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Physical chemistry

Single molecules filmed dancing on a table top

By ripping an electron away from a molecule and then slamming it back again, the motion of nuclei in a molecule has been tracked with extremely high temporal and spatial resolution. See Letter p.194

In 1909, the physicists Hans Geiger and Ernest Marsden shot α-particles (helium nuclei) at a thin gold foil1. Following the suggestion of Ernest Rutherford, they were looking for (and found) a substantial number of particles that were deflected by the foil at angles larger than 90° — rather than passing straight through it, as ideas about the structure of atoms predicted. According to Rutherford2, the discovery “was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” Rutherford concluded3 from these results that atoms have very compact nuclei, where all of their positive charge is concentrated, surrounded by much more loosely spaced electrons. The α-particles rebounded because they collided head-on with fully charged gold nuclei.

Ever since then, the scattering of charged particles has been used to study the ångström-scale structure of matter, effectively providing a range of 'cameras' for taking pictures of the microcosm. Reporting on page 194 of this issue, Blaga et al.4 describe an unusual variant of these cameras that has allowed them to study the motion of nuclei in single molecules of oxygen and nitrogen at sub-ångström spatial resolution, and with a temporal resolution of just a few femtoseconds (1 femtosecond is 10−15 s).

For molecules to be seen at their ångström-scale dimensions, they must be illuminated with a beam of some sort that has an ångström-scale wavelength. Pulses of X-rays are an option, but they must be very short and exceptionally bright to take snapshots of quickly moving nuclei in single molecules. Free-electron lasers can produce such X-ray pulses, but they require massive facilities that cost billions of dollars to build and maintain.

Short electron pulses are a much cheaper alternative to X-rays, and can be made using table-top-sized apparatus. Electrons are, of course, particles, but quantum mechanics tells us that they also have wave-like properties. When accelerated to about 3% of the speed of light, their wavelength becomes short enough to image objects at the ångström scale. What's more, because electrons are charged and have low mass, accelerating them with electric fields is quite straightforward. Electrons also scatter well, much better than X-rays, and thus provide bright signals for imaging. Extremely short pulses of electrons can therefore take stroboscopic 'snapshots' of moving nuclei in molecules, much like the frames of a movie. Indeed, this is the basis of the techniques of ultrafast electron diffraction5 and ultrafast electron microscopy6.

The charged nature of electrons, however, also presents a problem: repelled by their like charges, electrons do not want to stay together in groups. Ultrashort electron pulses containing many electrons therefore tend to become much longer by the time they reach their target during imaging experiments, reducing the temporal resolution that can be achieved. With very few electrons per pulse (ideally, just one), the pulses can stay short — but then the strength of the imaging signal plummets, making the observation of individual molecules exceptionally difficult. This is why ultrafast electron diffraction and ultrafast electron microscopy have failed to take freeze-frame pictures of individual molecules at a temporal resolution of a few femtoseconds. Blaga et al.4 have now accomplished this feat, remarkably using just one electron per pulse.

The authors' ultrafast electron camera is based on an unusual approach that was originally proposed about ten years ago7,8,9: the electron that is used to illuminate a molecule is pulled out from the molecule itself by a strong infrared laser field (Fig. 1). Thanks to the high intensity of the laser field (its field strength reaches several volts per ångström), a distance of less than 100 Å is sufficient to accelerate the electrons to several per cent of light speed. And because of the oscillating nature of the field, the accelerated electron can then be turned around and slammed back into the parent molecule.

Figure 1: Molecular movies.
figure1

Blaga et al.4 have used laser-induced electron diffraction to 'film' single molecules in motion. In the authors' technique, a molecule such as oxygen is irradiated with a strong infrared laWHser field, pulling away an electron (e) and causing the bond between the atoms in the molecule to stretch. The electron is accelerated in the laser field, then directed back towards the resulting ion. Collision of the electron with nuclei in the ion causes the electron to rebound and to generate a diffraction pattern — an electron spectrum. This spectrum contains information about the positions of the nuclei at the moment of collision. By performing a series of experiments in which the time delay between the electron's departure from, and collision with, the molecule is varied, a series of snapshots (spectra) of the molecular motion can be taken, much like the frames of a movie.

The removal of an electron from a molecule sets the molecule's nuclei in motion. Subsequent re-collision of the electron with the molecule takes a snapshot of what has happened since the electron was taken away. This snapshot takes the form of an electron spectrum — the patterns of minimum and maximum electron intensity that are generated by the diffraction of electron waves by nuclei, not unlike the patterns of ripples formed when water waves are disrupted by rocks.

Even with relatively few molecules in the focus of the laser beam, there is a good chance that the removed electron will hit the target, because it never leaves the vicinity of the parent molecule. This ensures a strong imaging signal. Furthermore, everything happens in a time roughly equal to the period of the laser field's oscillation cycle (a few femtoseconds for the radiation used in Blaga and colleagues' experiment), ensuring high temporal resolution.

To make any movie, snapshots must be taken at different time delays after the motion has begun. In Blaga and colleagues' case, the delay is that between the time the electron is pulled from a molecule and the time it returns. This delay is determined by the length of the laser cycle, and so the authors simply repeated their experiment at different wavelengths to obtain a series of frames at different times. Just like Rutherford before them, they observed electrons scattering at large angles as they hit nuclei almost head-on. The scattering angles and the diffraction patterns created by scattered electrons sensitively depend on the locations of the nuclei — even if the positions of the nuclei shifted by only a fraction of an ångström, the difference was clearly visible in the experiment.

So far, so good. But Blaga et al. had to overcome many challenges to reconstruct molecular motion from the measured electron signals. The main difficulty arose from the presence of the strong laser field — ironically, the same field that is integral to the inner workings of their camera. In their experiments, information about molecular structure is encoded in the observed electron spectra. But because each electron is accelerated by the laser field after scattering, the patterns become distorted. Extracting images from the electron spectra therefore requires an accurate theoretical analysis7.

The physics of laser-driven electron–molecule re-collision is rich and complex. The next step will be to perform a detailed analysis of all of the similarly complex information encoded in Blaga and colleagues' electron spectra, to deal with various side effects introduced by the strong laser field, and to extend the scope of the technique to larger molecules. This will involve a lot of work, but obtaining the ability to film the motion of electrons, holes (quasiparticles generated by the absence of electrons) and nuclei in isolated molecules, with sub-femtosecond, sub-ångström resolution, is worth the effort. No doubt Blaga et al. would agree with Lenin10 that “for us, cinematography is the most important form of art”.

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Correspondence to Misha Y. Ivanov.

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Ivanov, M. Single molecules filmed dancing on a table top. Nature 483, 162–163 (2012). https://doi.org/10.1038/483161a

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