[1] Vibrational excitation of ground-state NO through collisions with oxygen atoms produces NO(v = 1) in the lower thermosphere, representing a significant source of atmospheric cooling through the subsequent 5.3-mm radiative emission. A laser pumpprobe experiment has been used to measure the temperature dependence of the NO(v = 1)-O vibrational relaxation rate coefficient k O (v = 1) in the 295-825 K range, along with updated measurements of k O (v = 1,2) at room temperature. The experiment employed a continuous wave microwave source to form O atoms, combined with photolysis of a trace amount of added NO 2 to produce vibrationally excited NO. Oxygen atoms were detected through two-photon laser-induced fluorescence, cross-calibrated against a normalized O atom signal resulting from photolysis of a known concentration of NO 2 . No temperature dependence was observed for k O (v = 1) to within the uncertainty in the measurements. The measured room temperature value of k O (v = 1) = (4.2 ± 0.7) Â 10 À11 cm 2 s À1 is 75% larger than the value obtained previously in this laboratory, a significant difference at the 1s level. The present value is preferred owing to an improved experimental technique. The atmospherically relevant NO(v = 0)-O vibrational excitation rate coefficient can be derived from measured values of k O (v = 1) through detailed balance. The variable temperature measurements provide key information for aeronomic models of the lower thermospheric energy budget, infrared emission intensities, and neutral constituent densities.

[1] In the Earth's upper atmosphere, collisions with ambient O atoms efficiently excite the CO 2 [00 0 0] vibrational ground-state population to the first excited, [01 1 0] or n 2 , vibrational bend state. Subsequent relaxation of the n 2 population occurs through spontaneous emission of 15-mm radiation. Much of this radiation escapes into space, thereby removing ambient kinetic energy from the atmosphere. This cooling mechanism is especially important at altitudes between the mesopause and the lower thermosphere, approximately 80-120 km, where the O-atom density is relatively high and the kinetic temperature is rising. Laboratory measurements have been performed to better characterize the CO 2 (n 2 )-O vibrational relaxation rate coefficient k O (n 2 ). A 266-nm laser pulse photolyzed trace amounts of O 3 in a CO 2 -O 3 -rare gas mixture, simultaneously creating O atoms and raising the gas temperature to create a nonequilibrium CO 2 vibrational distribution. Transient diode laser absorption spectroscopy was used to monitor CO 2 vibrational level population reequilibration. A global nonlinear least squares fitting technique was used to interpret the kinetic data, yielding k O (n 2 ) = (1.8 ± 0.3) Â 10 À12 cm 3 s À1 . The result is in good agreement with previous laboratory measurements, with published k O (n 2 ) values in the (1.2-1.5) Â 10 À12 cm 3 s À1 range and at the low end of the (2-6) Â 10 À12 cm 3 s À1 range estimated from the analysis of upper atmospheric data.

The direction of the transition dipole moment of nitrobenzene between 230 and 250 nm was determined by orienting gas-phase molecules in a strong, uniform electric field. Oriented nitrobenzene was photodissociated with linearly polarized light, and the NO fragments were detected by resonantly enhanced multiphoton ionization (REMPI). When the polarization direction of the photolysis laser was perpendicular (rather than parallel) to the orientation field, a 44% enhancement in the NO signal was observed. This implies a predominantly perpendicular relationship between the transition dipole and the permanent dipole. However, the experimentally observed enhancement falls below that expected of a pure perpendicular transition, indicating the presence of a second potential-energy surface that is simultaneously accessed through a parallel transition. Quantitative analysis indicates that the parallel transition contributes 20% of the overall oscillator strength.

[1] Laboratory measurements of the quenching of CO 2 (n 2 ) by O atoms are presented over the 142-490 K temperature range relevant to the 75-120 km altitude region of the terrestrial atmosphere. The primary cooling mechanism in this region occurs when CO 2 is efficiently excited through collisions with ambient O atoms, populating the bending vibrational (n 2 ) modes. A significant fraction of the vibrationally excited CO 2 relaxes through spontaneous 15-mm emission that escapes into space, thereby removing kinetic energy from this region of the atmosphere and generating a local cooling effect. The rate coefficient for the vibrational relaxation of CO 2 (n 2 ) by O atoms, k O (n 2 ), is measured using transient diode laser absorption spectroscopy. A slight negative temperature dependence is observed for k O (n 2 ), with values ranging from 2.7 (AE0.4) Â 10 À12 cm 3 s À1 at 142 K to 1.3 (AE0.2) Â 10 À12 cm 3 s À1 at 490 K.