Recreating interstellar molecular synthesis on Earth
13 July 2012 (Volume 7 Issue 6)
Toshiyuki Azuma and researchers from the Atomic, Molecular & Optical Physics Laboratory are developing a world-first cryogenic vacuum ion storage ring in an effort to reproduce the synthesis of interstellar molecules including complex biotic molecules observed in interstellar molecular clouds
Atomic, Molecular & Optical Physics Laboratory
RIKEN Advanced Science Institute
Amino acids and nucleic acids are the building blocks of life. After the Big Bang, various astronomical objects and interstellar materials were formed in the universe during the gradual evolution of materials, which eventually led to the beginning of life on Earth. Scientists have already begun astronomical observations in a search for evidence of this remarkable process. Replicating the cryogenic formation of these molecules on Earth is also a significant challenge. Toshiyuki Azuma, chief scientist of the Atomic, Molecular & Optical Physics Laboratory at the RIKEN Advanced Science Institute, is currently heading an experimental program aimed at achieving just that.
The secret of dark nebulae
The stars of our galaxy make up the billions of points of light we call the Milky Way. Among the stars, however, are molecular clouds known as dark nebulae (Fig. 1). These interstellar clouds of dust and ice can be dense enough to appear dark and obscure our view of distant stellar objects, and it was not until radio astronomy was used in their observation that they were found to consist of a surprising range of complex molecules.
Figure 1: The Horsehead Nebula, a dark nebula in the Orion Constellation and the molecular model of glycine, an amino acid.enlarge image
Photo courtesy of NASA, NOAO, ESA and the Hubble Heritage Team (STScI/AURA)
“The density of molecules in an interstellar molecular cloud is comparatively low. They essentially consist of molecules sparsely distributed in a vacuum,” says Azuma. “Most of the molecules in these clouds are electrically neutral or positively charged, and the speed of intermolecular collisions is low, allowing chemical reactions that produce new molecules. I want to study these chemical reactions, although it has proved to be very difficult to achieve a sufficiently low collision energy to allow such reactions to be recreated experimentally on Earth.”
Manipulating molecules using electric fields
Experiments involving the collision of ions or molecules are not new. However, it has so far been impossible to achieve the level of control needed to recreate the slow collisions that occur in interstellar molecular clouds. The Radioisotope Beam Factory (RIBF) at RIKEN, which has been operating since 2006, is an accelerator facility that can collide any naturally occurring nucleus, ranging from hydrogen to uranium, and produce the most intense ion (or radioisotope) beam in the world. The facility is used to study how heavy elements are created, and to examine the nature and structure of atomic nuclei.
“In experiments with accelerators like the RIBF, the trajectory of a beam of nuclei is controlled by a powerful magnetic field,” says Azuma. “However, even the RIBF is unable to control the trajectory of a beam of large molecules consisting of many atoms because they are too heavy. Controlling the trajectory of such a beam of large molecules requires an impractically huge magnet. Conventional magnetic facilities have enabled us to control a beam of molecules consisting of up to three atoms, such as water molecules.”
In 1997, S. P. Møller of Aarhus University in Denmark developed an electrostatic ion storage ring in which a beam of large molecules is made to bend around a continuous circular path using an electric field generated by electrical poles. “We took it for granted that our molecular experiments would involve using a magnetic field, like in the RIBF. However, we could have used an electric field. When we recognized this possibility, it was a revelation, like Columbus’s egg.”
There are good reasons, however, why electric fields are not used in facilities like the RIBF. “An electric field cannot be used to control the trajectory of a high-speed beam. In experiments using a beam of nuclei like at the RIBF, the nuclei are accelerated almost to the speed of light, and controlling the trajectory of the high-speed beam requires a magnetic field. However, in experiments with a beam of atoms or molecules larger than nuclei, the beam is not accelerated to such high speeds. We believe Dr Møller recognized this when he used an electric field to control the trajectory of a low-speed beam of large molecules.”
The use of an electric field for controlling the trajectory of molecules also requires the molecules to have a positive or negative charge, or in other words, to be ionized. “A wonderful technique has already been developed for ionizing macromolecules such as proteins without damaging them by irradiating the macromolecules with laser light, a method known as matrix-assisted laser desorption. This technique was developed for mass spectrometers by Dr Koichi Tanaka of Shimadzu Corporation. The American chemist Dr J. B. Fenn also discovered that molecules can be ionized when a solution of the molecules is turned into a fine spray, a technique called the electro-spray method. Drs Tanaka and Fenn were awarded the 2002 Nobel Prize in Chemistry for their inventions.”
Japan’s High Energy Accelerator Research Organization constructed their own electrostatic ion storage ring in 2002. “I was working for Tokyo Metropolitan University at the time and decided to construct a third electrostatic ion storage ring which has an advantage over the previous two electrostatic rings. The entire unit is designed to be cooled in order to keep the stored molecular ions in the vibrationally cooled states.”
In 2003, Azuma and his laboratory team completed the world’s first cooled electrostatic ion storage ring, called the TMU E-ring, which can be cooled with liquid nitrogen to temperatures of 77 kelvin (K), or –196 °C (Fig. 2).
Figure 2: The TMU E-ring ion storage ring at Tokyo Metropolitan University
Experiments are performed using a ring of about 7.7 meters in circumference and a vacuum vessel cooled with liquid nitrogen to 77 K.
Photo courtesy of Tokyo Metropolitan University
Recreating interstellar chemical synthesis
Recent astronomical observations have revealed the existence of negatively charged molecules in interstellar molecular clouds, a result that is inconsistent with prevailing theory. “Negative molecular ions can be easily produced in water solution because they are stabilized by water molecules, but they are extremely unstable in a vacuum because there are no similar effects there.”
The negative molecular ions observed to date include C4H–, consisting of a single hydrogen (H) atom and four carbon atoms (C) with a net negative charge, as well as C6H–. “We have used the TMU E-ring to measure the stability of these molecular ions in a vacuum,” says Azuma. “We confirmed that both molecules were stable for periods of up to a few seconds while moving through the vacuum.”
In 2010, observations using the US Spitzer space telescope revealed the existence of C60 fullerene — a soccer-ball-shaped structure consisting of 60 carbon atoms — in interstellar space. The possibility of C60 as a product of interstellar molecular synthesis was discovered in 1985 by the UK chemist H. W. Kroto, who was awarded the 1996 Nobel Prize in Chemistry for his work. “Surprisingly, it was found to be possible for C60 to be created in space,” says Azuma. “It is considered that the soccer-ball-like structure of C60 forms through the cryogenic cooling of molecules produced at much higher temperatures of tens of thousands of degrees. We are studying the cooling process of C60 using the TMU E-ring by irradiating the ions with laser light to heat them to high temperatures then letting them cool in the cryogenic storage ring. These experiments enable us to reproduce part of the C60 formation process in space.”
Exploring the mystery of the origin of life in space
Japan, the US, and some European countries are jointly constructing a huge telescope in Chile called the Atacama Large Millimeter/submillimeter Array (ALMA). The first observations using ALMA were made in 2011 and full-scale observations will begin in 2013 once the facility is completed. One of the main purposes of ALMA is to find complex biotic molecules — the building blocks of life — in space. Amino and nucleic acids have been found in meteorites on Earth. To date however, it is unknown whether such molecules originate from molecular clouds in interstellar space or some other source.
“Amino or nucleic acids might have been produced in molecular clouds, but how? I want to reproduce the chemical syntheses that are occurring in cryogenic space at temperatures as low as 10 K in a laboratory here on Earth. I moved to RIKEN in 2009 to achieve this goal.”
Azuma and his laboratory team aimed to develop an electrostatic ion storage ring capable of cooling the entire vacuum vessel to just 4 K, or –269 °C — the temperature of liquid helium. “At first, I thought that the only difference between the TMU E-ring and the new storage ring would be a change from liquid nitrogen cooling to liquid helium. However, we began to understand that cooling the vacuum vessel to 4 K would be quite difficult compared with cooling to 77 K.”
Liquid helium has been used successfully in other research facilities where superconducting coils or small vacuum vessels have been cooled to cryogenic temperatures. “The vacuum vessel we are constructing has a circumference of about 3 meters. Such a large vessel has never been cooled using liquid helium, so instead we decided to use a cryogenic freezing machine to cool the vessel, which will allow us to get down to 10 K. We also had difficulty finding materials that could maintain a vacuum under such extreme conditions. However, after much testing we managed recently to identify appropriate materials.” The new electrostatic ion storage ring (Fig. 3) is due to be completed in 2012.
Figure 3: The cryogenic vacuum electrostatic ion storage ring
A molecular beam is produced and passed around a circular path (the storage ring) in a vacuum vessel cooled to 10 K. The system reproduces the conditions of molecular synthesis that occurs in interstellar molecular clouds. The ring has a perimeter of about 3 meters.
Now, research groups in Germany and Sweden are also constructing electrostatic ion storage rings that can be cooled to liquid helium temperatures, with completion soon to be finalized, and US and French researchers are planning to construct similar facilities. “Rival research will expand, and we will face increased competition in this field. However, at RIKEN we are surrounded by experts in the areas of superconductivity at liquid helium temperatures, mass spectroscopy for large ionized molecules, laser spectroscopic technology, low-temperature science, and accelerators — we expect to be able to use the support of these world-leading researchers to our advantage in the fields critical to our experiments. For example, we have introduced a cutting-edge laser technique capable of attosecond pulses of laser light developed by the Laser Technology Laboratory at the RIKEN Advanced Science Institute.” Azuma intends to use the technique to conduct molecular synthesis experiments that cannot be performed without it (Fig. 4).
Figure 4: Experiments using the electrostatic ion storage ring currently under development at RIKEN
A beam of molecules is repeatedly passed around a circular path, and the beam is broken by attosecond laser light. The fragments are detected and the shapes of the molecules are examined.
Azuma and his laboratory team plan to use the new facility to collide neutral molecules with positive ions at relatively low speed in a vacuum at temperatures near 10 K in an attempt to reproduce the conditions of interstellar molecular synthesis of amino and nucleic acids. “At cryogenic temperatures, we can also conduct extremely precise measurements of radio signal emissions from the amino and nucleic acids. With these data, astronomers at ALMA might have a better chance to discover amino and nucleic acids in molecular clouds. To start this discussion we have hosted symposiums to deepen our exchange with astronomers.”
In interstellar molecular clouds, chemical synthesis is considered to occur not only under vacuum conditions but also on the surface of floating ice particles. Naoki Watanabe of the Institute of Low Temperature Science at Hokkaido University and his laboratory team are conducting experiments aimed at reproducing such chemical reactions here on Earth. “Professor Watanabe is a graduate of Tokyo Metropolitan University, and we communicate with each other on a daily basis to advance our respective research activities.”
This research aims to clarify how a variety of molecules are being synthesized in space. “That will offer an important insight into how life might be created in space.”
The effect of a cryogenic vacuum on atoms and molecules
“I am interested in the primordial properties of atoms and molecules. We really do not know much about either. For example, we do not know how the first hydrogen molecule was created in space; the mechanism and speed of synthesis remain a mystery,” says Azuma.
Electrons and protons are thought to have been created in the Big Bang, and to have combined some 380,000 years later to form hydrogen atoms. “Assuming that these hydrogen atoms collided with each other to form a pair of hydrogen atoms, the standard hydrogen molecule, and that the first star was made of hydrogen molecules, the hydrogen atoms would have released part of their initial energy as light. However, this release of light energy is known to be difficult, so how were hydrogen atoms synthesized into hydrogen molecules in a cryogenic environment? What was the speed of that synthesis? These facts are not yet completely understood. We do not know much about primordial reactions involving large molecules such as amino acids or C60, or even those involving small molecules such as hydrogen molecules. I am planning to use the new facility to combine individual carbon atoms to form hydrocarbon molecules, each consisting of five to six carbon atoms (Fig. 5). Hydrocarbon molecules have been identified in astronomical observations, but we do not know how fast the hydrogen molecules are being synthesized in a cryogenic vacuum environment.”
Figure 5: Hydrocarbon synthesis in an interstellar molecular cloud
Azuma and his laboratory members are attempting to reproduce the synthesis of carbon–carbon-linked hydrocarbon molecules in interstellar space using the cryogenic vacuum electrostatic ion storage ring currently under development at RIKEN.
Reference: S. Yamamoto. How Molecules Were Born. Encouragement for Chemistry 1997 (Chikumashobo Ltd, Japan)
Conventionally, the properties of atoms and molecules have been studied in solutions or on substrates. “Even if the temperature of a solution is set exactly, the collisions among molecules that lead to chemical reactions can occur over a range of speeds and can also be affected by surrounding water molecules,” says Azuma. “In experiments using a substrate, the influence of the substrate cannot be eliminated. However, using an electrostatic ion storage ring, we can study reactions at a specific speed in a vacuum; by causing molecules to collide with each other at a specific energy level or by bombarding them with laser light or electrons at a specific energy level. In this way, we can measure the properties of atoms or molecules rigorously without the influence of extraneous factors. I think precise measurements of this kind could lead to the discovery of phenomena that have not been explainable by conventional theories.”
Azuma predicts that electrostatic ion storage rings will be made even more compact in the future. “In principle, they can be downsized to a compact facility that can be placed on a benchtop. At present, there are only a few groups in the world that have conducted experiments using electrostatic ion storage rings. If such compact facilities become available commercially, many researchers will be able to use the facility to conduct a variety of experiments. There are so many atoms, molecules, and chemical reactions for us to study.”