Femtosecond vibrational nuclear dynamics in the electronically excited state of the I2 molecule

Abstract

Dynamics of the nuclear motion in the bound electronic B-state of the I2 molecule was studied in the real time scale. Experiments were performed by the femtosecond “pump-probe” technique, which measured the dependence of the intensity of fluorescenceP(Δt) in the highly excited f-state on the time delay between pump and probe pulses. TheP(Δt) dependence observed has an oscillating character with a period of −300 fs. The pump pulse was generated by a femtosecond dye laser and amplified in a pulse dye laser amplifier; its spectral width was 5.6 nm, the wavelength in the center of the spectrum was 614 nm, and the duration was 90 fs. The probe pulse was generated in a KDP crystal due to duplication of the light frequency; its spectral width was 1.2 nm, the wavelength in the center of the spectrum was 307 nm, and the duration was 120 fs. TheP(Δt) dependence on the parameters of the probe and pump pulses was theoretically analyzed in terms of the quantum model based on the known energies of electronic vibrational-rotational states in the X-, B-, and f-terms of the iodine molecule. Experimental and calculatedP(Δt) plots at time delays of up to l.5 ps and time resolution of less than 100 fs were compared. Values of potentials in the X-, B-, and f-terms of the iodine molecule, spectra, and durations of pump and probe pulses are sufficient data for quantitative calculation of the experimentally obtainedP(Δt) plot.