Femtosecond Time-Resolved Spectroscopy Of Molecular Ultrafast Dynamics Under High Pressure

A high-pressure time resolved spectroscopy system is set up, which is the combination of the DAC high pressure generation technology and the femtosecond pump-probe technology. By using this system we study the ultrafast dynamics of LDS698 molecules and CdTe quantum dots excited by ultrashort laser pulse under high pressure and explore the pressure induced changes of the vibrational relaxation, rotational relaxation, energy transfer, charge transfer, localized electronic coupling and the chemical bonds.From the measured spectra under high pressure, two important classes of ultrafast relaxation processes are observed which include internal conversion, intramolecular vibrational redistribution and some intermolecular vibrational relaxation processes. The experiment results indicate that the rate constant for the fast internal conversion increases exponentially with pressure, which implies that the energy gap between the S1 and S2 states decreases with pressure based on the theoretical model proposed. The rate constant for the slow relaxation decreases with pressure, this maybe suggest that it is related to other relaxation processes in addition to IC (S1→S0). These relaxation processes strongly affected by the intermolecular interaction that likely can be attributed to such vibrational energy relaxations as intramolecular and intermolecular processes through the inter-molecular collision and/or H-bond interaction. When the pressure higher than the solidified pressure (~ 3 GPa), the observed relaxation of excited molecules is also expressed by a bi-exponential process which is attributed to an intramolecular energy relaxation with a fixed time constant and an intermolecular energy transfer with a pressure-dependent time constant. For interpreting this pressure effect on the intermolecular process, a corrected harmonic-oscillator model is proposed. The results demonstrate that under pressure the cooling of the excited molecules is through lattice vibration and thermal conduction in the molecular solid. The pressure effect on the molecular rotation relaxation is also studied, and the results indicate that the change of the molecular rotational dynamics is assigned to the pressure induced variations of the interaction between solute and solvent molecules and the solvent properties.The influence of pressure on the carrier relaxation dynamics in CdTe quantum dots is also obvious. In general, the carrier dynamics in quantum dots are very complicated, which involves three processes: the surface trapping of carriers, the carrier recombination in surface states and the lowest state of valence band and the relaxation of carriers from surface states to the deep trapping states. Since the surface trapping process is very fast (less than100fs), only the latter two processes are observed. Under the pressure these two processes have great changes, especially near 0.5 GPa and 1.5 GPa the abrupt changes have been observed, which suggests that CdTe quantum dots take place a series of phase transition from zinc-blende to cinnabar, then from cinnabar to rock-salt. The decrease of phase transition pressure in present work is attributed to the influence of ultrashort laser field. So through measuring the change of carrier relaxation dynamics we can explore the pressure-induced structure changes of matters.We firstly combine the femtosecond time-resolved transient absorption technology with the DAC high-pressure generation method. By using this new technology we study the ultrafast molecular dynamics under high pressure, and obtain the influence of pressure on their structure and properties. Therefore, this work opens a new window for gaining more knowledge of molecular system.