Absolute number densities of vibrationally excited N2 (A3Σu+) produced in a low pressure rf plasma

The absolute populations of the vibrational levels v = 0, 1, 3 and 6 of the A((3)Sigma(+)(u)) state of molecular nitrogen produced in a low pressure inductively coupled plasma have been determined as a function of plasma operating conditions. The measurements were conducted using diode laser-based cavity enhanced absorption spectroscopy on a selection of vibrational bands within the strong first positive system, B((3)Pi(g)) <- A((3)Sigma(+)(u)), and calibrated using cavity ring-down spectroscopy. At 25 mTorr and 200 W power applied to the discharge we find the populations of the v = 0, 1, 3 and 6 levels to be (1.31 +/- 0.16) x 10(11) cm(-3), (8.44 +/- 1.01) x 10(10) cm(-3), (2.83 +/- 0.34) x 10(10)cm(-3) and (5.27 +/- 0.63) x 10(9) cm(-3), respectively, corresponding to a vibrational temperature of 3600 +/- 150 K. The vibrational temperature of the A-state was determined to lie in the range 2900-3700 K for the operating conditions employed. The translational and rotational temperatures of each vibrational state probed were considerably colder (in the range 300-500 K) and show the translational and rotational modes to be equilibrated. In addition, we present the observation of the N-2(+) (X-2 Sigma(+)(g)) molecular ion in its vibrational ground state using both cavity enhanced absorption spectroscopy and the same technique in combination with wavelength modulation spectroscopy. At 10 mTorr and 400 W we measure a translational temperature of (431 +/- 80) K, which is the same as that for the A-state under the same conditions, and estimate the total population in v = 0 to be (1.26 +/- 0.15) x 10(9) molecules cm(-3). The combination of cavity enhanced and wavelength modulation methods is found to improve the sensitivity by approximately an order of magnitude in a plasma environment.