Is the hydronium ion unstable



10.09.2010 21:27

Hot water in cold comets

Dr. Bernold Feuerstein Press and public relations
Max Planck Institute for Nuclear Physics

Water is a main component of comets, which is why they are also known as "dirty snowballs". An international research group led by Andreas Wolf from the Max Planck Institute for Nuclear Physics in Heidelberg has now succeeded in deciphering the formation of water molecules in space in detail. Surprisingly, they discovered that the H2O molecules in the ultra-cold comets are formed as particles at 60,000 degrees Celsius. For their research, however, the physicists did not use a telescope, but a particle accelerator (Physical Review Letters, September 3, 2010).

In comets and interstellar clouds, the starting material of the water is the positively charged hydronium ion H3O +. This molecular ion can be detected from Earth with telescopes. Negatively charged electrons usually also fly around in cosmic clouds, so that collisions often occur. The hydronium ion becomes the neutral but unstable molecule H3O, which disintegrates immediately. "Nature offers three options for this," explains Andreas Wolf: Either H2O plus H, OH plus H2 or OH plus two H atoms is created. One goal of the current research is to find out with what frequency the three types of decay occur and water is formed.

Wolf and colleagues investigated this question by simulating electron attachment in the laboratory. For this they used the Heidelberg test storage ring, a kind of racing track with a circumference of 55 meters, on which charged particles race around guided by magnets.

The researchers feed hydronium ions into this ring, more precisely D3O +, i.e. hydronium ions from heavy hydrogen atoms, which are better suited for the experiments than ordinary hydrogen. At one point in the ring, electrons flow in from the outside, which accompany the ions on a nearly two-meter-long section and then leave the ring again. This happens with every revolution, i.e. several hundred thousand times per second.

In the electron bath - almost like in space - electrons with hydronium ions accumulate to form molecules that disintegrate immediately. The fragments are, however, electrically neutral. As a result, they do not react to the magnetic field and fly out of the storage ring. At this point, Wolf's group has installed a detector that registers the impacting particles. This sensitive device was developed in collaboration with colleagues from the Israeli Weizmann Institute in Rehovot.

With up to a thousand “images” per second, the detector registers the molecular masses and impulses of all decay products. The processes involved in the attachment of electrons and the subsequent break-up of the molecule can be precisely reconstructed from this data.

The first important result: When electrons are attached, 16.5 percent of the hydronium breaks down into water. "That is quite a high figure," says Wolf. "Electron attachment to hydronium ions is probably the most important water production path in interstellar clouds and comets."

Most often, namely 71 percent, the hydronium ion splits into the three components OH plus two H atoms. The researchers can now explain why this is so. When the electron attaches itself to the ion, binding energy is released. This absorbs the entire molecule and begins to vibrate, similar to a spiral spring that you tighten and let go. “To everyone's surprise, we found that the molecules vibrate with the maximum possible energy,” says Wolf. This means that every molecule is on the verge of tearing when electrons are attached and will break into three rather than two parts.

The high vibration energy can also be converted into a temperature. In doing so, the physicists come to 60,000 degrees Celsius: water comes out hot into the world.

The new findings have other implications as well. On the one hand, they are used as parameters in computer models with which the complex reaction network in interstellar clouds is calculated. On the other hand, they explain observations of some comets. In their infrared spectra, astronomers had found bands of “hot” water molecules, which can now be explained: The newly formed water molecules gradually release their vibrational energy, and with each further “de-excitation stage” they emit infrared radiation. And last but not least, the new measurement results provide unexpectedly detailed information about the electronic processes in a hydronium ion and thus serve as input for quantum mechanical models of these molecules.

Original publication:

H. Buhr, J. Stützel, MB Mendes, O. Novotný, D. Schwalm, MH Berg, D. Bing, M. Grieser, O. Heber, C. Krantz, S. Menk, S. Novotny, DA Orlov, A Petrignani, ML Rappaport, R. Repnow, D. Zajfman, and A. Wolf
Hot water molecules from dissociative recombination of D3O + with cold electrons
Physical Review Letters, September 3, 2010

Contact:

Prof. Dr. Andreas Wolf
Max Planck Institute for Nuclear Physics, Heidelberg
Tel .: +49 6221 516-503
Email: [email protected]

Dr. Henrik Buhr
Max Planck Institute for Nuclear Physics, Heidelberg
Tel .: +49 531 5926-208
Email: [email protected]


Additional Information:

http://link.aps.org/doi/10.1103/PhysRevLett.105.103202 Original publication
http://www.mpi-hd.mpg.de/blaum/members/molecular-qd/index.en.html Working group of Prof. Andreas Wolf


Features of this press release:
Physics / astronomy
supraregional
research results
German