Ph. Brechignac

Theory of mode specific vibrational predissociation: The HF dimer

Quantum mechanical close coupling calculations on (HF)2 vibrational predissociation are presented. The model considers vibrational excitation of the proton donor monomer. The other momomer is frozen at its equilibrium position. The linewidth (lifetime) and final state rotational distribution of the initially vibrationally excited HF were calculated using the artificial channel method. The potential surface was taken from Cournoyer and Jorgensen. The calculated linewidth is in good agreement with recent experimental measurements. This result is drastically different from what is obtained in the collinear model, merely reflecting the important influence of the rotational–vibrational coupling in the highly anisotropic systems. The final rotational distribution of the initially excited HF monomer is highly inverted. The possibility of a rotational in conjunction with a conventional vibrational HF laser is raised.

Resonant two-photon two-color photoionization (R2P2CI) spectra of aniline-Ar n clusters: Isomer strutures and solvent shifts

We present resonant two-photon two-color photoionization (R2P2CI) spectra of a series of Aniline-Arn complexes (n=1−6). An apparently anomalous blue shifted spectra for An-Ar3 is explained by a modified spectral shift additivity rule which assigns different shifts to different relative positions of the Ar with respect to aniline. Evidence is presented for the existence of several isomers of clusters withn≧2. It is shown that, by changing the nucleation conditions, it is possible to control the relative populations of the various isomers.

A new fitting law for rotational energy transfer

Abstract A new fitting law for the primary set of rate constants, kl→0, occuring in IOSA factorisation is presented. Assuming that the interaction potentials is dominated by a single term of the Legendre polynomial expansion, Pλ(cos θ), we find kλn→0 = c[(a/n) log(a/n) - a/n + 1]. The two fitting parameters a and c are expressed in terms of the range and strength of the model potential. The validity of this law is tested.

Pressure broadening of CO infrared lines perturbed by H2 and He

We present results of infrared linewidth measurements at low and room temperatures for CO diluted in H2 and He using a tunable diode laser in the 4.7 μm spectral region (CO fundamental band). Information on the intermolecular potential is obtained by comparison with results of dynamical calculations. In the case of CO–H2 pressure broadening (PB), data are compared to direct measurements of inelastic cross sections by infrared–infrared double resonance and to ab initio quantum calculations. In the case of CO–He, experimental PB coefficients are compared to theoretical cross sections derived from two ab initio potential energy surfaces.

Infrared absorption and predissociation of NO dimer

The infrared absorption spectrum of the ν4 fundamental band of (NO)2 has been recorded at 0.3 cm−1 resolution in a cell cooled down to 110 K. From the band contour analysis and comparison with a simulated spectrum accurate values of the rotational constants in the ν4 state are derived, showing a reduction of the van der Waals bond on vibrational excitation. Absorption measurements performed using the various lines emitted by an isotopic CO laser suggest an unresolved rotational fine structure. A lower limit of the time for energy redistribution of 50 to 100 ps is proposed. Also, infrared photopredissociation of (NO)2 has been directly observed in a crossed laser‐molecular beam experiment using bolometric detection of the beam attenuation. An upper limit of the dissociation lifetime can be set to 10−3 s.

Quantum calculation of vibrational states in the aniline-argon Van der Waals cluster

Theoretical calculations of vibrational intermolecular states of the aniline–argon van der Waals complex for J=0 are reported. A fully‐quantum method (LCHOP) was used in order to describe the van der Waals cluster. Results in the first two electronic states S0 (X 1A1) and S1 (A 1B2) are presented; in the S1 state a comparison with available experimental data is made. We introduce an additive repulsive interaction between N and Ar in the S1 state in order to account for the spectral features observed in larger clusters. Several parametrizations of this term in the potential are discussed with a view to applications to semiclassical simulation of the spectra of the larger An–Arn clusters.

Near-resonant V-V transfer rates for high-lying vibrational states of CO

Abstract Time-resolved double resonance measurements on high-lying states of CO are described.The approximations used for reducing the data are carefully discussed. Rates for excited state-excited state near resonant V-V processes are derived by means of the “reduced probabilities” per collision, implying V-V cross sections larger than the gas kinetic σ 0 . Deviations from a Sharma-Brau-like model are found for levels ν ⩾ 13. The results are compared to previous determinations of the cross sections. Owing to the importance of multi quantum transitions, it is suggested that a more complete theoretical treatment is warranted. b1]

High-resolution spectroscopy of aniline-rare gas Van der Waals complexes: results and comparison with theoretical predictions

Abstract Laser-induced fluorescence spectra are presented for the first allowed electronic transitions in the near-UV of the aniline-argon and aniline-neon 1:1 Van der Waals complexes, formed in a molecular beam. The experimental linewidth was of the order of 10−3 cm−1 due to residual Doppler broadening and lifetime contributions. Single rotational eigenstates were resolved and complete sets of spectral assignments obtained using a rigid-rotor Hamiltonian model. From the rotational constants we extract both structural and dynamical information on the different clusters. This information is better understood in the light of a comparison with the results of quantum calculations.

A,C, and D electronic states of the Ar–NO van der Waals molecule revisited: Experiment and theory

The A–X transition of ArNO has been reinvestigated by laser induced fluorescence (LIF) both in the bound–free and bound–bound region. The discrete part of the spectrum is at least two orders of magnitude weaker than the continuum part, indicative of a large change in geometry from the ground state. This very different configuration, both from the ground state and from the C and D states, can only be explained by strong interactions, induced by the perturbing argon atom, between the excited states of the van der Waals complex converging to the 3sσ,A, 3pπ,C, and 3pσ,D Rydberg states of NO. In order to quantitatively understand the observed structure of the A–X, C–X, and D–X excitation spectra, a global theoretical approach is proposed, based on ab initio calculations of the potential energy surfaces in the planar A′ and A″ symmetries, including a configuration interaction between the states of same symmetry. Small adjustments of the diabatic energy surfaces lead to a satisfactory agreement between the obser...

Vibrational kinetics in liquid nitrogen cooled 5% CO-He radio-frequency discharges

Abstract 5% CO-He radiofrequency discharges have been theoretically and experimentally studied in a liq N 2 -cooled reactor operated at 6 torr as a function of the residence time. The discharges are characterized by CO dissociation rates higher than those observed in uncooled discharges and by time-dependent non Boltzmann vibrational distributions. A purely vibrational mechanism can reasonably be considered the main CO dissociation channel. The importance of O atoms as the main deactivating species, when CO 2 and C are at low density because they are trapped by the cooled walls, is stressed.