Sub-picosecond imaging of short-lived molecular structures

When a photon is absorbed by a molecule, the energy of the photon is converted to chemical and mechanical energy, in the form of nuclear motion. As a result, the structure of the molecule changes. In many photochemical reactions, the timescale for such nuclear motion is on the order of a few hundred femtoseconds. For example, in the first steps of photosynthesis, or in the vision process of animals, light is converted into chemical energy on femtosecond timescales. To understand and eventually control such reactions, it is necessary to determine the 3D structure of the intermediate states, i.e., the path that the molecule takes between the initial and final states. This, however, poses a tremendous technological challenge because it requires an imaging method with femtosecond temporal resolution. The imaging method must also be able to capture the molecular structure with sub-angstrom spatial resolution.

With the development of femtosecond lasers, it has become possible to probe molecules on the relevant timescales.^1–3 Spatial information with the necessary resolution, however, is still not available. Laser-based probes can typically be used to measure changes in energy levels. From this information—with the aid of theoretical and computation models—the structure of a molecule can be inferred. Although, in principle, the structure and dynamics of isolated molecules in the gas phase can be captured with electron diffraction methods,^{4, 5} this technique is hampered by the random orientation of molecules that occurs in the gas phase. The information content of the diffraction patterns is therefore reduced, and the molecular structure is only accessible when the molecules are aligned.

Through our work, we have shown—by aligning molecules with a femtosecond laser pulse—that it is possible to capture a diffraction pattern from aligned molecules and to retrieve the molecular structure.⁶ In our method, we use the laser to apply an impulsive torque on the molecules. This is achieved through a non-resonant induced-dipole interaction. The molecules become aligned shortly after they receive a ‘kick’ from the laser pulse. Our methodology presents some advantages, i.e., the molecules remain aligned after the laser pulse has passed, and they can be probed outside the laser field (which can otherwise cause distortion of the measurements). Nonetheless, the alignment is short-lived and typically lasts about a picosecond. The transient nature of this alignment means that a resolution of a picosecond, or better, is required to capture the diffraction at the correct moment. The duration of electron pulses on a target is limited by the repulsive Coulomb force, which exists between the electrons that make up each laser pulse. We generate the electron pulses by shining a femtosecond laser pulse onto a photocathode and typically emit only a few thousand electrons per pulse so that we can minimize the pulse broadening. We then use a static electric field to accelerate the photoemitted electrons to their final kinetic energy. New technologies (i.e., that use relativistic electrons and compression cavities) have recently become available for ultrafast electron diffraction (UED) experiments.^{7, 8} We can therefore use these new technologies to further improve the temporal resolution of our approach.

In the first successful demonstration of our technique, we retrieved the structure of trifluoroiodomethane (CF₃I) from diffraction patterns of laser-aligned molecules.⁶ We achieved alignment of the molecules with the use of a 200fs laser pulse that had a wavelength of 800nm. We reached a resolution of 850fs in this diffraction experiment, which represented the first sub-picosecond UED experiment in the gas phase. Although it is straightforward to retrieve the structures of molecules when they are perfectly aligned, for the case of laser-induced alignment the angular distribution is fairly broad along the alignment axis. To successfully obtain the structure, we have thus developed a new retrieval algorithm. In this new method, we use a genetic algorithm to combine multiple diffraction patterns—each of which is only partially aligned—which correspond to different projections of the molecules.⁹

One of the important challenges that remains in fully implementing our molecule-alignment diffraction technique is to better characterize the effects of intense pulses on the alignment and structure of the molecules. As such, we have used UED to capture the structure of carbon disulfide (CS₂) molecules shortly after exposing them to the laser.¹⁰ We observed that the molecular alignment became saturated, and the molecular structure was distorted, when the laser intensity was increased beyond a certain threshold.

We have also used femtosecond laser mass spectrometry to independently measure the ionization of the molecules. We thus determined that structural deformation occurs at laser intensities below the ionization threshold. We attribute this structural deformation to excited electronic states that are reached by absorption of multiple photons. The lifetime of the excited states is less than a picosecond. In this process, the excited molecules had significantly longer bond lengths. In addition, the excitation caused bond breaking (dissociation) in a fraction of the molecules (see Figure 1). The position of the spots in our reconstructed images can be used to determine distances within the molecules. The reduced relative brightness of the upper spot in Figure 1(c), compared with Figure 1(b), indicates that a carbon–sulfur fragment was formed by dissociation of the CS₂ molecule (under high-intensity laser excitation). Furthermore, our ionization measurements—combined with our diffraction results—show that the dissociation of the molecule produced neutral fragments (i.e., additional evidence that the bond breaking is caused by electronic excitation rather than by ionization).

Figure 1. Images of carbon sulfide (CS₂) molecules. (a) Ball-and-stick model of the molecule (carbon and sulfur atoms are represented by the gray and orange balls, respectively). (b) Ultrafast electron diffraction (UED) image of the CS₂molecule, obtained under low-intensity laser excitation. The observed structure is similar to that of the molecule's ground state. The upper spot corresponds to the sulfur–sulfur molecular distance, and the lower spot indicates the carbon–sulfur distance. (c) UED image of the CS₂ molecule, obtained under high-intensity laser excitation. The distance between the atoms is longer in this case. In addition, the upper spot is dimmer, which indicates that a fraction of the molecules have dissociated.

We have shown that femtosecond electron pulses can be used to retrieve images of short-lived molecular structures.¹¹ In a series of tests, we have fully characterized the effects of intense laser pulses on model molecules. We observed alignment, deformation, dissociation, and ionization of the target molecules. We are now planning to conduct further experiments on larger molecules that will allow us to generalize the results of our technique.