By Saren Johnston
When chemists try to predict whether the chemicals they want to combine will create a reaction, they must first know if the reaction is energetically possible -- if there's enough "oomph" in the atoms to make the reaction go.
Knowing the ionization and bond dissociation energies for the molecules under investigation is critical to accurately predicting reactions, such as electron transfer or oxidation-reduction processes. And Cheuk Ng, an Ames Laboratory senior chemist and an Iowa State University distinguished professor of chemistry, has helped many a chemist make some successful and more accurate predictions. His research in vacuum ultraviolet (VUV) chemistry has provided some of the most accurate ionization and bond dissociation energy data for many molecular species.
Ng considers himself fortunate because he has a dual "playground" in which to investigate molecular dissociation (separation into two or more fragments, creating atoms, ions or radicals) and ionization processes in the VUV range of the spectrum -- photon energies in the 6-30 electron-volt region. There, in principle, any molecule can be ionized, a process in which a molecule or atom loses an electron, acquiring a net charge and making the transformation to an ion. To perform his experiments, Ng takes advantage of the third-generation synchrotron radiation source at the Chemical Dynamics Beam Line associated with the Advanced Light Source (ALS) at the Department of Energy's Lawrence Berkeley National Laboratory. Or, he may use the state-of-the-art high-resolution VUV laser apparatus at Ames Laboratory.
To be sure, Ng's research sounds weighty and complicated, but he would argue unequivocally that it is a fundamental thing of beauty. "When I look at the data and understand it, I feel unified with the universe and in harmony with nature," he says. "It's very basic research, but I'm excited about it because every day I learn something new. How much energy does it take to ionize or dissociate a specific molecule? This is a parameter that is fundamental to chemists. Once they know the energetics, they can use them to predict reaction rates. Those rates are needed for modeling problems in atmospheric, plasma and combustion chemistry."
Ng's work at the Advanced Light Source centers around the high-resolution VUV facility he designed for the Chemical Dynamics Beam Line. Specialized instrumentation at the VUV facility allows him to make both photoionization and photoelectron measurements. The spectroscopy he performs provides information on the ionization and dissocia-tion energetics of specific molecules of interest. Photoionization spectroscopy involves detecting the ion(s) produced when one or more electrons are removed from an atom or molecule by absorption of a VUV photon. Photoelectron spectroscopy offers a means of detecting the electron(s) liberated by photon-molecule interaction.
To design the VUV facility, a unique user facility that today serves chemical physics researchers throughout the world, Ng went back in time, in a manner of speaking. "I started to design our experimental station at Berkeley in the summer of 1993," he says. "I went back to live in a graduate student dorm, and I worked like a graduate student. Everybody asked me which group I was working in at Berkeley, and I told them I had graduated years ago," he says with a laugh, recalling his brief return to graduate school days.
In 1994, Ng sent graduate students Chia-Wei Hsu and Matt Evans to Berkeley to assemble the photoelectron/photoion apparatus he had designed for the experimental station. "These two students put the whole thing together. I only put together the mechanical design," says Ng modestly.
"In 1995, everything was up and working. Chia-Wei and Matt did all the performance checks for the apparatus, so whenever I traveled from Ames to visit the Berkeley experimental station, I only had to ask, 'How are things going?' I feel I've become a theoretical experimentalist," Ng jokes, but with a certain measure of seriousness. "I design a scheme and ask the students to try it and report the results to me."
The VUV facility is considered the best in the world for doing ion chemistry. "The photon energy resolution and energy range we can cover is unprecedented," says Ng. "Due to the development of several novel experimental schemes, the resolution we can achieve is at least 10 times better than what was available in previous synchrotron-based studies before the VUV facility existed."
Ng notes that they can measure the dissociation threshold and thus the bond dissociation energy of a molecular ion to the accuracy of less than 0.001 electron volts. "Many of the things we are doing now can be considered as breakthroughs in the field of VUV chemistry, and the facility coaxes us to do more," he says.
Some of Ng's work includes looking at radicals, or unstable molecules, to explore the energetics of combustion species for the Department of Energy. "Combustion is almost impossible to understand because it's a radical chain reaction," he says. "When one radical is formed, it initiates another reaction, so there are many, many intermediate species that have not even been detected and for which the energetics and structures are not known."
An aspect of Ng's research important to plasma chemistry involves looking at the reactivities of ions and radicals, which depend on their internal energy states. The state of a molecule is related to how the electrons and nuclei arrange in the molecule. Because of the high resolution capability of the VUV facility, Ng is able to select a given state of a molecular ion and then react it and look at how the internal excitation influences chemical reactivity.
"A molecule can move around -- that's what we call translational energy," says Ng. "A molecule can also vibrate -- vibrational energy, and rotate -- rotational energy. Electrons can also arrange differently with respect to the nuclei," he adds. "This arrangement gives rise to the specific electronic state of the molecule. So all of these things can influence the chemical reactivity of the molecular species. The study and ultimately the control of molecular reactivity are of great interest to chemists. So the VUV facility has opened up opportunities for future investigations in which scientists can really control the internal energy of a molecular ion to a very high degree. For VUV chemistry, I can confidently say that this facility is the best in the world. Many users have come here and told me the same thing. So it's better that they tell me than I say so," Ng notes with a smile.
His research is also helping advance knowledge in the field of atmospheric chemistry. The ionosphere, the outermost shield of the earth's atmosphere, is ionized and dissociated by solar ultraviolet and VUV radiation. In order to understand and model the chain reactions that follow the ionization and dissociation processes and cause such phenomena as the aurora borealis, Ng says scientists must first measure the reaction rates and determine how the ions are initially formed. "A lot of research work is needed," he says with conviction.
As remarkable as the VUV facility is, Ng can't spend all his time at the ALS. So, with the assistance of fellow chemist Chung-Lin Liao and postdoctoral fellow Yuxiang Mo, he also runs experiments on the high-resolution VUV laser instrumentation at Ames Laboratory.
"The VUV laser has an established resolution better than that we can achieve at the ALS, but it's very difficult to tune," says Ng. "For example, if you want to tune about .2 electron volts, it would take a few weeks for two students to work on it. At the ALS, we can tune through more than 10 times that range in one week, so the throughput or productivity comparison is no match at all. I don't think the laser can win in this case. The resolution is slightly worse at the ALS, but you don't lose that much chemistry and physics. It's really the ease of tunability that makes the synchrotron so attractive."
Ng explains that many of the techniques they have now developed for the ALS facility were previously laser-based. "Now that we have tapped into the synchrotron, I think the laser is gradually losing ground," he says. "But for optical resolution, the laser is still the best. Furthermore, some experiments are easier using VUV lasers."
Ng and Liao have many ongoing projects in their Ames Lab research quarters. They are focusing on VUV mass spectrometry, VUV and ion-etching reactions, and photoelectron spectroscopy of radicals.
"For the studies of radicals, the VUV laser is expected to do a better job than synchrotron radiation because most radicals can be prepared with high specificity by pulsed laser dissociation," explains Ng. "This pulsed radical source matches the pulsed nature of the VUV laser system."
Two students from Ng's group, Evans and Stephanie Stimson, finished their Ph.D. theses working at the VUV facility at the ALS, and Ng will soon be sending three or four more students to work there. "It's a tremendous educational instrument for our group," he says. "Many people in the world who are known to be experts in VUV chemistry come to the ALS, so these students have the opportunity to interact with them and learn from them, not just from me. Then, after spending four or five years working at the VUV facility, the students will already have connections with well-known scientists."
Ng says there is no better avenue for training students than basic research and that as he gets older the teaching part of his job becomes more significant.
"I often have students ask me why they have to learn certain things," he says. "To me, it's very obvious -- understanding is really the fulfillment in life. I tell my students, 'There are two ways to make you work hard -- one is to impress and one is to inspire. To impress means you apply yourself -- if I do this and work hard, I'll get a million bucks in maybe a year or so. Inspire means you don't talk about money -- you work night and day. That's a higher level of learning.'
"If I work night and day on a project, it must be fun," says Ng. "Fun is actually the key to being inspired. I would like my students to be inspired. The most important mission to me is not to produce a commercial product but to train students."
As Ng's research matures, the molecules he investigates at the VUV facility are becoming more complex. "We've been working on small molecules because we have to test the techniques we develop," he says. "But we're moving on to more complicated molecules related to combustion.
"The VUV is a user facility, so my job is to convince people that this research is exciting," he adds. "It has been quite a venture in the last year. People come from the United States, Japan, Germany and France because the chemistry is completely different compared to working in a lab. Everybody has his or her own expertise, and I pick up many ideas from users when they come. So to me, the whole world is like a playground. I don't feel the boundaries anymore."
For more information:
Cheuk Ng, (515) 294-4225
cyng@ameslab.gov
Current research funded by:
DOE Office of Basic Energy Sciences
Last revision: 12/17/99 sd
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