When a molecule reaches a point in its configuration space where the energies of two quantum states cross, it has a choice to make: It can continue along in the state it was in, or it can cross over to the other one. Understanding the dynamics of those choices—why some molecules end up in one state and some in the other—has been a long-standing challenge in chemical physics.
The choices are known to have implications for various biochemical processes, including how DNA protects itself from UV damage. But because they happen so fast, with simultaneous motion of electrons and atomic nuclei that cause the Born–Oppenheimer approximation to break down, they’re hard for theorists to confidently model. And experimental studies aren’t much easier.
Now Kristina Chang of the University of California, Berkeley; her advisers Daniel Neumark and Stephen Leone; and their colleagues have directly measured an important quantity: the time scale of the choice.
Their experiment, on the widely studied prototype molecule methyl iodide (CH3I), follows the course of many that have come before. With a UV laser pulse, the researchers pump a population of CH3I molecules into an excited state. From that state, the molecules reliably fall apart: Specifically, the carbon–iodine bond lengthens and then breaks. Shortly into the journey of self-destruction, the molecules encounter a choice. Some stay the course, from which the newly freed I atoms end up in a spin–orbit excited state. Others cross over to a different state, which yields ground-state I atoms. After a tunable time delay, the researchers blast the dissociating molecules with an ultrashort broadband pulse, which probes whether the molecules have made their choice yet: Are they all still in the same state, or have they split into two subpopulations with different energies?
But the sticking point, until now, has been the initial UV pump pulse. Because of the power and wavelength needed to excite the CH3I molecules, it previously wasn’t possible to make the pulses any shorter than 100 fs—far longer than the time scale that researchers are hoping to measure. Using a newly innovated technique involving frequency tripling in a nonlinear optical crystal, Chang and colleagues can get the pump pulse duration down to 20 fs, short enough that they can see a clear signature of molecules going their separate ways 15 ± 4 fs following excitation.
The results agree well with what theorists predicted for the system two years ago—a promising sign that theorists and experimenters alike are well on their way to understanding molecular choices. (K. F. Chang et al., J. Chem. Phys. 154, 234301, 2021.)
