VIBRATIONAL-RELAXATION OF NASCENT DIIODIDE IONS STUDIED BY FEMTOSECOND TRANSIENT RESONANCE IMPULSIVE STIMULATED RAMAN-SCATTERING (TRISRS) -EXPERIMENT AND SIMULATION

Impulsive stimulated Raman scattering performed with femtosecond pulse s on resonance with an electronic transition, comprises an all time do main Raman technique providing vibrational dynamics of the ground stat e chromophore. We report the application of this technique to record t ransient Raman responses of nascent diiodide ions, undergoing rapid vi brational relaxation following triiodide photodissociation in ethanol solution. Like other Fourier spectroscopic methods, this spectroscopy maximizes high simultaneous time and frequency resolution, making it w ell adapted, and in the present study exclusively capable, for recover ing vibrational dynamics of highly excited molecular populations in tr ansition. Master equation simulations of vibrational relaxation, coupl ed to quantum wave-packet representation of the light-matter interacti ons, are combined to provide a semiquantitative analysis of the experi mental results. Previous assignment of spectral narrowing of the nasce nt diiodide absorption to vibrational relaxation is bourn out by simul ations. Inherent limitations and fortitudes of the TRISRS method are t heoretically investigated and discussed.