可視近赤外数サイクルレーザーを用いた量子ビート分光によるシアニン色素分子中の超高速動力学

摘要

Femtosecond pump-probe technique was utilized to study the ultrafast dynamics betweenthe ground state (S0) and the first excited state (S1) of a cyanine dye molecule HDITC(1,1’,3,3,3’,3’- hexamethy l-4,4’,5,5’-dibenzo-2,2’-indotricarbocyanine ).The light source is a home-made Noncollinear Optical Parametric Amplification (NOPA),and the output can be achieved as short as 7fs with the broad spectral range from 550 to750nm which overlaps with the absorption of the first excited state (S1) of HDITC. Thesample induced pulse broadening was analyzed, and the effective pulse width wasconsidered to be 10fs which is short enough to detect the frequency as high as 1500cm-1.The electronic phase relaxation process and the vibrational modes on the excited statewere studied in the negative-time range where probe beam precedes pump beam. Molecularvibrations induced by impulsive excitation of femtosecond laser can be measured by usingthe high temporal resolution of the laser. Real-time energy exchange among the threecoupled vibrational modes in a cyanine dye molecule has been studied by using the pulselaser with a few optical-cycles. The analysis shows that the energy exchange is taking placebetween the coherent molecular vibration generated by the pump pulse both in the excitedstate and the ground state. We utilized the zeroth and the first moments of some spectralrange in the time-resolved spectrum to discuss the spectral changes in their transitionintensity and transition energy. The former and the latter are attributed to the imaginary andreal parts of the third order susceptibility of the stimulated Raman process.A new method to study mode coupling on the first excited state (S1) was developed and themodes coupling was considered to be the reason of slowing down the vibrational dephasingtime. This is because of the closed-loop coupling within the triplet of the vibrations. Incontrast, the other modes are coupled with many more modes, resulting in a loss of coherencebecause of inevitable coherence destruction associated with vibrational energy transfer to alower frequency.