Sequential Ionization Process Research Articles

Secondary ionization of pyrimidine nucleobases and their microhydrated derivatives in helium nanodroplets.

Radiation damage in biological systems by ionizing radiation is predominantly caused by secondary processes such as charge and energy transfer leading to the breaking of bonds in DNA. Here, we study the fragmentation of cytosine (Cyt) and thymine (Thy) molecules, clusters and microhydrated derivatives induced by direct and indirect ionization initiated by extreme-ultraviolet (XUV) irradiation. Photofragmentation mass spectra and photoelectron spectra of free Cyt and Thy molecules are compared with mass and electron spectra of Cyt/Thy clusters and microhydrated Cyt/Thy molecules formed by aggregation in superfluid helium (He) nanodroplets. Penning ionization after resonant excitation of the He droplets is generally found to cause less fragmentation compared to direct photoionization and charge-transfer ionization after photoionization of the He droplets. When Cyt/Thy molecules and oligomers are complexed with water molecules, their fragmentation is efficiently suppressed. However, a similar suppression of fragmentation is observed when homogeneous Cyt/Thy clusters are formed in He nanodroplets, indicating a general trend. Penning ionization electron spectra (PIES) of Cyt/Thy are broad and nearly featureless but PIES of their microhydrated derivatives point at a sequential ionization process ending in unfragmented microsolvated Cyt/Thy cations.

Exploration of strong-field double ionization of CS2 molecule in bichromatic counterrotating circularly polarized laser fields

By using classical ensemble method, we investigate the double ionization of CS2 molecule in linearly, the bichromatic counterrotating circularly polarized laser fields and the combination of bichromatic counterrotating circularly polarized laser fields and static field, respectively. The numerical results show that the ionization probability in the bichromatic counterrotating circularly polarized laser fields is about 2 order magnitude higher than that in linearly polarized laser field. When a static field is added, the ionization probability is the largest. Besides, the “knee” structure occurs at about 0.05PW/cm2 in linearly polarized laser field; whereas “knee” structure is disappeared in the bichromatic counterrotating circularly polarized laser fields and combined laser field. The corresponding momentum distribution of CS2 molecule presents a “finger-like” structure at about 0.05PW/cm2 in linearly polarized field. By analysing the energy distributions of double-ionized electrons versus time and corresponding trajectories, we find that, for linearly polarized case non-sequential double ionization (NSDI) is predominant at about 0.05PW/cm2, for bichromatic counterrotating circularly polarized laser fields, one electron ionizes after another which indicate sequential ionization process (SDI). When the static field is added, the two electrons undergoes a long pre-ionization process first and then ionizes one after another, and the pre-ionization process lasts longer than other two cases.

Two-photon double ionization of metastable1,3S1s2shelium

We investigate the process of two-photon double ionization (TPDI) of the metastable helium ${}^{1,3}S$ $1s2s$ states. The process has been simulated within a fully $ab$ $initio$ numerical framework, solving the time-dependent Schr\"odinger equation in full dimensionality for the two interacting electrons, in a $B$-spline-based methodology. The presence of doubly excited (autoionizing) states in the direct TPDI regime causes resonance-enhanced multiphoton ionization, and we demonstrate this effect by accessing the ${}^{1,3}P$ $2s2p$ doubly excited states. Fully converged theoretical calculations are presented, and a well-defined cross section is extracted for the direct TPDI process and compared in the context of its analogous process in the helium ground state. In addition, the resonance-enhanced two-photon double-ionization mechanism is explored, and we discuss how this process differs from both direct and sequential ionization processes.

Exploration of carrier-envelope phase dependence on the double ionization process of Ar and N2 in few-cycle laser fields

The double ionization dynamics of the Ar atom and the N2 molecule are investigated in few-cycle laser fields with different carrier-envelope phases (CEPs). The corresponding electron momentum distribution of Ar and N2 presents different patterns. The energy distribution of double ionized electron trajectories is also presented to further describe the collision ionization and recollision excitation sequential ionization process of double ionization. Moreover, the angular distribution of the Ar atom and the N2 molecule at the end of a pulse is also sensitive to the CEP. The numerical results indicate that the double ionization mechanisms of atoms and molecules can probably be controlled by adjusting carrier-envelope phase in few cycle fields.

Coulomb explosion of diatomic molecules in intense XUV fields mapped by partial covariance

Single-shot time-of-flight spectra for Coulomb explosion of N2 and I2 molecules have been recorded at the Free Electron LASer in Hamburg (FLASH) and have been analysed using a partial covariance mapping technique. The partial covariance analysis unravels a detailed picture of all significant Coulomb explosion pathways, extending up to the N4+–N5+ channel for nitrogen and up to the I8+–I9+ channel for iodine. The observation of the latter channel is unexpected if only sequential ionization processes from the ground state ions are considered. The maximum kinetic energy release extracted from the covariance maps for each dissociation channel shows that Coulomb explosion of nitrogen molecules proceeds much faster than that of the iodine. The N2 ionization dynamics is modelled using classical trajectory simulations in good agreement with the outcome of the experiments. The results suggest that covariance mapping of the Coulomb explosion can be used to measure the intensity and pulse duration of free-electron lasers.

Two-photon double ionization of the helium atom by ultrashort pulses

Two-photon double ionization of the helium atom was the subject of early experiments at FLASH and will be the subject of future benchmark measurements of the associated electron angular and energy distributions. As the photon energy of a single femtosecond pulse is raised from the threshold for two-photon double ionization at 39.5 eV to beyond the sequential ionization threshold at 54.4 eV, the electron ejection dynamics change from the highly correlated motion associated with nonsequential absorption to the much less correlated sequential ionization process. The signatures of both processes have been predicted in accurate ab initio calculations of the joint angular and energy distributions of the electrons, and those predictions contain some surprises. The dominant terms that contribute to sequential ionization make their presence apparent several eV below that threshold. In two-colour pump–probe experiments with short pulses whose central frequencies cause the sequential ionization process to dominate, a two-electron interference pattern emerges that depends on the pulse delay and the spin state of the atom.

Multiply Charged Intact Ions of Polyatomic Cyclic Molecules Generated by a Strong Laser Field

The interaction of strong laser fields (∼1016 W/cm2) with cyclic polyatomic molecules have been studied at λ = 790 and 395 nm. The induced ionization/fragmentation processes were analyzed by means of a time-of-flight mass spectrometer. The production of multiply charged intact ions is found to be related to molecular structure. Doubly charged intact ions were observed for aromatic molecules (aniline, furan, and pyridine) with significant abundance at both wavelengths used. No evidence for doubly charged species appeared in the mass spectra of the cycloketones. The single ionization mechanism is believed to be different at the wavelengths used. There is evidence that multiply charged ion production is taking place through sequential ionization processes, although the rescattering model cannot be excluded.

Threshold ionization energies of Au 2+, Au 3+, and Au 4+

Threshold ionization energies (IE) for the metal ions Au 2+, Au 3+, and Au 4+ have been determined experimentally by utilizing a sequential ionization process. No experimental IE values are available in the literature for charge states of Au higher than Au 2+. The experimental IE value for Au 3+ is higher by 4 eV and that for Au 4+ is in good agreement (within 0.6 eV) with Lisitzin's [1] calculated values. However, the present experimental IE values for Au 3+ and Au 4+ are found to be lower than the values calculated by Carlson et al- [2]. The Au 3+ value calculated by the extrapolation method is within 2.6 eV of the experimental Au 3+ IE value.

Multiple Ionization by Sequential Electron Impact

Positive ions can be trapped, in the space-charge of an electron beam or in a cyclotron resonance or a quadrupole trap, and be struck several times by electrons before the ions escape the trap or are neutralized. If the electron energy is sufficient, each electron-ion collision can result in an increase in the degree of ionization or excitation of the ion. Since the cross-sections for sequential ionization decrease quite slowly with increasing degree of ionization, the sequential ionization process is an efficient means of producing highly charged ions. This review concentrates on space-charge ion traps because both cyclotron resonance and quadrupole traps have been reviewed recently and because these types of ion trap are more suitable for ion-molecule collision studies. Ion trapping mechanisms are reviewed and the results of measurements of ionization and excitation cross-sections by sequential electron impact summarized. The effects of unintentional ion-trapping and the apdlications of trapped ion sources to partial pressure measurements are considered.