Arthur E. Bragg

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.

Comment on "Characterization of Excess Electrons in Water-Cluster Anions by Quantum Simulations"

The conclusion by Turi et al. (Reports, 5 August 2005, p. 914) that all experimental spectral and energetic data on water-cluster anions point toward surface-bound electrons is overstated. Comparison of experimental vertical detachment energies with their calculated values for (H2O) n– clusters with surface-bound and internalized electrons supports previous arguments that both types of clusters exist.

Time-resolved relaxation dynamics of Hgn− (11⩽n⩽16,n=18) clusters following intraband excitation at 1.5 eV

Electron-nuclear relaxation dynamics are studied in Hg(n) (-) (11 <or= n <or= 16,n = 18) using time-resolved photoelectron imaging. The excess electron in the anion uniquely occupies the p band and is excited intraband by 1.53 eV pump photons; the subsequent dynamics are monitored by photodetachment at 3.06 eV and measurement of the photoelectron images as a function of pump-probe delay. The initially excited state decays on a time scale of approximately 10 ps, and subsequent relaxation dynamics reveal a smooth evolution of the photoelectron spectra towards lower electron kinetic energy over 50-100 ps. Qualitatively, the relaxation process is captured by a simple kinetic model assuming a series of radiationless transitions within a dense manifold of electronic states. All the clusters studied show similar dynamics with the exception of Hg(11) (-) in which the initially prepared state does not decay as quickly as the others.

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.

Time-resolved intraband electronic relaxation dynamics of Hgn− clusters (n=7–13,15,18) excited at 1.0 eV

Time-resolved photoelectron imaging has been used to study the relaxation dynamics of small Hg(n) (-) clusters (n=7-13,15,18) following intraband electronic excitation at 1250 nm (1.0 eV). This study furthers our previous investigation of single electron, intraband relaxation dynamics in Hg(n) (-) clusters at 790 nm by exploring the dynamics of smaller clusters (n=7-10), as well as those of larger clusters (n=11-13,15,18) at a lower excitation energy. We measure relaxation time scales of 2-9 ps, two to three times faster than seen previously after 790 nm excitation of Hg(n) (-), n=11-18. These results, along with size-dependent trends in the absorption cross-section and photoelectron angular distribution anisotropy, suggest significant evolution of the cluster anion electronic structure in the size range studied here. Furthermore, the smallest clusters studied here exhibit 35-45 cm(-1) oscillations in pump-probe signal at earliest temporal delays that are interpreted as early coherent nuclear motion on the excited potential energy surfaces of these clusters. Evidence for evaporation of one or two Hg atoms is seen on a time scale of tens of picoseconds.

C6- electronic relaxation dynamics probed via time-resolved photoelectron imaging

Anion time-resolved photoelectron imaging has been used to investigate the electronic relaxation dynamics of C(6) (-) following excitation of the C (2)Pi(g)<--X (2)Pi(u) and 2 (2)Pi(g)<--X (2)Pi(u) 0(0) (0) transitions at 607 and 498 nm, respectively. Analysis of evolving photodetachment energy distributions reveals differing relaxation pathways from these prepared states. Specifically, the C (2)Pi(g) 0(0) level relaxes on a time scale of 620+/-30 fs to vibrationally hot ( approximately 2.0 eV) anion ground state both directly and indirectly through vibrationally excited levels of the intermediate-lying A (2)Sigma(g) (+) state that decay with a time scale of 2300+/-200 fs. In contrast, the 2 (2)Pi(g) 0(0) level relaxes much more quickly (<100 fs) to vibrationally hot ( approximately 2.5 eV) anion ground state directly and with transient population accumulation in the A (2)Sigma(g) (+), B (2)Sigma(u) (+), and C (2)Pi(g) electronic levels, as determined by spectral and time-scale analyses. This work also presents the experimental observation of the optically inaccessible B (2)Sigma(u) (+) state, which is found to have an electronic term value of 1.41+/-0.05 eV.

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.

Structural and solvent control of nonadiabatic photochemical bond formation: photocyclization of o-terphenyl in solution.

Elucidating the molecular dynamics that underlie photoinduced electrocyclization is a critical step toward controlling nonadiabatic photochemistry that enables bond formation. Here we present a comprehensive examination of the photochemical dynamics of o-terphenyl (OTP) in solution. Ultrafast transient absorption measurements demonstrate that OTP cyclizes upon 266 nm photoexcitation to form 4a,4b-dihydrotriphenylene (DHT) on a solvent-dependent time scale of 1.5-4 ps, considerably slower than the nonadiabatic cyclization of related diarylethenes. Correlations in these time scales versus bulk solvent properties reveal that mechanical rather than electrostatic solvent-solute interactions impact the excited-state relaxation rate, impeding nuclear dynamics leading toward the conical intersection for cyclization. In contrast, solvent-dependent mechanical interactions are observed to facilitate vibrational relaxation of DHT on time scales of 10-25 ps. DHT decays via thermally activated ring-opening with a lifetime of ∼46 ns in tetrahydrofuran, 12 orders of magnitude faster than dihydrophenanthrenes. We conclude that the differences in excited-state dynamics of OTP and diarylethenes and the relative stability of their cyclized products are determined by the relative strain induced by twisting the central carbon-carbon bond that bridges the terminal phenyl rings in each to enable bond formation. We relate these structure-dynamics relationships to the feasibility of photoinduced cyclodehydrogenation of o-arenes and design considerations for molecular photoswitches.