Tracking nuclear motion in single-molecule magnets using femtosecond X-ray absorption spectroscopy.

The development of new data storage solutions is crucial for emerging digital technologies. Recently, all-optical magnetic switching has been achieved in dielectrics, proving to be faster than traditional methods. Despite this, single-molecule magnets (SMMs), which are an important class of magnetic materials due to their nanometre size, remain underexplored for ultrafast photomagnetic switching. Herein, we report femtosecond time-resolved K-edge X-ray absorption spectroscopy (TR-XAS) on a Mn(III)-based trinuclear SMM. Exploiting the elemental specificity of XAS, we directly track nuclear dynamics around the metal ions and show that the ultrafast dynamics upon excitation of a crystal-field transition are dominated by a magnetically active Jahn-Teller mode. Our results, supported by simulations, reveal minute bond length changes from 0.01 to 0.05 Å demonstrating the sensitivity of the method. These geometrical changes are discussed in terms of magneto-structural relationships and consequently our results illustrate the importance of TR-XAS for the emerging area of ultrafast molecular magnetism.

Equatorial restriction of the photoinduced Jahn-Teller switch in Mn(iii)-cyclam complexes.

Ultrafast transient absorption spectra were recorded for solutions of [MnIII(cyclam)(H2O)(OTf)][OTf]2 (cyclam = 1,4,8,11-tetraazacyclotetradecane and OTf = trifluoromethanesulfonate) in water to explore the possibility to restrict the equatorial expansion following photoexcitation of the dxy ← dz2 electronic transition, often resulting in a switch from axial to equatorial Jahn-Teller distortion in MnIII complexes. Strong oscillations were observed in the excited state absorption signal and were attributed to an excited state wavepacket. The structural rigidity of the cyclam ligand causes a complex reaction coordinate with frequencies of 333, 368, 454 and 517 cm-1, and a significantly shorter compressed-state lifetime compared to other MnIII complexes with less restricted equatorial ligands. Complementary density functional theory quantum chemistry calculations indicate a switch from an axially elongated to a compressed structure in the first excited quintet state Q1, which is accompanied by a modulation of the axial tilt angle. Computed harmonic frequencies for the axial stretching mode (∼379 cm-1) and the equatorial expansions (∼410 and 503 cm-1) of the Q1 state agree well with the observed coherences and indicate that the axial bond length contraction is significantly larger than the equatorial expansion, which implies a successful restriction of the wavepacket motion. The weak oscillation observed around 517 cm-1 is assigned to a see-saw motion of the axial tilt (predicted ∼610 cm-1). The results provide insights into the structural perturbations to the molecular evolution along excited state potential energy surfaces of MnIII octahedral complexes and can be used to guide the synthesis of optically controlled MnIII-based single-molecule magnets.

Photoinduced Jahn-Teller switch in Mn(III) terpyridine complexes.

Ultrafast transient absorption spectra were recorded for [Mn(terpy)X3], where X = Cl, F, and N3, to explore photoinduced switching from axial to equatorial Jahn-Teller (JT) distortion. Strong oscillations were observed in the transients, corresponding to a wavepacket on the excited-state potential energy surface with oscillation frequency around 115 cm-1 for all three complexes. Multireference quantum chemistry calculations indicate that the reaction coordinate is a pincer-like motion of the terpyridine ligand arising from bond length changes in the excited state due to the JT switch. We observed long dephasing times of the wavepacket, with times of 620 fs for [Mn(terpy)Cl3], 450 fs for [Mn(terpy)F3], and 370 fs for [Mn(terpy)(N3)3]. The dephasing time of these coherences decreases with an increasing number of vibrational modes at lower energy than the mode dominating the reaction coordinate, suggesting they act as an effective bath to dissipate the excess energy obtained from photoexcitation.

A femtosecond magnetic circular dichroism spectrometer.

We describe the development of a broadband magneto-optical spectrometer with femtosecond temporal resolution. The absorption spectrometer is based on a white-light supercontinuum (∼320 to 750 nm) using shot-to-shot temporal and spectral referencing at 1 kHz. Static and transient absorption spectra using circularly polarized light are collected in a magnetic field. The difference spectra with respect to the external field direction give the static and transient magneto-optical Faraday rotation (magnetic optical rotary dispersion) and ellipticity (magnetic circular dichroism) spectra. An achromatic quarter-wave plate is used, and the impact of the deviation from ideal retardance on the spectra is discussed. Results from solution-based and thin-film samples are used to demonstrate the performance and wide applicability of the instrument. The sensitivities for the static and time-resolved data were found to be 5 and 0.4 mdeg, respectively. The method presents a simple way to measure magneto-optical spectra using a transient absorption spectrometer and an electromagnet.

Ultrafast photoinduced dynamics in Prussian blue analogues.

New magnetic materials and methods for controlling them are needed to improve data storage technologies. Recent progress has enabled optical detection and manipulation of spins in molecule-based magnets on the femtosecond timescale, which is promising for both increasing the read/write speed but also the data storage density. Experimental developments in femtosecond X-ray free-electron lasers (XFELs) and magneto-optics, in combination with theory advances, have opened up several new avenues to investigate molecule-based magnets. This review discusses the literature concerning ultrafast photoinduced dynamics in Prussian blue analogues (PBAs), which are molecule-based magnets. In PBAs spin-flips and lattice distortions can happen on the 100 fs timescale, which in some analogues lead to photoinduced changes in the long-range magnetic order. The literature and themes covered in this review are of relevance for ultrafast optical control of new multifunctional materials.

Vibrational coherences in manganese single-molecule magnets after ultrafast photoexcitation.

Magnetic recording using femtosecond laser pulses has recently been achieved in some dielectric media, showing potential for ultrafast data storage applications. Single-molecule magnets (SMMs) are metal complexes with two degenerate magnetic ground states and are promising for increasing storage density, but remain unexplored using ultrafast techniques. Here we have explored the dynamics occurring after photoexcitation of a trinuclear µ3-oxo-bridged Mn(III)-based SMM, whose magnetic anisotropy is closely related to the Jahn-Teller distortion. Ultrafast transient absorption spectroscopy in solution reveals oscillations superimposed on the decay traces due to a vibrational wavepacket. Based on complementary measurements and calculations on the monomer Mn(acac)3, we conclude that the wavepacket motion in the trinuclear SMM is constrained along the Jahn-Teller axis due to the µ3-oxo and µ-oxime bridges. Our results provide new possibilities for optical control of the magnetization in SMMs on femtosecond timescales and open up new molecular-design challenges to control the wavepacket motion in the excited state of polynuclear transition-metal complexes.

Super-atom molecular orbital excited states of fullerenes.

Super-atom molecular orbitals are orbitals that form diffuse hydrogenic excited electronic states of fullerenes with their electron density centred at the centre of the hollow carbon cage and a significant electron density inside the cage. This is a consequence of the high symmetry and hollow structure of the molecules and distinguishes them from typical low-lying molecular Rydberg states. This review summarizes the current experimental and theoretical studies related to these exotic excited electronic states with emphasis on femtosecond photoelectron spectroscopy experiments on gas-phase fullerenes.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Relative Photoionization Cross Sections of Super-Atom Molecular Orbitals (SAMOs) in C60.

The electronic structure and photoinduced dynamics of fullerenes, especially C60, is of great interest because these molecules are model systems for more complex molecules and nanomaterials. In this work we have used Rydberg Fingerprint Spectroscopy to determine the relative ionization intensities from excited SAMO (Rydberg-like) states in C60 as a function of laser wavelength. The relative ionization intensities are then compared to the ratio of the photoionization widths of the Rydberg-like states, computed in time-dependent density functional theory (TD-DFT). The agreement is remarkably good when the same photon order is required to energetically access the excited states. This illustrates the predictive potential of quantum chemistry for studying photoionization of large, complex molecules as well as confirming the assumption that is often made concerning the multiphoton excitation and rapid energy redistribution in the fullerenes.

Dynamics of thermal electron emission from highly excited C60.

Gas-phase fullerenes emit thermal electrons after femtosecond laser excitation in the wavelength range 400-800 nm. We have used angular-resolved photoelectron spectroscopy (PES) to study the influence of the laser's electric field on the dynamics of the thermally emitted electrons. The laser field introduces an asymmetry in the thermal electron distributions with respect to the laser polarization direction, which was confirmed by carrying out experiments at different wavelengths. A simple model could reproduce the trends in measured apparent temperatures in the PES. The asymmetry effect was exploited in a pump-probe experiment to estimate the time scale for thermal electron emission. It was found that, when 400 nm, 120 fs laser pulses of 2 TW cm(-2) intensity are used, thermal electrons are emitted up to ca. 300 fs after the peak of the laser pulse. The pump-probe scheme should be applicable to a wider range of complex molecules and clusters showing thermal electron emission on a femtosecond time scale.

Hot electron production and diffuse excited states in C70, C82, and Sc3N@C80 characterized by angular-resolved photoelectron spectroscopy.

Angular-resolved photoelectron spectroscopy using wavelength-tuneable femtosecond laser pulses is presented for a series of fullerenes, namely, C70, C82, and Sc3N@C80. The photoelectron kinetic energy distributions for the three molecules show typical thermal electron spectra with a superimposed peak structure that is the result of one-photon ionization of diffuse low-angular momenta states with electron density close to the carbon cage and that are related to so-called super atom molecular orbitals. Photoelectron angular distributions confirm this assignment. The observed structure is less prominent compared to the thermal electron background than what was observed in C60. It can be concluded that hot electron emission is the main ionization channel for the larger and more complex molecules for these excitation conditions.

Probing rapidly-ionizing super-atom molecular orbitals in C60: a computational and femtosecond photoelectron spectroscopy study.

Super-atom molecular orbitals (SAMOs) are diffuse hydrogen-like orbitals defined by the shallow potential at the centre of hollow molecules such as fullerenes. The SAMO excited states differ from the Rydberg states by the significant electronic density present inside the carbon cage. We provide a detailed computational study of SAMO and Rydberg states and an experimental characterization of SAMO excited electronic states for gas-phase C(60) molecules by photoelectron spectroscopy. A large band of 500 excited states was computed using time-dependent density functional theory. We show that due to their diffuse character, the photoionization widths of the SAMO and Rydberg states are orders of magnitude larger than those of the isoenergetic non-SAMO excited states. Moreover, in the range of kinetic energies experimentally measured, only the SAMO states photoionize significantly on the timescale of the femtosecond laser experiments. Single photon ionization of the SAMO states dominates the photoelectron spectrum for relatively low laser intensities. The computed photoelectron spectra and photoelectron angular distributions are in good agreement with the experimental results.

Probing excited electronic states and ionisation mechanisms of fullerenes.

Fullerenes are interesting model systems for probing the complex, fundamental electron dynamics and ionisation mechanisms of large molecules and nanoparticles. In this Tutorial Review we explain how recent experimental and theoretical advances are providing insight into the interesting phenomenon of thermal electron emission from molecular systems and the properties of hydrogenic, diffuse, excited electronic states, known as superatom molecular orbitals, which are responsible for relatively simple, well-resolved structure in fs laser photoelectron spectra of fullerenes. We focus on the application of velocity map imaging combined with fs laser photoionisation to study angular-resolved photoelectron emission.

Angular-resolved photoelectron spectroscopy of superatom orbitals of fullerenes.

Photoelectron angular distributions from both C(60) and C(70) were recorded for low laser intensity femtosecond and picosecond pulses. Rich structure is seen for electron kinetic energies that lie below the photon energy. Strong, broad peaks are observed for photoelectron energies corresponding to single-photon ionization of so-called superatom molecular orbitals (SAMOs). The very simple angular distributions measured for these peaks, the close similarity of the spectra observed from C(60) and C(70), and the comparison with time-dependent density functional theory provide strong support for the SAMO hypothesis.