P. Beaud

Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex

X-ray absorption spectroscopy is a powerful probe of molecular structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. We investigated the ultrafast formation of the lowest quintet state of aqueous iron(II) tris(bipyridine) upon excitation of the singlet metal-to-ligand-charge-transfer (1MLCT) state by femtosecond optical pump/x-ray probe techniques based on x-ray absorption near-edge structure (XANES). By recording the intensity of a characteristic XANES feature as a function of laser pump/x-ray probe time delay, we find that the quintet state is populated in about 150 femtoseconds. The quintet state is further evidenced by its full XANES spectrum recorded at a 300-femtosecond time delay. These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which we identify as a simple 1MLCT→3MLCT→5T cascade from the initially excited state. The time scale of the 3MLCT→5T relaxation corresponds to the period of the iron-nitrogen stretch vibration.

Large-Amplitude Spin Dynamics Driven by a THz Pulse in Resonance with an Electromagnon

Ultrafast Manipulation Multiferroic materials commonly show both magnetism and ferroelectricity, such that the electric field can be used to manipulate the magnetic order, and vice versa. Kubacka et al. (p. 1333, published online 6 March) used a strong terahertz electromagnetic pulse in resonance with an electromagnon—an excitation based on both electric and magnetic ordering—to control the spin dynamics of the multiferroic TbMnO3 on a sub-picosecond time scale and induce the rotation of the spin-cycloid plane of the material. The electric field of an electromagnetic pulse exerts ultrafast control on the spin dynamics of the multiferroic TbMnO3. Multiferroics have attracted strong interest for potential applications where electric fields control magnetic order. The ultimate speed of control via magnetoelectric coupling, however, remains largely unexplored. Here, we report an experiment in which we drove spin dynamics in multiferroic TbMnO3 with an intense few-cycle terahertz (THz) light pulse tuned to resonance with an electromagnon, an electric-dipole active spin excitation. We observed the resulting spin motion using time-resolved resonant soft x-ray diffraction. Our results show that it is possible to directly manipulate atomic-scale magnetic structures with the electric field of light on a sub-picosecond time scale.

High resolution femtosecond coherent anti-Stokes Raman scattering: Determination of rotational constants, molecular anharmonicity, collisional line shifts, and temperature

In this paper we present high resolution spectroscopy performed with femtosecond coherent anti-Stokes Raman scattering (CARS). After a theoretical treatment of the issue, specific experimental configurations will be introduced. Transients from nonresonant rotational and vibrational CARS on di- and polyatomics in the gas phase were analyzed. Rotational and vibrational constants and pressure-dependent line shifts are determined with high accuracy. The method is suitable for precise measurement of temperatures. We present thermometry on combustion relevant species like H2 and N2 and discuss the reliability and accuracy of the data.

Flame thermometry by femtosecond CARS

The potential of time-resolved coherent anti-Stokes Raman scattering (CARS) for thermometry studies in combustion processes using the isotropic Q-branch of nitrogen is discussed. Despite the increasing complexity of the femtosecond (fs)-CARS transients with rising temperature, due to the high number of contributing levels, the time-resolved CARS signal can be simulated with excellent correspondence to the experiment. The feasibility of flame thermometry with fs-CARS is demonstrated for an atmospheric pressure methane/air flame.

Femtosecond Dynamics of the Collinear-to-Spiral Antiferromagnetic Phase Transition in CuO

We report on the ultrafast dynamics of magnetic order in a single crystal of CuO at a temperature of 207 K in response to strong optical excitation using femtosecond resonant x-ray diffraction. In the experiment, a femtosecond laser pulse induces a sudden, nonequilibrium increase in magnetic disorder. After a short delay ranging from 400 fs to 2 ps, we observe changes in the relative intensity of the magnetic ordering diffraction peaks that indicate a shift from a collinear commensurate phase to a spiral incommensurate phase. These results indicate that the ultimate speed for this antiferromagnetic reorientation transition in CuO is limited by the long-wavelength magnetic excitation connecting the two phases.

Structural and Magnetic Dynamics of a Laser Induced Phase Transition in FeRh

We use time-resolved x-ray diffraction and magneto-optical Kerr effect to study the laser-induced antiferromagnetic to ferromagnetic phase transition in FeRh. The structural response is given by the nucleation of independent ferromagnetic domains (τ(1)~30 ps). This is significantly faster than the magnetic response (τ(2)~60 ps) given by the subsequent domain realignment. X-ray diffraction shows that the two phases coexist on short time scales and that the phase transition is limited by the speed of sound. A nucleation model describing both the structural and magnetic dynamics is presented.

Ultrafast electron emission from metallic nanotip arrays induced by near infrared femtosecond laser pulses

The authors explore the impact of femtosecond light pulses on the field-emission properties of single-gate molybdenum field-emitter arrays with nanometer scale tip apex. Despite the small fraction of the emission area, we observed a single-photon photoelectric current from the emitter tips on top of the dc field-emission current under the irradiation of 50fs laser pulses at a wavelength of 800nm with an external quantum efficiency up to ∼2×10−7 and the emitter tip quantum efficiency of ∼10−2. The result indicates that metallic field-emitter arrays are promising for applications that require high-brightness short electron beams.

8-TW 90-fs Cr:LiSAF laser.

Ultrashort 90-fs 8-TW Fourier-transform-limited pulses are generated with chirped pulse amplification in a Cr3+:LiSrAlF6 laser system incorporating a regenerative amplifier and three additional double-pass amplifiers with increasing aperture up to 25 mm.

Determination of rotational constants in a molecule by femtosecond four-wave mixing

Femtosecond time-resolved DFWM experiments were performed on CHCl3 and SO2. The recurrences observed originate from the preparation of rotational coherences within the sample. The determination of rotational constants for a symmetric and an asymmetric top are shown. The simulation of the transients for the symmetric top allows the extraction of the main moment of inertia B and the centrifugal terms perpendicular to the molecule axis. For the asymmetric top, all three rotational constants can be derived. In addition to the transients at 1/2(B + C), two additional types of asymmetry transients at 1/4C and 1/4A arise from this experiment. The first origins from the levels at low K and the second from the levels at high K in the high-J limit. Copyright

Collision induced rotational energy transfer probed by time-resolved coherent anti- Stokes Raman scattering

We show that the technique of femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy provides a powerful tool for the investigation of collision-induced linewidths and the validation of rotational energy transfer (RET) models. The fs-CARS method is applied to the N2–N2 collision system, and a comparison between the commonly used exponential gap (ECS-E), power gap (ECS-P), frequency corrected (EFCS), and the recently proposed angular momentum and energy corrected (AECS) variants of the ECS model is presented. As result we show that the AECS scaling law requires only two free parameters, and is appropriate for the determination of RET rates from the measured fs-CARS signals. The AECS model is also applied to the more complex C2H2–C2H2 collision system. As vibrational energy transfer and dephasing is not negligible in this case, the model has to be modified by introducing a vibrational relaxation factor. With this modification the fs-CARS signals from acetylene can be described s...