Photon Factory Activity Report 2004 #22 Part B (2005)
Atomic and Molecular Science

20A/2003G151

Photoabsorption Cross Section Measurements of CO2 and N2
Glenn Stark1,*, Kouichi Yoshino2, Kenji Ito3
1Physics Dept., Wellesley College, Wellesley, MA, USA 02481
2
Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA, USA 02138
3
KEK-PF, Tsukuba, Ibaraki 305-0801, JapanIntroduction
Our laboratory research program is organized aroundthe measurement of VUV molecular photoabsorption
cross sections with the highest practical resolution. It is
explicitly designed to support current efforts to interpret,model, and understand observations of planetaryatmospheres. In this user report, we describe newmeasurements on carbon dioxide (CO2) and molecular
nitrogen (N2).
Carbon Dioxide
Carbon dioxide is the principal constituent of theatmospheres of Mars and Venus. The photodissociationof CO2 by ultraviolet solar radiation initiates theproduction of non-thermal atoms that may escape these
atmospheres. The analyses of recent high-quality VUVobservations of emission features in the Martian
atmosphere and the modeling of non-thermal escapemechanisms from the Martian and Venusian atmospheres
are limited by poorly and incompletely characterized CO2
photoabsorption cross sections. In 2002 we initiated newlaboratory measurements, at the Photon Factory, of CO2
photoabsorption cross sections in the 106 to 120 nm
region. In this reporting cycle, we completed ourmeasurements of the 106 – 120 nm region at 295 K and
195 K. A manuscript detailing our results is inpreparation.
CO2 photoabsorption cross sections were measured in
the 106 to 120 nm region at a resolution of 0.05 Å. All
measurements were carried out on the 3-meter normalincidence vacuum monochromator on the BL-20A beam
stline. A 1200 l/mm grating was used in the 1 order toachieve the desired resolution of 0.05 Å. A 12-cm
stainless steel absorption cell, mounted behind the exit slitof the monochromator, could be cooled to 195 K byimmersion in a dry ice-methanol slush. LiF windows
were employed on the cell. Our new results indicate
significant spectral structure in the CO2 absorption crosssection that was not resolved in previous, lowerresolution results [1]. There is also clear evidence ofsystematic underestimation of peak absorption crosssections for the strongest CO2 features in the earlier
measurements – a result of inadequate instrumental
resolution.

8Users' Report

Molecular Nitrogen
We are measuring fundamental parameters of theabsorption spectrum of N2 in the 80 to 100 nm spectral
region to support the analyses of occultation and airglow
observations of the N2-rich atmospheres of Titan, Triton,and Pluto. High-resolution absorption measurements areused to determine band and line oscillator strengths, andline widths. In this reporting cycle, we continued ourPhoton Factory measurement program on thisatmospherically important molecule. In particular, wefocused on the determination of accurate line widths for theunusually broad b(3) – X(0) transition at 97.2 nm. The
spectroscopic details of this band are needed to guideongoing theoretical efforts to understand the mechanisms ofpredissociation in the molecule.
A supersonic jet apparatus was used on the BL-20A
beam line to cool the nitrogen molecules to approximately
20 K. With the rotational cooling provided by the expansion
jet, the line shapes of the lowest rotational levels of the b(3)state were directly observed for the first time. Preliminary
analyses of the measurements reveal a much larger line
width for the lowest-J levels than is seen for the higher-Jlevels accessed in a room temperature measurement. This
trend has been theoretically predicted in a recent coupled
1
Schrödinger equation model of the interacting Π levels ofN2 [2]. A manuscript describing the spectroscopy of the14b(3) level of N2 is now being prepared.

References[1] R.S. Nakata et al., Sci. Light 14, 54 (1965).
[2] B. R. Lewis et al., J. Chem. Phys. 122, 144302 (2005).
* gstark@wellesley.edu