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X-Rays Put Molecules Into a Spin

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X-Rays Put Molecules Into a Spin COMMENTARY COMMENTARY X-rays put molecules into a spin John T. Costelloa,b,1 and Steven T. Mansonc The past decade has witnessed the emergence of a electronic and rotational spectroscopies in a combined new X-ray light source technology that has been under way to elicit information about ultrafast molecular rota- development for some decades: the so-called self- tion. The method involves photoionizing an inner shell amplified spontaneous emission-based free-electron of a molecule with X-rays, thereby ejecting photoelec- laser (FEL). Such lasers provide pulses with durations trons. Owing to the conservation of momentum, the − from just below 1 fs (10 15 s) to over 100 fs and with photoelectron release is accompanied by the recoil of energies from a few microjoules to a few millijoules, the molecular ion left behind. At high X-ray photon permitting the study of, for example, nonlinear optical energy, the excess energy transferred in this recoil is effects (1), ultrafast processes (2), coherent diffractive translated partially into ultrafast molecular rotation; in imaging of biological systems such as protein crystals this study, the transfer is on the femtosecond timescale (3, 4), and mimivirus (5). After the opening of the first (Fig. 1). In effect, the dynamics of the molecular rotation extreme-UV to soft X-ray user facility, namely FLASH in are sensitive to the recoil, which in turn is dependent on the Deutsches Elektronen-Synchrotron at Hamburg, the energy of the photoelectron released in the first Germany (6), a number of other machines covering (ionization) step. the extreme-UV (FERMI FEL-1, DALIAN), soft X- In a traditional pump-probe experiment, these ray (FERMI FEL-2, FLASH1/FLASH2), and hard X-ray dynamics would be initiated by the ultrashort X-ray spectral ranges (LCLS, PAL-XFEL, SACLA, and XFEL) pulse and probed by a second pulse to which a have been opened to users from a wide range of sci- variable delay could be applied. However, it has been entific and engineering disciplines. Many more are well established that X-ray FEL pulses exhibit an under construction (e.g., SwissFEL and the Shanghai inherent time jitter on the order of 100 fs (FWHM) FEL), with an up-to-date list available in ref. 7. In when synchronized to other optical lasers (9). One can PNAS, C ´eolinet al. (8) focus on an experimental meth- overcome this limitation by splitting each individual X- odology aimed at the study of ultrafast molecular ro- ray pulse into two parts and optically delaying one tation; the rotation is initiated by X-ray pulses, from a with respect to the other (10, 11) or by creating two synchrotron, that are photoabsorbed by the molecule time-delayed electron bunches from a single electron and probed by electrons emitted from the molecule. bunch using a slotted foil (12, 13), which results in the Below, the innovative nature of the study is explored; formation of a pair of corresponding time-delayed X- the basic premise of the experiment is explained, ray pulses (14). along with why this study is important and likely to However, C ´eolinet al. (8) show that ultrafast rota- be widely used in the future. tional dynamics can be probed by another ionization To begin with, molecules are interesting beasts process, the Auger effect, in which the inner atomic because they have three modes of energy storage and shell hole created by the X-ray photoionization pro- transfer: electronic, vibrational, and rotational transi- cess is filled by the relaxation of an outer-shell elec- tions. The transitions among the states of each mode tron, with the excess energy created being used to occur, in general, at vastly different energies and rather further ionize the atom, resulting in the emission of different timescales. Traditionally, experimental inves- an Auger electron (15, 16). While the motion of mo- tigations employed electronic, vibrational, or rotational lecular electrons occurs on the attosecond timescale spectroscopies individually to explore molecular phe- (17), the Auger decay (and associated electron emis- nomena, although certain studies did combine two of sion) takes place on the femtosecond scale, which is of these methods. C ´eolin et al. (8) describe a method with the same order of magnitude as the ultrafast molecu- a twist, employing the differing characteristics of lar rotation. The process is statistical, with an average aSchool of Physical Sciences, Dublin City University, Dublin 9, Ireland; bNational Centre for Plasma Science & Technology, Dublin City University, Dublin 9, Ireland; and cDepartment of Physics and Astronomy, Georgia State University, Atlanta, GA 30303 Author contributions: J.T.C. and S.T.M. wrote the paper. The authors declare no conflict of interest. Published under the PNAS license. See companion article on page 4877. 1To whom correspondence should be addressed. Email: [email protected]. Published online February 19, 2019. 4772–4773 | PNAS | March 12, 2019 | vol. 116 | no. 11 www.pnas.org/cgi/doi/10.1073/pnas.1900971116 Downloaded by guest on September 30, 2021 Fig. 1. Photoemission initiated ultrafast molecular rotation probed by a delayed and Doppler-shifted Auger electron. lifetime between the ejection of the photoelectron and the ap- method yields not only a measurement of the ultrafast rotational pearance of the Auger electron known as the Auger (core-hole) state of a molecule, but also a way to control which rotational state lifetime, which, by the Heisenberg Uncertainty Principle, results in the molecular ion ends up in by simply varying the X-ray photon a finite kinetic energy width as measured by an electron spectrom- energy. There are, however, some challenges for the technique. eter. This lifetime can be used as a sort of clock to probe any For example, at high photon energies, the rotational dynamics changes in the timescales of ultrafast processes, such as the rota- may become nonlinear, which will have to be accounted for. tional dynamics reported in C ´eolinet al.’s paper. In short, the method developed by C ´eolinet al. (8) to both This is the essence of the method: The Auger electron line study and control the ultrafast rotation of molecular ions uses a shape is asymmetric owing to the rotation of the molecule in the combination of inner-shell X-ray photoionization and Auger elec- time between the recoil (when the photoelectron is emitted) and tron spectroscopy. The technique provides a clever twist on the the emission of the Auger electron (filling of the inner-shell hole)— usual pump-probe experiment: Although the pump (the pulse of that is, the core-hole lifetime. This asymmetry depends on how X-ray photons) is generated from an external source, the probe fast the molecule is rotating in that time interval, essentially a (the Auger electron) is ejected from the target molecule itself. rotational Doppler effect. Thus, monitoring the Auger line shape The effect is quite fundamental and should be borne in mind for allows the determination of the rotational state of the molecular Auger electron studies on molecules at high photon energies, ion after the photoionization process. In addition, by varying even for larger molecules. Finally, although carbon monoxide is the energy of the X-ray photons, the photoelectron energy is investigated in the C ´eolinet al. (8) report, this method, by its also varied, which leads to the differing rotational velocities and, nature, should be general and useful for a wide variety of small hence, different rotational states of the molecular ion. Thus, this molecular systems. 1 Berrah N, et al. (2010) Non-linear processes in the interaction of atoms and molecules with intense EUV and X-ray fields from SASE free electron lasers (FELs). J Mod Opt 57:1015–1040. 2 Beyerlein KR, et al. (2018) Ultrafast nonthermal heating of water initiated by an X-ray free-electron laser. Proc Natl Acad Sci USA 115:5652–5657. 3 Opara NL, et al. (2018) Demonstration of femtosecond X-ray pump X-ray probe diffraction on protein crystals. Struct Dyn 5:054303. 4 Wiedorn MO, et al. (2018) Megahertz serial crystallography. Nat Commun 9:4025. 5 Ekeberg T, et al. (2015) Three-dimensional reconstruction of the giant mimivirus particle with an x-ray free-electron laser. Phys Rev Lett 114:098102. 6 Ackermann W, et al. (2007) Operation of the free electron laser FLASH in the water window. Nat Photonics 1:336–342. 7 Neyman PJ, et al. Free electron lasers in 2017. 38th International Free Electron Laser Conference (JACoW Publishing, Santa Fe, NM). Available at accelconf.web. cern.ch/AccelConf/fel2017/papers/mop066.pdf. Accessed January 27, 2019. 8 C´eolin D, et al. Recoil-induced ultrafast molecular rotation probed by dynamical rotational Doppler effect. Proc Natl Acad Sci USA 116:4877–4882. 9 Schulz S, et al. (2015) Femtosecond all-optical synchronization of an X-ray free-electron laser. Nat Commun 6:5938. 10 Fang L, et al. (2017) X-ray pump–probe investigation of charge and dissociation dynamics in methyl iodine molecule. Appl Sci 7:529. 11 Inoue I, et al. (2016) Observation of femtosecond X-ray interactions with matter using an X-ray-X-ray pump-probe scheme. Proc Natl Acad Sci USA 113:1492–1497. 12 Ding Y, et al. (2015) Generating femtosecond X-ray pulses using an emittance-spoiling foil in free-electron lasers. Appl Phys Lett 107:191104. 13 Christoph Bostedt C, et al. (2016) Linac Coherent Light Source: The first five years. Rev Mod Phys 88:015007. 14 Hoffmann MC, et al. (2018) Femtosecond profiling of shaped x-ray pulses. New J Phys 20:033008. 15 Young L, et al. (2010) Femtosecond electronic response of atoms to ultra-intense X-rays.
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