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Introduction
Despite their apparent transparency and high symmetry, molecular nitrogen and oxygen have a large number of absorption bands distributed throughout the infrared and near infrared. Most of these absorptions are driven by binary collisions, which induce a transient electric dipole moment matrix element for infrared or near infrared absorption. An example of such a process is: ![${\rm O}_2({\rm v}=0)+{\rm M}\rightarrow[{\rm O}_2({\rm v}=0)+{\rm M}]^\dagger$](eq1.gif){kind=small}
![$[{\rm O}_2({\rm v}=0)+{\rm M}]^\dagger +h\,\nu\rightarrow {\rm O}_2({\rm v}=1)+{\rm M}~,$](eq2.gif){kind=small}
which is forbidden in the absence of the collision partner M. Here, the  denotes a metastable
collision complex.These absorptions obscure sensor windows and contribute to the radiative heating and photophysics of the atmosphere. Moreover, they are not sufficiently quantitatively understood to be included in general climate models, yet they contribute significantly more to the approximately 80 W/m2 atmospheric solar absorption than the 2.5 W/m2 radiative forcing of anthropogenic CO2, CH4, etc. which are postulated to be responsible for the greenhouse effect. We note that the continuum absorptions are also most likely responsible for a significant fraction of the 10 to 30 W/m2 discrepancy between the predicted and observed atmospheric absorption. The dominant atmospheric absorptions are from H2O and CO2, however to obtain reliable global climate models and to assess the effect of humans on the atmosphere, it is critical to have accurate measurements on even the weak atmospheric absorptions. Our laboratory measurements use a high-resolution (0.004 cm-1) Fourier-Transform InfraRed (FTIR) spectrometer to obtain spectral parameters for these weak magnetic dipole and collision-induced absorptions. To achieve the long absorption pathlengths found in the atmosphere, particularly at dawn and dusk, we use a high-pressure long-pathlength multipass optical cell (White cell). Such a cell is essential for observing these weak features. As an example, in the case of collision-induced absorption where the signal is proportional to the square of the pressure, an atmospheric pathlength of 10 km at 1 atm is equivalent to a laboratory pathlength of 100 m at 10 atm. 1.27 µm Band of O2 The 1.27 µm band of O2 has both a sharp magnetic-dipole component and a broad collision-induced component. The continuum and sharp structure is seen in absorption against the solar background [Mlawer et al. J. Geophys. Res. 103, 3859 (1998)]. Emission at 1.27 µm is also seen at dusk from O2(a1Δg) produced by solar photolysis of ozone. The Einstein A coefficient for this emission corresponds to an upper state lifetime of 1.24(3) hr [Lafferty et al. App. Optics 37, 2264 (1998)].  The 1.27 µm emission is used to monitor the ozone concentration in the upper atmosphere (mesophere). The Solar Mesosphere Explorer Satellite (SME) and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Satellite (TIMED) take this approach. Since the percentage error in the Einstein A coefficient leads to the same percentage error in the retrieved O3 concentration, it is critical to have accurate values for the absorption coefficient (Einstein B coefficient) from which the A coefficient is derived. Our results together with measurements by Newman et al. [J. Chem. Phys. 110, 10749 (1999)] have led to an agreed upon value for the Einstein A coefficient. Also, our value for the continuum absorption is 37 % greater than the value determined by [Mlawer et al. J. Geophys. Res 103, 3859 (1998)] from their atmospheric measurements. A comparison of the observed and calculated laboratory spectra is also shown below.  6.4 µm Band of O2 The 6.4 µm, collision-induced band of O2 overlaps the very weak electric quadrupole Our laboratory measurements of the collision-induced absorption for pure oxygen samples are shown below followed by a comparison with other work. In particular, note the improved measurement precision over previous studies. 
Simultaneous Vibrational Absorption A rigorous description of the atmospheric spectrum must also consider the possibility of even weaker collisional events, called simultaneous or double transitions. The intensity for such a process is approximately an order of magnitude weaker than the single transitions. An example of such a simultaneous collision-induced absorption process is shown by:
CO2(v3=0) + N2(v=0) + h
ν →
CO2(v3=1) + N2(v=1) at
4680 cm-1
 Because of the weakness of these absorptions they are frequently severely blended with features from stronger bands. This is illustrated on the right by our study of the above CO2-N2 simultaneous transition, which overlaps a low frequency wing of a CO2 band. Despite this overlap, good quality spectra and precise values for the absorption coefficients are obtained, as shown below. Our value disagrees by 30 % from earlier (1950's) values and by 15 % from our theoretical calculations.  References
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