Femtosecond photoelectron spectroscopy of the I 2 − anion: Characterization of the à ′ 2 Π g,1/2 excited state The X 2 ⌺ u ϩ state of the I 2 Ϫ anion has been fit to a Morse potential using data from two techniques: conventional and femtosecond photoelectron spectroscopy ͑FPES͒. Conventional photoelectron spectroscopy is used to determine the adiabatic electron affinity of I 2 as well as the well depth and equilibrium nuclear geometry of I 2 Ϫ . In the FPES experiment, the pump pulse induces coherent nuclear motion on the ground state of I 2 Ϫ by resonant impulsive stimulated Raman scattering ͑RISRS͒, and the vibrational frequency of the anion is determined from the resulting oscillatory structure in the time-dependent photoelectron spectra. We find the electron affinity ͑EA͒ of I 2 to be 2.524Ϯ0.005 eV, the well depth (D e ) for I 2 Ϫ to be 1.014Ϯ0.005 eV, the equilibrium internuclear separation (R e ) to be 3.205Ϯ0.005 Å, and the vibrational frequency to be 110Ϯ2 cm Ϫ1 . These values for the I 2 Ϫ potential parameters differ significantly from previous results.

In this paper we describe a new semiclassical method for simulating femtosecond pump-probe photoelectron spectroscopy, and its implementation to study the excited state photodissociation dynamics of the I 2 Ϫ anion. Our algorithm involves a forward-backward ͑FB͒ semiclassical ͑SC͒ initial value representation ͑IVR͒ method for calculating the time dependent photodetachment spectrum P(⑀,⌬t) as a function of the kinetic energy ⑀ of the photodetached electron and the delay time ⌬t between the pump and probe pulses. We describe the radiation-chromophore interaction perturbatively to first order in both pulse fields, assuming the Condon approximation for the electronic transition dipole moments. Our computed spectra are in excellent agreement with full quantum mechanical simulations.

Anion femtosecond photoelectron spectroscopy was used to follow the dynamics of the I 2 − (Ar) 6 and I 2 − (Ar) 20 clusters subsequent to photodissociation of the I 2 − chromophore. The experiments showed that photodissociation of the I 2 − moiety in I 2 − (Ar) 6 is complete by approximately 200 femtoseconds, just as in bare I 2 − , but also that attractive interactions between the departing anion fragment and the solvent atoms persisted for 1200 femtoseconds. Photodissociation of I 2 − (Ar) 20 results in caging of the I 2 − followed by recombination and vibrational relaxation on the excited à 2 Π g ,3/2 and the ground X̃ 2 Σ u + states; these processes are complete in 35 and 200 picoseconds, respectively.