Real-time spectroscopy of isomerization process in azobenzene

Summary form only given. Azobenzene and its derivatives are known as typical molecules showing the trans-cis photoisomerization and are studied for basic understanding of the fundamental chemistry. There are two mechanisms which are proposed for photoisomerization process of azobenzene and its derivatives. One is the rotation of phenyl ring around the azo bond, and the other is the in-plane inversion of phenyl ring at one of the two N-atoms. We have investigated the real-time dynamics of molecular vibration in the excited state of trans-DMAAB [methyl yellow; 4-(dimethylamino)azobenzene] in dimethyl-sulfoxide solution by pump-probe spectroscopy using sub-5-fs visible pulse from a noncollinear optical parametric amplifier (NOPA). The chirped pulse excitations were performed to study separately the wave-packet dynamics in the excited and ground states. The positively chirped (PC) and negatively-chirped (NC) pulse excitation cases mainly reflect the wave packets in the excited state and the ground state, respectively. The chirped pulses were prepared by changing the insertion amount of a prism pair in the NOPA. The traces of the normalized transient transmittance change [/spl Delta/T/T(t)] of DMAAB at 570 nm (/spl pi/-/spl pi/* breaching is dominant) induced by the PC- and NC pulse excitations, respectively are shown. Figures representing their Fourier power spectra are presented. The oscillating components of /spl sim/ 1400 and /spl sim/1150 cm/sup -1/ are attributed to the N=N and C-N stretching modes, respectively.