Diode laser measurements of the Ar 3p⁵4s¹excited states in an inductively coupled RF plasma

Abstract

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.

Neutral gas depletion mechanisms in dense low-temperature argon plasmas

“…k m = 3.7 × 10 −14 exp(−15 eV/k B T e ) m 3 s −1 [25] for the excitation to the metastable state and k i = 2 × 10 −13 exp(−5 eV/k B T e ) m 3 s −1 [25] for the ionization and k t = 2 × 10 −13 m 3 s −1 [25] for the transfer to the resonant state of metastable atoms by electron impact. The radiative lifetime of the resonant Ar * ( 3 P 1 ) state is a few nanosecond; however, due to resonant radiation trapping, the effective radiative lifetimes reach a few microsecond and consequently the density of Ar * ( 3 P 1 ) atoms becomes almost comparable to that of the metastable state [26,27] and consequently the reverse process (electron impact transfer from 3 P 1 to 3 P 2 ), which has 5/3 times larger rate coefficient, partly compensates the depletion of the Ar * ( 3 P 2 ) state. Therefore, the overall depletion rate by population transfer must be smaller than k t .…”

Pressure broadening of 772.376 and 772.421 nm argon lines and kinetics of argon metastable atoms

“…In low pressure plasmas, the measured [Ar(1s3)]/[Ar(1s5)] density ratio was usually closer to 0.1 63,64,65,66 , rather than to the statistical value (2J3+1)/ (2J5+1)=0.2, where J3=0 and J5=2 are the total angular momentum of 1s3 and 1s5 sates, respectively. The observed enhancement of this ratio with increasing argon pressure, reaching 0.3 at atmospheric pressure, can be related to both smaller reaction coefficient of Ar*(1s3) atoms in 3-body collision with 2 Ar atoms (Eq.…”

Density of atoms in Ar*(3p54s) states and gas temperatures in an argon surfatron plasma measured by tunable laser spectroscopy

“…8 for a surfatron plasma with the same radius, in which the n(1s 3 )/n(1s 5 ) ratio was rising from 0.1 at 0.26 millibars to 0.35 at 3.7 millibars. Bakowski et al 35 also reported a density ratio near 0.1 in an ICP plasma source. For the n(1s 4 )/n(1s 5 ) ratio, one observes a small deviation from the predicted statistical equilibrium.…”