Rotation–vibration state resolved unimolecular dynamics of highly vibrationally excited CH3O (X̃ 2E). I. Observed stimulated emission pumping spectra

Submillimeter-Wave Spectroscopy of the CH3O Radical

The methoxy radical, CH3O, has long been studied experimentally and theoretically by spectroscopists because it displays a weak Jahn-Teller effect in its electronic ground state, combined with a strong spin-orbit interaction. In this work, we report an extension of the measurement of the pure rotational spectrum of the radical in its vibrational ground state in the submillimeter-wave region (350-860 GHz). CH3O was produced by H-abstraction from methanol using F atoms, and its spectrum was probed in absorption using an association of source-frequency modulation and Zeeman modulation spectroscopy. All the observed transitions together with available literature data in ν = 0 were combined and fit using an effective Hamiltonian allowing to reproduce the data at their experimental accuracy. The newly measured transitions involve significantly higher frequencies and rotational quantum numbers than those reported in the literature (f < 860 GHz and N ≤ 15 instead of 372 GHz and 7, respectively), which results in significant improvements in the spectroscopic parameters determination. The present model is well constrained and allows a reliable calculation of the rotational spectrum of the radical over the entire microwave to submillimeter-wave domain. It can be used with confidence for future searches of CH3O in the laboratory and in the interstellar medium.

Unified Description of the Jahn-Teller Effect in Molecules with Only Cs Symmetry: Cyclohexoxy in Its Full 48-Dimensional Internal Coordinates

The two lowest potential energy surfaces of cyclohexoxy which are coupled by conical intersections and the spin-orbit interaction are determined in the full 48-dimensional internal coordinate space using a feedforward neural network to fit a diabatic potential energy matrix. The electronic structure data are obtained at the multireference configuration interaction with single- and double-excitation level. Underlying parallels between these coupled surfaces and those of the alkoxy radicals methoxy and isopropoxy are established. Earlier work by Dillon and Yarkony is extended. While the parallels would have been challenging to appreciate using the concept of the Jahn-Teller active modes, they are readily seen in terms of two internal modes centered at the conical intersection: g the energy difference gradient vector and h the interstate coupling gradient vector. In other words, g and h vectors provide a unified description of the Jahn-Teller effect in molecules exhibiting C3v and quasi-C3v symmetries. A spectral simulation in the full 48-vibrational-internal coordinate space is reported. This spectrum is obtained using recently developed algorithms designed to increase the size of the systems that can be treated with a time-independent vibronic coupling approach.

Jet-cooled laser-induced fluorescence spectroscopy of isopropoxy radical: vibronic analysis of B̃-X̃ and B̃-Ã band systems

Recently we published [ Liu et al. J. Chem. Phys. 2013 , 139 , 154312 ] an analysis of the rotational structure of the B̃-X̃ origin band spectrum of isopropoxy, which confirmed that the double methyl substitution of methoxy to yield the isopropoxy radical only slightly lifted the degeneracy of the former's X̃(2)E state. Additionally the spectral results provided considerable insight into the relativistic and nonrelativistic contributions to the experimental splitting between the components of the (2)E state. However, left unexplained was how the Jahn-Teller (JT) vibronic coupling terms within methoxy's (2)E state manifest themselves as pseudo-Jahn-Teller (pJT) vibronic coupling between the Ã(2)A″ and X̃(2)A' levels of isopropoxy. To cast additional light on this subject we have obtained new isopropoxy spectra and assigned a number of weak, "forbidden" vibronic transitions in the B̃-X̃ spectrum using new electronic structure calculations and rotational contour analyses. The mechanisms that provide the nonzero probability for these transitions shed considerable information on pJT, spin-orbit, and Coriolis coupling between the à and X̃ states. We also report a novel mechanism caused by pJT coupling that yields excitation probability to the B̃ state dependent upon the permanent dipole moments in the B̃ and à or X̃ states. By combining a new B̃-à and the earlier B̃-X̃ rotational analyses we determine a much improved value for the experimental Ã-X̃ separation.

State-resolved spectroscopy of high vibrational levels of water up to the dissociative continuum

We summarize here our experimental studies of the high rovibrational energy levels of water. The use of double-resonance vibrational overtone excitation followed by energy-selective photofragmentation and laser-induced fluorescence detection of OH fragments allowed us to measure previously inaccessible rovibrational energies above the seventh OH-stretch overtone. Extension of the experimental approach to triple-resonance excitation provides access to rovibrational levels via transitions with significant transition dipole moments (mainly OH-stretch overtones) up to the dissociation threshold of the O-H bond. A collisionally assisted excitation scheme enables us to probe vibrations that are not readily accessible via pure laser excitation. Observation of the continuous absorption onset yields a precise value for the O-H bond dissociation threshold, 41 145.94 ± 0.15 cm(-1). Finally, we detect long-lived resonances as sharp peaks in spectra above the dissociation threshold.

Communication: Feshbach resonances in the water molecule revealed by state-selective spectroscopy

We employ triple-resonance vibrational overtone excitation to access quasibound states of water from several fully characterized bound states of the molecule. Comparison of the measured dissociation spectra allows a rigorous assignment of rotational quantum numbers J, nuclear spin and parity, and a tentative vibrational characterization of the observed resonances. Their asymmetrical shapes (Fano profiles) reflect interference of dipole moments for transitions to these resonances with that to the dissociative continuum. The assignments and Fano profile parameters of the resonances stand as a benchmark for the extension of accurate quantum-mechanical calculations to activated complexes of water. The narrow widths of some of these resonances indicate that water molecules may survive for as long as up to 60 ps in states above the dissociation threshold. We consider the possible implication of such long-lived states for the kinetics of water dissociation and the OH+H association reaction.

Slow electron velocity-map imaging of negative ions: applications to spectroscopy and dynamics

Anion photoelectron spectroscopy (PES) has become one of the most versatile techniques in chemical physics. This article briefly reviews the history of anion PES and some of its applications. It describes efforts to improve the resolution of this technique, including anion zero electron kinetic energy (ZEKE) and the recently developed method of slow electron velocity-map imaging (SEVI). Applications of SEVI to studies of vibronic coupling in open-shell systems and the spectroscopy of prereactive van der Waals complexes are then discussed.

Probing chemical dynamics with negative ions

Experiments are reviewed in which key problems in chemical dynamics are probed by experiments based on photodetachment and/or photoexcitation of negative ions. Examples include transition state spectroscopy of biomolecular reactions, spectroscopy of open shell van der Waals complexes, photodissociation of free radicals, and time-resolved dynamics in clusters. The experimental methods used in these investigations are described along with representative systems that have been studied.

Unimolecular processes in CH2OH below the dissociation barrier: O–H stretch overtone excitation and dissociation

The OH-stretch overtone spectroscopy and dynamics of the hydroxymethyl radical, CH(2)OH, are reported in the region of the second and third overtones, which is above the thermochemical threshold to dissociation to H+CH(2)O (D(0)=9600 cm(-1)). The second overtone spectrum at 10 484 cm(-1) is obtained by double resonance IR-UV resonance enhanced multiphoton ionization (REMPI) spectroscopy via the 3p(z) electronic state. It is rotationally resolved with a linewidth of 0.4 cm(-1) and displays properties of local-mode vibration. No dissociation products are observed. The third overtone spectra of CH(2)OH and CD(2)OH are observed at approximately 13 600 cm(-1) by monitoring H-atom photofragments while scanning the excitation laser frequency. No double resonance REMPI spectrum is detected, and no D fragments are produced. The spectra of both isotope analogs can be simulated with a linewidth of 1.3 cm(-1), indicating dissociation via tunneling. By treating the tunneling as one dimensional and using the calculated imaginary frequency, the barrier to dissociation is estimated at about 15 200 cm(-1), in good agreement with theoretical estimations. The Birge-Sponer plot is linear for OH-stretch vibrations 1nu(1)-4nu(1), demonstrating behavior of a one-dimensional Morse oscillator. The anharmonicity parameter derived from the plot is similar to the values obtained for other small OH containing molecules. Isomerization to methoxy does not contribute to the predissociation signal and the mechanism appears to be direct O-H fission via tunneling. CH(2)OH presents a unique example in which the reaction coordinate is excited directly and leads to predissociation via tunneling while preserving the local-mode character of the stretch vibration.

Explorations of conical intersections and their ramifications for chemistry through the Jahn-Teller effect

Much recent progress has been made theoretically and computationally towards understanding the importance of conical intersections for chemical reactions. Nonetheless, experimental characterization of conical intersections has proven extremely difficult with one striking exception: the Jahn-Teller conical intersection. This article overviews the fundamental similarity of a variety of conical intersections and demonstrates how the spectroscopy of Jahn-Teller active molecules can be used to characterize them. Specific results are reviewed for four representative Jahn-Teller active molecules, C5H5, C6H6+, Ag3 and CH3O.

The dynamics of "stretched molecules": experimental studies of highly vibrationally excited molecules with stimulated emission pumping

We review stimulated emission pumping as used to study molecular dynamics. The review presents unimolecular as well as scattering studies. Topics include intramolecular vibrational redistribution, unimolecular isomerization and dissociation, van der Waals clusters, rotational energy transfer, vibrational energy transfer, gas-surface interactions, atmospheric effects resulting from nonequilibrium vibrational excitation, and vibrational promotion of electron transfer.

EXPERIMENTAL STUDIES OF RESONANCES IN UNIMOLECULAR DECOMPOSITION

▪ Abstract  In recent years we have witnessed tremendous progress in our understanding of unimolecular reactions on a fully state-resolved level. Here we describe recent state-resolved experimental studies of resonances in unimolecular reactions, focusing on the transition from isolated to overlapping resonances. Depending on the well depth and extent of intramolecular vibrational energy redistribution, the resonances can exhibit properties ranging from mode- and state-selective to statistical behavior. In the statistical limit the resonances are usually overlapped, and interference effects may become prominent. We use recent studies of HCO, HFCO, and CH3O to examine the transition from mode-selective to statistical behavior in the isolated regime. Experimental and theoretical studies of NO2, including photofragment yield spectra, fully resolved NO quantum state distributions, and decomposition rates are used to examine unimolecular decomposition in the regime of overlapping resonances.