Aeronomy Measurements in Professor Castle's Lab
Kinetics of vibrational energy transfer processes that participate in the dominant cooling mechanisms of Earth's upper atmosphere
Our experiments are designed to support NASA's TIMED (thermosphere ionosphere mesosphere energetics and dynamics) mission. Specifically, our laboratory measurements will be useful for interpreting data collected by the SABER (sounding of the atmosphere using broadband emission radiometry) instrument. The TIMED mission launched on Deceber 7th, 2001 and continues to gather data. More information on this mission can be found on the SABER outreach website.
Our Research Laboratory
| Mass flow controller system |  |
InSb and MCT infrared detectors |
| Mid-infrared, cw tunable diode laser | 5-11 Hz pulsed Nd:YAG laser | |
| Soda lime-filtered vacuum pumps | jacketed, 1-m pathlength, coolable/heatable reaction cell | |
| Hg lamp, monochromator, and ultraviolet PMT for ozone monitoring | LabView/PC-interfaced digital oscilloscope for data acquisition |
Aeronomy Project Descriptions
1) Quenching of CO2(n2) by O Atoms
CO2(n2)–O vibrational energy transfer (VET) is a key contributor to CO2 15-mm emission intensity and thus to upper atmospheric cooling in the 75-120 km altitude range. A 266 nm laser pulse photolyzes O3, producing O atoms and initiating a temperature jump, while transient diode laser absorption spectroscopy is used to monitor the CO2(n2) level population. By measuring the population decay rate as a function of O-atom quencher concentration, we can extract the relevant rate coefficient. This measurement will be performed over a range of temperatures important to the upper mesosphere and lower thermosphere. Recent improvements in the experiment have mitigated vibrational cascading effects, and the development of a powerful global kinetic fitting routine to allow the simultaneous determination of the appropriate rate parameters from a large body of data. Predictions of upper atmospheric density and temperature are sensitive to the input value of the CO2(n2) + O relaxation rate coefficient as well as its temperature dependence. Aeronomic models imply that increasing CO2 levels from anthropogenic sources will cause the thermosphere to cool and contract over time. The model results are supported by analyses of satellite orbital motion data over the past 40 years, which are consistent with a few percent thermospheric density decrease per decade. This has important implications for spacecraft drag and orbital longevity. It also provides an interesting connection between a molecular-level parameter, the CO2–O VET efficiency, and the macroscopic effects of atmospheric density and temperature.
2) Laboratory Measurements of Ozone – M Vibrational Energy Transfer
In preliminary work, we explore using a temperature-jump method, similar to what has been used in our ongoing CO2(n2)-O energy transfer studies, to measure vibrational energy transfer efficiencies in O3-M encounters, where M=O2, N2, or O. A lingering concern in the analysis of NASA’s TIMED/SABER data involves the 9.6-mm channel, where the observed radiance is dominated by intense emission from the O3(n3) asymmetric stretch level. Hot band emission trailing to longer wavelengths is also present, arising from vibrationally excited O3 initially populated by O + O2 three-body recombination. Poor knowledge of the relevant collisional quenching rate coefficients constitutes one of the most significant deficiencies in the non-LTE models used to retrieve ozone densities from SABER data. Specifically, accurate rate parameters for the relaxation of vibrationally-excited O3 by the major atmospheric species in the mesosphere and lower thermosphere, N2, O2, and O, are required. The O3(v)-O2, N2 quenching rate coefficients derived from existing laboratory measurements vary over a substantial range, and there exists only a single published measurement of O-atom quenching coefficients. The proposed method involves a slow-flowing, dilute mixture of O3 in Xe bath gas. A 266 nm laser pulse is used to dissociate a small fraction of the O3, forming O atoms and stimulating a modest temperature increase. The O3 vibrational level populations redistribute according to the new temperature, and the excited vibrational level populations are monitored via transient diode laser absorption spectroscopy as they return to equilibrium. Rate parameters are determined by effectively plotting the redistributions rates against the quencher concentration.