All 253 vibrational levels in the ground electronic state of 12C2H2 with assigned rotational structure reported in the literature from absorption, stimulated emission pumping, and dispersed laser induced fluorescence spectroscopic investigations are gathered. They cover the range up to 18 915 cm−1. Some 219 of these energies are simultaneously fitted using the same so-called Cluster model based on the emergence of three constants of the motion, as previously used to deal with the vibrational energy levels up to 12 000 cm−1 [Abbouti Temsamani and Herman, J. Chem. Phys. 103, 5931 (1995)]. Thirty-nine vibrational constants are produced. The rms value of the fit is 0.81 cm−1. Principal rotational constants are predicted for all levels, which satisfactorily compare with the experimental results. Problems are demonstrated to concern a fraction of the 34 remaining levels only. Thus, the adequacy of the model is fully confirmed. The remaining problems are discussed and globally attributed to problems of a vibration–rotation nature. A comprehensive listing of calculated vibrational energies for all k sublevels up to 15 000 cm−1, including predicted Bv values and assignments is made available.

The vibrational energy pattern in acetylene (iv) : updated global vibration constants for 12c2h2

The non-Markovian approach developed in the companion paper [Hughes et al., J. Chem. Phys. 131, 024109 (2009)], which employs a hierarchical series of approximate spectral densities, is extended to the treatment of nonadiabatic dynamics of coupled electronic states. We focus on a spin-boson-type Hamiltonian including a subset of system vibrational modes which are treated without any approximation, while a set of bath modes is transformed to a chain of effective modes and treated in a reduced-dimensional space. Only the first member of the chain is coupled to the electronic subsystem. The chain construction can be truncated at successive orders and is terminated by a Markovian closure acting on the end of the chain. From this Mori-type construction, a hierarchy of approximate spectral densities is obtained which approach the true bath spectral density with increasing accuracy. Applications are presented for the dynamics of a vibronic subsystem comprising a high-frequency mode and interacting with a low-frequency bath. The bath is shown to have a striking effect on the nonadiabatic dynamics, which can be rationalized in the effective-mode picture. A reduced two-dimensional subspace is constructed which accounts for the essential features of the nonadiabatic process induced by the effective environmental mode. Electronic coherence is found to be preserved on the shortest time scale determined by the effective mode, while decoherence sets in on a longer time scale. Numerical simulations are carried out using either an explicit wave function representation of the system and overall bath or else an explicit representation of the system and effective-mode part in conjunction with a Caldeira–Leggett master equation.

Effective-mode representation of non-Markovian dynamics: a hierarchical approximation of the spectral density. II. Application to environment-induced nonadiabatic dynamics.

We have recorded the absorption spectrum of pyrrole (C5H5N) and furan (C4H4O) on a broad spectral range in the infrared region and under various experimental conditions, using a Fourier transform interferometer. The resolution was good enough to reliably identify band shapes. We have performed the vibrational assignment of the vibration–rotation bands observed between 500 and 12,000 cm−1 for pyrrole, and 500 and 9000 cm−1 for furan. The assignment of the fundamental bands is supported by ab initio calculations at the MP2 level of approximation, considering both band positions and absolute absorption intensities. These calculations also allowed the normal mode coordinates to be drawn for the two species and their geometrical structure to be calculated. Vibrational mode numbering and symmetries are updated to latest conventions in the literature. In the combination and overtone spectral ranges, the band shape information guided the assignment, which could be achieved for most of the numerous observed bands. An extensive set of vibration constants was obtained for each species. The results from the literature available on each species are considered at each step of the investigation.

The vibrational spectrum of pyrrole (C4H5N) and furan (C4H4O) in the gas phase

Photodissociation studies of vibrationless ground state molecules pervade diverse areas of chemical physics, while those of rovibrationally excited molecules are expected to have even more impact due to the additional fascinating possibilities they offer and the new horizons they open. Photodissociation of rovibrationally excited species involves a double-resonance scheme in which a photodissociative transition is initiated from an excited rovibrational state that might substantially affect the intensity and wavelength dependence of the photoabsorption spectrum. In favourable cases, when the energy is disposed in vibrations that are strongly coupled to the reaction coordinate, this pre-excitation might influence photodissociation pathways and lead to selective bond cleavage. In other cases it might influence the branching ratio between different fragments by altering the photodissociation dynamics. Moreover, the photodissociation of rovibrationally excited species can serve as a sensitive means for detect...

Controlling bond cleavage and probing intramolecular dynamics via photodissociation of rovibrationally excited molecules

The evolution of high-resolution spectroscopic investigation of the vibration–rotation energy states of acetylene in its ground electronic state is presented, focusing on advances co-authored by the ULB group. The emergence of a global picture accounting for all available spectroscopic fingerprints, at their full accuracy, is highlighted. The contributions of this research to various topics is illustrated, including instrumental developments, local mode trends, quantum vs. classical correspondence, energy vs. time approaches and nucleation processes.

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The acetylene ground state saga

We have performed the rovibrational analysis of the absorption spectrum of 12C2D2 between 5150 and 8000 cm−1, recorded by Fourier transform absorption spectroscopy, and between 12 800 and 16 600 cm−1, recorded by intracavity laser absorption spectroscopy. Respectively 10 and 9 bands are reported for the first time in each range. Improved or new rovibrational parameters were obtained for 34 vibrational levels altogether. The vibrational energies we obtained, together with those reported in the literature, were taken into account to model the vibrational energy pattern in 12C2D2(X 1Σg+). The analysis was performed in successive steps, inferring each time suitable parameters. The 44/55, 11/33, 12/33, and 1/244 quartic order anharmonic resonances were introduced during the procedure. They altogether define vibrational clusters which are characterized by only two dynamical constants of motion, Ns=V1+V2+V3 and k=l4+l5.

A. Pisarchik

Papers

The Vibrational Energy Levels in Acetylene (III): 12C2D2