Angular Distributions Of Fragment Ions Research Articles

Dissociative ionization of benzene in intense laser fields of picosecond duration

Mass spectrometry experiments have been performed to probe the morphology of dissociative ionization of a ring molecule benzene when it is immersed in a laser field of 35-ps duration whose intensity is in the range ${10}^{13} {\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2}.$ The field-induced fragmentation pattern is grossly different from that obtained by electron impact. The intense laser field clearly gives rise to much more fragmentation. Covariance mapping has been applied to study field-induced formation of highly charged molecular ions. Results indicate that the overall fragmentation dynamics is complex and is likely to involve both fast ladder switching as well as multiple ionization of fragment molecules at some intermediate steps in the ladder. Angular distributions of fragment ions have been measured. Anisotropic distributions indicate that some fragments are produced from precursors that are strongly aligned along the direction of the laser polarization vector while at least one fragment is formed from a precursor whose angular distribution is markedly isotropic. The nature of the precursors remains to be discovered.

Renner–Teller effect and Rydberg-valence mixing in the N and O K-edge photoabsorption spectra of N2O

We have measured high-resolution angle-resolved ion-yield spectra of the O and N 1s excited N2O molecules (Nt–Nc–O) and investigated them with the help of ab initio quantum chemical calculations. The peak width of the Nc 1s→π* transition is larger than that of the Nt 1s→π* one. This mainly arises from different populations of bending vibrations excited through the Renner–Teller effect, which breaks the degeneracy of the 1s→π* excited states by bending the linear molecule. The angular distribution of fragment ions emitted after the Auger decay is also affected by the Renner–Teller effect. That is, fragment ions significantly lose the information of the Π symmetry of the linear molecule. For the 1s→Rydberg excitations no Renner–Teller effect is observed and the angular distribution is directly related to the Σ and Π symmetries. Furthermore, it is found that the peak intensities of the O and Nt 1s→nsσ Rydberg transitions are unusually large. The N2O molecule has two σ* orbitals, σs*(8σ) and σp*(9σ), below and above the ionization threshold and the 1s→σs* excited states are mixed with the O and Nt 1s–nsσ Rydberg excited states; on the other hand, the Nc 1s→σs* transition is almost dipole-forbidden because of the orbital symmetry.

Charge separation reaction dynamics from pepipico using a position‐sensitive detector

AbstractThe combination of a position‐sensitive detector with the photoelectron–photoion–photoion coincidence (PEPIPICO) technique allows complete determination of the laboratory momenta and angles of ions in coincidence. The technique yields angular distributions of fragment ions, high resolution kinetic energy distributions for two‐body reactions and Newton diagrams for three‐body dication dissociation. The reactions SF→F++SF+F2 and cyclo‐C4F→CF+C2F+CF are shown to follow sequential mechanisms as determined previously. The technique's potential for elucidating more complex reactions is illustrated for SO→O++S++O and CF3I2+→CF+I++F, which show variations in mechanism as a function of total kinetic energy release.

Observation of anisotropic angular distribution of ionic fragments in the dissociation of CO2+

Angular distributions of fragment ions (${\mathrm{C}}^{+}$+${\mathrm{O}}^{+}$) produced in dissociative double photoionization of carbon monoxide have been studied in the photon energy region 39--100 eV using a photoion-photoion coincidence technique and a source of synchrotron radiation. The asymmetry parameter \ensuremath{\beta} ranges from 0.69 at 39 eV to -0.19 at 100 eV. The observations of anisotropy in the dissociative double photoionization from the valence orbitals of CO are discussed in relation to the symmetry considerations involved in the transition.

Anisotropic angular distribution of fragment ions in dissociative double photoionization of OCS.

Angular distributions of fragment ions produced in the dissociative double photoionization of OCS have been studied in the photon energy region 37--100 eV with the use of synchrotron radiation and photoion-photoion coincidence. The asymmetry parameter \ensuremath{\beta} ranges from 0.01 to 0.48 for the three major dissociation channels ${\mathrm{OC}}^{+}$+${\mathrm{S}}^{+}$, ${\mathrm{O}}^{+}$+${\mathrm{S}}^{+}$+C, and ${\mathrm{C}}^{+}$+${\mathrm{S}}^{+}$+O of ${\mathrm{OCS}}^{2+}$. The observation of an anisotropy in the direct double photoionization from valence orbitals is discussed in relation to the symmetry consideration involved in the transition and the dynamics of the dissociation.

Anisotropic dissociation of NO in K-shell excited states.

The angular distributions of fragment ions produced from NO in N K-shell or O K-shell excited states have been measured with a rotatable time-of-flight mass spectrometer. The asymmetry parameter (β) derived from the measured angular distribution is −0.75 at 399.7 eV (N 1s→π*). The β parameter for transitions of the N 1s electron to Rydberg orbitals changes with the symmetry of the transitions

Angular distribution of fragment ions formed by K-shell excitation of N2 molecule

Angular distributions of the fragment ions produced by the K-shell excitation of N2 molecule have been studied by use of a rotatable time-of-flight (TOF) mass spectrometer. The TOF spectra of N+ and N2+ have been measured at 0, 30, 54.7 and 90' by a photoelectron-photoion coincidence method. Asymmetry parameters obtained for N+ and N2+ are equal. The asymmetry parameters at the photon energies corresponding to the transitions from 1s to the discrete molecular orbitals (2p pi g, 3s sigma g and so forth) are consistent with the values expected from the symmetry of the transition. In the region of the shape resonance (around 419 eV), the values of these parameters are smaller than those obtained by theoretical prediction.

Angular distribution measurement of fragment ions from a molecule using a new beamline consisting of a Grasshopper monochromator

Optical characteristics of a new beamline consisting of a premirror, a Grasshopper monochromator, and a refocusing mirror have been investigated. The intensity of the monochromatic soft x-ray was estimated to be about 108 photons/(s 100 mA) at 500 eV with the storage electron energy of 600 MeV and the minimum slit width. This slit width provides a resolution of about 500. Angular distributions of fragment ions from an inner-shell excited nitrogen molecule have been measured with a rotatable time-of-flight mass spectrometer by using this beamline.

The dissociative ionisation of hydrogen by 5-25 keV protons: energy spectra and angular distribution of fragment ions

The energy and angular distributions of fragment protons produced in H+-H2 collisions have been investigated within the energy range 5-25 keV. The observed energetic protons arise from at least four repulsive states, the 2p sigma u, 2p pi u and 2s sigma g states of H2+ and the unbound H+H+ state. No significant contribution from autoionisation is observed. Analysis of the angular distributions has been performed to distinguish the separate degenerate channels available to the ejected electron. The results show a remarkable similarity to photoionisation data.

The variation of translational energy release in collision-induced dissociations of polyatomic ions with initial kinetic energy and with observation angle. I. Theoretical considerations

Angle-resolved mass spectrometry has hitherto focused upon variations in relative yields of fragment ions formed in collision-induced dissociations. This paper investigates the sequence of assumptions which must be made in order to interpret observed variations in peak widths, as the observation angle is varied, in terms of energy deposited in the projectile ion by the collision. Some of these assumptions, such as the nature of the collisional excitation mechanism, and the relationship of the energy thus deposited to the total internal energy of those ions sampled by the fragmentation process, are common to both types of experiment. Other assumptions, such as the manner in which the total internal energy of the reacting ion is partitioned into translational energy of the fragments, are peculiar to the peak width measurements. The latter, however, are free from ambiguities of interpretation arising from convolution of the collisional scattering with differences in angular distributions of fragment ions arising only from differences in translational energy release. For both types of experiment the situation is complex, and interpretation requires considerable care.

The angular distribution of fragment ions produced from polyatomic ion collisions

Energy released to daughter ions from collision-induced dissociation of high energy (≈ keV) CH 4 +, CH 3NO 2 + and CS 2 + is measured. Fragmentation mechanics are developed to enable calculation of the angular distribution of fragments due to energy released. Subsequent plots of fragment-ion relative abundance vs. laboratory angle fit published experimental data, even though collisional scatter is presumed negligible.

Angular distribution of fragment ions arising from the collision-induced decomposition of polyatomic primary ions

The angular distributions of the relative intensities of fragment ions produced by the collision-induced decomposition of a number of selected ions have been measured. The distributions have been discussed in terms of contributions from the scattering angle and from the release of internal energy as translational energy of the fragment ions. The relative importance of the two contributions appears to vary with the ion undergoing fragmentation but in general, it is possible to vary the average internal energy of the decomposing ion by varying the angle at which products are observed.

Measurement of the angular distribution and energy of ionic fragments from photodissociation of molecular ions

A theoretical analysis of the trapping efficiency of photofragment ions formed by polarized and unpolarized radiation is developed for the ion cyclotron resonance cell. The analysis outlines a prescription for estimating the kinetic energy and anisotropy of the angular distribution of fragment ions and is found to agree well with experiments performed on the H 2 + molecular ion. With a photodissociation wavelength of 5790 Å the cross section for formation of H + from H 2 + is found to be 6.8 × 10 −19 cm 2 and the average kinetic energy of the protons was estimated as 0.7 eV. In the light of these results use of the ICR technique for such measurements is evaluated.