In this paper, we present an optical study of the state of N2 produced in an inductively coupled plasma. The operation of the discharge was characterized using ion flux measurements and broadband optical emission, and a clear change from capacitively to inductively coupled behaviour was observed with increasing applied power. The typical ion flux at 100 W and 10 mTorr was found to be 1.8 × 1018 m2 s−1, from which a ion density of ∼1.5 × 109 cm−3 was inferred. Diode laser cavity enhanced absorption spectroscopy (CEAS) was used to probe the state via the band at 686 nm. P33 band head spectra were used to determine both the translational (Ttr) and rotational (Trot) temperatures of the molecules at the v = 0 level. These were found to be in equilibrium but dependent on plasma parameters; in a 10 mTorr discharge, Trot ≈ Ttr, varying from ∼300 K at 5 W to ∼450 K at 400 W applied power. Absolute number densities in individual spin–rotation states were determined by calibrating the CEAS technique using the cavity ringdown time to measure the mirror reflectivity. The overall population in the v = 0 level was found to be (1.19 ± 0.07) × 1010 cm−3 under typical conditions of 100 W radio frequency power and 10 mTorr pressure, corresponding to a discharge efficiency for the production of this level of ∼10−5. A kinetic scheme is presented to account for the pressure and power dependence of the A-state concentration in the v = 0 level.

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Diode laser absorption spectroscopy has been used to study the four lowest energy excited states of atomic argon (1s2–1s5) arising from the 3p54s1 electronic configuration and produced in an inductively coupled RF discharge. Three of the states (1s2, 1s4 and 1s5) were detected using wavelength modulation spectroscopy with diode laser radiation in the wavelength range 425–430 nm, obtained by frequency doubling the output of a near infrared diode laser in a KNbO3 crystal. The 1s3 state was probed using direct absorption spectroscopy with the fundamental output of a diode laser operating at 795 nm. The 1s5 state dominates the total population of this manifold with a proportion of around 48%, which is essentially independent of plasma power and pressure. Translational temperatures varied with power but were essentially pressure independent. For example, at 20 mTorr total pressure and 400 W applied power, the temperature was found to be 384 ±15 K. The variations in concentrations of all four 1sn states with plasma power and pressure are reported and suggests that a statistical equilibrium between them has been established. The results are rationalized by utilizing a simple kinetic model, to determine the dominant production (electron impact excitation from the ground state) and loss (intra-manifold quenching and radiative decay) processes. Fast redistribution of population in the states, mainly via electron impact excitation to higher lying electronic states and subsequent radiative decay back down, in combination with modified radiative rates for the two radiative states as a consequence of radiation imprisonment, can partially account for the observed relative trends.

Radiation at 308 nm has been obtained by frequency doubling the output of a commercial diode laser cooled to 165 K. A single pass through a crystal of LiIO converted 1 mW of 616 nm radiation to 50 pW of UV, and this was used to detect the 3 OH radical in absorption in a flow tube. Possible extensions of the method for detection of OH in the atmosphere are discussed. q

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