Experimental and theoretical investigation of the spectroscopy and dynamics of multiply charged CO cations

Atomic autoionization in the photo-dissociation of super-excited deuterated water molecules fragmenting into D+ + O+ + D

We present the relaxation dynamics of deuterated water molecules via autoionization, initiated by the absorption of a 61 eV photon, producing the very rare D+ + O+ + D breakup channel. We employ the COLd target recoil ion momentum spectroscopy method to measure the 3D momenta of the ionic fragments and emitted electrons from the dissociating molecule in coincidence. We interpret the results using the potential energy surfaces extracted from multi-reference configuration interaction calculations. The measured particle energy distributions can be related to a super-excited monocationic state located above the double ionization threshold of D2O. The autoionized electron energy shows a sharp distribution centered around 0.5 eV, which is a signature of the atomic oxygen autoionization occurring in the direct and sequential dissociation processes of D2O+* at a large internuclear distance. In this way, an O+ radical fragment and a low-energy electron are created, both of which can trigger secondary reactions in their environment.

Charge symmetric dissociation of doubly ionized N2 and CO molecules

We report a comparative study of the features in dissociative double ionization by high energy electron impact of N2 and CO molecules. The ratio of cross-section of charge symmetric dissociative ionization to non-dissociative ionization (CSD-to-ND ratio) and the kinetic energy release (KER) spectra of dissociation are experimentally measured and carefully corrected for various ion transmission losses and detector inefficiencies. Given that the double ionization cross sections of these iso-electronic diatomics are very similar, the large difference in the CSD-to-ND ratios must be attributable to the differences in the evolution dynamics of the dications. To understand these differences, potential energy curves (PECs) of dications have been computed using multi-reference configuration interaction method. The Franck-Condon factors and tunneling life times of vibrational levels of dications have also been computed. While the KER spectrum of N2 (++) can be readily explained by considering dissociation via repulsive states and tunneling of meta-stable states, indirect dissociation processes such as predissociation and autoionization have to be taken into account to understand the major features of the KER spectrum of CO(++). Direct and indirect processes identified on the basis of the PECs and experimental KER spectra also provide insights into the differences in the CSD-to-ND ratios.

A combined electron-ion spectrometer for studying complete kinematics of molecular dissociation upon shell selective ionization

A combined electron-ion spectrometer has been built to study dissociation kinematics of molecular ions upon various electronic decay processes ensuing from ionization of neutral molecules. The apparatus can be used with various ionization agents. Ion time-of-flight (ToF) spectra arising from various electronic decay processes are acquired by triggering the ToF measurement in coincidence with energy analyzed electrons. The design and the performance of the spectrometer in a photoionization experiment is presented in detail. Electron spectra and ion time of flight spectra resulting from valence and 2p1∕2 ionization of Argon and those from valence ionization of CO are presented to demonstrate the capability of the instrument. The fragment ion spectra show remarkable differences (both kinematic and cross sectional) dependent on the energy of the ejected electron, corresponding to various electron loss and decay mechanisms in dissociative photoionization of molecules.

Spectroscopy and metastability of CO22+ molecular ions

The spectroscopy and metastability of the carbon dioxide doubly charged ion, the CO(2) (2+) dication, have been studied with photoionization experiments: time-of-flight photoelectron photoelectron coincidence (TOF-PEPECO), threshold photoelectrons coincidence (TPEsCO), and threshold photoelectrons and ion coincidence (TPEsCO ion coincidence) spectroscopies. Vibrational structure is observed in TOF-PEPECO and TPEsCO spectra of the ground and first two excited states. The vibrational structure is dominated by the symmetric stretch except in the TPEsCO spectrum of the ground state where an antisymmetric stretch progression is observed. All three vibrational frequencies are deduced for the ground state and symmetric stretch and bending frequencies are deduced for the first two excited states. Some vibrational structure of higher electronic states is also observed. The threshold for double ionization of carbon dioxide is reported as 37.340+/-0.010 eV. The fragmentation of energy selected CO(2) (2+) ions has been investigated with TPEsCO ion coincidence spectroscopy. A band of metastable states from approximately 38.7 to approximately 41 eV above the ground state of neutral CO(2) has been observed in the experimental time window of approximately 0.1-2.3 mus with a tendency towards shorter lifetimes at higher energies. It is proposed that the metastability is due to slow spin forbidden conversion from bound excited singlet states to unbound continuum states of the triplet ground state. Another result of this investigation is the observation of CO(+)+O(+) formation in indirect dissociative double photoionization below the threshold for formation of CO(2) (2+). The threshold for CO(+)+O(+) formation is found to be 35.56+/-0.10 eV or lower, which is more than 2 eV lower than previous measurements.

Vibrational branching ratios in photoionization of CO and N2

We report results of experimental and theoretical studies of the vibrational branching ratios for CO 4sigma(-1) photoionization from 20 to 185 eV. Comparison with results for the 2sigma(u)(-1) channel of the isoelectronic N2 molecule shows the branching ratios for these two systems to be qualitatively different due to the underlying scattering dynamics: CO has a shape resonance at low energy but lacks a Cooper minimum at higher energies whereas the situation is reversed for N2.

Auger electron emission from fixed-in-space CO

We have measured the angular distribution of carbon K-Auger electrons from fixed in space, core-ionized, CO molecules in coincidence with the kinetic energy release of the C+ and O+ fragments. We find a very narrow ejection of Auger electrons in the direction of the oxygen and an oscillatory diffraction pattern. Even for similar electron energies, the angular distribution strongly depends on the symmetry of the final state.