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Angular distributions of photoelectrons from both C and O K-shells of the fixed-in-space CO molecule have been measured using the angle-resolved photoelectron-photoion coincidence technique. The measurements have been performed at several photon energies from the ionization thresholds up to about 30 eV above them, where the σ* shape resonances occur. Experimental results are compared with the multiple-scattering calculations of Dill et al (1976 J. Chem. Phys. 65 3158) and with our new calculations in the relaxed-core Hartree-Fock approximation. Our calculations are in a better agreement with the experimental data though numerical discrepancies remain. The experimental angular distributions are fitted by the expansion in Legendre polynomials containing up to ten terms and the extracted parameters are compared with the corresponding theoretical values.

Using the experimental angular distributions of photoelectrons from the K-shells of an oriented CO molecule reported in a companion paper, we have performed a so-called complete experiment and determined 18 dynamical parameters (ten moduli of transition moments and eight phase differences) for the O K-shell, and 16 dynamical parameters (nine moduli of transition moments and seven phase differences) for the C K-shell, and compared them with the results of our calculations in the relaxed-core Hartree-Fock (RCHF) approximation. The agreement between theory and experiment is only qualitative, therefore the model has to be improved by including electron correlations. From the analysis of experimental data we proved that the σ * shape resonance is due to not only the f-wave, as was widely believed earlier, but is due to approximately equal contributions of three partial waves with 1 l 3 for the C K-shell, and four partial waves with 0 l 3 for the O K-shell, with a rather substantial contribution of other partial waves with l 5. From the analysis of the transition moments determined from the experiment it follows that several Cooper minima are likely to exist in partial photoionization cross sections, in particular, in the C 1sσ → εsσ and in the O 1sσ → εdσ transitions.

The general form of the molecular frame photoelectron angular distributions ͑MFPADs͒ for linear groundstate molecules ionized by linearly polarized light (n ) is reported. A comparison between computed and measured MFPADs as a function of the polar and azimuthal emission angles is presented for photoionization of NO leading to the c 3 ⌸ state of NO ϩ . The importance of the azimuthal dependence of the MFPADs for the determination of the symmetry of the states involved in the excitation and of the underlying photoionization dipole matrix elements is demonstrated.

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The reaction paths of dissociation and the mechanisms of electronic relaxation of the ethylene cation have been calculated ab initio. Internal rotation is shown to bring about radiationless transition of states ? 2A and ? 2B2 to the ground state. Two competing channels are available for the first excited ? 2B3: it can either undergo internal conversion to the ground state ? 2B3 or lose a hydrogen atom by simple bond cleavage and give C2H+3+H fragments. The competition is governed by nonadiabatic interaction around a conical intersection. The coupling matrix element 〈?‖∂/∂q‖?〉 has been calculated and is shown to obey the linear model of nonadiabatic interaction quite well. The ground state ? 2B3 cannot eliminate directly a hydrogen molecule. It must first undergo rearrangement to a 2E state of a CH3CH+ structure via a bridged structure. Jahn–Teller interaction takes place at the doubly degenerate 2E state. Dissociation of CH3CH+ via 1,1-hydrogen elimination is then allowed and leads to C2H+2+H2. There is a small (possibly inexistent) energy barrier along this path. The complicated nature of this multistep reaction path brings about energy redistribution among many vibrational degrees of freedom and provides a plausible explanation for the success of statistical theories of unimolecular reactions (RRKM and QET).

The dynamical information (ten dipole matrix elements and eight phase differences) has been deduced from the measured angular distributions of photoelectrons from O K shell of oriented CO molecules near the ionization threshold in the region of a sigma(*) shape resonance. Light polarization parallel and perpendicular to the molecular axis has been used. An important contribution of six lsigma partial waves with 0

The geometrical structure of the low-lying states of CO+2 has been calculated ab initio. The C̃ 2Σ+g/2A1 state is found to be slightly bent in its equilibrium geometry. A new assignment of the vibrational structure of the corresponding band in the photoelectron spectrum is suggested. State C̃ is predissociated by two competitive channels. One of them leads to O++CO, the other to CO++O. The mechanism of these predissociations involves a slow, rate-determining, intersystem crossing to a bent ã 4B1 state. The population of state ã has a choice between dissociating to O++CO fragments and undergoing a further, much faster, intersystem crossing to the ground state X̃ which dissociates to CO++O. Since radiationless transitions between X̃ and ã are relatively rapid, the state which is lower in energy (i.e., X̃) has a much larger population than the other (i.e., ã). Hence, the CO++O channel prevails as soon as it is energetically accessible. The rate-determining step of both processes is the intersystem crossing between states C̃ and ã. Its rate constant is estimated by a statistical method due to Zahr, Preston, and Miller, recasted in a simpler form. A value of about 4×107 s−1 is obtained.