Population Measurement of the 3p 5 4s Conﬁguration Levels in an Argon Microwave Plasma at Atmospheric Pressure I. SANTIAGO and M. D. CALZADA* Grupo de Espectroscopı ´a de Plasmas, Ediﬁcio A.Einstein (C-2), Campus de Rabanales, Universidad de Co ´rdoba, 14071 Co ´ rdoba, Spain Metastable atoms, due to their intrinsic properties, are very useful to characterize plasma and to control the scientiﬁc and technological applications carried out with it. This paper describes the implementation of a method to determine the densities of the argon 3p 5 4s metastable and resonant levels in a microwave plasma at atmospheric pressure. Because the method is based on the self-absorption of the radiation emitted by the plasma, no external radiation source is needed. Using this method, the populations of the four levels of the 3p 5 4s argon conﬁguration were found to be in the range 10 10 –10 12 cm 3 , in agreement with the values found in the literature. Index Headings: Argon metastable levels; Microwave plasmas; Atmo- spheric pressure; Radiation self-absorption; Atomic emission spectros- copy; AES. INTRODUCTION The discharges created and maintained by a surface wave (SWDs) belong to the category of high-frequency (HF) discharge. The main characteristics of SWDs are their great stability and reproducibility in a broad range of experimental conditions, the use of relatively low power, and the absence of electrodes for coupling the energy to the discharge. Due to these properties, these plasmas have been extensively studied and used in many applications of scientiﬁc and technological interest such as microelectronics, treatment of surfaces, or even the sterilization of medical and veterinary instruments. 1–3 To ensure that the industrial and technological applications of this type of plasma are made with maximum effectiveness, it is necessary to characterize the discharge. This characterization consists of determining the value of its characteristic parameters, such as the temperatures and the population densities of the different species it contains. In this way, the internal kinetics of the discharge can be studied exhaustively. Among the different species in the plasma, the metastable atoms play a very important role. Because they are not radiatively connected with the ground state, they have a long half-life time, which implies that they can serve as an energy reservoir, causing excitation, ionization, and dissociation processes. 4,5 Atoms in a metastable state can be considered as an effective intermediate state for the stepwise excitation and ionization of the atom because only a small amount of energy (,5 eV) is needed for further excitation or ionization. This energy is lower than the amount that is needed to excite or ionize the atoms from the ground state (15.75 eV in argon). This is important because most electrons in some industrial plasmas have energies below 4 eV. It has been veriﬁed that the value of the effective excitation cross-section of certain higher levels of argon starting from the metastable levels may reach several orders of magnitude higher than the effective excitation cross-section from the ground state. 6 Moreover, depending on the experimental conditions, the contribution to ionization from the metastable levels to total ionization may be of great importance, and this stepwise ionization may even permit a decrease in the energy necessary to maintain the discharge. 7,8 As a result, metastable levels have a signiﬁcant inﬂuence on the population distribution of the excited states of the plasma atoms, causing important changes in the population of those levels, even in the case in which the metastable density represents a small percentage of the total plasma species population. Thus, it is possible to say that metastable levels intervene in discharge creation mechanisms although they also do so in plasma extinction mechanisms due to their long half- life. 9–11 Consequently, determining the density of metastable levels is especially useful to ﬁnd out the degree of plasma thermodynamic equilibrium and the microscopic balances that permit control of its excitation kinetics. For instance, in a plasma in partial local thermodynamic equilibrium (pLTE), the concentration of metastable atoms allows us to specify the ionizing or recombining character of the plasma. 12,13 All this implies that the experimentally measured metastable level populations can help to evaluate the validity of certain existing and future theoretical discharge models. 14,15 The metastable states can also be of importance in the excitation and ionization of analytes, both atomic and molecular ones, introduced into the discharge. 4,5,9,16–21 They are used in some types of detectors for the spectrochemical analysis of certain atoms and molecules, improving their performance. 22–24 Metastables can participate actively in processes like surface treatment, their intervention being fundamental for the formation and deposition of thin layers of material in the manufacture of electronic devices 16 and in the pumping of laser levels. 5 Also, determining the populations of these metastable species permits the determination of some parameter values, which are basic for the functioning of inductive ﬂuorescent lamps (Philips QL lamp), a special type of electrodeless lamp. 25,26 However, the characteristics of the metastable levels can also make their presence pernicious in certain industrial processes, so that it is essential to control the populations of these species. 27 This work aims to describe the implementation of an experimental method to estimate the absolute densities for the four ﬁrst excited levels of argon, belonging to the 3p 5 4s conﬁguration (Fig. 1), in a plasma column created by a surface wave at atmospheric pressure. This method is based on the self- absorption of radiation generated by the 4p ! 4s transition in Ar. Two levels of the 3p 5 4s conﬁguration are metastables, 3 P 2 and 3 P 0 , with energies of 11.55 and 11.71 eV above the ground state, and two are resonant levels that can decay to the ground state by emission of radiation, 3 P 1 and 1 P 1 , with energies of 11.62 and 11.83 eV, respectively. In atmospheric pressure discharges, the difference of behavior between radiative and Received 21 July 2006; accepted 7 May 2007. * Author to whom correspondence should be sent. E-mail: fa1cazal@uco. es. Volume 61, Number 7, 2007 APPLIED SPECTROSCOPY 725 0003-7028/07/6107-0725$2.00/0 Ó 2007 Society for Applied Spectroscopy