Photofragment velocity map imaging was used to study the H atom elimination mechanism in the first excited state of pyrrole at l ¼ 243.1 nm. Two major channels were observed. The first one (76%) produces very fast H atoms and appears to be due to a rapid direct N-H bond breaking in the excited electronic state. The respective H atom kinetic energy distribution has a strong narrow peak at high energies, showing that %72% of the available energy is transferred into relative fragment translation. The observed angular recoil distribution which is described by an anisotropy parameter of b ¼ À0.37 AE 0.05 indicates that the excited optical transition is preferentially perpendicular with respect to the N-H dissociation coordinate. From the maximal kinetic energy release, the value of the N-H bond dissociation energy was found to be D 0 (N-H) ¼ (32 400 AE 400) cm À1 . The other channel (24%) leads to much slower H atoms with a very broad kinetic energy distribution, consistent with subsequent unimolecular decay reactions of the molecules in the ground electronic state after internal conversion. This conclusion was supported by similar experiments for N-methylpyrrole which showed only H atoms from the second channel and no fast component. The results corroborate the conclusion that the lowest electronic state of pyrrole has ps* anti-bonding character and is repulsive with respect to the stretching of the N-H bond.
The H and D atom elimination mechanisms in the photodissociation of jet cooled pyrrole and pyrrole-d1 have been studied by photofragment velocity map imaging. The molecules were excited to the 1 1A2 (pi sigma*) state at lambda = 243 nm and to the 1 1B2 (pi pi*) state at lambda = 217 nm. H/D atoms were detected by (2 + 1) resonance enhanced multiphoton ionization (REMPI) at lambda = 243 nm. The analysis of the images and the resulting translational energy distributions from the 1 1A2 state demonstrates the existence of two decay pathways, fast mode-specific cleavage of the NH bond in the excited state (channel A) and internal conversion (IC) to the electronic ground state (S0) followed by unimolecular decomposition of the vibrationally hot S0 molecules (channel B). The angular distributions of the H/D atoms from the direct dissociation in the excited state are strongly anisotropic, whereas the decay of the S0 molecules leads to spatially isotropic distributions. The results at lambda = 217 nm indicate that the 1 1B2 state undergoes an ultrafast radiationless transition to 1 1A2 followed by the abovementioned direct mode-specific NH bond fission on the 1 1A2 potential energy surface (channel A') or conversion to S0 and subsequent unimolecular decomposition (channel B'). The latter pathway may also be initiated by a direct relaxation from 1 1B2 to S0. The anisotropy parameter of beta approximately -1 for the direct NH bond fission at lambda = 217 nm is in accordance with the expectations for a perpendicular electronic excitation and a dissociation lifetime that is short compared to the rotational period of the molecules. The fast decay dynamics of both excited electronic states can be rationalized with reference to the theoretically predicted conical intersections between the pi pi*, pi sigma*, and S0 potential energy surfaces and the antibonding nature of the pi sigma* potential energy surface with respect to the NH bond [A. L. Sobolewski, W. Domcke. C. Dedonder-Lardeux and C. Jouvet, Phys. Chem. Chem. Phys. 2002, 4, 1093].
Infrared spectra of CH 2 OH have been recorded in the molecular beam using a combination of depletion and double resonant ionization detected IR (DRID-IR) spectroscopy via the 3p z Rydberg state. With DRID-IR spectroscopy, IR transitions are detected by exciting CH 2 OH in a selected vibrational level in the ground state to a Franck-Condon favorable level in 3p z from which it is ionized. Rotationally resolved spectra of the fundamental CH symmetric stretch (ν 3 ), CH asymmetric stretch (ν 2 ), OH stretch (ν 1 ), and the first OH-stretch overtone (2ν 1 ) are obtained. The rovibrational structure is analyzed with the aid of ab initio calculations and asymmetric rotor simulations. The OH and CH symmetric stretch fundamentals are hybrid a/b-type bands. On the other hand, a pure b-type transition is observed for the CH asymmetric stretch fundamental. The spectrum of the first overtone of the OH stretch is rotationally resolved, indicating that intramolecular vibrational redistribution (IVR) is not extensive. The rotational linewidth, however, is ∼0.8 cm -1 , greater than the laserlimited linewidth observed for the fundamental transitions (∼0.4 cm -1 ). Reasons for the increase in linewidth are discussed. † Part of the special issue "Richard Bersohn Memorial Issue".
Laser Raman spectroscopy (LRS) has been proposed for in situ characterization of molecular species in planetary surface exploration, and three laser Raman spectrometers are included in the science payloads of two under‐development missions to Mars (ESA‐ExoMars2018 and NASA‐Mars2020). We report the first rover test of a laser Raman spectrometer developed for flight, the Mars Micro‐beam Raman Spectrometer (MMRS) in the Atacama Desert (Chile). The MMRS was integrated on the Zoë rover and analyzed subsurface samples brought up by a 1 m drill and delivered by a carousel. The MMRS demonstrated robust performance over 50‐km traverse on rugged terrains. From MMRS data, igneous minerals, carbonates, sulfates and carbonaceous materials were unambiguously identified. Quantified distributions of major minerals and carbonaceous materials are extracted from MMRS results, which can be used to imply the regional geological evolution, and potential bioactivities. MMRS in the field performed as well as an LRS laboratory instrument when MMRS was focused satisfactorily. The discovery of stable γ‐anhydrite, in large quantity (20% in a sample), in the Atacama soils raises an important question of its stability in the field in a natural environment that is worth further laboratory experimental investigation. Copyright © 2015 John Wiley & Sons, Ltd.
The ZCC model invokes vibrational channel specific "detachment orbitals" and attributes this behavior to coupling of the electronic and nuclear motion in the parent anion. The spatial extent of the model detachment orbital is dependent on the final state of O 2 : the higher the neutral vibrational excitation, the larger the electron binding energy. Although vibronic coupling is ignored in most theoretical treatments of PADs in the direct photodetachment of molecular anions, the present findings clearly show that it can be important. These results represent a benchmark data set for a relatively simple system, upon which to base rigorous tests of more sophisticated models.
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