Pirouetting Molecules Can Help Us Learn About the History of the Universe

O God! Can I not grasp
Them with a tighter clasp?
O God! can I not save
One from the pitiless wave?
(from “A Dream Within a Dream” by Edgar Allan Poe)

In their natural environment, molecules behave like the balls in a lottery machine. They spin, tumble and crash into each other many millions of times per second.

Just like an airstream agitates the balls in a lottery machine, ubiquitous thermal radiation also gives molecules energy that keeps them bouncing around and spinning chaotically.

So, as you can imagine, it’s quite challenging to study squirmy molecules in their restless chaos!

Imagine how much more we can learn if we can keep an individual molecule in one place. We need to isolate it from the bustle of the crowd and hold onto it tightly to counteract the thermal radiation that attempts to rotate it. This is like overcoming the airstream agitation in the lottery machine by holding onto a single ball with a firm grip, so it becomes easy to read its number.

Manipulating a lottery machine will likely get you in trouble, but luckily, our team is only studying how to apply this conceptual idea to molecules in our lab at NIST.

Atoms are the smallest unit of matter, and molecules are a group of these atoms bonded together.

When we remove an electron from an atom or a molecule, it becomes positively charged. Once positively charged, it is what scientists call an atomic ion or a molecular ion. We can confine ions inside a vacuum chamber in a cage made of electromagnetic fields, known as an ion trap. The vacuum inside our apparatus guarantees that few other particles collide with the trapped ions.

Because ions are charged, they strongly repel each other. We set up our experiments by trapping a single atomic ion and a molecular ion right next to each other. When one ion moves back and forth in the trap, the repelling force pushes the other ion, so they both move back and forth in concert.

Cooling and control of individual atomic ions with lasers started in the late 1970s with seminal contributions by Nobel laureate David Wineland, who founded our group at NIST. Researchers have refined the control of atomic ions greatly in the last 50 years.  

As a result, we can laser-cool the back and forth of the atomic ion until it stops moving. At that point, the molecular ion goes along and stops moving back and forth as well.

You Spin Me Right ’Round

The molecule can also spin in place, and that rotational motion is not picked up by the atomic ion. Thermal radiation can drive the molecular rotation and heat it up. This is similar to how your skin heats up when absorbing infrared radiation from the sun.

You may think that the molecule can rotate at any rate, but that is not true. Quantum mechanics dictates that the molecule's rotational energy changes in discrete steps.

So, when the molecule heats, it climbs the rungs of a ladder of energies. We call each rung of this ladder a quantum state. Due to the random nature of the thermal radiation, this is not a directed climb. Instead, the molecule is driven randomly through hundreds of different rotational states.

I’ll Be Watching You

To pick up information on the molecule’s rotational state, our team had to come up with a few more tricks that used the same co-trapped atomic ion that already did the cooling. It can pick up the molecule’s back-and-forth motion but not rotation.

By applying laser pulses with just the right frequency, we can change the rotational state and make the molecule wiggle back and forth. The crucial point is that the laser frequency only fits with one pair of rotational states, like a key in a lock.

When we apply a certain laser light “key” and then the atom picks up that the molecule is going back and forth, we know that the molecule was initially in the one rotational state. Afterward, it’s in the corresponding rotational state of the pair where the “key fits.”

When the wiggling atom signals that the molecule is in a known state, we can seize the moment and apply further laser pulses to study the rotational level structure. This technique allows us to view the molecule with unprecedented precision and detail.

However, while we try to find out as much as possible about the molecule, the thermal radiation continues to randomly drive the rotation. At some point, we lose track of the state. It randomly wanders around again, passing through many other states until it returns by chance to one of the few states we watch with our “key” laser pulses.

Credit: NIST

(Top) Magnified image of the photons scattered by two calcium ions that are approximately 10 micrometers apart (a tenth of a human hair). 

(Middle) A calcium hydride molecular ion on the left and a calcium ion on the right. 

(Bottom) A calcium ion on the left and calcium hydride molecular ion on the left after we have manipulated the trap to switch ion positions. 

In contrast to the calcium ions, the molecules cannot be made visible by a resonant laser and therefore appear as dark places encircled by the light blue dashed lines. We know they are there because we can make them and the bright calcium ions trade places. Since positively charged ions strongly repel each other, we can still learn about the molecular ion by sensing its motion with the calcium ion and then encoding what the calcium ion senses into the brightness with which it appears. The calcium ion appearing dark or bright is the “Morse code” with which it relays information about the molecule to us, human observers. For these images, the calcium ions are in their bright state because otherwise, they will not be visible either.
 

Until recently, we spent more time waiting for the rotation to return to a state where the key fits than doing experiments, and the rate at which we could learn about the molecule was rather slow. Imagine a school where the students are at recess approximately 93% of the time and are only in class 7% of the time. The students may enjoy this, but they wouldn’t learn much! We wanted to learn more quickly and grew impatient waiting for the return of our wandering states.

This motivated us to think about how we can keep the rotational state from wandering away in the first place. It turns out that when the molecule randomly climbs up and down that state ladder, it is extremely unlikely to skip rungs.

So instead of patiently waiting for its return after the rotational state escaped, we now rapidly “change the key” to laser pulses at frequencies that check on the rungs immediately above or below our original key state. In most instances, we can catch the molecule on one of these adjacent rungs before it escapes any further. We then return the runaway with a microwave pulse of exactly the right frequency and duration back to the state we want to study.

With this “intercept and return” approach, we can learn lessons about the molecule 65% of the time and “recess time” is down to 35%. A sequence of experiments that previously took an hour can now be done in about six minutes.

In principle, this strategy can be extended over the next-nearest rungs to further increase “classroom time” and is only limited by very rare occasions where the molecule does not change the quantum state of its rotation but rather how it “vibrates.”

Molecular vibration is an entirely different form of motion, where the atoms in the molecule start to oscillate relative to one another. Think of this vibration as the molecule “breathing,” except the molecule itself becomes longer and shorter during the vibration process. A change in vibration requires substantially more energy than altering how the molecule spins around. Thermal radiation has very few photons with enough energy to change the vibration, so it happens very rarely.

It All Started With the Big Bang

One main motivation for our work is that the methods we have developed on just one particular “guinea pig” molecular ion, the positive ion of calcium hydride, will also work for many other small molecular ions. Indeed, a growing community of researchers has picked up our methods and started to apply them to other molecular ions.  

There are lots of molecular ions in the vast interstellar gas clouds where stars are born, but we know very little about them. That’s because it has been difficult to get undisturbed glimpses of them in a laboratory.

We now have brand-new tools to help us unlock the currently unknown properties of many charged molecules. This is important for science because researchers who examine such clouds can use the molecular “fingerprints” we produce to compare with the fingerprints of molecules their telescopes receive from outer space. This allows researchers to identify molecules that are active at the “crime scene” during the formation of stars.

Faraway interstellar clouds represent the early days of the universe and provide glimpses into its history. There’s still a lot we don’t know, but hopefully, our research can help unlock some of those celestial mysteries.

The team that worked on this research at NIST included Yu Liu, Julian Schmidt, Zhimin “Cheryl” Liu, David R. Leibrandt, “James” Chin-wen Chou and me. We combined many different skills in a strong and cohesive team to achieve this success.

Initially, our approach was new and untested, and funding was not readily available. We started with equipment that was repurposed from retired setups or borrowed from other labs.

We are grateful to all the colleagues who helped us in this initial phase. We are particularly grateful to the whole Ion Storage Group, but also to others at NIST, in particular Scott Diddams and Tara Fortier. Philipp Plessow from the Karlsruhe Institute of Technology in Germany provided invaluable help in understanding the theory behind our initial observations.

What’s Next?

We are now working to extend control to molecular vibration and to learn even more about molecules. This requires different laser sources to provide the more energetic laser “light keys” required to unlock vibrational transitions.

We have already succeeded in creating one particular vibrational key and are now designing and building a more universal laser “keychain” that should allow us to unlock nearly all vibrational states of many molecules.

When vibrational energy is added, the bond between the vibrating atoms is softened. When vibrational energy is removed, the bond becomes stronger. Breaking and making bonds is what chemistry is all about.

We hope to open a new frontier in chemical reaction research, state by state and molecule by molecule.