X-rays put molecules into a spin

The past decade has witnessed the emergence of a new X-ray light source technology that has been under development for some decades: the so-called self-amplified spontaneous emission-based free-electron laser (FEL). Such lasers provide pulses with durations from just below 1 fs (10⁻¹⁵ s) to over 100 fs and with energies from a few microjoules to a few millijoules, permitting the study of, for example, nonlinear optical effects (1), ultrafast processes (2), coherent diffractive imaging of biological systems such as protein crystals (3, 4), and mimivirus (5). After the opening of the first extreme-UV to soft X-ray user facility, namely FLASH in the Deutsches Elektronen-Synchrotron at Hamburg, Germany (6), a number of other machines covering the extreme-UV (FERMI FEL-1, DALIAN), soft X-ray (FERMI FEL-2, FLASH1/FLASH2), and hard X-ray spectral ranges (LCLS, PAL-XFEL, SACLA, and XFEL) have been opened to users from a wide range of scientific and engineering disciplines. Many more are under construction (e.g., SwissFEL and the Shanghai FEL), with an up-to-date list available in ref. 7. In PNAS, Céolin et al. (8) focus on an experimental methodology aimed at the study of ultrafast molecular rotation; the rotation is initiated by X-ray pulses, from a synchrotron, that are photoabsorbed by the molecule and probed by electrons emitted from the molecule. Below, the innovative nature of the study is explored; the basic premise of the experiment is explained, along with why this study is important and likely to be widely used in the future.

To begin with, molecules are interesting beasts because they have three modes of energy storage and transfer: electronic, vibrational, and rotational transitions. The transitions among the states of each mode occur, in general, at vastly different energies and rather different timescales. Traditionally, experimental investigations employed electronic, vibrational, or rotational spectroscopies individually to explore molecular phenomena, although certain studies did combine two of these methods. Céolin et al. (8) describe a method with a twist, employing the differing characteristics of electronic and rotational spectroscopies in a combined way to elicit information about ultrafast molecular rotation. The method involves photoionizing an inner shell of a molecule with X-rays, thereby ejecting photoelectrons. Owing to the conservation of momentum, the photoelectron release is accompanied by the recoil of the molecular ion left behind. At high X-ray photon energy, the excess energy transferred in this recoil is translated partially into ultrafast molecular rotation; in this study, the transfer is on the femtosecond timescale (Fig. 1). In effect, the dynamics of the molecular rotation are sensitive to the recoil, which in turn is dependent on the energy of the photoelectron released in the first (ionization) step.

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Fig. 1.

Photoemission initiated ultrafast molecular rotation probed by a delayed and Doppler-shifted Auger electron.

In a traditional pump-probe experiment, these dynamics would be initiated by the ultrashort X-ray pulse and probed by a second pulse to which a variable delay could be applied. However, it has been well established that X-ray FEL pulses exhibit an inherent time jitter on the order of 100 fs (FWHM) when synchronized to other optical lasers (9). One can overcome this limitation by splitting each individual X-ray pulse into two parts and optically delaying one with respect to the other (10, 11) or by creating two time-delayed electron bunches from a single electron bunch using a slotted foil (12, 13), which results in the formation of a pair of corresponding time-delayed X-ray pulses (14).

However, Céolin et al. (8) show that ultrafast rotational dynamics can be probed by another ionization process, the Auger effect, in which the inner atomic shell hole created by the X-ray photoionization process is filled by the relaxation of an outer-shell electron, with the excess energy created being used to further ionize the atom, resulting in the emission of an Auger electron (15, 16). While the motion of molecular electrons occurs on the attosecond timescale (17), the Auger decay (and associated electron emission) takes place on the femtosecond scale, which is of the same order of magnitude as the ultrafast molecular rotation. The process is statistical, with an average lifetime between the ejection of the photoelectron and the appearance of the Auger electron known as the Auger (core-hole) lifetime, which, by the Heisenberg Uncertainty Principle, results in a finite kinetic energy width as measured by an electron spectrometer. This lifetime can be used as a sort of clock to probe any changes in the timescales of ultrafast processes, such as the rotational dynamics reported in Céolin et al.’s paper.

This is the essence of the method: The Auger electron line shape is asymmetric owing to the rotation of the molecule in the time between the recoil (when the photoelectron is emitted) and the emission of the Auger electron (filling of the inner-shell hole)—that is, the core-hole lifetime. This asymmetry depends on how fast the molecule is rotating in that time interval, essentially a rotational Doppler effect. Thus, monitoring the Auger line shape allows the determination of the rotational state of the molecular ion after the photoionization process. In addition, by varying the energy of the X-ray photons, the photoelectron energy is also varied, which leads to the differing rotational velocities and, hence, different rotational states of the molecular ion. Thus, this method yields not only a measurement of the ultrafast rotational state of a molecule, but also a way to control which rotational state the molecular ion ends up in by simply varying the X-ray photon energy. There are, however, some challenges for the technique. For example, at high photon energies, the rotational dynamics may become nonlinear, which will have to be accounted for.

In short, the method developed by Céolin et al. (8) to both study and control the ultrafast rotation of molecular ions uses a combination of inner-shell X-ray photoionization and Auger electron spectroscopy. The technique provides a clever twist on the usual pump-probe experiment: Although the pump (the pulse of X-ray photons) is generated from an external source, the probe (the Auger electron) is ejected from the target molecule itself. The effect is quite fundamental and should be borne in mind for Auger electron studies on molecules at high photon energies, even for larger molecules. Finally, although carbon monoxide is investigated in the Céolin et al. (8) report, this method, by its nature, should be general and useful for a wide variety of small molecular systems.