Alison V. Davis

Time-resolved photoelectron imaging of the photodissociation of I2

Time-resolved photoelectron imaging is presented as a new method for the study of anion dynamics. Time-dependent photoelectron energy spectra and angular distributions are extracted from images taken during the dissociation of I2− at 793 nm, and used to follow in detail the dissociation dynamics from 0–1 ps.

Photodissociation of gas phase I3− using femtosecond photoelectron spectroscopy

The photodissociation dynamics of gas phase I3− following 390 nm excitation are studied using femtosecond photoelectron spectroscopy. Both I− and I2− photofragments are observed; the I2− exhibits coherent oscillations with a period of 550 fs corresponding to ∼0.70 eV of vibrational excitation. The oscillations dephase by 4 ps and rephase at 45 and 90.5 ps on the anharmonic I2− potential. The gas phase frequency of ground state I3− is determined from oscillations in the photoelectron spectrum induced by resonance impulsive stimulated Raman scattering. The dynamics of this reaction are modeled using one- and two-dimensional wave packet simulations from which we attribute the formation of I− to three-body dissociation along the symmetric stretching coordinate of the excited anion potential. The photodissociation dynamics of gas phase I3− differ considerably from those observed previously in solution both in terms of the I2− vibrational distribution and the production of I−.

Time-resolved study of the symmetric SN2-reaction I−+CH3I

Time-resolved photoelectron spectroscopy of negative ions has been applied to study the title reaction as a model system for gas phase SN2 reactions. Starting from the precursor cluster I2−⋅CH3I, the interaction of the reactants I− and CH3I is initiated by a pump pulse and the subsequent dynamics are observed with a delayed probe pulse used to detach the excess electron and measure their photoelectron spectra. Using two different pump photon energies, which lead to different amounts of internal energy available to the reaction complex, a number of dynamical features have been observed. For small internal excitation, the reactants only form stable, albeit vibrationally excited, I−⋅CH3I complexes. However, with increased internal excitation, complexes are formed that exhibit biexponential decay back to I− and CH3I reactants with time scales of 0.8 and 10 ps. Similar dynamics are expected for entrance channel complex formed in the first step of a gas phase SN2 reaction.

Excited-state detachment dynamics and rotational coherences of C2− via time-resolved photoelectron imaging

Abstract Time-resolved photoelectron imaging (TRPEI) is used to investigate the effect of time-evolving alignment on the photoelectron angular distribution (PAD) from anion photodetachment. The B ← X 0 0 0 transition in C 2 − is pumped with a femtosecond laser pulse at 541 nm and probed by femtosecond photodetachment at 264 nm. The pump pulse produces rotational coherences in the upper state that exhibit partial and full revivals, as evidenced by modulation of the PAD anisotropy moments. From these, one can extract the excited state rotational constant of C 2 − and information regarding the molecular frame PAD.

Vibrational relaxation in I2−(Ar)n (n=1,2,6,9) and I2−(CO2)n (n=1,4,5) clusters excited by femtosecond stimulated emission pumping

Vibrational relaxation dynamics in I2−(Ar)n (n=1,2,6,9) and I2−(CO2)n (n=1,4,5) clusters are studied using femtosecond stimulated emission pumping (fs-SEP) in conjunction with femtosecond photoelectron spectroscopy. fs-SEP generates coherently excited I2− within the cluster; results are reported here for excitation energies of 0.57 and 0.75 eV. The time-dependent PE spectra track relaxation of the clustered I2− through coherent intensity oscillations observed at short times (<10 ps) and shifts of the photoelectron spectra that can be seen out to several hundred picoseconds. The relaxation rates depend on the cluster type and excitation energy: the overall time scale in I2−(CO2)n clusters is relatively independent of both, but in I2−(Ar)n clusters the time scale generally increases with cluster size and decreases with excitation energy. The observed dynamics for I2−(CO2) and several of the I2−(Ar)n clusters directly probe the time scale for solvent evaporation.

Vibrational relaxation in clusters: Energy transfer in I2−(CO2)4 excited by femtosecond stimulated emission pumping

Vibrational relaxation dynamics in I2−(CO2)4 clusters are monitored by femtosecond stimulated emission pumping in conjunction with femtosecond photoelectron spectroscopy. Femtosecond pump and tunable dump pulses coherently excite the I2− within the cluster with vibrational energies ranging from 0.57 to 0.86 eV; the subsequent dynamics are monitored via the time-dependent photoelectron spectrum, and are compared to those resulting from excitation of bare I2−. Two observables are used to follow the vibrational relaxation from the vibrationally excited I2− to the surrounding solvent molecules. From 0 to 4 ps, relaxation is apparent through a time-dependent increase in the oscillation which is monitored at its inner turning point. At longer times, out to ∼100 ps, shifts in the photoelectron spectra are used to determine the vibrational energy content of the I2−. Indirect evidence is presented for early rapid energy loss during the first half-oscillation of the wave packet across the potential.

Femtosecond stimulated emission pumping: Characterization of the I2− ground state

Femtosecond stimulated emission pumping in combination with femtosecond photoelectron spectroscopy is used to characterize the potential energy function of the I2−(X 2Σu+) ground state up to vibrational energies within 2% of the dissociation limit. The frequency and anharmonicity of this state are measured at a series of vibrational energies up to 0.993 eV by coherently populating a superposition of ground state vibrational levels using femtosecond stimulated emission pumping, and monitoring the resulting wave packet oscillations with femtosecond photoelectron spectroscopy. The dissociative I2−(A′ 2Πg,1/2) state is used for intermediate population transfer, allowing efficient population transfer to all ground state levels. Using the measured frequencies and anharmonicities, the X 2Σu+ state has been fit to a modified Morse potential with the β-parameter expanded in a Taylor series, and the bond length, well depth, and υ=0–1 fundamental frequency set equal to our previously determined Morse potential [J....

Comment on `Iodine effect on the relaxation pathway of photoexcited I−(H2O)n clusters' [Chem. Phys. Lett. 335 (2001) 475]

a Department of Chemistry, University of California at Berkeley, Berkeley, CA 94720-1460, USA b Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA c Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA d Institut f€ur Physikalische Chemie und Elektrochemie I, Heinrich-Heine-Universit€at D€usseldorf, Universit€atstrasse 1, 40225 D€usseldorf, Germanyt

Electron solvation dynamics in I−(NH3)n clusters

Femtosecond photoelectron spectroscopy (FPES) is used to monitor the dynamics associated with the excitation of the charge-transfer-to-solvent (CTTS) precursor states in I-(NH3)n = 4-15 clusters. The FPE spectra imply that the weakly bound excess electron in the excited state undergoes partial solvation via solvent rearrangement on a time scale of 0.5-2 ps, and this partially solvated state decays by electron emission on a 10-50 ps time scale. Both the extent of solvation and the lifetimes increase gradually with cluster size, in contrast to the more abrupt size-dependent effects previously observed in I-(H2O)n clusters.

Femtosecond stimulated emission pumping:: Dynamics of vibrational energy loss in excited I2−(CO2)4 clusters

Abstract Femtosecond stimulated emission pumping in conjunction with femtosecond photoelectron spectroscopy is used to monitor dynamics within I2−(CO2)4 following coherent vibrational excitation of the I2− chromophore. Femtosecond pump and dump pulses create an I2− wavepacket with 0.53 eV vibrational energy. Subsequent evolution of this wavepacket is monitored via its time-dependent photoelectron spectrum. At this energy, the vibrational frequency of the embedded I2− is 80 cm−1, 7 cm−1 higher than that of bare I2−. As the I2− loses energy to the CO2 molecules, the wavepacket frequency increases linearly at a rate of 3.8 cm−1/ps during the initial 3 ps of coherence. The rate of energy transfer can be determined either by this increase in vibrational frequency or by the shift of the measured photoelectron spectrum. No solvent evaporation during the first 7 ps is observed, but three CO2 molecules evaporate on a microsecond time scale.