Low-Frequency Relaxation Oscillations in Capacitive Discharge Processes

Low-frequency （2.72-3.70 Hz） relaxation oscillations at 100 mTorr at higher absorbed power were observed from time-varying optical emission of the main discharge chamber and the periphery. We interpret the low frequency oscillations using an electromagnetic model of the slot impedance with parallel connection variational peripheral capacitance, coupled to a circuit analysis of the system including the matching network. The model results are in general agreement with the experimental observations, and indicate a variety of behaviours dependent on the matching conditions.     

Instability Parameters of Optical Oscillation Frequency in Plasma Central Discharge and Periphery Region

We have observed relaxation oscillations in a capacitive discharge in Ar gas, connected to a peripheral ground chamber. The plasma oscillations observed from time-varying optical emission from the main discharge chamber show, for example, a high frequency （75.37kHz） relaxation oscillation, at lOOmTorr and 8 W absorbed power, and a low frequency （2.72Hz） relaxation oscillation, lOO mTorr and 325 W absorbed power. Time-varying optical emission intensity and plasma density are also detected with a Langmuir probe. The theoretical result agrees well with experiments.     

Electronic states and transitions of the TeX (X = C1, Br,I) radicals

Ab initio multireference configuration interaction calculations including spin-orbit coupling have been carried out for the first time for valence electronic states of the TeX (X = Cl, Br, I) radicals and compared with the results for the isovalent TeF and IO systems obtained earlier at a similar level of theoretical treatment. The calculated spectroscopic constants are in good agreement with experimental data in the rare cases when the latter are available. It is shown that the X² II(σ²π⁴π?³) ground state bonding becomes consistently weaker down the TeX group (calc. D_e, = 25480cm^?1 for TeF, 12 100cm^?1 for Tel) due to the more covalent character of bonding in the heavier radicals. The first excited state, A ⁴Σ^? (π?→ σ?), is calculated to be bound in all systems. It is split into Ω 1/2 and 3/2 components, with regular ordering in the Franck-Condon region, opposite to that of the ground X²II state. For larger internuclear distances, the A₁ ⁴Σ^? _1/2 state undergoes an avoided crossing with X₂ ²II_1/2, which causes a shoulder in the X₂ potential curve and also leads to a crossing between the A₁, and A₂ curves and large distinctions in their vibrational frequencies. The π? → σ? B²Σ^?, C²δ, and 1 ²Σ⁺ states are calculated to lie next in energy. They are all bound in the lightest of the TeX radicals, TeF, but successively lose their bonding character down the group. In contrast to oxygen monohalides, the 2²II(σ²π³ π?⁴) state has a repulsive potential curve. Electric dipole transition moments and radiative lifetimes have also been calculated for the low lying bound states in all systems. Most of them are found to be quite weak. The A^1,2 → X^1,2 spectra are dominated by parallel contributions, with the A₂ → X₁ being the strongest one. The T values for this transition are quite similar and lie in the 17–30 μs range. Radiative lifetime values for the B → X^1,2 transitions demonstrate very irregular behaviour for various, TeX radicals, due to strong admixture of A⁴Σ^? character to the X^1,2 states near the B²Σ^? potential minimum. The A^{1,2 4}Σ^? _1/2,3/2 and B²Σ^? _1/2 states of TeX (X = Cl, Br, I) still await their experimental observation.     

Long-lived diatomic dications: potential curves and radiative lifetimes for CaBr2+ and CaI2+ including relativistic effects

Relativistic effective core potential calculations have been employed in the framework of a spin–orbit configuration interaction to compute the lowest-lying electronic states of the CaBr²⁺ and CaI²⁺ dications, and the results are compared with the data for the isovalent CaCl²⁺ system studied earlier. The ground X²Π state in all three dications arises from a strong polarization of X(²P°)(X= Cl, Br or I) by the Ca²⁺(¹S) ion and is bound by 0·96–1·55eV with respect to the corresponding diabatic dissociation limits. It is split by the spinorbit interaction into the X₁ ²Π_3/2 and X₂ ²Π_1/2 components, with the energy splittings calculated to be 647 cm^-1(CaCl²⁺), 2115 cm^-1(CaBr²⁺) and 3544 cm^-1(CaI²⁺). The X₁ and X₂ states are found to be thermodynamically stable in CaCl²⁺ and CaBr²⁺, while in CaI²⁺ the lowest dissociation limit, Ca⁺+(²S)+ I^{+ 3}P₂), lies 1700 and 5200 cm^-1 lower than the X₁ and X₂ minima respectively. The X₁ and X₂ states in CaI²⁺ are extremely long-lived, however, owing to the high and very broad potential barriers to dissociation. The first electronic excited state, A²σ⁺, is also bound in all the above systems, although it is pre-dissociated in CaBr²⁺and CaI²⁺ at large internuclear distances. All other low-lying electronic states of CaX²⁺ are repulsive. Electric-dipole moments are calculated for the A→ X₁, X₂ transitions. The corresponding radiative lifetimes are found to be very long in CaCl²⁺ : τ(A→X₁) = 19·3 ms and τ(A→X₂) = 9·9 ms (the values are given for ν' = 0), and become very significantly shorter for CaBr²⁺ and CaI²⁺ because of the stronger spin-orbit interaction in the heavier systems. This effect is especially noticeable for the A →X₂ transitions, for which the values are computed to be 364 μs in CaBr²⁺ and 50·3 μs in CaI²⁺. The theoretical data obtained should aid in the future spectroscopic detection of these species. To data no experiment of this type has been successfully carried out for any of the thermodynamically stable diatomic dications.