100 picosecond diffraction catches structural transients of laser-pulse triggered switching in a spin-crossover crystal

We study by 100?picosecond X-ray diffraction the photo-switching dynamics of single crystal of the orthorhombic polymorph of the spin-crossover complex [(TPA)Fe(TCC)]PF(6), in which TPA = tris(2-pyridyl methyl)amine, TCC(2-) = 3,4,5,6-Cl(4)-Catecholate(2-). In the frame of the emerging field of dynamical structural science, this is made possible by using optical pump/X-ray probe techniques, which allow following in real time structural reorganization at intra- and intermolecular levels associated with the change of spin state in the crystal. We use here the time structure of the synchrotron radiation generating 100?picosecond X-ray pulses, coupled to 100?fs laser excitation. This study has revealed a rich variety of structural reorganizations, associated with the different steps of the dynamical process. Three consecutive regimes are evidenced in the time domain: 1)?local molecular photo-switching with structural reorganization at constant volume, 2)?volume relaxation with inhomogeneous distribution of local temperatures, 3)?homogenization of the crystal in the transient state 100?μs after laser excitation. These findings are fundamentally different from those of conventional diffraction studies of long-lived photoinduced high spin states. The time-resolution used here with picosecond X-ray diffraction probes different physical quantities on their intrinsic time-scale, shedding new light on the successive processes driving macroscopic switching in a functionalized material. These results pave the way for structural studies away from equilibrium and represent a first step toward femtosecond crystallography.     

100 Picosecond Diffraction Catches Structural Transients of Laser‐Pulse Triggered Switching in a Spin‐Crossover Crystal

We study by 100 picosecond X‐ray diffraction the photo‐switching dynamics of single crystal of the orthorhombic polymorph of the spin‐crossover complex [(TPA)Fe(TCC)]PF₆, in which TPA=tris(2‐pyridyl methyl)amine, TCC^2?=3,4,5,6‐Cl₄‐Catecholate^2?. In the frame of the emerging field of dynamical structural science, this is made possible by using optical pump/X‐ray probe techniques, which allow following in real time structural reorganization at intra‐ and intermolecular levels associated with the change of spin state in the crystal. We use here the time structure of the synchrotron radiation generating 100 picosecond X‐ray pulses, coupled to 100 fs laser excitation. This study has revealed a rich variety of structural reorganizations, associated with the different steps of the dynamical process. Three consecutive regimes are evidenced in the time domain: 1) local molecular photo‐switching with structural reorganization at constant volume, 2) volume relaxation with inhomogeneous distribution of local temperatures, 3) homogenization of the crystal in the transient state 100 µs after laser excitation. These findings are fundamentally different from those of conventional diffraction studies of long‐lived photoinduced high spin states. The time‐resolution used here with picosecond X‐ray diffraction probes different physical quantities on their intrinsic time‐scale, shedding new light on the successive processes driving macroscopic switching in a functionalized material. These results pave the way for structural studies away from equilibrium and represent a first step toward femtosecond crystallography.     

六棱柱型金属团簇化合物Mo2Ag4S8(dppy)4的合成与非线性光学性质

新合成了一种六棱柱型金属团簇化合物Mo2Ag4S8(dppy)4, 并用X射线单晶衍射结构分析方法测定了其晶体结构. 应用Z-scan技术研究了该金属团簇化合物在波长为532 nm的纳秒激光脉冲作用下的三阶非线性光学特性. 为了研究Mo2Ag4S8(dppy)4的非线性起源, 应用泵浦探测技术研究了该金属团簇化合物在波长为532 nm的皮秒激光脉冲作用下的光学非线性响应. 实验结果表明金属团簇化合物Mo2Ag4S8(dppy)4具有自聚焦特性和非线性吸收特性; 非线性折射率n2, 非线性吸收系数β和非线性极化率χ(3)分别为1.45×10－10 esu, 6.2×10－10 m/W及2.1×10－11 esu. 实验结果表明金属团簇化合物Mo2Ag4S8(dppy)4的光学非线性响应是非线性吸收和非线性散射共同作用的结果.     

Transient absorption spectra of pinacyanol and cyanine photoisomers obtained with a sync-pumped picosecond dye laser and independently tunable probe light

We have measured the absorption difference spectra and quantum yields of photoisomers of pinacyanol iodide and 1,1′-diethyl-4,4′-cyanine iodide on a picosecond time scale using syne-pumped picosecond dye laser pulses for excitation and independently tunable analyzing light.     

Step-scan near-infrared Fourier-transform Raman spectroscopy with 100 picosecond laser pulses

Time-resolved Fourier-transform Raman scattering with picosecond excitation is reported for the first time. The resonance Raman spectrum of 9,10-diphenylanthracene in its excited single state was obtained by Raman excitation at 1064 nm with 100 ps pulses, following photoexcitation at 355 nm. The implementation and characterization of time-resolved FT-Raman spectroscopy with a step-scan interferometer is discussed. FT-Raman spectra generated with continuous and mode-locked beams in the continuous-scan mode of the interferometer are compared with step-scan FT-Raman spectra generated with 2 kHz pulses.     

The Coordination Behaviour of CuI Photosensitizers Bearing Multidentate Ligands Investigated by X-ray Absorption Spectroscopy

A systematic series of four novel homo- and heteroleptic Cu^I photosensitizers based on tetradentate 1,10-phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9-position (X=SMe or OMe) were designed. Their solid-state structures were assessed by X-ray diffraction. Cyclic voltammetry, UV-vis absorption, emission and X-ray absorption spectroscopy were then used to determine their electrochemical, photophysical and structural features in solution. Following, time-resolved X-ray absorption spectroscopy in the picosecond time scale, coupled with time-dependent density functional theory calculations, provided in-depth information on the excited state electron configurations. For the first time, a significant shortening of the Cu−X distance and a change in the coordination mode to a pentacoordinated geometry is shown in the excited states of the two homoleptic complexes. These findings are important with respect to a precise understanding of the excited state structures and a further stabilization of this type of photosensitizers.     

Photoproperties of isolated cis and trans stilbene molecules

A detailed account is given of the experimental approach to measuring transient spectra of dilute gases using picosecond pulses. The picosecond continuum generated by Nd:glass laser pulses is used to probe gaseous samples and spectra are recorded in a double beam arrangement. The pump and probe pulses interact with the sample over a few centimeters by means of a dielectric waveguide. Picosecond time resolved spectra, relative fluorescence quantum yield measurements, and fluorescence spectra are reported for trans-stilbene under collision free conditions. The lifetime of the optically prepared states at 265 nm and 287 nm are 15 ps and 55 ps respectively, measured by the decay of the transient absorption. The deuteration effect is less than 20%. The variation of the fluorescence yield with vibrational energy excess in the excited state of trans is fitted to these lifetime measurements to yield the variation of nonradiative decay due to twisting of trans-stilbene. Cis-stilbene is suggested to twist in less than 1 ps. Consideration of the spectral results yields new information about the isomerization of stilbene, in particular that there exists a barrier to twisting in the isolated molecule and that vibrational energy redistribution at the trans configuration is probably not complete on the time scale of our experiments. A pictorial model for discussing constant energy relaxation phenomena is introduced.     

Instrumentation of kinetic spectroscopy-20: Cherenkov emission in a ‘foil’ of optical fibers to measure the excitation in pulse radiolysis

Cherenkov emission in a “foil” of optical fibers is utilized for measuring the excitation in pulse radiolysis experiments, i.e., the charge of the pulses of high-energy electrons which penetrate the foil perpendicularly. Because of the inherent response of Cherenkov emission, this method can be applied for the detection of electron pulses with durations as short as a few picosecond or longer. High sensitivity and high time resolution as well as excellent precision are provided by this method.     

Ultrafast spin-state photoswitching in a crystal and slower consecutive processes investigated by femtosecond optical spectroscopy and picosecond X-ray diffraction

We report the spin state photo-switching dynamics in two polymorphs of a spin-crossover molecular complex triggered by a femtosecond laser flash, as determined by combining femtosecond optical pump-probe spectroscopy and picosecond X-ray diffraction techniques. The light-driven transformations in the two polymorphs are compared. Combining both techniques and tracking how the X-ray data correlate with optical signals allow understanding of how electronic and structural degrees of freedom couple and play their role when the switchable molecules interact in the active crystalline medium. The study sheds light on crossing the border between femtochemistry at the molecular scale and femtoswitching at the material scale.     

Picosecond X-ray absorption spectroscopy of a photoinduced iron(II) spin crossover reaction in solution

In this study, we perform steady-state and time-resolved X-ray absorption spectroscopy (XAS) on the iron K-edge of [Fe(tren(py)3)](PF6)2 dissolved in acetonitrile solution. Static XAS measurements on the low-spin parent compound and its high-spin analogue, [Fe(tren(6-Me-py)3)](PF6)2, reveal distinct spectroscopic signatures for the two spin states in the X-ray absorption near-edge structure (XANES) and in the X-ray absorption fine structure (EXAFS). For the time-resolved studies, 100 fs, 400 nm pump pulses initiate a charge-transfer transition in the low-spin complex. The subsequent electronic and geometric changes associated with the formation of the high-spin excited state are probed with 70 ps, 7.1 keV, tunable X-ray pulses derived from the Advanced Light Source (ALS). Modeling of the transient XAS data reveals that the average iron-nitrogen (Fe-N) bond is lengthened by 0.21+/-0.03 A in the high-spin excited state relative to the ground state within 70 ps. This structural modification causes a change in the metal-ligand interactions as reflected by the altered density of states of the unoccupied metal orbitals. Our results constitute the first direct measurements of the dynamic atomic and electronic structural rearrangements occurring during a photoinduced FeII spin crossover reaction in solution via picosecond X-ray absorption spectroscopy.     

Use of ultrashort laser pulses for desorption from semiconductor surfaces and nonresonant post-ionization of sub-monolayers

Summary The system aspects of a completely optical surface-sensitive time-of-flight mass spectrometer are presented. Using picosecond laser pulses for non-thermal desorption of neutrals from semiconductor surfaces and their optical post-ionization, a laser based mass spectrometry of extremely high sensitivity has been developed and tested. Initial experimental results are presented concerning the desorption yield from semiconductor surfaces, the saturation of the post-ionization and the useful yield of the system.     

Ultrafast electron diffraction: oriented molecular structures in space and time.

The technique of ultrafast electron diffraction allows direct measurement of changes which occur in the molecular structures of isolated molecules upon excitation by femtosecond laser pulses. The vectorial nature of the molecule-radiation interaction also ensures that the orientation of the transient populations created by the laser excitation is not isotropic. Here, we examine the influence on electron diffraction measurements--on the femtosecond and picosecond timescales--of this induced initial anisotropy and subsequent inertial (collision-free) molecular reorientation, accounting for the geometry and dynamics of a laser-induced reaction (dissociation). The orientations of both the residual ground-state population and the excited- or product-state populations evolve in time, with different characteristic rotational dephasing and recurrence times due to differing moments of inertia. This purely orientational evolution imposes a corresponding evolution on the electron scattering pattern, which we show may be similar to evolution due to intrinsic structural changes in the molecule, and thus potentially subject to misinterpretation. The contribution of each internuclear separation is shown to depend on its orientation in the molecular frame relative to the transition dipole for the photoexcitation; thus not only bond lengths, but also bond angles leave a characteristic imprint on the diffraction. Of particular note is the fact that the influence of anisotropy persists at all times, producing distinct differences between the asymptotic "static" diffraction image and the predictions of isotropic diffraction theory.     

Level decay and orientational kinetics of the rhodamine B monomer and dimer

The population kinetics and the rotational diffusion of the rhodamine B monomer and dimer were measured by using picosecond pulses from a mode-locked Nd : YAG laser to induce and time resolve the concentration-dependent transient absorption saturation of various aqueous solutions of this organic dye.     

Accelerators and lasers as sources of pulsed excitation present status

The past quarter of a century has seen considerable improvement in time resolution for pulse radiolysis and laser photolysis experiments, made possible by advanced excitation sources. To document the present status, three installations for pulse radiolysis are discussed: a picosecond linac (Tokyo), a nanosecond Van de Graaff (Delft), and a high current accelerator (ód). Also indicated are future experimental possibilities, in particular the use of a high current photoemission electron gun. Available lasers for excitation with nanosecond pulses are summarized and a picosecond facility which permits optical and conductivity observations with subnanosecond time resolution is described.     

DEPENDENCY OF APPARENT RELATIVE QUANTUM YIELD OF ISORHODOPSIN TO RHODOPSIN ON THE PHOTON DENSITY OF PICOSECOND LASER PULSE

Abstract— The quantum yield of bleaching of isorhodopsin relative to that of rhodopsin was measured by irradiation of both pigments with a steady light source or a picosecond laser pulse. The each pigment was prepared by incubation of 11- cis or 9 -cis retinal with opsin, respectively. The ratio of isorhodopsin to rhodopsin in the quantum yield was estimated to be 0.37 using irradiation with a steady light, while with a weak picosecond laser pulse (excitation photon density: below 20 μ.J/1.8 mmφ), it was estimated to be 0.39. Both values are in good agreement with each other. On the other hand, excitation with a strong picosecond laser pulse (above 20 μ.J/1.8 mmφ) produced a larger ratio than 0.39, indicating that saturation effects can be easily observed by irradiation with strong picosecond laser pulses.     

Picosecond laser study of the collisionless photodissociation of dimethylnitramine at 266 nm

We report a picosecond pump-probe study of the UV photolysis of gaseous dimethylnitramine. After photolysis by picosecond laser pulses at 266 nm, efficient monophotonic collision-free photodissociation occurs within 6 ps. NO₂ fragments are formed in the ground electronic state and in a fluorescent excited state; the quantum yields for both of these channels are estimated.     

Picosecond photon echoes detected by optical mixing

Picosecond photon echoes are shown to be easily detected by optical mixing. The synchronized picosecond excitation and probe pulses are generated by amplifying pulses from two dye lasers, synchronously pumped by a mode-locked argonion laser. The technique is used to study optical dephasing in the organic mixed crystal of pentacene in p-terphenyl.     

Crystal thin film of bis (imidazole) silver(I) nitrate for all optical switching application

The single crystal of a silver complex bis (imidazole) silver(I) nitrate (Ag(C₃H₄N₂)₂(NO₃), BISN) has been obtained and characterized by X‐ray single‐crystal diffraction. Its crystal thin film was prepared using direct growth on quartz substrates. The surface morphology of the thin film was studied by atomic force microscopy (AFM). The nonlinear optical (NLO) properties of the thin film were investigated by using closed‐aperture Z‐scan technique with 20 picosecond (ps) pulses at wavelength 532 nm. Using time‐dependent density‐functional theory (TDDFT) with the basis set LanL2DZ, its linear and NLO properties were calculated.     

Excited state absorption dynamics in metal cluster polymer [WS4Cu3I(4-bpy)3]n solution

The nonlinear absorptive property of a novel metal cluster [WS(4)Cu(3)I(4-bpy)3]n in DMF solution is studied by using an open-aperture Z-scan technique with picosecond and nanosecond laser pulses at the wavelength of 532 nm. The experimental results show that the cluster has strong nonlinear absorption under the 8 ns pulse excitation and a relatively weak nonlinear absorptive property under the picosecond pulse excitation. The picosecond pump-probe response of the metal cluster is similar to that of C60 solution, which implies that the nonlinear mechanisms are the same for the two materials. By using the rate-equation model, the experimental data are theoretically simulated; several optical parameters of the cluster, especially the lifetime of the higher excited singlet state of the cluster, are obtained.