Review of “Molecules in superfluid helium nanodroplets: Spectroscopy, structure, and dynamics,” edited by Alkwin Slenczka and Jan Peter Toennies, Volume 145, Topics in Applied Physics, Springer Publishing Co., New York, 2022

A. Slenczka, Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Regensburg, Germany. J. Peter Toennies, Max-Planck-Institut für Dynamik and Selbstorganisation and Universität Göttingen, Göttingen, Germany. A brief preface gives the historical background of the field. Cryogenic matrices as molecular sample holders were first introduced in the 1950s by George Pimentel, Herbert Broida, and coworkers for stabilizing reactive species in low-temperature, nonreactive solids and studying them using a variety of techniques, including optical and infrared spectroscopy. In 1977, the seeded beam method was introduced by Lennard Wharton, Donald Levy, and Richard Smalley as an alternative to matrix isolation. In this method, gas phase molecules were cooled to low temperatures by expanding them in an excess of inert gas into a vacuum, and then probed by optical methods, usually with lasers. Then, over a period of several years, studies of helium expansions by mass spectrometry revealed the presence of large clusters of He atoms, the possibility that these clusters might be doped with other species, and the discovery that single molecules could be inserted into droplets where they were free to rotate. A dramatic early discovery was the finding in 1992 by Fröchtenicht and Vilesov in Göttingen that SF6 in droplets of several thousand He atoms exhibited well-resolved rotational lines of the P- and R-branches of an infra-red (IR) spectrum centered around a sharp Q-branch. As this compendium shows, superfluid helium nanodroplets (HNDs) have opened up a wide, new era of high-resolution spectroscopy, resulting from the exposure of a large variety of species to the low-temperature environment of the droplets and its gentle nature. Spectroscopic studies of the doped droplets have provided a new perspective on the physics and chemistry of atoms, molecules (both inorganic and organic), and even large biomolecules, also in the form of clusters and large aggregates. New information has been obtained about the reactions and structures of radicals and ions, and the photoexcitation dynamics of large molecules. At the same time, the observed spectra have provided rare insight into the microscopic many-body coherent quantum physics of the helium superfluid. For example, the first evidence that small finite-sized objects with even less than 100 helium atoms can be superfluid comes from this new spectroscopy. This is the first book in 25 years that provides an overview of this field. My hope is that the reader will find as much enjoyment in reading about these findings as I have in my review of these impressive accomplishments. The fact that the book is an “open-access” book will greatly improve its availability to all. Reviewed here are the history of superfluidity in helium, of helium as a cryomatrix for spectroscopy, the thermodynamic properties of liquid helium, formation and characterization of nanodroplets (including a detailed discussion of isentropes and size distributions), physical properties of nanodroplets, and evidence for superfluidity in droplets. I particularly enjoyed reading about the role that spectroscopy played in providing information about these properties. Thus, as shown in Figure 1.15, the rotationally resolved vibration–rotation spectrum of OCS exhibits a Boltzmann distribution with a rotational temperature of 0.37 K, in excellent agreement with the results for SF6 in 4He droplets, as mentioned before. Additionally, it was found that the spacing of the rotational lines in these spectra is 2.8 times smaller than the corresponding spacing in the spectrum of the bare molecule, clearly indicating that the embedded molecule has a moment of inertia that is larger by the same amount. This phenomenon has been observed in several systems, which suggests that a number of He atoms are attached to the bare molecule and rotate with it. The data on these and other systems strongly suggest, but do not prove, that the droplets are superfluid. The first direct unequivocal experimental evidence that 4He droplets are indeed superfluid came in 1996 from electronic excitation spectrum of the S1 ← S0 transition of the glyoxal (C2H2O2) molecule embedded in a 5000-atom droplet, which exhibits a clearly peaked phonon wing, a hallmark of superfluidity (see Figure 1.16). Discussed here are the properties of small He clusters containing two or three He atoms. The clusters were prepared by expanding gaseous helium into vacuum through a nozzle with a 5 μm orifice. Effective cluster formation with yields up to 6% was found at nozzle temperatures below 30 K; they were sorted by mass using a transmission grating, as diffraction by the grating depends on their de Broglie wavelengths (a technique pioneered by the Toennies group). Then, in a Coulomb explosion experiment, the cluster is ionized by an fs laser pulse, and its constituents fly apart due to their mutual Coulomb repulsion. The momenta or kinetic energy that these charged molecular fragments (e.g., atomic ions) gain during this process depends on the initial distances between the atoms in the neutral molecule. Thus, by measuring the momenta of charged fragments, it is possible to determine the structure of the molecule. This chapter discusses the work on a mass spectrometric detection of HNDs performed by members of the research group of Paul Scheier at the University of Innsbruck, as well as by many other workers in the field. The first report on a mass spectrometric study of small helium clusters was made in 1975 by van Deursen and Reuss in Nijmegen. Later, in 1983, Stephens and King discovered magic numbers in small He cluster ions, finding n = 7, 10, 14, 23, and 30 to be anomalously abundant in free jet expansions. Then, in 1990, Scheidemann and coworkers, working in the Toennies group, demonstrated the HNDs ability to capture various foreign atoms and molecules, including Ne, Ar, Kr, H2, O2, H2O, CH4, and SF6. Large clusters (e.g., (H2O)18) also were observed, though it was not clear whether the clusters resided in or on the droplets. Subsequent studies by this and other groups revealed the possibility that chemical reactions might be occurring inside the droplets, stimulating much interest. The basic setup of an HND source, a pick-up region, and a mass spectrometer is proving to be a simple but efficient way to investigate pure HNDs, dopant clusters, ionization mechanisms, and chemical reactions inside HNDs. Described in this chapter are infrared (IR) studies of organic radicals and carbenes in HNDs, both in zero field and in the presence of applied electric or magnetic fields. Many of the species studied are products of prototype gas-phase reactions relevant to both combustion and atmospheric chemistry. A nice example is provided by an early study of the thermal dissociation of n-butyl nitrite in a pyrolysis source, which leads to the production of the propargyl radical (C3H3), nitric acid (HNO3), and formaldehyde (CH2O), all detected with a quadrupole mass spectrometer and the idler output from a continuous-wave optical parametric oscillator, counter-propagating to the droplet beam. Detailed studies of the IR spectra of these species, and similar bands of other small molecules in the presence of applied electric or magnetic fields, make possible the determination of their electric dipole moments and magnetic g-factors. Even for some larger species that do not exhibit resolved rotational structure, permanent dipole moments and vibrational transition moments can be determined by measuring the electric field dependence of the band intensity. More recent studies have employed new pyrolysis sources, making possible studies of the tunneling dynamics in the vinyl radical, and radical plus molecular oxygen reactions in HNDs. Investigations, by means of electronic spectroscopy, of mainly organic molecules embedded in helium droplets, of van der Waals clusters generated inside the droplets, and of chemical reactions taking place inside the droplets are described in the three main sections of this chapter. Systems studied include glyoxal (described earlier in this review), benzene and other polycyclic hydrocarbons, phthalocyanine, porphyrins, pyrromethene dyes, and van der Waals clusters of some of these species. Interesting findings include a doubling of the zero-phonon line (ZPL) in the spectrum of tetracene; similar origin band splittings in the spectra of other molecules (e.g., porphin) remain only partly explained. Studied chemical reactions include Ba with N2O, photolysis of iodomethane, and the excited state proton transfer reaction in 3-hydroxyflavone. All these experiments reveal a significant influence of the gentle, yet polarizable, helium environment on the appearance of the spectra, including solvation and the superfluid characteristics of the HNDs. Determination of the conformational, dynamic, and physicochemical properties of biologically important oligosaccharides, glycopeptides, and glycoproteins remains an important goal for science in the 21st century. In this report from the Fritz Haber Institute (FHI) in Berlin, we learn that substantial progress toward meeting this goal is being made using HNDs to both capture and cool biomolecular ions and to study them using IR action spectroscopy. In the experiment (see Figure 6.2), ions are generated by electro-spray ionization and transferred into high vacuum by two ion guides, where they are monitored by a time-of-flight mass spectrometer. Once the ions of interest are isolated by the quadrupole mass filter and the ion current is optimized, they are injected into a hexapole ion trap. The ion trap is traversed by a beam of HNDs that can pick up trapped ions and thermalize them to 0.4 K. The droplets then transport the embedded ions into the detection region, where they are irradiated by an IR laser beam produced by the FHI free-electron laser. Typical examples reveal the full power of this technique for the IR spectra of mono, di, tri, and (naturally occurring) tetrasaccharides, all of which are distinguishable over the region 950–1650 cm−1. The next challenge faced by researchers in this field is to develop theoretical methods that can be used to determine the structural candidates that might be responsible for the observed IR spectra. Reviewed here are the results of recent coherent diffraction imaging (CDI) experiments designed to study HND shape deformations, the configurations of dopant nanostructures, and their dynamic behavior following excitation by an intense laser pulse. Necessary parameters for CDI have been reached with recent technological developments of fourth-generation XUV and X-ray light sources, briefly described here. Images from static imaging are collected with one single pulse of the light beam and represent the instantaneous state of the droplet. These images describe the size and shape of a droplet, and, in the case of small scattering measurements of doped droplets, the different configurations of dopant nanoclusters assembled inside a droplet. Examples of pure and Xe-doped droplets are shown in Figure 7.5. Following in the footsteps of Ken Hedberg (an early pioneer in gas-phase electron diffraction at Oregon State), Prof. Kong and her research group have been developing a method called serial single-molecule electron diffraction imaging (SS-EDI) for determining the structures of large biological molecules and nanomaterials. The experiment begins with electrospray ionization to produce ions for doping into superfluid HNDs. The cooled ions are then aligned by an elliptically polarized laser field and subjected to radiation by high-energy electrons. The diffraction patterns from isolated molecules embedded in the droplets are accumulated as the sample is refreshed in repetitive pulses for the desired signal-to-noise (S/N) ratio. Three-dimensional information is obtained from diffraction images collected from different orientations of the sample achieved by different polarizations of the alignment laser beam. The first part of this chapter introduces some of the central concepts of laser-induced alignment of isolated molecules, helpfully illustrated by numerical and experimental examples (see Figure 9.1). Alignment of molecules describes a situation where one (or more) molecular axes of the rotating molecule is (are) confined with respect to a laboratory axis. Typically, the alignment axis is the most polarizable axis in the molecule. In the present experiments, alignment is produced by a nonresonant interaction of an applied laser field with this axis, which causes the molecule to rotate toward the direction in which this interaction is maximized. Although 3D alignments are possible using elliptically polarized pulses, this chapter discusses only the 1D case. Even in this “simple” case, it is possible to study or exploit the ubiquitous spatial dependence of a molecule's interaction with other molecules, atoms, or polarized light. Typical droplet experiments with such an arrangement (see Figure 9.5 for a diagram of a typical setup) are discussed next, which include the production of IHe+ ions from Coulomb exploded solvated I2 molecules using a ps laser, double ionization of I2 molecules and their explosion into 2I+, also produced by the ps laser, and nonadiabatic alignments using an fs laser. Other, more recent experiments on larger molecular dimers are also described, including experiments which show that molecules in HNDs can be aligned by laser pulses that are much shorter than the intrinsic rotational period of the molecules. Two regimes are identified; when the laser pulses are weak (long), the alignment dynamics can be accurately described by effective rotational constants and inhomogeneous broadening is taken into account; when the laser pulses are strong (short), the rotational dynamics during the first few ps is essentially as fast as for the isolated molecule, perhaps because weakly bonded He atoms are evaporated by the rapid motion. Being able to control the motion in this way may lead to many new imaging possibilities. In Freiburg, Stienkemeier and his group have used femtosecond (fs) pump–probe photoionization experiments to investigate the dynamics of atomic and molecular dopants, including coherent wavepacket dynamics and long-lived vibrational coherences of molecules attached to and immersed in HNDs. The experiments are performed in the several-fs time domain in order to access information about a wide variety of processes occurring in HNDs. In early experiments, the authors used time-correlated single-photon counting (TCSPC) techniques to study a number of atomic species in or on the surface of HNDs. In later experiments, the group has used fs pump–probe schemes to study exciplex formation in alkali monomers, dimers, and trimers; in a few instances, interactions of the He droplet with the vibrating molecules, such as In2Hen. causing dephasing and relaxation, were detected. More recently, TCSPC spectroscopy experiments made it possible to follow the time-course of reactions involving both In–Hen+ and In2+ in this system. Several new directions in droplet spectroscopy are also described in this chapter, such as the interference of different pathways leading to the same final state (called wave packet interferometry), and, more generally, coherent multidimensional spectroscopy, which combines the resolution advantage of wave-packet interferometry with the extended sensitivity to dynamics known from a variety of 2D spectroscopies, as in 2D-NMR, and so forth. The interplay of dopant and helium bath dynamics, and the development of new XUV and X-ray FEL facilities, will make possible many new studies of coherent motion in atomic and molecular systems in the future. This chapter describes the formation of metal and metal oxide nanoparticles within helium droplets, providing a unique medium for the synthesis of new materials, first exploited by several groups in the 1990s (see the references in this chapter) and now encompassing a large variety of different metals, all of which show similar structures. Typically, HNDs are doped in pickup regions containing resistively heated ovens, downstream of the Hen source. Spherical sub-10 nm particles are formed in smaller HNDs, whereas nanorods and nanowire structures are formed within large droplets. Figure 11.4 shows some examples. The fact that all of these materials (including alloys) have similar structures shows that their sizes and shapes are determined by the synthetic environment. Several different types of experiments have been performed using these materials, as they are readily deposited on various substrates. These include studies of several plasmonic materials, including Cu, Ag, and Au in HNDs, using nanoparticles deposited on glass coverslips. An important application of these materials is surface-enhanced Raman spectroscopy, a technique that exploits the enhancement of electromagnetic fields on or near the particles. Figure 11.19 shows a typical example; Ag nanoparticles, which exhibit a very strong plasmon resonance, give rise to the strongest signals and have been studied extensively by several groups, including that of Moskovits in Toronto and Santa Barbara. Electron-energy loss spectroscopy, IR, and laser-induced fluorescence spectroscopy have also been used to characterize these systems. And more recently, several groups have studied the properties of metal core–transition metal oxide shell nanoparticles, such as Ag@ZnO, which consist of Ag cores having a diameter of 3 nm and uniform ZnO shells having a thickness of 1.3 nm surrounding them (see Figure 11.28). These materials and others like them will be useful in many future plasmonic and photocatalysis experiments. The monograph concludes with an Appendix listing the relevant review articles that have been previously published, each with a short description of its content. None. The authors have no conflict of interest to declare. This Critique has been internally reviewed by Professors Gerard Meijer and Bretislav Friedrich. The peer review history for this article is available at https://publons.com/publon/10.1002/ntls.20220063. None. The authors confirm that they have followed the ethical policies of the journal.

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