries are estimated to be 7.5 s-1 (65-75 mV/dec), 3.2 s-1 (65-85 mV/dec), and 0.1 s-1 (120-160 mV/dec), respectively. We also demonstrate that the catalytic activity of sulfur vacancies strongly depends on the density of the vacancies and the local crystalline structure in proximity to the vacancies. Unlike edge sites, whose catalytic activity linearly depends on the length, sulfur vacancies show optimal catalytic activities when the vacancy density is in the range of 7-10%, and the number of sulfur vacancies in high crystalline quality MoS2 is higher than that in low crystalline quality MoS2, which may be related with the proximity of different local crystalline structures to the vacancies.}, number={51}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Li, Guoqing and Zhang, Du and Qiao, Qiao and Yu, Yifei and Peterson, David and Zafar, Abdullah and Kumar, Raj and Curtarolo, Stefano and Hunte, Frank and Shannon, Steve and et al.}, year={2016}, month={Dec}, pages={16632–16638} }
 @article{martin_zafar_caughman_isler_bell_2016, title={Applications of Doppler-free saturation spectroscopy for edge physics studies}, volume={87}, ISSN={["1089-7623"]}, DOI={10.1063/1.4961287}, abstractNote={Doppler-free saturation spectroscopy provides a very powerful method to obtain detailed information about the electronic structure of the atom through measurement of the spectral line profile. This is achieved through a significant decrease in the Doppler broadening and essentially an elimination of the instrument broadening inherent to passive spectroscopic techniques. In this paper we present the technique and associated physics of Doppler-free saturation spectroscopy in addition to how one selects the appropriate transition. Simulations of Hδ spectra are presented to illustrate the increased sensitivity to both electric field and electron density measurements.}, number={11}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, author={Martin, E. H. and Zafar, A. and Caughman, J. B. O. and Isler, R. C. and Bell, G. L.}, year={2016}, month={Nov} }
 @article{zhang_zafar_coumou_shannon_kushner_2015, title={Control of ion energy distributions using phase shifting in multi-frequency capacitively coupled plasmas}, volume={117}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4922631}, DOI={10.1063/1.4922631}, abstractNote={Control of ion energy distributions (IEDs) onto the surface of wafers is an ongoing challenge in microelectronics fabrication. The use of capacitively coupled plasmas (CCPs) using multiple radio frequency (rf) power sources provides many opportunities to customize IEDs. In dual-frequency CCPs using a fundamental frequency and its second harmonic, varying the relative voltages, powers, and phases between the fundamental and second harmonic biases have demonstrated potential as control mechanisms for the shape of the IEDs. In this paper, we report on computational and experimental investigations of IED control in dual-frequency and triple-frequency CCPs where the phase between the fundamental and second harmonic frequency voltage waveform is used as a control variable. The operating conditions were 5–40 mTorr (0.67–5.33 Pa) in Ar and Ar/CF4/O2 gas mixtures. By changing the phase between the applied rf frequency and its second harmonic, the Electrical Asymmetric Effects was significant and not only shifted the dc self-bias but also affected plasma uniformity. When changing phases of higher harmonics, the energies and widths of the IEDs could be controlled. With the addition of a 3rd high-frequency source, the plasma density increased and uniformity improved. Computed results for IEDs were compared with experimental results using an ion energy analyzer in systems using rf phase locked power supplies.}, number={23}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Zhang, Yiting and Zafar, Abdullah and Coumou, David J. and Shannon, Steven C. and Kushner, Mark J.}, year={2015}, month={Jun}, pages={233302} }