The role of Rydberg states in photoionization of NO2 and (NO+, O-) ion pair formation induced by one VUV photon

Fragmentation and rearrangement of Breslow intermediates: branches to both radical and ionic pathways

Breslow intermediates are the key species in N-heterocyclic carbene-catalyzed reactions to promote the C-C bond formation. As the fragmentation and rearrangement of Breslow intermediates terminate the catalytic cycle of N-heterocyclic carbene, two mechanisms under debate have been proposed in terms of the radical channel and the ionic route. Theoretical calculations demonstrate herein that ionic and radical characteristics can coexist, depending on the protonation state of the hydroxyl group in Breslow intermediates: radicals are merely generated in the enol system, while both ionic and radical species exist in the enolate system with a lower barrier. Complete pathways for thiamin analogue and N-allyl benzothiazole Breslow intermediates are exclusively constructed considering experimental conditions. The growing population of the enolate under higher pH values rationalizes the increased rate of the fragmentation of thiamin. The fragmentation products of thiamin, namely pyrimidine and ketone, are the thermodynamic products, while the tertiary alcohol is both the kinetic and thermodynamic product for N-allyl benzothiazole Breslow intermediate via a Claisen-like rearrangement. Other NHCs used to synthesize tertiary alcohols could form the enolate due to the base, followed by the production of stable radicals and recombination to form tertiary alcohols. It is concluded that specific protonation states and chemical structures of NHCs account for the distinct mechanisms.

Resolving the F2 bond energy discrepancy using coincidence ion pair production (cipp) spectroscopy

Coincidence ion pair production (cipp) spectra of F₂ were recorded on the DELICIOUS III coincidence spectrometer in the one-photon excitation region of 125 975-126 210 cm^-1. The F⁺ + F^- signal shows a rotational band head structure, corresponding to F₂ Rydberg states crossing over to the ion pair production surface. Spectral simulation and quantum defect analysis allowed the characterization of five new molecular Rydberg states (F₂**): one Π and four Σ states. The lowest-energy Rydberg state spectrum observed (T₀ = 125 999 cm^-1) lacked some of the predicted rotational structure, which allowed an accurate determination of the ion pair production threshold of 15.6229₄± 0.0004₃ eV. Using the well-known atomic fluorine ionization energy and electron affinity, this number leads to a ground state F-F dissociation energy of 1.6012₉± 0.0004₄ eV. Photoelectron photoion coincidence (PEPICO) experiments were also carried out on F₂ and the dissociative photoionization threshold to F⁺ + F was determined as 19.0242 ± 0.0006 eV. Using the atomic fluorine ionization energy, this can be converted to an F₂ dissociation energy of 1.6013₂± 0.0006₂ eV, further confirming the cipp-derived value above. Because the two experiments were independently energy-calibrated, they can be averaged to 1.6013₀± 0.0003₆ eV and this value can be used to derive the fluorine atom's 0 K heat of formation as 77.25₁± 0.01₇ kJ mol^-1. This latter is in excellent agreement with the latest Active Thermochemical Table (ATcT) value but improves its accuracy by almost a factor of three.

Distinguishing resonance symmetries with energy-resolved photoion angular distributions from ion-pair formation in O2 following two-photon absorption of a 9.3 eV femtosecond pulse

We present a combined experimental and theoretical study on the photodissociation dynamics of ion-pair formation in O₂ following resonant two-photon absorption of a 9.3 eV femtosecond pulse, where the resulting O⁺ ions are detected using 3D momentum imaging. Ion-pair formation states of Σg-3 and ³Π_g symmetry are accessed through predissociation of optically dark continuum Rydberg states converging to the B Σg-2 ionic state, which are resonantly populated via a mixture of both parallel-parallel and parallel-perpendicular two-photon transitions. This mixture is evident in the angular distribution of the dissociation relative to the light polarization and varies with the kinetic energy release (KER) of the fragmenting ion pair. The KER-dependent photoion angular distribution reveals the underlying two-photon absorption dynamics involved in the ion-pair production mechanism and indicates the existence of two nearly degenerate continuum resonances possessing different symmetries, which can decay by coupling to ion-pair states of the same total symmetry through internal conversion.

High resolution vibronic state-specific dissociation of NO2+in the 10.0–15.5 eV energy range by synchrotron double imaging photoelectron photoion coincidence

Low-lying electronic states of NO₂⁺are prepared with vibrational structures and their state-specific dissociation mechanisms are unraveled with high-resolution i²PEPICO.

Spectral dependence of photoemission in multiphoton ionization of NO2 by femtosecond pulses in the 375-430 nm range

We investigate the multiphoton ionization of NO₂ using tunable (430-375 nm) femtosecond pulses and photoelectron-photoion coincidence momentum spectroscopy. In order to understand the complex electronic and nuclear photodynamics at play following absorption of three to five photons, we also report extended photoionization calculations using correlated targets and coupled channels. Exploring the multiphoton dissociative ionization (MPDI) and multiphoton ionization (MPI) processes over such a broad energy range enables us to lend further support to our work carried out around 400 nm of a femtosecond laser [S. Marggi Poullain et al., J. Phys. B: At., Mol. Opt. Phys., 2014, 47, 124024]. Two excitation energy regions are identified and discussed in terms of the proposed reaction pathways, highlighting the significant role of Rydberg states, such as the [R*(6a₁)^-1, 3pσ] Rydberg state, in the NO₂ multiphoton excitation and photoionization. These new results support our previous assumption that different bent and linear geometries of the NO₂⁺(X¹Σ_g) ionic state contribute to the MPDI and MPI, consistent with the reported calculations which reveal an important vibronic coupling characterizing the photoemission. Remarkably, the strong anisotropy of the recoil frame photoelectron angular distribution (RFPAD) previously observed at 400 nm appears as a fingerprint across the whole explored photon energy range.

A tandem time-of-flight spectrometer for negative-ion/positive-ion coincidence measurements with soft x-ray excitation

We present a newly constructed spectrometer for negative-ion/positive-ion coincidence spectroscopy of gaseous samples. The instrument consists of two time-of-flight ion spectrometers and a magnetic momentum filter for deflection of electrons. The instrument can measure double and triple coincidences between mass-resolved negative and positive ions with high detection efficiency. First results include identification of several negative-ion/positive-ion coincidence channels following inner-shell photoexcitation of sulfur hexafluoride (SF6).