Time Resolved Fluorescence from Parity Mixed Rotational Energy Levels: Collisons vs. Electric Field Effects.

Recently, a laser-induced fluorescence technique was developed for measuring plasma electric fields in situ and non- intrusively. This is an important diagnostic tool in the study of plasma chemical processes utilized in the manufacturing of microelectronic devices and in the study of fundamental plasma physics of glow discharges. The technique relies upon preparation of a specific parity level in a dipolar diatomic molecule; in the presence of a finite electric field, the excited parity level mixes with a nearby level of opposite parity resulting in the appearance of nominally forbidden emission. The ratio of "forbidden" to "allowed" intensities is then used as a measure of the local electric field amplitude. One difficulty in applying this technique to measure electric fields, but an interesting process to study in its own right, is the collisional mixing that occurs between the two parity levels. We describe time-resolved and spectrally-resolved laser-induced fluorescence measurements from the parity or LAMBDA doublet levels of the [EQ]A sup 1 PI[EN] state of [EQ]BC [EN] radicals formed in an rf discharge through [EQ]BC sub 3[EN]. By making such measurements, we show how the effects of collisional mixing can be discerned from the effects of electric field mixing. A set of rate equations for population transfer which include the effects of both field and collisional mixing are derived and compared to the more general theory of Alexander. The collisional mixing is determined in the low-field region of the plasma to yield a parity-changing collision rate: [EQ]k sub ef = 4.9, 2.5, and 2.4 times 10 sup 6 sec sup -1 Torr sup -1[EN] for [EQ]J prime = 5, 11, and 18[EN], respectively, as well as a phenomenological quenching rate and the radiative rate. These rate constants are then used in the rate equations to determine the extent of electric field mixing which takes place in the strong-field sheaths of the plasma. The variation of the field strength in the sheaths as a function of pressure is then examined.