Femtosecond Dynamics of Photoinduced Phase Transitions

在这个项目开始之前，光诱导相变的超快动力学还没有得到很好的理解。我们利用时间分辨发光、反射率和拉曼光谱研究了这些现象在皮秒和飞秒区域的动态行为。研究的材料有：(1)准一维铂配合物，(2)镍配合物，(3)钒青铜，(4)二价离子自旋交叉配合物，(5)量子准电性。主要成就如下：在准一维卤素桥铂配合物中，我们利用飞秒发光光谱研究了在光致晶格重排的中间态中起重要作用的自捕获激子。波包的运动第一次以电影的形式被捕获。基于卤素离子对衰变动力学的依赖性，揭示了非辐射跃迁的重要性。在Ni-Br配合物中，我们发现了具有较大Stokes…More位移的发光，这表明激发态中电子-晶格相互作用的重要性，尽管人们认为电子相关比基态中电子-声子相互作用更重要。我们首次将飞秒光谱应用于自旋阶梯体系α′-NaV_2O_5。在自旋间隙能的两倍附近发现了一种新的模式。光激发引起的电荷无序过程表现为反射变化，响应速度快于热效应。光致相变或电荷无序被认为是造成这种现象的原因。这些研究为这种材料的动力学研究打开了大门。在含二价铁离子的自旋交叉配合物中，我们发现了一个不能通过热过程产生的新相。通过调节激发光子的能量，可以选择性地产生特定的自旋态。通过紫外光子照射，发现SrTiO_3和KTaO_3具有巨大的介电响应。少

The ultrafast dynamics of the photoinduce phase transitions has not been well understood before starting this project. We studied the dynamic behavior of these phenomena in pico-and femto-second regions, by using time-resolved luminescence, reflectance and Raman spectroscopy. The materials studied are (1)quasi-one-dimensional platinum complexes, (2)nickel complexes, (3)vanadium bronze, (4)divalent ion spin-cross-over complexes, and (5)quantum paraelectrics.Followings are the main achievements.1.In quasi-one-dimensional halogen-bridged platinum complexes, we investigated the self-trapped excitons, which are important as the intermediate states of photoinduced lattice rearrangements, by using femtosecond luminescence spectroscopy. The motion of the wave-packets is captured as a movie for the first time. Importance of the non-radiative transitions has been revealed, based on the halogen-ion dependence of the decay dynamics.2.In Ni-Br complexes, we found a luminescence with a large Stokes … More shift, which shows the importance of electron-lattice interactions in the excited state, although it has been believed that the electron correlation is more important than the electron-phonon interaction in the ground state.3.We applied femtosecond spectroscopy to the spin ladder system, α'-NaV_2O_5 for the first time. Elementary excitations were coherently generated and a new mode was found near the twice of the spin gap energy. The charge disordering process induced by photo-excitation has been observed as a reflectance change and a response faster than thermal effect was found. Photo-induced phase transition or charge disordering is suggested to be responsible for this phenomenon. These investigations opened the doorway to dynamical studies in this material.4.In spin crossover complexes containing divalent Fe ions, we found a new phase that cannot be produced through thermal process. By tuning the excitation photon energy, specific spin states are generated selectively.5.A giant dielectric response was found in SrTiO_3 and KTaO_3, by irradiating with ultraviolet photons. Less