Laboratory measurements and theoretical calculations of O2 A band electric quadrupole transitions

Abstract

Frequency-stabilized cavity ring-down spectroscopy was utilized to measure electric quadrupole transitions within the O 16 2 A band, b Σ1 g+ ←X Σ3 g- (0,0). We report quantitative measurements (relative uncertainties in intensity measurements from 4.4% to 11%) of nine ultraweak transitions in the ON, OP, SR, and ST branches with line intensities ranging from 3× 10-30 to 2× 10-29 cmmolec. -1. A thorough discussion of relevant noise sources and uncertainties in this experiment and other cw-cavity ring-down spectrometers is given. For short-term averaging (t<100 s), we estimate a noise-equivalent absorption of 2.5× 10-10 cm-1 Hz-1/2. The detection limit was reduced further by co-adding up to 100 spectra to yield a minimum detectable absorption coefficient equal to 1.8× 10-11 cm-1, corresponding to a line intensity of ∼2.5× 10-31 cmmolec. -1. We discuss calculations of electric quadrupole line positions based on a simultaneous fit of the ground and upper electronic state energies which have uncertainties <3 MHz, and we present calculations of electric quadrupole matrix elements and line intensities. The electric quadrupole line intensity calculations and measurements agreed on average to 5%, which is comparable to our average experimental uncertainty. The calculated electric quadrupole band intensity was 1.8 (1) × 10-27 cmmolec. -1 which is equal to only ∼8× 10-6 of the magnetic dipole band intensity. © 2009 The American Physical Society.