Chemistry in Color

David Pacchioli
December 01, 1995
red tube and red highlighted metal coil

On a long broad table-top in a basement room in Davey Lab there sits an experimental set-up of considerable complexity.

An argon laser, encased in a black box, feeds its blue-green beam into a second laser a foot away, through which a stream of orange dye is being pumped by a plastic tube. A similar pair of lasers holds down the far side of the table. From each set, guided through a bewilderment of directional shifts by a compounded assortment of lenses, mirrors, prisms, and baffles, issues a hair-thin beam of light, on its way to the center of the array, where each one enters - one from north, one from south - into a four-armed intersection of heated stainless-steel tube, three inches in diameter. This cross-shaped heat pipe is ringed by two metal bands. Its four arms are capped tight with Pyrex windows.

There's a bit of a Rube-Goldberg quality, altogether, in this gathering of hardware; what with all the miscellaneous optics, all the chrome and metal and glass. But what grabs your eye irresistably is the sheer beauty of the light. At their sources, the laser beams are vivid, pure: aqua here, sunset there, ruby across the way. On their courses, they dwindle to filaments, but amazingly retain their iridescence: Interrupt one with a piece of paper and a sphere of brilliance splashes there. Take the paper away and it resumes, slicing its perfect sparkling line through space, absolutely silent. The effect is magical.

A digital gauge gives the temperature inside the heat pipe at 466 degrees centigrade. A small mirror held up to the pipe's west window reveals what's inside: a silvery lump - sodium. Above it, bisecting the circle, is a thick band of bright orange. Again, it is the quality of color that seems remarkable. Nor is it of mere aesthetic interest. The orange band, graduate student Ray Hoobler explains, is vaporized sodium atoms: released from the partly molten mass, they fill the center of the chamber; they have absorbed the laser light passing through and are re-emitting it, fluorescing as they release energy. The color corresponds to a precise frequency; and frequency is at the heart of what is going on here.

The idea of using lasers to control chemical reactions has been around since the '60s, almost as long as lasers themselves. It's an idea that takes advantage of a basic concept in quantum mechanics: Both light and matter exhibit the properties of waves.

"A lot of this work," Ray Hoobler acknowledges with a grin, "is being done by physicists." Hoobler, clean-cut, in his late twenties, the doting father of a seven-month-old baby girl, is a Ph.D. candidate in the lab of chemistry professor Robert Bernheim, who has been working at the boundary between the two disciplines for three decades.

Molecules are never still. They vibrate, each element at its own characteristic frequency. If these molecules absorb additional energy in the form of light or heat, their vibrations intensify. They become excited.

This influx of energy is the basis of a chemical reaction. In classical terms, the energy would come in the form of a diffuse heat. "For 150 years," says Hoobler, "chemistry was basically confined to beaker and flame." Fine, as far as it goes. But when you put a beaker to the flame, everything starts vibrating. It's not a very selective process. "You're limited to what the molecules want to do," says Hoobler.

The last few decades have seen development of chemical catalysts, agents which hasten reaction. But catalysts too, are all-or-nothing in their effect. Most industrial chemistry is still done by what amounts to brute force, a practice that results in large amounts of waste.

What if you could pinpoint the energy required for a particular reaction, applying it only to selected atoms and not to others? Then you could direct a reaction in a much subtler way. This is the promise lasers hold for chemistry.

A laser can be tuned to emit concentrated energy of a precise frequency, "connecting" with particular molecules whose atoms vibrate at the same frequency. Once this connection is made, the laser can be used to influence the activity of these atoms, exciting them in order to alter a reaction. Currently, there are several types of laser chemistry being developed: competing, as it were.

One or more of these may someday have a huge impact on industries like pharmaceuticals, where fine control can spell the difference between an inert compound and a biologically active one. But the chemists involved say that prospect is a long way off.

Most of the work that has been done so far has been on simple gas phase reactions involving isolated individual molecules. Bernheim's team recently passed a milestone when they showed how lasers can be used to change the point of equilibrium of a bulk chemical reaction.

In the central "cell" of the heat pipe, Hoobler explains, he heats a chunk of lithium (like sodium, a simple metal) until it liquifies. The heat triggers a classical transformation: lithium atoms are freed from the solid phase into vapor, where they become highly reactive. Bouncing around within the confines of the cell, some few of these atoms will collide with each other; if conditions are right, these will bond to form two-atom molecules called dimers. Some of the dimers, in turn, will be knocked apart into their component atoms.

At last, an equilibrium is reached. The vapor that remains contains a set ratio of thousands of lithium atoms (Li) to each lithium dimer (Li2).

Bernheim's team can alter this ratio, using a laser beam. "What we're doing is shifting the chemical equilibrium," says Hoobler. "Eliminating bonding, encouraging anti-bonding. We end up with more atoms, fewer dimers."

To accomplish this change, they first have to set up a frame of reference. Hoobler points to the two rings around the pipe. "They contain copper wire," he says. "When the current is switched on, they produce a magnetic field." Like the poles of the earth, the field creates an axis, drawing all the electrons in the atoms to "spin" either up or down.

"Here's where it gets quantized," Hoobler says. That is, an electron's energy is increased or diminished not over a continuum, but in discrete units, or quanta: these correspond to steps, or energy levels, which orbit the nucleus. "You have to add so much to get to the next level. You can't work up to it. It's like the notes on a piano, as opposed to a violin."

At the lowest energy level, ground state, electrons are closest to the nucleus. When an electron absorbs a quantum of energy, from heat or light, it "jumps" to a higher level. This is called a quantum jump, and the atom is said to be in an excited state.

Eventually, though, the excitement wears off. The electrons jump back down, emitting their excess energy as light of a particular frequency. (This process is the principle behind electric lights.)

Here's where the spin comes in. Each energy level, Hoobler explains, has associated with it two magnetic sub-levels, one slightly higher in energy and one slightly lower. Switch on a magnetic field and electrons in the ground state will jump up to one of these sub-levels or down to the other, pretty much randomly. Those atoms which jump to the higher sub-level are said to "spin up." To the lower, they "spin down."

Once the atoms are thus deployed, they are exposed to a circularly polarized laser beam, carefully tuned to their frequency.

The laser beam acts to "orient" the magnetized atoms. In effect, it "pumps" them all into either the "up"or the "down" sub-level. "We can choose which sub-level they go to," says Hoobler, "depending on the experiment."

Since bonding - the formation of dimers - occurs only when atoms from spin-up and spin-down collide, if there aren't any atoms in one of these sub-levels, there can't be any bonding.

In the lab, the outcome of the experiment becomes visible on a computer screen at the far end of the table. Dual plots, representing a laser beam's scan through the center of the heat cell, show changes in fluorescence thrown by the dimers that are present: less intense fluorescence means fewer dimers. The lines run roughly parallel, climbing and dropping together, but they are distinctly separate. The bottom plot, recorded with the magnetic field switched on, shows clearly the depressing effect of orientation on the formation of two-atom molecules.

Bernheim made the original prediction that is the basis for Ray Hoobler's experiment way back in 1965.

"I had done a lot of work on what was called optical pumping," he recalls. "I very early realized that one could affect chemistry by looking at the interaction of matter with electromagnetic radiation."

Subsequently, interest in atomic orientation sprang up around the world. "Back then," Bernheim says, "it was treated as an academic exercise. This was before lasers were commonly available to chemists."

In the '70s, once lasers came into wide use, laser-controlled chemistry really took off. Early practitioners attempted to attack specific bonds in large molecules.

"There was enormous interest," Bernheim says. "However, a lot of it has gone unrealized. The problem was people didn't have a detailed understanding of the energy-level structures of the molecules they were investigating." Because of the high degree of interdependence that exists between chemical bonds, laser energy imprecisely applied will simply be redistributed throughout the whole molecule. "In effect," says Hoobler, "you're just heating the molecule, same as you would with a Bunsen burner."

Acquiring a thorough knowledge of a molecule's energy levels, and its kinetics would mean painstaking groundwork. That's where molecular spectroscopy comes in.

If you take a burning element and pass the light of its flame through a prism, that light will separate to produce a spectrum: a unique pattern of narrow colored lines, each line corresponding to a specific frequency, or energy level separation. Taking a molecule's spectrum to identify its energy levels is the first step in molecular spectroscopy.

The second step, as Bernheim explains, is interpreting what you find. "You assign the spectra to different quantum numbers that describe energy levels, creating a structure." One of the continuing projects of Bernheim's career has been his efforts to create such a structure for the lithium dimer. The results of this work fill a long yellowed chart on a wall in the lab opposite the laser set-up: a record of long years of work by Bernheim and chemistry department colleague Peter Gold. "We now have a comprehensive fundamental understanding of this important molecule," Bernheim says.

A few years ago, Bernheim decided to return to laser-controlled chemistry. "You sometimes like to go back and explore the ideas of your youth," he says, smiling, "and we were finally in a position to do something about it." The development of tunable lasers made the experiments a lot easier. Charting the energy levels of larger molecules, however, presents far more complicated problems than the earlier work on lithium.

When electrons get excited and ascend to higher energy levels, an atom vibrates. The molecule it is a part of not only vibrates, it rotates too. For all these vibrations and rotations there are corresponding - and overlapping - energy levels. It's a lot to figure out.

Twenty years ago, experimenters had the idea to try to use a laser to affect a reaction involving the six-atom molecule methyl isocyanide. "This is the archetypical unimolecular reaction," Hoobler explains, "the one in all the textbooks. It's just a rearrangement process, dependent on temperature." Add a litle heat, in other words, and a carbon and a nitrogen atom within the molecule will switch places.

"The idea was to use a laser to excite only a particular vibrational mode in this molecule and see whether you could change the rate of this transformation." Trouble was, no one had done the spectroscopy, so the optimum laser frequencies to be used were unknown. When Bernheim got interested in this experiment, Hoobler says, "We had to go back and map all the energy levels before we could do the experiment." Another of Bernheim's Ph.D. students, Linh Le, has recently completed the assignment and interpretation of the spectra.

A new approach to laser chemistry, touted in other labs, exploits that property of laser light known as coherence.

Conventional light sources produce incoherent light: The excited tungsten atoms of a light-bulb filament release their glow independently, resulting in a jumble of waves. The active atoms in lasers, however, are perfectly in synch; the light produced is coherent.

Coherent light displays the property of interference, which refers to the way waves combine. Coherent control uses ultra-short laser pulses (as short as a femtosecond, 10-14 second) to excite molecules so that they interfere with one another, influencing a reaction.

Its supporters hope that this approach will allow them to scale up laser chemistry and work on larger molecules. But Bernheim regards coherent control dubiously. "They're using laser pulses so intense they push nuclei around - actually changing energy levels, changing the geometry of the molecules involved." Coherent control is a also a lot more complicated to pull off, Hoobler says. "Our approach is easier to visualize. You have a particular bond, a particular frequency - you just have to match the laser frequency to the frequency of the bond.

"With coherent control, there's a ton of quantum mechanics in there. It's a lot more involved. The pulse has to be the right length and shape. The sequence of pulses has to be staggered at a particular rate. . . ." Practitioners often require the use of computer models to obtain rough estimates before they can undertake an experiment. "This modeling capability can be an advantage," Hoobler acknowledges, but it also makes things a lot more expensive.

And you still need to know your energy levels. You still need to be able to determine just what you're aiming your laser at. Which leaves laser chemistry somewhat stuck right now at the level of very small molecules. "It's been kind of a quagmire," Hoobler acknowledges.

"It was a big step to move up to water."

Monodeuterated water, that is: A water molecule with one of its hydrogens (an H) replaced by a heavy water atom (deuterium, or D). Studies have shown, Hoobler reports, that "you can hit these two bonds" - that between H and O, and that between O and D - "with different frequencies, and break the bonds selectively."

This experiment has been done successfully in other labs using both mode-selective and coherent-control approaches. Still, says Hoobler, "it's a long way from pharmaceutical-type chemistry."

Recently, Hoobler's goal has been to try to extend the results obtained with lithium atoms to another, slightly more complex, reaction, a metamorphosis involving two species of atom: sodium and hydrogen. Moving to the blackboard in his office, he writes it out:
Na + H NaH.

Altering this reaction, he says, "would be a significant extension of what we've done so far."

The first obstacle is to produce the free hydrogen atom required to fulfill the first half of the equation. It takes a lot of energy, Hoobler explains, to break down the hydrogen gas molecule, H2, into just the atom H. "Typically," he says, "you would hit it with an electric charge, which blows the H2 apart."

"But you can also get it this way:"

"Na* + H2 NaH + H."

"The asterisk means sodium in an excited state."

"Now, if you use this reaction as a source of free hydrogen, then you should be able, using the laser, to orient your sodium and your hydrogen and prevent them from reacting with each other or themselves."

He steps away from the board, wipes his hands.

"If this works we'll have free hydrogen atoms sitting around in our heat cell, which is a neat prospect. In fact, there are some people at Princeton who are actually working on this as a possible energy source." (The idea: once the magnetic field and hence the orientation was "turned off," the collected hydrogen atoms would react like gangbusters to get back to the stability of being hydrogen gas. "This process", Hoobler says, "gives off an awful lot of energy.")

"Our interest is just to extend the current process to a more complex molecule."

Early attempts have not been successful. "The problem is," says Hoobler, "I haven't been able to tell whether I'm generating free hydrogen or not. Hydrogen is very transparent to detection - it isn't easy to see, at least in the small amounts that we'd be getting."

Hoobler has, however, identified one oversight that was holding up the works. "The mistake I was making," he says, "- and I could kick myself for not realizing this - is that orientation defeats the process of the first step. Once all the sodium atoms are in one magnetic sub-level, there's no way to excite them.

"Without excited sodium atoms, there's no reaction."

He rectified the problem by incorporating a fresh alteration into the experimental set-up, splitting the laser beam before it enters the heat cell.

"I'll be taking half of the beam and circularly polarizing it, to take care of orientation," he explains. "The other half will be constantly promoting atoms to the excited state."

This time, if all goes well, Hoobler should notice when he peers into the center of the cell a slight increase in the intensity of the fluorescent orange band - an indirect indication of the presence of hydrogen atoms.

"It will be a pretty subtle effect," he acknowledges. "But I'll be able to tell."

Ray J. Hoobler received his Ph.D. in chemistry in December 1995. He is now completing a postdoctoral fellowship at the Joint Institute for Laboratory Astrophysics in Boulder, Colorado; e-mail hooblerr@jila.colorado.edu. Robert A. Bernheim, Ph.D., is professor of Chemistry in the Eberly College of Science, 152 Davey Laboratory, University Park, PA 16802; e-mail r5b@psu.edu; 814-865-3642. Funding for the above research was provided by the National Science Foundation and by the Petroleum Research Fund of the American Chemical Society.

Last Updated December 01, 1995