Dinamiche ultraveloci di transizione di spin fotoindotte su singolo cristallo FeII(PM-AZA)2(NCS)2 studiate al femtosecondo con misure di reflettività mediante pump-probe al femtosecondo.

In the present work, our goal is to present a detailed study of the ultrafast dynamic after femtosecond photoexcitation. We are able to draw a preliminary conclusion on the ultrafast dynamics of the Photo Induced Phase Transition (PIPT) on FeII(PM-AZA)2(NCS)2 single crystal by underlining the occurrence of an intermediate intersystem crossing driving the spin conversion into an excited HS state in less than 1 ps. We can also demonstrate the dynamical step involved at different time scale by extrapolating the time constants of each process. In order to investigate such dynamics, we need an ¿investigation probe¿ with time resolution shorter than the process under investigation. In such a case, to probe the ultrafast electronic photo excitation we need devices with femtosecond resolution. The manuscript will be divided as follow: I will give a brief introduction on photoinduced phase transitions. In chapter 2 I mainly focus spin crossover compounds resuming the work already done in literature and describing easily the theoretical aspect of the spin conversion. In the third part I will introduce the FeII(PM-AZA)2(NCS)2 single crystal and its thermal spin conversion underlining the structural, magnetic and optical properties change. In the chapter 4 is descripted the time resolved pump-probe technique with the experimental set up used in the ultrafast studies. In chapter 5, I will present the data of the ultrafast photoswitching focusing to the 0-10 ps range. Demonstration of several different mechanisms occurring in the ultrafast time scale will be presented, giving preliminary conclusion in the end.