Eur. Phys. J. D 60, 479–487 (2010) DOI: 10.1140/epjd/e2010-00279-0 Regular Article T HE EUROPEAN P HYSICAL JOURNAL D Spectroscopic diagnostics and electric field measurements in the near-cathode region of an atmospheric pressure microplasma jet B.N. Sismanoglu 1, a , K.G. Grigorov 1, 2 , R.A. Santos 3 , R. Caetano 1 , M.V.O. Rezende 1 , Y.D. Hoyer 1 , and V.W. Ribas 1 1 Departamento de F´ ısica, Instituto Tecnol´ogico de Aeron´autica, Comando-Geral de Tecnologia Aeroespacial, P¸ca Marechal Eduardo Gomes 50, 12 228-900, S˜ao Jos´ e dos Campos, SP, Brazil 2 Institute of Electronics, Bulgarian Academy of Science, 72 Tzarigradsko Chaussee, Sofia 1784, Bulgaria 3 EMEF Carlos Chagas, Av. Osvaldo Valle Cordeiro 337, 03 584-000, S˜ao Paulo, SP, Brazil Received 20 July 2010 / Received in final form 15 September 2010 Published online 20 October 2010 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2010 Abstract. Linear Stark splitting of the H β Balmer line components and spatially resolved optical emission spectroscopy (OES) measurements were used to estimate the electric field gradient in the cathode sheath region (∼70 μm long) of an atmospheric pressure direct current argon flow-stabilized microplasma jet. Also, plasma parameters in the negative glow region were investigated by both OES and electrical diagnostics. The microplasma jet was operated for current ranging from 10 to 110 mA. OH (A 2 Σ + , v =0 → X 2 Π, v ′ = 0) rotational bands at 306.357 nm and also the Ar 603.213 nm line were used to determine the gas temperature, which ranges from 600 to 1000 K. Electron number density, ranging from 4.1 × 10 14 to 8.5 × 10 14 cm -3 , was determined through analysis of the H β line. Electron excitation temperature was also measured from the ratio of two Mo lines (8500-18 000 K) and from Boltzmann-plot of Ar 4p-4s and 5p-4s transitions (11 000–13 500 K). 1 Introduction Atmospheric pressure microplasmas are stable, uniform and homogeneous glow discharges [1–4], with dimensions less than 1 mm. The non-thermal equilibrium is one important characteristic of these plasmas and the gas temperature T g less than 1000 K allows many applica- tions [3–8]. Direct current (dc) microplasma jets are similar to the well-known microhollow cathode discharges [4,9,10], because they present approximately the same current- voltage characteristic curves [11]. The usual configuration consists of a capillary tube used as a cathode and molybde- num metal as anode. At atmospheric pressure the cathode- tube microplasma jet operates in a regime which is similar to the normal dc glow mode with current ranging from 2 to 20 mA [11]. Moreover, using the metal tube as anode and a molybdenum foil as cathode the discharge operates in abnormal mode, now at higher currents. Atmospheric pressure dc glow discharges at low cur- rent has been studied [1,12]. Staack et al. [13] have in- vestigated atmospheric pressure dc argon microplasma generated between a wire anode and a flat cathode sur- face, achieving T g ∼ = 630 K for current of 3.5 mA with 0.4 mm electrode spacing. This microplasma is similar to a e-mail: bogos@ita.br this present work, but in their case the discharge current was lower and the electrode spacing shorter. In our exper- iment, a new microplasma jet was implemented to work at relatively low gas temperatures even operating at high currents, due to the high flow used (0.7 ℓ min -1 ). The aim of this work is to study the anode-tube mi- croplasma jet for application to surface treatment, pat- terning and modification, thin film growth and analyti- cal chemistry. This plasma is also a low-cost and effective way to minimize the film surface treatment costs because it takes place at atmospheric pressure and has low power consumptions of about 8 W for 30 mA. The optical and electrical characterization of the plasma jet has been re- ported in previous work [14], where Cu electrode was used as cathode instead of molybdenum. In this present work the electrical performance of the microplasma jet have been studied by plotting current- voltage diagrams and Paschen’s curve. In this configura- tion, the capillary tube (tungsten-carbide with inner di- ameter of 520 μm) remains at low temperature even at high currents. However, the increase of the current, from 10 to 110 mA, assists a flow of Mo atoms from the hot cathode surface to the plasma. The analysis of the ra- diation emitted from these sputtered and excited atoms could be used to determine the electron excitation tem- perature (T exc ) in the negative glow region. The presence of high electron number density (n e ) has enabled the use