Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010mwac.meet..t03k&link_type=abstract
"Midwest Astrochemistry Meeting 2010, held 5-6 November at the University of Illinois at Urbana-Champaign. http://midwest.astroc
Astronomy and Astrophysics
Astronomy
Scientific paper
The dissociative recombination (DR) of H3+ with free electrons is the dominant destruction mechanism for H3+ in the diffuse interstellar medium. Due to the relevance of H3+ in interstellar chemical networks, the H3+ DR process has been studied extensively by a number of groups around the world. Unfortunately, the H3+ DR rate coefficient has had a tumultuous history with a large scatter among various experimental techniques. In the last decade or so the storage ring rate coefficient has become the accepted reference value]. State-of-the-art theoretical calculations agree well with the storage ring results on the absolute scale of the rate coefficient, while distinct discrepancies remain for the exact shape and energy dependence of the rate cofficient curve. These discrepancies raise questions regarding the temperatures of the ion sources that are used to produce the H3+ ions for storage ring experiments and - more specifically - the internal excitation of the stored ions during the DR measurements.
To address some of these issues we have performed new H3+ DR measurements at the ion storage ring TSR utilizing a supersonic expansion ion source. The ion source has been characterized by continuous wave cavity ring-down spectroscopy. We present high-resolution DR rate coefficients for different nuclear spin modifications of H3+ combined with precise fragment imaging studies of the internal excitation of the H3+ ions inside the storage ring. The measurements resolve for the first time changes in the energy dependence between the ortho-H3+ and para-H3+ rate coefficients at low center-of-mass collision energies. Analysis of the imaging data indicates that the stored H3+ ions may have higher rotational temperatures than previously assumed, most likely due to collisional heating during the extraction of the ions from the ion source. We present an overview of possible origins of the heating process and how to avoid it in future experiments.
Berg Max H.
Bing Dennis
Buhr Henrik
Crabtree Kyle N.
Krantz Claude
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