Characterization of Ultrashort Pulses And Ultrafast Dynamics of Molecules In the Liquid Phase

Sammanfattning: This thesis reports an alternative method for fully acomplete characterization of the UV pump pulse and white lightcontinuum probe pulse, which is commonly used in the pump-probeexperiments to investigate the ultrafast dynamics of molecules.Besides that, real dynamics of infrared cyanine dye molecules(Methyl-DOTCI) and photo-induced dissociation of trihalide ions(I3-or I2Br-) solvated in different organic solvent has alsobeen carefully investigated.The traditional way to measure the time profile ofultrashort laser pulse like autocorrelation and FROG (frequencyresolved optical gating) fails in the ultraviolet region due tothe nonlinear crystal's absorption edge. We developed analterative simple method to explore the time-domain spectrum ofultraviolet-region ultrafast laser pulse by taking theadvantage of fast-chirped white light continuum. The method wasimplemented by cross-correlating ultraviolet pulse with awhite-light continuum in a two-photon absorption pump-probetransient absorption experiment. The high chirp rate of thecontinuum, in combination with frequency-resolved detection,effectively makes it act like a ?-pulse. The spectral andtemporal characteristics of ~150 fs laser pulses centered at387.5nm, generated by second-harmonic generation (SHG) of aregenerative amplified mode-locked Ti:sapphire laser pulses ina non-linear BBO (b-Barium Borate) crystal, were investigatedexperimentally. The phase of SHG pulses were also obtainedthrough information from both spectral and temporal profiles.In order to test our method, theoretically calculated timespectrum were compared with experimented ones.Molecular dynamics in liquid phase is much more complex thanits counterpart in gas phase. Besides electronic transition andnuclear rotation and vibration, it also involves solute-solventinteraction. With the advanced weapons of transient absorptionspectroscopy, the various intra- and intermolecular relaxationprocesses of cyanine dye molecules (Methyl-DOTCI) solved indifferent solvents, such as methanol, acetone, and ethyleneglycol has been probed. The electronic relaxation time constantfrom the excited state SN down to the first excited state S1 isobtained as 10 ps for both 390 nm and 260 nm excitationwavelength. For the more viscous solvent ethylene glycol therelaxation is slightly slower (t ~12 ps) relative to the othersolvents. The vibrational cooling constant of Methyl-DOTCIvaries with solvents: 6 ps is determined for Methyl-DOTCI inethylene glycol, 7 ps for the case of methanol, and 10 ps forboth acetone and acetonitrile cases. The time difference withsolvent can be rationalized through the formation ofhydrogen-bond between dye solute and methanol/ethylene glycolsolvents.Femtosecond transient absorption measurement is presentedfor the photodissociation of trihalide anion (I3-and I2Br-) in methanol and acetonitrile solution. Theultrafast (below 300fs) transient state dynamics are revealedthough interpreting transient absorption spectrum. Real-timeobservation of the wavepacket motion at these very early stagesof the reaction is made possible through wavelength selectiveprobing. The effect of solvent and solute geometry on coherentvibration, vibration cooling, and geminate recombinationprocesses of di-iodide ions are also investigated . Thevibration relaxing constants have been determined as 2 ps forall three solutions.