Removal rate coefficients for NO(B 2Π) in the v=2 and 3 levels are measured at 230 K for seven colliders: NO, N2O, CO2, O2, N2, Ar, and He. These measurements are the first below room temperature and are compared to earlier 295 K measurements. These NO(B 2Π) vibrational levels differ from each other in that the v=2 level is unperturbed, and the v=3 level is significantly perturbed by the v=12 level of the a 4Π state. Although there are large variations in removal rate coefficients between the two B 2Π vibrational levels, the effect of reducing the temperature on the removal rate coefficients is modest, the largest effects occurring with the least effective colliders, He and Ar.

The temperature dependence of the thermally averaged collisional removal cross section of OH (X 2∏, v=10) by O2, N2O, and CO2 is measured between 220 and 310 K using a two-laser pump–probe technique and a specially designed vacuum-isolated flow cell. OH molecules are generated in v=6–9 by the reaction of hydrogen atoms and ozone. The (10,7) vibrational transition is excited with pulsed near-infrared laser light to create a population of OH (v=10) molecules. The temporal evolution of the v=10 population is monitored as a function of collider gas pressure by a time-delayed ultraviolet laser pulse. The probe step uses laser-induced fluorescence by exciting the B 2∑+–X 2∏ (0,10) transition and detecting the fluorescence from the B 2∑+–A 2∑+ (0,6–8) transitions. From 310 to 223 K, the OH (v=10) removal cross section increases by 35±21, 33±14, and 58±48 percent for the colliders O2, N2O, and CO2, respectively. This inverse temperature dependence is typical of a loss mechanism governed by long-range attractive forces.

Measurement of the rate coefficient for collisional removal of O 2 ( X Σ g − 3 , υ = 1 ) by O ( P 3 )The solar H Lyman-␣ line is, through O 2 photodissociation, an important source of O( 1 D) production throughout the mesosphere and lower thermosphere. To ascertain the energy balance in this altitude region, it is necessary to know the O( 1 D) yield across the solar H Lyman-␣ feature, since H Lyman-␣ absorption by O 2 at ϳ80 km accounts for a substantial fraction of the solar radiation absorbed in the mesosphere. An earlier laboratory study had provided a value of 0.44Ϯ0.05 for the O( 1 D) yield at the center of the solar H Lyman-␣ line, where the profile shows a minimum in intensity due to strong self-reversal of the line. Using tunable laser radiation, we have determined the O( 1 D) yield from O 2 photodissociation across the entire H Lyman-␣ profile from 121.2 to 121.9 nm, at a spectral resolution of 0.0015 nm ͑1 cm Ϫ1 ). The results reveal a strongly wavelength-dependent window in the O( 1 D) yield, the origins of which are explained using calculations based on a coupled-channel Schrödinger-equations model of the O 2 photodissociation. The calculations, which show significant isotopic dependence near H Lyman-␣, predict that the depth of the quantum-yield window will increase significantly as the temperature is lowered.

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