Benjamin J. McCall
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LABORATORY PROJECTS - BERKELEY
Storage-Ring Dissociative Recombination Measurements
Our detection of abundant H3+ in diffuse clouds
(more details)
has led to an enigma in diffuse cloud chemistry. One possible
solution to this enigma has been that the value of the rate coefficient
for dissociative recombination (DR) of H3+,
measured in various laboratory experiments in the past few decades,
was not applicable to interstellar conditions. In particular, some
had questioned whether rotationally cold H3+ ions
might have a substantially slower DR rate than the ions probed by
laboratory measurements.
This question led me into a fruitful collaboration with Mats Larsson at
Stockholm University and Alex Huneycutt (a graduate student in Rich
Saykally's group). Because we often use supersonic expansion discharges
to produce cold ions for laboratory spectroscopy, we decided to adapt
this technology for storage-ring measurements of
H3+ DR. In Berkeley, we developed a supersonic ion
source for H3+ and spectroscopically characterized
it using cavity ringdown laser absorption spectroscopy [using a YAG-pumped
dye laser which was Raman shifted into the infrared using gaseous hydrogen].
By probing the H3+ fundamental band, we confirmed
that this source produced ions with a rotational temperature of ~30 K.
We then took this source to the CRYRING ion-storage ring at the Manne Siegbahn Laboratory in Stockholm, where we performed dissociative recombination cross-section measurements. These measurements provide the first direct measurement of the H3+ dissociative recombination rate under "interstellar conditions," and have ruled out a very low value of this rate coefficient as the cause of the overabundant H3+ in diffuse clouds.
Reference:
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B. J. McCall, A. J. Huneycutt, R. J. Saykally, T. R. Geballe,
N. Djuric, G. H. Dunn, J. Semaniak, O. Novotny, A. Al-Khalili,
A. Ehlerding, F. Hellberg, S. Kalhori, A. Neau, R. Thomas,
F. Osterdahl, and M. Larsson
Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+-e- recombination rate Nature, 422, 500 (2003) |
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