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[Perspective] Detecting molecular hydrogen on Enceladus

Science

Planetary bodies with global oceans are prime targets in the search for life beyond Earth owing to the essential role of liquid water in biochemical reactions that sustain living organisms. In addition to water, life requires energy and a source of essential chemical elements (C, H, N, O, P, and S). Although there is compelling evidence for liquid water and many of the essential elements on several ice-covered planetary bodies in our solar system and beyond, direct observation of energy sources capable of fueling life has, to this point, remained elusive. On page 155 of this issue, Waite et al. (1) report that recent flybys of the ice-covered saturnian moon Enceladus by the Cassini spacecraft reveal the presence of molecular hydrogen (H2) in jets of vapor and particles ejected from a liquid water ocean through cracks in the ice shell. The abundance of H2 along with previously observed carbonate species suggests a state of chemical disequilibria in the Enceladus ocean that represents a chemical energy source capable of supporting life.


Rotary and linear molecular motors driven by pulses of a chemical fuel

Science

Many biomolecular motors catalyze the hydrolysis of chemical fuels, such as adenosine triphosphate, and use the energy released to direct motion through information ratchet mechanisms. Here we describe chemically-driven artificial rotary and linear molecular motors that operate through a fundamentally different type of mechanism. The directional rotation of [2]- and [3]catenane rotary molecular motors and the transport of substrates away from equilibrium by a linear molecular pump are induced by acid-base oscillations. The changes simultaneously switch the binding site affinities and the labilities of barriers on the track, creating an energy ratchet. The linear and rotary molecular motors are driven by aliquots of a chemical fuel, trichloroacetic acid.


The molecular wagon that stays on track

Science

Ever since the 2016 Nobel Prize in Chemistry was awarded “for the design and synthesis of molecular machines,” growing attention has been aimed at connecting and embedding molecular machines into hierarchically more complex systems for sophisticated applications. Such connections would, for example, enable the targeted delivery of molecules over larger distances. An important question that arises in this context is what kind of transporter would do such a job, over what distances, and with what degree of precision. On page 957 of this issue, Civita et al. ([ 1 ][1]) found a distinct example of a bromine-terminated terfluorene molecule on a metal surface that can be sent and received deliberately across more than a hundred nanometers with atomic precision. The bromine-terminated terfluorene molecules can be moved easily on a Ag(111) support by applying electric fields between the surface and a scanning tunneling microscopy (STM) tip. The close-packed rows of the metal support essentially act as railway tracks for the molecular wagons. To characterize this movement on the single-molecule level, the authors had to sense the molecule at specific start and end points. This required bringing the molecule on the desired track and locally inducing motion along that track. The beauty of the experiment by Civita et al. lies in the successful application of a two-probe STM setup: Each tip provides a separate probe and manipulation device at the start and end points of the movement. In addition, each tip can individually generate local fields. The experiment thus constitutes a near-ideal sender–receiver setup where atomic-scale cargo is transmitted. Previous experiments have already demonstrated that molecules can be accelerated along a surface using an STM tip ([ 2 ][2]) and that preferential diffusion directions can be accessed by rotating the molecules ([ 3 ][3], [ 4 ][4]), but they have never reached this long-distance precision and high level of motion control. The interaction of the bromine-terminated terfluorene molecule with the silver surface is unusually well suited for this application. The linear molecule consists of three stiff fluorene units linked by rotationally flexible junctions and is not bonded strongly to a particular adsorption site on the surface. The reason lies in lateral methyl distancers that lift the molecule, while guiding it with its long axis along the track of the close-packed rows. However, keeping the molecule safely on the track is only achieved by the additional terminal bromine substituents, which add a further interaction to the support atoms. Furthermore, the molecule–surface system has a permanent dipole moment pointing out of the surface that is the handle to induce lateral movement. The authors convincingly show that the electric fields can be used to induce the movement from afar (150 nm) by both repulsion and attraction. The fortunate combination of all these properties leads to the particular mobility observed here, constituting a seminal quality and thus opening up important questions with regard to the design of hierarchical nanomachineries. An interesting part of the experiment is that all molecules initially adsorb in locked-in, static orientations, upon deposition at low temperatures. Only after rotation by STM manipulation do the molecules snap to the track and become mobile. They reach a state with low lateral diffusion barriers along their long axis, which might point to incommensurability. One could speculate that the difficulty of dissipating the excess adsorption energy in this orientation is what keeps the molecules from adsorbing in the highly mobile orientation in the first place. Additionally, this might be the reason for the concomitant facile diffusion. The role of rotational flexibility along the long axis and of internal vibrational modes for the molecules' mobility will be interesting to assess. From previous studies, we know that these aspects influence the long jumps in the diffusion path ([ 5 ][5]) and modulate the degree of incommensurability that can lead to particular diffusion behaviors, such as, for example, Lévy flights ([ 6 ][6]). In Civita et al. 's experimental setup, such an influence can be studied by systematic structural variation of the molecules. The two-tip setup, however, reaches its time resolution limit when it comes to understanding in more detail the acceleration and maximal velocities that the wagons can reach. For quantitative access to the thermally activated diffusion properties, helium spin-echo measurements would be the way forward ([ 7 ][7]). However, observations of this type require a considerable population of molecules moving along tracks in an equilibrium state, at temperatures where immobilization at steps and island edges can be overcome. Civita et al. show that the possibility exists to reliably move molecules along tracks. Moving closer toward nanomachineries will require implementation into more-complex systems, opening up several challenges. Similar track–molecule systems have to be realized on functionalized supports. Also, reversible cargo attachment to the nanowagons and an adequate handover process need to be established. And easily addressable or alternative acceleration mechanisms that can be actioned by nanomachines have to be explored. Nanomachines might generate the required local fields or induce chemical reactions that steer the cargo transport by energy release into distinct degrees of freedom. One might even envisage the use of light to photostimulate reactions or to create optically rectified fields in plasmonic nanoreactors that control the reactant supply. The possibilities are boundless—exciting times lie ahead. 1. [↵][8]1. D. Civita et al ., Science 370, 957 (2020). [OpenUrl][9][Abstract/FREE Full Text][10] 2. [↵][11]1. S. W. Hla, 2. K. F. Braun, 3. B. Wassermann, 4. K. H. Rieder , Phys. Rev. Lett. 93, 208302 (2004). [OpenUrl][12][CrossRef][13][PubMed][14] 3. [↵][15]1. R. Otero et al ., Nat. Mater. 3, 779 (2004). [OpenUrl][16][CrossRef][17][PubMed][18][Web of Science][19] 4. [↵][20]1. B. Vasić et al ., Nanoscale 10, 18835 (2018). [OpenUrl][21] 5. [↵][22]1. K. D. Dobbs, 2. D. J. Doren , J. Chem. Phys. 97, 3722 (1992). [OpenUrl][23] 6. [↵][24]1. W. D. Luedtke, 2. U. Landman , Phys. Rev. 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High-precision molecular measurement

Science

The hydrogen molecular ion (H2 + ≡ p+ + p+ + e−) is the simplest molecule with two protons bound by an electron. Historically, it was the first molecule to be studied by using quantum mechanics, and it remains on the short list of experimentally accessible molecules for which a truly precise theoretical understanding is possible. However, several characteristics make precision optical spectroscopy of H2+ a formidable challenge in laboratory experiments. Hydrogen deuteride molecular ion (HD+ ≡ p+ + d+ + e−), in which one of the protons of H2+ is replaced by a deuteron ([ 1 ][1]–[ 3 ][2]), has an asymmetric dipolar structure that allows numerous vibrational-rotational transitions. These “rovibrational” transitions (νrv) have exceptionally narrow relative widths of less than 10−13 and occur at much higher rates when compared to the even narrower H2+ transitions. On page 1238 of this issue, Patra et al. report two frequencies with a precision of 2.9 parts per trillion and determine the mass ratio between the proton and electron ([ 2 ][3]). Quantum electrodynamics (QED) is the relativistic quantum field theory that describes the electromagnetic interaction, which is among the four known fundamental interactions of nature. QED reveals the forces that act between the bound electron, proton, and deuteron in HD+ that arise from an infinite series of elementary processes of progressively higher complexity. These involve the exchange of virtual photons that exist as transient quantum fluctuations of the underlying electromagnetic field. The fluctuations can temporarily transform into pairs of virtual electrons and positrons that immediately annihilate back into photons. This gives rise to minute but measurable changes in the structure of HD+. Some processes involving multiple virtual particles that cause sub–parts-per-billion scale shifts in the HD+ energies have taken a decade to calculate ([ 2 ][3]–[ 5 ][4]). Despite these difficulties, QED remains the most stringently tested part of the standard model. The authors compared their measured HD+ frequencies with the results of QED calculations. Under the assumption that there are no deviations from QED predictions, the authors determined the proton-to-electron mass ratio M p/ m e with a precision of 21 parts per trillion. This value lies within 30 and 350 parts per trillion of other experiments that instead measured the characteristic motions of a proton ([ 6 ][5]) or a H2+ ion ([ 7 ][6]) confined within the magnetic fields of ion traps. The result is also in excellent agreement with the ratio determined to a similar precision by a recent measurement carried out in Düsseldorf ([ 3 ][2]) of several hyperfine components of a HD+ rotational transition. So high a consistency between multiple experiments at the forefront of precision measurements is unusual. The experiment required samples of the reactive HD+ ions to be isolated in an ultrahigh-vacuum environment and cooled to temperature T ≈ 10 mK to minimize the Doppler broadenings of the spectral resonances due to the thermal motions. The authors achieved this by first confining a cloud of beryllium ions (Be+) in the oscillating electric field of a radiofrequency ion trap. The Be+ ions were irradiated with an ultraviolet laser beam, so that higher-velocity ions would scatter more laser photons. This velocity-selective scattering eventually cooled an ensemble of ≈1000 Be+ ions into the ordered structure of a so-called “Coulomb crystal” ([ 8 ][7]). The HD+ ions were suspended in the center of the crystal and allowed to thermalize (see the figure). The ions were then irradiated with two counterpropagating laser beams with infrared frequencies ν1 and ν2 that excited the HD+ transition when the sum ν1 + ν2 was tuned to νrv. The motion of each HD+ ion in the trap was strongly confined within its own micrometer-sized volume, which allowed the observation of particularly narrow spectral lines. Although the early pioneers ([ 1 ][1]) realized the potential of HD+ experiments to eventually determine the physical constants, the numerous degrees of freedom in a three-body molecule made the theoretical evaluation vastly complicated. At the time, the HD+ molecular frequencies were typically calculated with parts-per-million scale precision. This appeared to limit any determination of the proton-to-electron mass ratio to a similar precision. Development of computational techniques based on variational trial functions that included the molecular degrees of freedom occurred in the 1980s. These techniques were used to study muonic molecular heavy hydrogen ions [(ddµ)+ ≡ d+ + d+ + µ− and (dtµ)+ ≡ d+ + t+ + µ−] to estimate some of the reaction rates relevant for the possibility of energy production by muon-catalyzed fusion. The methods were used to calculate the transition frequencies of neutral antiprotonic helium atoms ( p ̄He+ ≡ p ̄ + He2+ + e−) ([ 4 ][8], [ 5 ][4]), which eventually allowed the determination of the antiproton-to-electron mass ratio to a precision of 8 parts in 1010 ([ 9 ][9]). Advances in the calculations and measurements of the HD+ frequencies ([ 2 ][3]–[ 4 ][8]) cumulated in the 2 parts per 1011 determination of the M p/ m e ratio. Several advances in fundamental physics could result from these observations. Other physical constants such as the Rydberg constant, the charge radii of protons and deuterons ([ 10 ][10]–[ 13 ][11]), and the deuteron-to-electron mass ratio ([ 14 ][12]) may eventually be determined. The charge radii are especially interesting, as deviations of up to 4% have been reported among the results of a few experiments ([ 10 ][10]–[ 13 ][11]). Some of these physical constants until recently could only be precisely determined on the basis of either the elegant simplicity of a single proton confined in an ion trap ([ 6 ][5]) or two-body systems, such as atomic hydrogen (H ≡ p+ + e−) ([ 10 ][10]–[ 12 ][13]), muonic hydrogen and deuterium atoms (µH ≡ p+ + µ− and µD ≡ d+ + µ−) ([ 13 ][11]), or hydrogenic carbon ions (12C5+ ≡ 12C6+ + e−) ([ 15 ][14]). Upper limits have also been set on phenomena that may cause deviations from the predictions of QED like the possible existence of a fifth fundamental force that may act between the constituent particles of HD+ ions ([ 3 ][2]). 1. [↵][15]1. W. H. Wing, 2. G. A. Ruff, 3. W. E. Lamb, 4. J. J. Spezeski , Phys. Rev. Lett. 36, 1488 (1976). [OpenUrl][16] 2. [↵][17]1. S. Patra et al ., Science 369, 1238 (2020). 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