A key question concerning the three-body fragmentation of polyatomic molecules is the distinction of sequential and concerted mechanisms, i.e., the stepwise or simultaneous cleavage of bonds. Using laser-driven fragmentation of OCS into ${\mathrm{O}}^{+}+{\mathrm{C}}^{+}+{\mathrm{S}}^{+}$ and employing coincidence momentum imaging, we demonstrate a novel method that enables the clear separation of sequential and concerted breakup. The separation is accomplished by analyzing the three-body fragmentation in the native frame associated with each step and taking advantage of the rotation of the intermediate molecular fragment, ${\mathrm{CO}}^{2+}$ or ${\mathrm{CS}}^{2+}$, before its unimolecular dissociation. This native-frame method works for any projectile (electrons, ions, or photons), provides details on each step of the sequential breakup, and enables the retrieval of the relevant spectra for sequential and concerted breakup separately. Specifically, this allows the determination of the branching ratio of all these processes in ${\mathrm{OCS}}^{3+}$ breakup. Moreover, we find that the first step of sequential breakup is tightly aligned along the laser polarization and identify the likely electronic states of the intermediate dication that undergo unimolecular dissociation in the second step. Finally, the separated concerted breakup spectra show clearly that the central carbon atom is preferentially ejected perpendicular to the laser field.

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Native Frames: Disentangling Sequential from Concerted Three-Body Fragmentation.

We present in this paper new and updated calculations of the ionization equilibrium for all the elements from H to Ni. We collected for these elements all the data available in the literature for the ionization and radiative plus dielectronic recombination rates. In particular, the dielectronic rates have been fitted with a single formula and the related coefficients are tabulated. Our results are compared with previous works.

Ionization Balance for Optically Thin Plasmas: Rate Coefficients for all Atoms and Ions of the Elements H to Ni and implication for the calculated X-ray spectrum

We have studied a solid-to-plasma transition by irradiating Al foils with the FLASH free electron laser at intensities up to 10(16) W/cm(2). Intense XUV self-emission shows spectral features that are consistent with emission from regions of high density, which go beyond single inner-shell photoionization of solids. Characteristic features of intrashell transitions allowed us to identify Auger heating of the electrons in the conduction band occurring immediately after the absorption of the XUV laser energy as the dominant mechanism. A simple model of a multicharge state inverse Auger effect is proposed to explain the target emission when the conduction band at solid density becomes more atomiclike as energy is transferred from the electrons to the ions. This allows one to determine, independent of plasma simulations, the electron temperature and density just after the decay of crystalline order and to characterize the early time evolution.

Decay of Cystalline Order and Equilibration during the Solid-to-Plasma Transition Induced by 20-fs Microfocused 92-eV Free-Electron-Laser Pulses

We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. Roadmap III focusses on heavy particles: with zer ...

https://iopscience.iop.org/article/10.1088/1361-6455/ab26ea/pdf

Roadmap on photonic, electronic and atomic collision physics: III. Heavy particles: with zero to relativistic speeds

The three-body fragmentation dynamics of ${{\mathrm{CH}}_{4}}^{2+}$ dications induced by single-electron capture of slow (3-keV/u) $\mathrm{A}{\mathrm{r}}^{8+}$ ions is investigated. The experiment is performed on a newly built, highly charged ion collision platform which consists of an electron cyclotron resonance ion source and a cold target recoil ion momentum spectroscopy (COLTRIMS) setup. Using the COLTRIMS methodology, the complete kinematical information is determined for two three-body breakup channels, ${{\mathrm{CH}}_{4}}^{2+}\ensuremath{\rightarrow}{\mathrm{H}}^{+}+{{\mathrm{CH}}_{2}}^{+}+\mathrm{H}$ and ${{\mathrm{CH}}_{4}}^{2+}\ensuremath{\rightarrow}{{\mathrm{H}}_{2}}^{+}+\mathrm{C}{\mathrm{H}}^{+}+\mathrm{H}$. Then analyzing the complete kinematics with the Dalitz plot, very different fragmentation mechanisms (e.g., sequential and/or concerted pathway) are clearly identified for the two channels. To confirm the existence of some possible fragmentation pathways, we also simulate corresponding Dalitz plots employing a simple classical mechanical model. For the ${\mathrm{H}}^{+}+{{\mathrm{CH}}_{2}}^{+}+\mathrm{H}$ channel, the dependence of the fragmentation pathway on its kinetic energy release is studied, which reflects the different nature of the corresponding states of ${{\mathrm{CH}}_{4}}^{2+}$ dications. Furthermore, the kinetic energy ratio of two ionic fragments is analyzed to infer the three-body fragmentation mechanism of ${{\mathrm{CH}}_{4}}^{2+}$ dications.

Three-body fragmentation of methane dications produced by slow A r 8 + -ion impact

We have studied the phenomenon of electromagnetically induced transparency (EIT) of 87Rb vapor at room temperature in a magnetic field with an arbitrary angle to the laser propagation direction. Rather than exposing atoms to a parallelled or transverse magnetic field as usual, in our work, we apply a magnetic field (up to 45 Gauss) with an arbitrary angle to the laser propagation direction and the spectra become much more complex. More EIT dips are observed due to the Zeeman splitting on the D 2 line of 87Rb in a -type configuration. With a 5 Torr N2 buffer gas in the thermal 2 cm vapor cell, the state has a very short effective lifetime, corresponding to a large energy broadening, which removes the velocity selective optical pumping effect almost completely and keeps the high resolution EIT spectrum for the energy splitting of 87Rb in magnetic fields. The shifting of the EIT resonances with the strength of the applied magnetic field coincides well with the theory based on a full matrix Hamiltonian combined with a spectral decomposition method. Our work can be extended to measure the magnetic field vector in space. The effects of the detuning of the probe and coupling beams on the spectral lines are also investigated.

Electromagnetically induced transparency of 87Rb in a buffer gas cell with magnetic field

An experimental investigation of the breakup of ${(\mathrm{C}{\mathrm{O}}_{2})}^{3+}$ induced by $\mathrm{N}{\mathrm{e}}^{4+}$ ion impact at incident energies of 1.12 MeV was performed. By analyzing the momentum distributions and the kinetic energies of the three fragment ions, the nonsequential and sequential dissociation mechanisms are verified. In contrast to highly charged ion impact, two different sequential decay pathways were observed in the present experiment. One pathway originates from the primary cation ${(\mathrm{C}{\mathrm{O}}_{2})}^{3+}$ populated into $^{4}\mathrm{\ensuremath{\Sigma}}^{+}$ states by collision charge exchange and its daughter cation ${(\mathrm{CO})}^{2+}$ populated into the two excited states ($^{3}\mathrm{\ensuremath{\Pi}}$ and $X{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$) by the first fragmentation step, resulting in a lower KER peak. The other pathway originates from the primary cation ${(\mathrm{C}{\mathrm{O}}_{2})}^{3+}$ locating at $^{6}\mathrm{\ensuremath{\Pi}}$ state, and its daughter cation ${(\mathrm{CO})}^{2+}$ populated into the metastable excited states $^{3}\mathrm{\ensuremath{\Pi}}$, $X{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$, and $^{3}\mathrm{\ensuremath{\Sigma}}^{+}$, leading to the higher KER peak. Our work is a breakup experiment of ${(\mathrm{C}{\mathrm{O}}_{2})}^{3+}$ presenting the initial states of the parent cation ${(\mathrm{C}{\mathrm{O}}_{2})}^{3+}$ and the metastable states of $\mathrm{C}{\mathrm{O}}^{2+}$ ion simultaneously.

Observation of two sequential pathways of ( C O 2 ) 3 + dissociation by heavy-ion impact

The absolute rate coefficients for dielectronic recombination (DR) of sodiumlike krypton ions were measured by employing the electron-ion merged-beam technique at the heavy-ion storage ring CSRm at the Institute of Modern Physics in Lanzhou, China. The measured DR spectrum covers the electron-ion collision energy range of 0--70 eV, encompassing all of the DR resonances due to $3s\ensuremath{\rightarrow}3p$ and part of the DR resonances from $3s\ensuremath{\rightarrow}3d(\mathrm{\ensuremath{\Delta}}n=0)$ and $3s\ensuremath{\rightarrow}4l(\mathrm{\ensuremath{\Delta}}n=1)$ core excitations. A series of peaks associated with DR processes have been identified by the Rydberg formula. The experimental DR results are compared with the theoretical calculations using a relativistic configuration interaction flexible atomic code and the distorted-wave collision package autostructure. A very good agreement has been achieved between the experimental results and the theoretical calculations by considering the strong mixing among the low-energy resonances in both calculations. The experimentally derived DR spectrum is then convolved with a Maxwellian-Boltzmann distribution to obtain the temperature dependent plasma recombination rate coefficients and compared with previously available results from the literature. The present experimental result yields a precise plasma rate coefficients at the low temperature range up to $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and the calculated data by Altun et al. [Z. Altun, A. Yumak, N. R. Badnell, S. D. Loch, and M. S. Pindzola, Astron. Astrophys. 447, 1165 (2006)] provide reliable plasma rate coefficients at high temperature range above $2\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$.

Absolute rate coefficients for dielectronic recombination of Na-like Kr 25 +

Two-electron transfer and ionization mechanism in 80-keV/u Ne 8 + on He collisions

https://iopscience.iop.org/article/10.1088/1361-6455/ab7424/pdf

Han-Mu Wang

Papers

Electromagnetically induced transparency of 87Rb in a buffer gas cell with magnetic field

Observation of two sequential pathways of ( C O 2 ) 3 + dissociation by heavy-ion impact

Absolute rate coefficients for dielectronic recombination of Na-like Kr 25 +

Two-electron transfer and ionization mechanism in 80-keV/u Ne 8 + on He collisions

Electron-ion recombination rate coefficients of carbon-like Ar 12+