Ultrafast relaxation of photoexcited superfluid He nanodroplets.

The relaxation of photoexcited nanosystems is a fundamental process of light-matter interaction. Depending on the couplings of the internal degrees of freedom, relaxation can be ultrafast, converting electronic energy in a few fs, or slow, if the energy is trapped in a metastable state that decouples from its environment. Here, we study helium nanodroplets excited resonantly by femtosecond extreme-ultraviolet (XUV) pulses from a seeded free-electron laser. Despite their superfluid nature, we find that helium nanodroplets in the lowest electronically excited states undergo ultrafast relaxation. By comparing experimental photoelectron spectra with time-dependent density functional theory simulations, we unravel the full relaxation pathway: Following an ultrafast interband transition, a void nanometer-sized bubble forms around the localized excitation (He[Formula: see text]) within 1 ps. Subsequently, the bubble collapses and releases metastable He[Formula: see text] at the droplet surface. This study highlights the high level of detail achievable in probing the photodynamics of nanosystems using tunable XUV pulses.

Resolving multi-exciton generation by attosecond spectroscopy.

We propose an experimentally viable attosecond transient absorption spectroscopy scheme to resolve controversies regarding multiexciton (ME) generation in nanoscale systems. Absence of oscillations indicates that light excites single excitons, and MEs are created by incoherent impact ionization. An oscillation indicates the coherent mechanism, involving excitation of superpositions of single and MEs. The oscillation decay, ranging from 5 fs at ambient temperature to 20 fs at 100 K, gives the elastic exciton-phonon scattering time. The signal is best observed with multiple-cycle pump pulses.

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.

Photofragment translation spectroscopy of ClN3 at 248 nm: determination of the primary and secondary dissociation pathways.

Photofragmentation translational spectroscopy was used to identify the primary and secondary reaction pathways in the KrF laser (248 nm) photodissociation of chlorine azide (ClN(3)) under collision-free conditions. Both the molecular channel producing NCl (X (3)Sigma,a (1)Delta) + N(2) and the radical channel producing Cl ((2)P(J)) + N(3) were analyzed in detail. Consistent with previously reported velocity map ion imaging experiments [N. Hansen and A. M. Wodtke, J. Phys. Chem. A 107, 10608 (2003)] a bimodal translational energy distribution is seen when Cl atoms are monitored at mz = 35(Cl(+)). Momentum-matched N(3) counterfragments can be seen at mz = 42(N(3) (+)). The characteristics of the observed radical-channel data reflect the formation of linear azide radical and another high-energy form of N(3) (HEF-N(3)) that exhibits many of the characteristics one would expect from cyclic N(3). HEF-N(3) can be directly detected by electron-impact ionization more than 100 mus after its formation. Products of the unimolecular dissociation of HEF-N(3) are observed in the mz = 14(N(+)) and mz = 28(N(2) (+)) data. Anisotropy parameters were determined for the primary channels to be beta = -0.3 for the NCl forming channel and beta = 1.7 and beta = 0.4 for the linear N(3) and HEF-N(3) forming channels, respectively. There is additional evidence for secondary photodissociation of N(3) and of NCl.

Observation of large water-cluster anions with surface-bound excess electrons.

Anionic water clusters have long been studied to infer properties of the bulk hydrated electron. We used photoelectron imaging to characterize a class of (H2O)n- and (D2O)n- cluster anions (n

Experimental and theoretical study of the infrared spectra of BrHI- and BrDI-.

Gas phase vibrational spectra of BrHI- and BrDI- have been measured from 6 to 17 microm (590-1666 cm(-1)) using tunable infrared radiation from the free electron laser for infrared experiments in order to characterize the strong hydrogen bond in these species. BrHI-.Ar and BrDI-.Ar complexes were produced and mass selected, and the depletion of their signal due to vibrational predissociation was monitored as a function of photon energy. Additionally, BrHI- and BrDI- were dissociated into HBr (DBr) and I- via resonant infrared multiphoton dissociation. The spectra show numerous transitions, which had not been observed by previous matrix studies. New ab initio calculations of the potential-energy surface and the dipole moment are presented and are used in variational ro-vibrational calculations to assign the spectral features. These calculations highlight the importance of basis set in the simulation of heavy atoms such as iodine. Further, they demonstrate extensive mode mixing between the bend and the H-atom stretch modes in BrHI- and BrDI- due to Fermi resonances. These interactions result in major deviations from simple harmonic estimates of the vibrational energies. As a result of this new analysis, previous matrix-isolation spectra assignments are reevaluated.

Hydrated electron dynamics: from clusters to bulk.

The electronic relaxation dynamics of size-selected (H2O)n-/(D2O)n[25 eaq-(s(dagger)) internal conversion lifetime.

Transition state spectroscopy of the I + HI reaction in clusters: photoelectron spectroscopy of IHI-.Arn (n = 1-15).

We have investigated effects of solvation on the transition state spectroscopy and dynamics of the I + HI reaction by measuring the anion photoelectron (PE) spectra of the clusters IHI-.Arn (n = 1-5). Argon clustering results in a successive shift of the PE spectra to lower electron kinetic energies with increasing cluster size. It also leads to significant vibrational cooling in the PE spectra and facilitates the observation of features associated with symmetric stretch vibrations and hindered rotational motions of the transition state complex IHI. The shifts in electron binding energy suggest that the first six argon atoms form a ring around the waist of the IHI- anion, just as in I2-.Arn. The spacing of the antisymmetric stretch features evolves with cluster size and is attributed at least in part to perturbation of the IHI- geometry in larger argon clusters. Intensities of features due to hindered rotation are enhanced for larger clusters, possibly due to solvent perturbation of the neutral transition state region.

Time-resolved photoelectron spectroscopy of molecules and clusters.

Time-resolved photoelectron spectroscopy (TRPES) has become a powerful new tool in studying the dynamics of molecules and clusters. It has been applied to processes ranging from energy flow in electronically excited states of molecules to electron solvation dynamics in clusters. This review covers experimental and theoretical aspects of TRPES, focusing on studies of neutral and negatively charged species.

The transition state of the f + h2 reaction.

The transition state region of the F + H(2) reaction has been studied by photoelectron spectroscopy of FH(2)(-). New para and normal FH(2)(-)photoelectron spectra have been measured in refined experiments and are compared here with exact three-dimensional quantum reactive scattering simulations that use an accurate new ab initio potential energy surface for F + H(2). The detailed agreement that is obtained between this fully ab initio theory and experiment is unprecedented for the F + H(2) reaction and suggests that the transition state region of the F + H(2) potential energy surface has finally been understood quantitatively.