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January 3, 2002

Electronic newsroom



Brookhaven Scientists Develop New Imaging Method

Approach shows promise in helping to understand challenging molecules

UPTON, NY – Scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have developed a novel imaging method, known as “ion pair imaging spectroscopy,” that may help them better understand the properties of previously hard-to-study molecules.

“Ion imaging is a subject that has received a lot of attention in recent years as a powerful means of studying a variety of fundamental chemical events,” said chemist Arthur Suits, the lead Brookhaven researcher on the project, which was featured in the December 21 issue of Science. “What we have developed here is a new tool to look at ions that, in certain cases, offers significant advantages over other methods.”

Typically, scientists learn about the properties of ions by using one of two methods. In photoelectron spectroscopy, scientists start with a neutral molecule and use light to eject one of its electrons. By determining the energy of that electron, researchers are then able to map the structure of the remaining positively charged ion. Alternately, scientists can expose the ions themselves to laser light and determine aspects of the ion’s structure by seeing how much light is absorbed at different colors.

Velocity map of a methyl ion

A velocity map image of a methyl ion obtained using the ion pair imaging spectroscopy technique. Each of the rings represents a single rotational / vibrational energy level for the ion.

Both of these approaches have drawbacks, however. While photoelectron spectroscopy is a powerful tool, it cannot be used to study a variety of fundamental ions that lack a neutral precursor or that possess a very different geometry from that precursor. Using absorbed laser light to determine ion structure can give very precise information, but can be very challenging for several reasons. It is difficult to prepare the required “cold” beam of ions containing little internal energy, and it is also not easy to find lasers that operate in the wavelength regions needed for some of these studies. As a result, little experimental data exist for many fundamental ion systems.

In an effort to determine the structure of some of these systems, Suits and his colleagues tried looking at the ions directly. “Our method is comparable to photoelectron spectroscopy, but instead of ejecting an electron and looking at its energy to determine the energy levels of the ion left behind, we eject a negatively charged ion and use its energy to determine the energy levels of the positively charged ion left behind,” said Suits. “We worried that because the negative ion is so much heavier than an electron, this could cause rotation of the ions, which would blur the images. Instead, we find we can actually see the energy of individual rotations of the ions, giving us even more information about their properties.”

For the experiments described in Science, Suits introduced a beam of methyl chloride gas ions into a vacuum chamber, used a vacuum ultraviolet laser to add energy, then projected the resulting methyl ion onto an imaging detector. The researchers used a technique called “velocity map imaging” to obtain the high resolution needed to determine the structure and energy levels of the ion.

Suits, who is also an associate professor in the Chemistry Department at Stony Brook University, chose methyl chloride for this initial research because ejecting the chloride ion would give information on the positively charged methyl ion, one of the simplest hydrocarbon ions. The research team now wants to use their technique on other species of ejected ions, including hydrogen, to determine its range of usefulness.

“The big question now is just how general this will be,” said Suits. “If we are able to go to higher energies and use hydride as the negative ion, that would make this a much more general probe for many ion species.”

Such spectroscopic information on ions is important in developing thermochemical scales (used in predicting chemical reactions) and for testing theoretical methods widely used in quantum chemistry, as well as in understanding astrochemistry (the study of chemical interactions between interstellar gases and dust), where ions play a major role.

Other collaborators on the project include Xianghong Liu, from Brookhaven’s Chemistry Department, and Richard Gross, a graduate student in chemistry at Stony Brook. The work was funded by the U.S. Department of Energy, which supports basic research in a variety of scientific fields.

Related links: Abstract and  full text of the ion imaging paper which appeared in Science magazine.


The U.S. Department of Energy's Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies. Brookhaven also builds and operates major facilities available to university, industrial, and government scientists. The Laboratory is managed by Brookhaven Science Associates, a limited liability company founded by Stony Brook University and Battelle, a nonprofit applied science and technology organization.