A molecular road movie

The development of instantaneous photography in the 19th century brought new understanding of human and animal movements that were too rapid to observe with the naked eye. Suddenly, it was possible to see with certainty that all four hooves of a galloping horse left the ground at once. Similarly, a central theme of modern physical chemistry is the observation of chemical reactions at the molecular level and in real time. So-called molecular movies allow researchers to view the intermediate steps of a reaction, not just its results (see Physics Today, August 2003, page 19 , and April 2011, page 13 ).

In the conventional view of chemical reactions, atoms follow straight, predictable paths, and one knows exactly where to find them. In a photodissociation reaction, in which a molecule breaks apart upon excitation with light, that simple picture gives rise to two basic types of reaction pathway: Either a single bond stretches until its breaks, or the atoms efficiently rearrange to form a new chemical bond while breaking two others. For the dissociation of formaldehyde (H₂CO), those pathways are represented by the green and blue arrows, respectively, in figure 1. They’re easily distinguished because they lead to different sets of products: H and HCO in the first case, H₂ and CO in the second.

Figure 1.

Sometimes, however, atoms slip loose and wander off into the wild. Their destinations are unknown, and their trajectories can be all over the map. Tracking their unpredictable paths is not easy, even when using the best time-resolved spectroscopic methods. We need to turn to powerful techniques sensitive to single molecules.

Roaming atoms

The elusive phenomenon now termed “roaming” surprised researchers when they discovered it in the dissociation of H₂CO in 2004. It is illustrated in figure 1 by two example trajectories shown in yellow and red. In each case, the molecule undergoes a hindered dissociation that strays from the conventional paths. Molecular fragments orbit each other at large interatomic distances for hundreds of femtoseconds before finally going their separate ways. Along the roaming path, one H atom might detach from the molecule, roam around the remaining HCO fragment, and then abstract the other H atom to yield H₂ and CO—but via a different path than the conventional one. (See the article by Joel Bowman and Arthur Suits, Physics Today, November 2011, page 33 .)

Roaming is now known to occur in many molecular systems. It’s generally observed indirectly, through the spectroscopic imprint it leaves on the reaction products. When H₂CO dissociates into H₂ and CO, roaming reveals itself through the distributions of H₂ vibrational states and CO rotational states. However, such indirect detection provides little information about the exact molecular pathway and the dynamics of the moving fragments. Even time-resolved experiments have, until now, been limited to the indirect observation of the outcome of processes that involve roaming. To observe roaming in real time, we must expand the concept of the molecular movie into a molecular-road movie: to identify not just the individual moving fragments but also the different roads they travel.

Directly observing roaming in dissociating H₂CO is challenging because of the unpredictability of the process. Each roaming fragment follows its own path, no two of which are alike. Furthermore, although the roaming process itself lasts just a picosecond or less, it could take up to tens of nanoseconds to get started, so at any given instant, only a tiny fraction of molecules might be caught in the act of roaming. Finally, many of the molecules still follow the conventional, direct dissociation pathways, which cross the roaming pathways and can therefore be difficult to distinguish. We needed a way to extract the roaming dynamics hidden behind the overwhelming statistical background.

Explosive imaging

We chose Coulomb-explosion imaging, an established technique in which a short, intense laser pulse ejects several of a molecule’s electrons at the same time. The ionic fragments repel one another and fly apart, and from their trajectories toward a detector, we can reconstruct the molecular geometry at the time of ionization. Not only is the technique sensitive to single molecules, but we can also isolate specific channels from the background by using ion correlations and thus select statistical events. At the Advanced Laser Light Source, headed by François Légaré, we have filmed the roaming dynamics of deuterated formaldehyde (D₂CO), which we chose over H₂CO for experimental reasons.

We can capture a snapshot of an ensemble of dissociating molecules at any desired moment with femtosecond precision by adjusting the time delay between the photoexcitation pulse and the ionization pulse and then correlating the signals from the D⁺, D⁺, and CO⁺ ion fragments. Supported by the theory groups of Michael Schuurman, Paul Houston, and Joel Bowman, we distinguished the possible pathways by plotting the kinetic energy released in the Coulomb explosion against the angle between the two deuterons’ momentum vectors as they fly apart. We cannot sharply separate all the regions, because deuterons undergoing conventional dissociation must pass through the same configurations as roaming ones. But for the most part, we can classify the pathways according to the outlines in figure 2, whose colors correspond to the trajectories in figure 1.

Figure 2.

The gray circle marks the region of undissociated D₂CO molecules. Before ionization, their atoms are bound together, so in the Coulomb explosion, the charged fragments strongly repel one another for a large kinetic energy release.

The blue circle indicates molecules following the conventional dissociation pathway to D₂ + CO: The two deuterons are near one another but far from the CO fragment, so they fly apart in nearly opposite directions. The green ellipse represents the conventional pathway to D + DCO: The departing D atom is already almost gone, so its repulsion in the Coulomb explosion is small, and its momentum direction is almost uncorrelated with the other D atom’s.

The red and yellow dashed curve shows molecules in the act of roaming, with one deuteron roaming around the remaining DCO fragment. That configuration leads to a larger kinetic-energy release than the conventional D + DCO pathway but a smaller D–D angle than the D₂ + CO pathway.

With this work, we generalized the definition of roaming to include processes recognized by their transient features, not just their outcomes. Because we catch the roamers on the road, we can detect them whether they complete the roaming path or end up in a conventional dissociation channel. Previous measurements of roaming in H₂CO relied on the quantum states of CO and H₂, so they were sensitive only to roaming pathways that yield those products, like the yellow trajectory in figure 1. But by tracking roaming populations over time, we found that some roamers, in fact, dissociate into D + DCO. Their trajectories, exemplified by the red path, have been invisible until now. The results introduce a new paradigm for detecting roaming dynamics and, more broadly, for detecting weak statistical dynamics hidden under a large stochastic background.