Overtone Excitation Research Articles

Energy dependence of vibrational excitation of adsorbed CO and OH

To assess the excitation mechanism of vibrationa modes of adsorbates by electron we have measured loss spectra for CO on Ni (100) and OH on NiO (111) as a function of primary electron energy. We find that the dipole interaction describes the data for the CO system but fails completely for the the OH system. We find the loss intensity in OH to be more than an order of magnitude larger than dipole estimates at low energies. This is accompanied by a strong overtone and intrinsic NiO phonon satellite excitation.

Coherent spectroscopy of anharmonic molecular vibrations which disobey the alternative forbiddenness

A new variant of coherent spectroscopy is proposed for investigating the excitation of molecular vibration overtones which do not obey the alternative forbiddenness. Estimates obtained for the CH4 and HCl molecules show that the resonant susceptibility describing an optical process of the type ωα=2ω+ωL (ω is the frequency of infrared pumping radiation tuned to the frequency Ω of a molecular vibration and ωL is the frequency of a probe beam) is considerably greater than the nonresonant background susceptibility. A distinguishing feature of this process is the absence (in an isotropic medium) of the competing process of repeated scattering of a probe beam by the fundamental harmonic of the molecular vibration.

Vibrational relaxation of the N2O ν1 mode

The vibrational relaxation of the ν1 symmetric stretch mode of N2O was studied at room temperature following an overtone excitation via a DF laser. The ν1 and ν3 (asymmetric stretch) fluorescence at 7.8 and 4.5 μ were, respectively, monitored as a function of time. A V–V equilibration time constant of 0.062 μsec⋅atm between the ν1 and ν2 (bending) modes was measured. The vibrational relaxations out of the ν2 and ν3 modes were measured to be 1.3 and 2.1 μsec⋅atm, respectively, with good agreement compared to previous measurements reported in the literature.

Excited states of gaseous ions. V. The predissociation of the h 2o + ion

The B̃ 2 state of H 2O + is predissociated twice. First, by the ã 4B 1 state, giving OH + + H fragments via spinorbit coupling interaction. Secondly, by a 2A state, giving H + OH fragments via spin-orbit coupling and Coriolis interactions. A vibrational analysis of the photoelectron band of the B̃ state of H 2O + and D 2O + is carried out. This provides the vibrational frequencies of the H 2O +, D 2O + and HDO + ions, as well as a vibrational assignment of the peaks. The H 2O + ion in its B̃ 2B 2 state is found to have a OH bond length of 1.12 A and a valence angie of 78°. In order to describe the unimolecular fragmentation process, a distinction is introduced between the totally symmetric, optically active vibrational modes, and the antisymmetric ones which are coupled to the continuum. The former are supplied with photon or electron impact energy, but only the latter are chemically efficient. The dynamics of the dissociation process depends therefore on the couplings among normal modes. This is studied in the framework of two models. In Model 1, it is assumed that, as a result of the anharmonicity of the potential energy surface, only even overtones of the antisymmetric vibration are excited by Fermi resonance. In Model II, excitation of the odd overtones is provided by vibronic coupling. Model II is in better agreement with experiment than Model I. Calculated and experimental results have been compared on the following points: isotopic shift on the appearance potential of OH + and OD + ions, shapes of the photoionization curves, fragmentation pattern with 21 eV photons, presence of a unimolecular metastable transition, production of O + ions. All the vibrational levels situated above the dissociation asymptote are totally predissociated. Autoionization is shown in this case to contribute only to the formation of molecular H 2O + ions, and not to that of the OH + fragments. For 21 eV electrons, the contribution due to direct ionization is calculated to represent about 25% of the total cross section, the rest being due to autoionization.

Use of direct overtone excitation in the measurement of the HF(v=2) deactivation rate

This letter reports the demonstration of direct optical overtone excitation of the second vibrational level of hydrogen fluoride. A unique variation of the laser-excited-fluorescence technique was employed in the experiment to achieve a significant nonequilibrium population of HF(v=2) within a room-temperature mixture of HF and argon. The HF(v=2) level was excited by a pulse from a temperature-tuned Nd:YAlO3 laser, and the resultant fluorescence was monitored. The Δv=1 fluorescence from the mixture revealed the presence of a double exponential decay. The preliminary data allow the interpretation of the two exponentials as representing the v=2→v=1 and v=1→v=0 collisional deactivations. This in turn has led to an approximate value for the ratio of the time constants of the processes of τ1/τ2=2.2±0.4.