An ultrafast X-ray source in laboratory format
In nature, some processes occur so quickly that even the blink of an eye is very slow in comparison. Many basic physical, chemical and biological reactions take place on the ultrafast time scale of a few femtoseconds (10−15 s) or even attoseconds (10−18 s). In molecules, elementary particles, such as electrons or photons, move in a mere 100 attoseconds (10−16 s). When electrons in a molecule jump from one atom to another, chemical bonds dissolve and new ones arise within a fraction of a femtosecond. The ability to track processes of this kind on the atomic scale in real time is one of the key reasons for development of major new research facilities such as the SwissFEL free electron laser. Now, researchers from the ETH Zurich and the University of Geneva have found a way to study ultrafast processes of this kind in the laboratory, using a soft X-ray source.Reference: Pertot, Y., C. Schmidt, M. Matthews, A. Chauvet, M. Huppert, V. Svoboda, A. von Conta, A. Tehlar, D. Baykusheva, J.-P. Wolf and H. J. Wörner (2017). Time-resolved x-ray absorption spectroscopy with a water window high-harmonic source. Science. (10.1126/science.aah6114) Pertot-2017 (1.13 MB)
January 17, 2017. More >>
Using X-rays to produce a movie of the photosynthesis reaction
Reference: Nango, E., A. Royant, M. Kubo, T. Nakane, C. Wickstrand, T. Kimura, T. Tanaka, K. Tono, C. Song, R. Tanaka, T. Arima, A. Yamashita, J. Kobayashi, T. Hosaka, E. Mizohata, P. Nogly, M. Sugahara, D. Nam, T. Nomura, T. Shimamura, D. Im, T. Fujiwara, Y. Yamanaka, B. Jeon, T. Nishizawa, K. Oda, M. Fukuda, R. Andersson, P. Båth, R. Dods, J. Davidsson, S. Matsuoka, S. Kawatake, M. Murata, O. Nureki, S. Owada, T. Kameshima, T. Hatsui, Y. Joti, G. Schertler, M. Yabashi, A.-N. Bondar, J. Standfuss, R. Neutze and S. Iwata (2016). A three-dimensional movie of structural changes in bacteriorhodopsin. Science 354: 1552 (10.1126/science.aah3497) Nango-2016 (1.52 MB)
January 3, 2017. More >>
Chemically Modified Insulin Is Available More Rapidly
Reference: El Hage, K., V. Pandyarajan, N. B. Phillips, B. J. Smith, J. G. Menting, J. Whittaker, M. C. Lawrence, M. Meuwly and M. A. Weiss (2016). Extending Halogen-Based Medicinal Chemistry to Proteins: Iodo-Insulin as a Case Study. J. Biol. Chem. (10.1074/jbc.M116.761015) El-Hage-2016 (2.42 MB).
November 14, 2016. More >>
Evidence that cations structure the hydrogen-bond network in water
Reference: Shalit, A., S. Ahmed, J. Savolainen and P. Hamm (2016). Terahertz echoes reveal the inhomogeneity of aqueous salt solutions. Nat. Chem. advance online publication. (10.1038/nchem.2642) Shalit-2016 (2.84 MB).
October 31, 2016. More >>
An Ultrafast Method to Track the Movement of Light and Electrons in Nanostructured Surface
Reference: Lummen, T. T. A., R. J. Lamb, G. Berruto, T. LaGrange, L. Dal Negro, F. J. García de Abajo, D. McGrouther, B. Barwick and F. Carbone (2016). Imaging and controlling plasmonic interference fields at buried interfaces. Nature Commun. 7: 13156. (10.1038/ncomms13156, http://www.nature.com/articles/ncomms13156 - suppl-info) Lummen-2016 (1.7 MB).
October 25, 2016. More >>
A new technique opens up advanced solar cells
Reference: Causa, M., J. De Jonghe-Risse, M. Scarongella, J. C. Brauer, E. Buchaca-Domingo, J.-E. Moser, N. Stingelin and N. Banerji (2016). The fate of electron–hole pairs in polymer:fullerene blends for organic photovoltaics. Nature Commun. 7: 12556. (10.1038/ncomms12556) Causa-2016 (26.44 MB)
September 2, 2016. More >>
Is there a material limit for high-speed electronics?
Reference: Lucchini, M., Sato, S. A., Ludwig, A., Herrmann, J., Volkov, M., Kasmi, L., Shinohara, Y., Yabana, K., Gallmann, L., and Keller, U. (2016). Attosecond dynamical Franz-Keldysh effect in polycrystalline diamond. Science 353, 6302, 916-919. DOI: 10.1126/science.aag1268, Lucchini-2016 (1.87 MB).
August 26, 2016. More >>
Attosecond Delays in Molecular Photoionization
Reference: Huppert, M., Jordan, I., Baykusheva, D., von Conta, A., and Wörner, H. J. (2016). Attosecond Delays in Molecular Photoionization. PhysRevLett.117.093001 Huppert-2016 (585 KB)
August 22, 2016. More >>
Catching proteins in the act with a lipidic cubic phase injector
Reference: Nogly, P., et al. (2016). Lipidic cubic phase injector is a viable crystal delivery system for time-resolved serial crystallography. Nature Commun. 7: 12314. 10.1038/ncomms12314 Nogly-2016 (1.87 MB).
August 22, 2016. More >>
Computer Simulation Renders Transient Chemical Structures Visible
Using computational chemistry, it is possible to characterize the motion of individual atoms of a molecule. Today, the latest simulation techniques allow scientists to quantitatively describe the dynamics of molecules and systems containing hundreds of thousands of atoms. These techniques are important, above all, for characterizing molecular states that are difficult to observe directly in experiments due to their short lifetime. Here, computer simulations are a source of valuable complementary insight.
Reference: Soloviov, M., A. K. Das and M. Meuwly (2016). Structural Interpretation of Metastable States in Myoglobin–NO. Angew. Chem. Int. Ed.: n/a-n/a. (10.1002/anie.201604552) Soloviov-2016 (2.28 MB)
July 14, 2016. More >>
Laser vaporization of cirrus-like ice particles with secondary ice multiplication
Reference: Matthews, M., F. Pomel, C. Wender, A. Kiselev, D. Duft, J. Kasparian, J.-P. Wolf and T. Leisner (2016). Laser vaporization of cirrus-like ice particles with secondary ice multiplication. Sci. Adv. 2. (10.1126/sciadv.1501912) Matthews-2016 (896 KB).
May 20, 2016. More >>
First measurement of multi-harmonics generation from a single nanoparticle
Reference: Schmidt, C., J. Riporto, A. Uldry, A. Rogov, Y. Mugnier, R. L. Dantec, J.-P. Wolf and L. Bonacina (2016). Multi-Order Investigation of the Nonlinear Susceptibility Tensors of Individual Nanoparticles. Sci. Rep. 6: 25415. (10.1038/srep25415) Schmidt-2016 (707 KB)
May 3, 2016. More >>
Dynamical Symmetries of Atoms and Molecules revealed by Bicircular High-Harmonic Spectroscopy
Reference: Baykusheva, D., M. S. Ahsan, N. Lin and H. J. Wörner (2016). Bicircular High-Harmonic Spectroscopy Reveals Dynamical Symmetries of Atoms and Molecules. Phys. Rev. Lett. 116: 123001. (10.1103/PhysRevLett.116.123001) Baykusheva-2016 (703 KB).
March 25, 2016. More >>
Electron transfer dynamics observed over 8 orders of magnitude in time
Charge transfer mechanisms lay at the heart of chemistry and biochemistry. Proton coupled electron transfers (PCET) are central in biological processes such as photosynthesis and in the respiratory chain, where they mediate long range charge transfers. These mechanisms are normally difficult to harness experimentally due to the intrinsic complexity of the associated biological systems. Metal-peptide cations experience both electron and proton transfers upon photo-excitation, proving an amenable model system to study PCET.
Reference: MacAleese, L., S. Hermelin, K. El Hage, P. Chouzenoux, A. Kulesza, R. Antoine, L. Bonacina, M. Meuwly, J.-P. Wolf and P. Dugourd (2016). Sequential Proton Coupled Electron Transfer (PCET): Dynamics Observed over 8 Orders of Magnitude in Time. J. Am. Chem. Soc. (10.1021/jacs.5b12587) MacAleese-2016 (807 KB).
March 23, 2016. More >>
Ptychographic reconstruction of attosecond pulses (free software)
In March 2016, the paper was choosen for inclusion in OSA Spotlight on Optics. From the OSA letter: "Spotlight on Optics (Spotlight) showcases research produced in our journals-research and information that would be impossible without your talent and contribution. Your paper is in excellent company. Only two papers are highlighted from our respective journals each month from among the scores of fine articles published."
In the context of this publication, the authors offer a minimal example MATLAB code that demonstrates the capabilities of this method based on four example data sets. Use of this software is free under the condition, that you include a reference to the original publication (below) whenever you make use of it.
Download software (10.56 MB).
November 4, 2015. More >>
Ionization Charge Dynamics Tracked
Led by ETH Zurich’s Hans Jakob Wörner, the researchers used a technique called high harmonic generation in which a laser pulse causes an electron to tunnel out and away from an atom—in this case, primarily the iodine of iodoacetylene. When the electron and hole recombine, the process releases a burst of attosecond-duration X-rays. If the molecule is perpendicular to the laser polarization field when it is ionized, the hole initially localizes on the iodine. The hole then delocalizes over the molecule before localizing on the carbons. If the molecule is parallel to the laser polarization field, the hole localizes mostly on the carbons.
Kraus, P.M., Mignolet, B., Baykusheva, D., Rupenyan, A., Horný, L., Penka, E.F., Grassi, G., Tolstikhin, O.I., Schneider, J., Jensen, F., Madsen, L.B., Bandrauk, A.D., Remacle, F., and Wörner, H.J. (2015) Measurement and laser control of attosecond charge migration in ionized iodoacetylene. Science 350, 790-795 (10.1126/science.aab2160) Kraus-20151 (1.64 MB)
October 22, 2015. More >>
Dissecting the electronic dynamics of a photovoltaic material
Santomauro, F.G., Lübcke, A., Rittmann, J., Baldini, E., Ferrer, A., Silatani, M., Zimmermann, P., Grübel, S., Johnson, J.A., Mariager, S.O., Beaud, P., Grolimund, D., Borca, C., Ingold, G., Johnson, S.L., and Chergui, M. (2015) Femtosecond X-ray absorption study of electron localization in photoexcited anatase TiO2. Scientific Reports 5, 14834 (10.1038/srep14834)
October 8, 2015. More >>
Tracking a biological process with atomic specificity
Silatani M, Lima FA, Penfold TJ, Rittmann J, Reinhard M, Rittmann-Frank H, Borca C, Grolimund D, Milne CJ, Chergui M. NO binding kinetics in Myoglobin investigated by picosecond Fe K-edge absorption spectroscopy. PNAS 05 October 2015. DOI: 10.1073/pnas.1424446112.
October 6, 2015. More >>
Understanding single-photon ionization dynamics - is the Wigner time delay valid?
Cirelli, Keller and co-workers demonstrate that the Wigner time delay (related to the electron wave packet group delay) can correctly explain the “classical trajectory” of the center of an electron wave packet only up to a certain level. We are able to show experimentally that the Wigner time delay can reproduce correctly the general trend of the measured delays but it does not capture all the observed features.
Sabbar, M., Heuser, S., Boge, R., Lucchini, M., Carette, T., Lindroth, E., Gallmann, L., Cirelli, C., and Keller, U. (2015) Resonance Effects in Photoemission Time Delays. Phys Rev Lett 115, 133001 (10.1103/PhysRevLett.115.133001)
September 23, 2015. More >>
Are the laws of optics still valid at the ultimate scaling limits of electronic and optoelectronic devices?
The laws of optics describing phenomena such as reflection or refraction are very well tested and established. However, they essentially describe the macroscopic and quasi-static response of matter to the electromagnetic light fields. While this view provides the correct description for most applications, the question arises whether the same optics laws can also be transferred to atomic length and time scales, which represent the ultimate scaling limits of electronic and optoelectronic devices. The authors answer this question by probing a metal surface with atomic length and attosecond time resolution.
Lucchini, M., Castiglioni, L., Kasmi, L., Kliuiev, P., Ludwig, A., Greif, M., Osterwalder, J., Hengsberger, M., Gallmann, L., and Keller, U. (2015) Light-Matter Interaction at Surfaces in the Spatiotemporal Limit of Macroscopic Models. Phys Rev Lett 115, 137401 (10.1103/PhysRevLett.115.137401).
September 22, 2015. More >>
Spintronics just got faster
Auböck, G., and Chergui, M. (2015) Sub-50-fs photoinduced spin crossover in [Fe(bpy)3]2+. Nature Chem. (DOI: 10.1038/NCHEM.2305)
July 20, 2015. More >>
Plasmonic Tipless Pyramid arrays for Cell Poration
Courvoisier, S., Saklayen, N., Huber, M., Chen, J., Diebold, E.D., Bonacina, L., Wolf, J.P., and Mazur, E. (2015) Plasmonic Tipless Pyramid arrays for Cell Poration. Nano Lett 15, 4461-4466 (10.1021/acs.nanolett.5b01697)
June 16, 2015. More >>
How long does it take to remove electrons from noble metal surfaces?
The energy dependence of the photoemission delays deviates considerably from the expectations based on a simple model using scattering theory and ballistic transport. The observed deviation highlights the importance of final state effects in the photoemission dynamics from solids – a contribution that was neither accessible nor considered in earlier studies.
Locher, R., Castiglioni, L., Lucchini, M., Greif, M., Gallmann, L., Osterwalder, J., Hengsberger, M., and Keller, U. (2015) Energy-dependent photoemission delays from noble metal surfaces by attosecond interferometry. Optica 2, 405-410 (10.1364/OPTICA.2.000405)
April 23, 2015. More >>
Electron transfer challenges fluorescence resonance analysis
Tryptophan is an amino acid, one of the building blocks of proteins. It is used extensively to study how proteins change their 3D structure, and also how they interact with other proteins and molecules. This is studied with a fluorescence technique called FRET, which measures the transfer of energy from tryptophan to another molecule. But in some cases, FRET data could be distorted because tryptophan transfers an electron instead of energy. Using a unique spectroscopic technique, scientists at EPFL have now confirmed for the first time that this is indeed the case. The study, which has far-reaching implications for the effectiveness of FRET, is published in PNAS.Monni, R., Al Haddad, A., van Mourik, F., Auböck, G., and Chergui, M. (2015) Tryptophan-to-heme electron transfer in ferrous myoglobins. Proc Natl Acad Sci USA (10.1073/pnas.1423186112)
April 20, 2015. More >>
The first ever photograph of light as both a particle and wave
Quantum mechanics tells us that light can behave simultaneously as a particle or a wave. However, there has never been an experiment able to capture both natures of light at the same time; the closest we have come is seeing either wave or particle, but always at different times. Taking a radically different experimental approach, EPFL scientists have now been able to take the first ever snapshot of light behaving both as a wave and as a particle. The breakthrough work is published in Nature Communications.
Piazza, L., Lummen, T.T.A., Quiñonez, E., Murooka, Y., Reed, B.W., Barwick, B., and Carbone, F. (2015) Simultaneous observation of the quantization and the interference pattern of a plasmonic near-field. Nat Commun 6, 6407 (10.1038/ncomms7407).
March 2, 2015. More >>
Experimental Demonstration of a Soft X-Ray Self-Seeded Free-Electron Laser.
The Linac Coherent Light Source has added a self-seeding capability to the soft x-ray range using a grating monochromator system. We report the demonstration of soft x-ray self-seeding with a measured resolving power of 2000–5000, wavelength stability of 10−4, and an increase in peak brightness by a factor of 2–5 across the photon energy range of 500–1000 eV. By avoiding the need for a monochromator at the experimental station, the self-seeded beam can deliver as much as 50-fold higher brightness to users.
Ratner, D., Abela, R., Amann, J., Behrens, C., Bohler, D., Bouchard, G., Bostedt, C., Boyes, M., Chow, K., Cocco, D., Decker, F.J., Ding, Y., Eckman, C., Emma, P., Fairley, D., Feng, Y., Field, C., Flechsig, U., Gassner, G., Hastings, J., Heimann, P., Huang, Z., Kelez, N., Krzywinski, J., Loos, H., Lutman, A., Marinelli, A., Marcus, G., Maxwell, T., Montanez, P., Moeller, S., Morton, D., Nuhn, H.D., Rodes, N., Schlotter, W., Serkez, S., Stevens, T., Turner, J., Walz, D., Welch, J., and Wu, J. (2015) Experimental Demonstration of a Soft X-Ray Self-Seeded Free-Electron Laser. Phys Rev Lett 114, 054801 (10.1103/PhysRevLett.114.054801)
Thomas Feurer and co-workers demonstrate resonant electric field enhancement structures, which concentrate the incident electric field in sub-diffraction size volumes and show an electric field enhancement as high as ~14,000 at 50 GHz. These values have been confirmed through a combination of near-field imaging experiments and electromagnetic simulations.
January 27, 2015. More >>
With the advent of optical systems delivering high-energy few-cycle pulses on the long-wavelength side of the visible spectrum around 3.4 µm, light-matter-interaction can now be studied in an area where the magnetic field component of the light pulses can be expected to play a measurable role. Thus far, this component could be neglected in the region of the parameter space where the majority of strong field ionization experiments take place. The authors showed that beyond this region the electron dynamics is altered by the magnetic field component of the light as well as the ion’s Coulomb force onto the escaping electron. Thus the so-called “Dipole Approximation” fails.
December 15, 2014. More >>
How quickly does a quantum particle tunnel through a barrier? This fundamental question has been hotly debated (as time is not a quantum operator) since the early days of quantum mechanics. Conclusive experiments were not possible. In modern ultrafast science, the reconstruction of electron dynamics, e.g., in several recent Science and Nature papers, implicitly relies on instantaneous tunneling time. Our experimental resolution shows tunneling time is neither instantaneous nor deterministic; most existing theory fails. Moreover, the time-scales involved significantly impact dynamics of valence shell electrons – and hence the chemical properties of molecules, with implications likely even for molecular biology research.
November 17, 2014. More >>
August 3, 2014. More >>
Thomas Feurer and co-workers report a unique two-dimensional spectrogram measurement of the relative X-ray/optical delay. This easily scalable relative delay measurement already surpasses previous techniques by an order of magnitude with its sub-1 fs temporal resolution and opens up the prospect of time-resolved X-ray measurements to the attosecond community.
July 6, 2014. More >>
March 6, 2014. More >>
Malignant human cell lines labelled by harmonic nanoparticles are targeted with a biophotonics approach based on the nonlinear optical process of second harmonic generation. The method enables independent imaging and therapeutic action, selecting each modality by simply tuning the excitation laser wavelength from infrared to visible. In particular, the generation of deep ultraviolet radiation at 270 nm allows direct interaction with nuclear DNA in the absence of photosensitizing molecules.
January 30, 2014. More >>
State of the art femtosecond electron microscopy experiments on a Praseodimium-doped bi-layered manganite helps to unravel the details of the response of diferent orbitals to photo-induced structural distortions. Carbone and co-workers show the dynamical response of the electronic structure of a Pr-doped manganite in a very broad spectroscopic range (more then 60 eV) together with the dynamical response of the crystal obtained in diffraction.
January 21, 2014. More >>
Jacques Moser and co-workers show using transient laser spectroscopy and microwave photoconductivity measurements that primary charge separation in hybrid organic–inorganic solid-state solar cells occurs at both junctions, with TiO2 and the hole-transporting material, simultaneously, with ultrafast electron and hole injection taking place from the photoexcited perovskite over similar timescales. Charge recombination is shown to be significantly slower on TiO2 than on Al2O3 films.
January 19, 2014. More >>
Peter Hamm and co-workers present two-dimensional Raman-terahertz (THz) spectroscopy as a multidimensional spectroscopy directly in the far-IR regime. The method is used to explore the dynamics of the collective intermolecular modes of liquid water at ambient temperatures that emerge from the hydrogen-bond networks water forming.
December 17, 2013. More >>
August 29, 2013
Using three-dimensional infrared (3D-IR) spectroscopy, Peter Hamm and co-workers have investigated the vibrational dynamics of isotope-diluted ice Ih. .
Perakis, F., Borek, J., and Hamm, P. (2013) Three-dimensional infrared spectroscopy of isotope-diluted ice Ih. J Chem Phys 139, 014501 (DOI: 10.1063/1.4812216).
August 11, 2013
A terahertz laser developed at the Paul Scherrer Institute makes it possible to control a material’s magnetisation at a timescale of picoseconds (0.000 000 000 001 seconds).
C. Vicario, C. Ruchert, F. Ardana-Lamas, P.M. Derlet, B. Tudu, J. Luning and C.P. Hauri (2013) Offresonant magnetization dynamics phase-locked to an intense phase-stable THz transient. Nature Photonics, Advance Online publication, 11 August 2013
Majed Chergui and co-workers: Solving electron transfer in waterJuly 2, 2013
EPFL scientists have shown how a solvent can interfere with electron transfer by using unprecedented time resolution in ultrafast fluorescence spectroscopy: paper in Nature Communications.
Fabrizio Messina, Olivier Bräm, Andrea Cannizzo, Majed Chergui. Real-time observation of the charge transfer to solvent dynamics. Nature Communications, 2013; 4 DOI: 10.1038/ncomms3119
Bill Pedrini, Rafael Abela, Bruce Patterson and co-workers: new paper in Nature CommunicationsApril 3, 2013
B. Pedrini, A. Menzel, M. Guizar-Sicairos, V.A. Guzenko, S. Gorelick, C. David, B.D. Patterson & R. Abela published in Nature Communications: Two-dimensional structure from random multiparticle X-ray scattering images using cross-correlations. DOI: 10.1038/ncomms2622.
Hans Jakob Wörner / Jean-Pierre Wolf and co-workers: Direct Amplitude Shaping of High Harmonics in the Extreme UltravioletFebruary 18, 2013
Foundations for the first coherent control experiments of core and valence electrons on attosecond timescales
In the framework of our MUST-collaboration, we recently demonstrated direct shaping of attosecond pulse trains after their generation using a reflective micromirror array based on micro-electro-mechanical-system (MEMS) technology.
Majed Chergui and co-workers: An ultraviolet analogue of 2D NMRFebruary 15, 2013
Unravelling electron and energy transfer processes of amino-acid residues in bio-systems
Recently, the group of Prof. Chergui has implemented the first experimental set-up for 2D UV spectroscopy and in a recent article in Science, they demonstrated its capabilities in the case of heme proteins.
Gebhard Schertler and co-workers: A glimpse inside the control centres of cell communication(from the PSI website, February 14, 2013)
Researchers detect characteristic constructional features in a family of sensors that process signals in the human body and control physiological processes.
The cells within the human body continually communicate with one another in order to fulfil their various tasks. For that purpose, they are equipped with sensors with which they receive signals from their environment. Sensors on cell surfaces are known as receptors. Numerous processes taking place within our body – such as sight, smell or taste – are performed by an important family of receptors known as G protein-coupled receptors (GPCR).
More >> (external link opening a new window)
Eric Vauthey and co-workers: Bimolecular Photoinduced Electron TransferJuly 2, 2012
Electron transfer processes are ubiquitous chemical reactions, involved for example in the conversion of light into chemical energy in the photosynthetic apparatus of plants or into electricity in photovoltaic devices. Additionally, electron transfer is the simplest chemical reaction and, as such, it has attracted much attention from theoreticians. Using ultrafast spectroscopy, we could observe the initial stages of the reaction in viscous environments and evidence the complex interplay of diffusion and reaction that requires state-of-the-art theoretical models to be correctly analysed.
Majed Chergui and co-workers: A Setup for Ultrafast Broadband Two-Dimensional UV SpectroscopyJune 8, 2012
Inspired by NMR techniques, the implementation multidimensional spectroscopies in the infrared regime (vibrational multidimensional spectroscopy) over the last 20 years made it possible to obtain molecular dynamical and structural information way beyond conventional (one-dimensional) time-resolved techniques. More recently the spectral range of these techniques was extended to the visible (electronic multidimensional spectroscopies).
Peter Hamm and co-workers: Towards 2D Raman-THz spectroscopyMarch 7, 2012
Water is a complex liquid due to the fast dynamics of the hydrogen-bond network that is responsible for its peculiar properties. We know from ultrafast vibrational spectroscopy that the memory time of water at room temperatures, i.e. the typical time a given water molecule stays in its hydrogen bond environment, is a few picoseconds at most. These studies concentrate on the high-frequency OH-stretch vibration of water and make use of the fact that its vibrational frequency is a relatively sensitive probe of the strength of hydrogen bonding of a given OH group to its environment.
Markus Meuwly and Peter Hamm: Temperature dependence of the heat diffusivity of proteinsNovember 2, 2011
In a combined experimental–theoretical study, we have investigated the transport of vibrational energy from the surrounding solvent into the interior of a heme protein, the sperm whale myoglobin double mutant L29W-S108L (left Figure ), and its dependence on temperature between 20 and 70 K .