Scientists at the US Department of Energy’s Brookhaven National Laboratory have developed an 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. ‘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 colours.
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. Consequently, 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.’
Suits introduced a beam of methyl chloride gas ions into a vacuum chamber, used a vacuum ultraviolet laser to add energy, and 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 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 said to be 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, where ions play a major role.