Glimpsing resonances as F and H₂ react

The reaction of fluorine atoms with hydrogen molecules has long provided a window into the subtle effects of quantum mechanics on chemical dynamics. Kim et al. now show that the system still has some secrets left to reveal. The authors applied photodetachment to FH₂⁻ anions and their deuterated analogs. This allowed them to intercept the reaction trajectory in the middle and thereby uncover unanticipated weakly bound resonances. Theoretical calculations explain these observations and predict additional similar features that have yet to be seen.

Science, this issue p. 510

Abstract

Electron transfer from valence to conduction band states in semiconductors is the basis of modern electronics. Here, attosecond extreme ultraviolet (XUV) spectroscopy is used to resolve this process in silicon in real time. Electrons injected into the conduction band by few-cycle laser pulses alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ~450-attosecond step rise time provides an upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band-gap modifications due to lattice motion, which occurs on a time scale of 60 ± 10 femtoseconds, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field–induced electron tunneling.

Relaxing in a Water Jet

High-energy irradiation of liquid water and its solutes can transiently liberate electrons, which act as potent chemical reductants, but they are challenging to characterize precisely. Seeking to bridge the gap between liquid and gas, Elkins et al. (p. 1496) report results from photoelectron spectroscopy of hydrated electron dynamics in a liquid jet. The results reveal a very rapid transition from the electronic excited state to the ground state, prior to full relaxation of the solvent shell.

Abstract

The three-body dissociation of ${\text{I}}_2^{-}$(CO₂) following excitation of the ${\text{I}}_2^{-}$ chromophore to the repulsive A′²Π_g,1/2 and $B{}^2{\mathrm{\Sigma }}_{g\text{,}1/2}^{+}$ electronic states at 1.72 and 3.21 eV has been investigated using fast beam photofragment translational spectroscopy. The translational energy distributions for three-body dissociation provide a direct measurement of the CO₂ binding energy, yielding a value of 218 ± 10 meV. These distributions are vibrationally resolved and show that some CO₂ is produced with bend excitation. Dalitz plots show that the dominant three-body decay mechanism is asynchronous–concerted decay, in which the two bond cleavages are distinct but nearly simultaneous events.

Abstract

Excited state dynamics of acetonitrile cluster anions, $({\text{CH}}_3\text{CN}{)}_n^{-}$, were investigated using time-resolved photoelectron imaging (TRPEI) for 20 ⩽ n ⩽ 50. The clusters were excited and then photodetached with femtosecond pump and probe pulses at 790 and 395 nm, respectively. Excited state lifetimes varied between 200 and 270 fs over this size range, showing no obvious size trend. Experimental evidence indicates that we are exciting ‘isomer II’ clusters in which the excess electron is valence-bound to a solvated anionic dimer core. The absence of an obvious size-dependence in the excited state lifetimes is consistent with such a structure.

Abstract

We report a systematic study of the photoelectron spectroscopy of hydrated electrons in liquid water jets using multiple precursors and photodetachment wavelengths. Hydrated electrons were generated in and detached from liquid microjets using two photons from a single nanosecond laser pulse at 266 or 213 nm. Solutions of 50 to 250 mM potassium hexacyanoferrate(II) or potassium iodide were used to provide precursor anions. All of our experimental conditions yield similar results, giving a mean vertical binding energy of 3.6 ± 0.1 eV at a temperature of ∼280 K, a slightly higher value than in recent reports.

Abstract

The degree of electronic and nuclear coupling in the Cl + H₂ reaction has become a vexing problem in chemical dynamics. We report slow electron velocity-map imaging (SEVI) spectra of ClH₂^– and ClD₂^–. These spectra probe the reactant valley of the neutral reaction potential energy surface, where nonadiabatic transitions responsible for reactivity of the Cl excited spin-orbit state with H₂ would occur. The SEVI spectra reveal progressions in low-frequency Cl·H₂ bending and stretching modes, and are compared to simulations with and without nonadiabatic couplings between the Cl spin-orbit states. Although nonadiabatic effects are small, their inclusion improves agreement with experiment. This comparison validates the theoretical treatment, especially of the nonadiabatic effects, in this critical region of the Cl + H₂ reaction, and suggests strongly that these effects are minor.

Abstract

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.

Abstract

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 ≤ 200 molecules) with vertical binding energies that are significantly lower than those previously recorded. The data are consistent with a structure in which the excess electron is bound to the surface of the cluster. This result implies that the excess electron in previously observed water-cluster anions, with higher vertical binding energies, was internally solvated. Thus, the properties of those clusters could be extrapolated to those of the bulk hydrated electron.