Ultrafast predissociation and coherent phenomena in CS2 excited by femtosecond laser pulses at 194-207 nm

The ultrafast predissociation dynamics of CS2 molecules excited to the B-1(2)((1)Sigma(u)(+)) electronic state by femtosecond laser pulses with 6.0-6.4 eV photon energy has been studied in pump-probe experiments. The analysis of the time-dependent ion signals has revealed lifetimes decreasing from 620 fs down to 180 fs for tuning the excitation wavelength from 207 nm to 194 nm. A nearly constant plateau at about 200 nm in the energy dependence of the lifetime reflects the barrier of the transition from the bent to a quasilinear geometry of the excited CS2 molecule. If two vibrational bands of the B-1(2)((1)Sigma(u)(+)) state were excited simultaneously by the femtosecond laser pulses we observed quantum beats with a modulation frequency corresponding to the energy difference of the two modes. Thus, the coherent excitation process is directly visualized despite the ultrafast decay of the excited molecular states due to predissociation. (C) 1999 American Institute of Physics. [S0021-9606(99)00736-9].