Unravelling the Electronic State of NO2 Product in Ultrafast Photodissociation of Nitromethane

Nitrogen Reduction to Ammonia on a Fe16 Nanocluster: A Computational Study of Catalysis

The sequence of elementary steps leading to reductive ammonia formation from N2 and H2 catalyzed by a Fe16 cluster is studied using generalized gradient approximation density functional theory and an all-electron basis set of triple-ζ quality. The computational methods are validated by comparison to experimental data such as binding energies where possible. First, the associative and dissociative attachment of N2 to Fe16 is considered, followed by exploration of the pathways leading to distal (Fe16-N-NH2) and enzymatic (NFe16-NH2) formation of an amino group. Next, the pathways leading to NH3 formation in both distal and enzymatic cases are examined. Two mechanisms for NH3 detachment have been discovered. An interesting peculiarity of the pathways is that they often proceed with total spin fluctuations, which are related to the rupture and formation of bonds on the surface of the catalyst over the course of the reactions. The reaction Fe16 + N2 + 2H2 → Fe16NH + NH3 is found to be exothermic by 1.02 eV (93.8 kJ/mol).

Coulomb Explosion Dynamics of Multiply Charged para-Nitrotoluene Cations

This work explores Coulomb explosion (CE) dissociation pathways in multiply charged cations of para-nitrotoluene (PNT), a model compound for nitroaromatic energetic molecules. Experiments using strong-field ionization and mass spectrometry indicate that metastable cations PNT²⁺ and PNT³⁺ undergo CE to produce NO₂⁺ and NO⁺. The experimentally measured kinetic energy release from CE upon formation of NO₂⁺ and NO⁺ agrees qualitatively with the kinetic energy release predicted by computations of the reaction pathways in PNT²⁺ and PNT³⁺ using density functional theory (DFT). Both DFT computations and mass spectrometry identified additional products from CE of highly charged PNT^q+ cations with q > 3. The dynamical timescales required for direct CE of PNT²⁺ and PNT³⁺ to produce NO₂⁺ were estimated to be 200 and 90 fs, respectively, using ultrafast disruptive probing measurements.

Ultrafast Dynamics of Nitro-Nitrite Rearrangement and Dissociation in Nitromethane Cation

We report new insights into the ultrafast rearrangement and dissociation dynamics of nitromethane cation (NM⁺) using pump-probe measurements, electronic structure calculations, and ab initio molecular dynamics simulations. The "roaming" nitro-nitrite rearrangement (NNR) pathway involving large-amplitude atomic motion, which has been previously described for neutral nitromethane, is demonstrated for NM⁺. Excess energy resulting from initial population of the electronically excited D₂ state of NM⁺ upon strong-field ionization provides the necessary energy to initiate NNR and subsequent dissociation into NO⁺. Both pump-probe measurements and molecular dynamics simulations are consistent with the completion of NNR within 500 fs of ionization with dissociation into NO⁺ and OCH₃ occurring ∼30 fs later. Pump-probe measurements indicate that NO⁺ formation is in competition with the direct dissociation of NM⁺ to CH₃⁺ and NO₂. Electronic structure calculations indicate that a strong D₀ → D₁ transition can be excited at 650 nm when the C-N bond is stretched from its equilibrium value (1.48 Å) to 1.88 Å. On the other hand, relaxation of the NM⁺ cation after ionization into D₀ occurs in less than 50 fs and results in observation of intact NM⁺. Direct dissociation of the equilibrium NM⁺ to produce NO₂⁺ and CH₃ can be induced with 650 nm excitation via a weakly allowed D₀ → D₂ transition.

Methyl substitution effects on the non-adiabatic dynamics of benzene: lifting three-state quasi-degeneracy at conical intersections

Previously, theoretical calculations on the non-adiabatic dynamics of benzene from the S₂ state have indicated that the S₂/S₁ and S₁/S₀ conical intersections (CIs) facilitate ballistic nuclear wavepacket motion from S₂ to S₀ (fast channel) and branching to S₁ (slow channel). In this paper, we present time-resolved photoelectron spectra of benzene and its methyl-derivatives (toluene and o-xylene) measured with a vacuum-UV laser, which clearly reveal both the fast and slow channels. The extremely short propagation time of the wavepacket between the two CIs of benzene indicates that the two are in close proximity to each other, while methyl substitution extends the propagation time and decreases the branching ratio into the fast channel. The results suggest that the quasi-degeneracy of the three states in benzene is lifted by the geometrical shifts of the CIs by methyl substitution.

Probing ultrafast dynamics during and after passing through conical intersections

Time-resolved photoelectron spectroscopy using vacuum-UV probe pulses enables observing ultrafast dynamics during and after passing through conical intersections (CIs). The ring-puckering CI plays a prominent role following the ππ* photoexcitation of furan. More than 90% of the excited molecules safely return to the original ground state, while the remaining 10% transforms into isomers after passing through the puckering CI.

UV-Driven Harmonic Generation for Time-Resolved Photoelectron Spectroscopy of Polyatomic Molecules

A single-order harmonic pulse in the vacuum-ultraviolet (VUV) is highly desirable for time-resolved photoelectron spectroscopy (TRPES) of polyatomic molecules. A high-power 9th harmonic of a Ti:sapphire laser (hv = 14 eV) is obtained using a UV driving laser at 270 nm (the 3rd harmonic). We describe our recent efforts to develop VUV TRPES combined with UV-driven harmonic generation, and present a few representative results from our recent TRPES studies.