Lei Xia

Positive/negative ion velocity mapping apparatus for electron-molecule reactions

In molecular dissociative ionization by electron collisions and dissociative electron attachment to molecule, the respective positively and negatively charged fragments are the important products. A compact ion velocity mapping apparatus is developed for the angular distribution measurements of the positive or negative fragments produced in the electron-molecule reactions. This apparatus consists of a pulsed electron gun, a set of ion velocity mapping optic lenses, a two-dimensional position detector including two pieces of micro-channel plates, and a phosphor screen, and a charge-coupled-device camera for data acquisition. The positive and negative ion detections can be simply realized by changing the voltage polarity of ion optics and detector. Velocity sliced images can be directly recorded using a narrow voltage pulse applied on the rear micro-channel plate. The efficient performance of this system is evaluated by measuring the angular distribution of O(-) from the electron attachments to NO at 7.3 and 8.3 eV and O(+) from the electron collision with CO at 40.0 eV.

Orientation effect in the low-energy electron attachment to the apolar carbon tetrafluoride molecule.

Stereodynamics of the molecular fragmentation by energetic particle impact provides essential information about the interaction between the projectile and target, which has been the central topic of chemical reactions. Versatile information on the collision stereodynamics may degrade seriously by measuring the spherically averaged cross-sections because of the randomly oriented target molecules, but can benefit from the oriented target molecules that are prepared prior to the collision. Up to date, the polar molecules in gas phase can be oriented or aligned by an electrostatic field or in a linearpolarized laser field. Except for some sophisticated methods (e.g., the stimulated Raman pumping), the alignment or orientation of apolar molecules (without permanent dipole moment) is still a challenge in experiments. For the randomly oriented molecules in some cases, a similar effect of alignment or orientation could be derived from the significant anisotropy of the polar-angle-resolved differential cross-sections (DCS). However, the orientation effect is scarcely observed in slow collision reactions. During the slow approaching of the projectile, the spatial anisotropy of the projectile–target interaction potential is usually believed to be averaged out because of the molecular rotations of the target. In contrast to the above-mentioned conventional wisdom, the orientation effect in the low-energy electron attachment to the apolar molecule CF4 is observed in our anion velocity image mapping experiments. The CF4 molecule is randomly oriented in a field-free environment during attachment. Up to now, only some simple linear apolar molecules (H2, D2, N2, and CO2) were investigated for the orientation effect in medium(dozens of electronvolts) or high-energy (several hundreds to more one thousand electronvolts) electron collisions. In this work, the low-energy (no more than 6 eV) electron attachment to CF4 shows that electron capture can be strongly influenced by the target molecular orientation, leading to specific distributions of the fragment momenta. In the impulsive dissociation, the axial-recoil approximation has been well validated. The intermediate state of the dissociative precursor usually follows the fragment trajectories parallel or perpendicular to the initial molecular axis. A temporary negative ion CF4 formed by electron attachment is just this type of dissociative precursor and may decay through two complementary dissociation pathways, CF4 !F + CF3 (channel a) and F + CF3 (channel b) with appearance energies of 3.7 and 4.4 eV, respectively. The yield efficiency curves of the F and CF3 fragments produced in these two dissociative electron attachment (DEA) processes were recorded and a broad band in each curve was found. At the lower energies of about 6 eV, channels a and b were suspected to experience the common resonance state T2 of CF4 , while the lower resonance state A1 was mainly responsible for the autodetachment CF4 !e + CF4. Although the DEA dynamics has been partly revealed by angular distributions (at 6.7 eV for F and 6.1 eV for CF3 ) of the anionic fragments, there are a lot of uncertainties because those measurements were performed in the small angular q range of 15 to 1108. Such limitation is due to the spatial restriction of the rotating ion detector used in their turn-table arrangement. The full picture of DEA dynamics should be promising by using more advanced experimental techniques. The state-of-the-art anion velocity image mapping method recently developed in our group and by others can realize measurements of both the DCS of the anionic fragment in a full angular range (q = 0–3608) and the kinetic energy distributions simultaneously. The time-sliced images (azimuth angle f of about 08) of the F velocity were recorded at electron energies of 5.3, 5.5, 6.0, and 7.0 eV (Figure 1); while the images of CF3 were recorded at 5.5, 6.0, 6.5, and 7.0 eV (Figure 2). Because of the different kinetic energies of the anionic fragments and the small size of the detector (its effective diameter is 40 mm), the images were contracted or enlarged by biasing the electrode voltages for the faster F and the slower CF3 anions. The kinetic energies or velocities obtained from the images were calibrated with values available in the literature. As shown in Figure 1, the central higher intensity of F indicates that most anions have low kinetic energies (0– 0.5 eV, increasing slightly at higher attachment energies) and the larger image size implies a higher kinetic energy or a larger velocity of the F ion. At the outer ring, which corresponds to kinetic energies higher than 0.8 eV, a remarkable anisotropy of the F distribution is observed although the ion intensities are much weaker (less than 10 % of the central intensity). The high-momentum distribution appears partially on the image recorded at 6.0 eV but completely disappears at 7.0 eV. The distribution is suspected to be located outside the detector, which is due to its large momentum value or another dissociation channel. The images of the CF3 momentum are distinctly different from those of F . As shown in Figure 2, the low kinetic-energy components are not observed for the CF3 [*] L. Xia, X.-J. Zeng, H.-K. Li, B. Wu, Prof. Dr. S. X. Tian Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical Physics University of Science and Technology of China Hefei, Anhui 230026 (China) E-mail: sxtian@ustc.edu.cn

Communication: Imaging the indirect dissociation dynamics of temporary negative ion: N2O− → N2 + O−

We reported an imaging study of the dissociation dynamics of temporary negative ion N(2)O(-) formed in the low-energy electron attachment, e(-) + N(2)O → N(2)O(-) → N(2) + O(-). With the help of ab initio molecular dynamics calculations, the evolution of momentum distributions of the O(-) fragment in terms of the electron attachment energy is identified as the result of a competition between two distinctly different indirect pathways, namely, climbing over and bypassing the energy ridge after the molecular structure bending. These two pathways prefer leaving the N(2) fragment at the high vibrational and rotational states, respectively.

Communication: State mixing by spin-orbit coupling in the anionic chloroiodine dissociations

Three spin-orbit states, 1(2)Π1/2, 2(2)Π3/2, and 2(2)Π1/2, of chloroiodine anion (ICl(-)) formed by low-energy electron attachment in the Franck-Condon region are associated with the dissociative limits of I(-) ((1)S0) and Cl ((2)P3/2) or Cl(*) ((2)P1/2) fragments. Within the adiabatic scheme, the presumptive Π-symmetry of the fragment angular distributions is dramatically changed to be the Π-Σ mixing symmetry, due to the significant spin-orbit interaction effect on the electronic state couplings of ICl(-). The present experimental approach also enables us to separate the contributions of different electronic states from the mixed states, providing a crucial method for quantitatively evaluating the configuration-interaction wavefunctions.

Dissociative Electron Attachment to 1,2-Dichlorobenzene using Mass Spectrometry with Phosphor Screen

Anion mass spectrometry is developed on the basis of our home-made anion velocity map imaging apparatus. The Cl− product efficiency curve for dissociative electron attachment to 1,2-dichlorobenzene is obtained from 0.2 eV to 8 eV, meanwhile the sliced images of this anion are recorded at 1.2 and 6.0 eV corresponding to two peak positions of the product efficiency curve.

Two- and Three-Body Dissociation Dynamics of Temporary Negative Ion NF3–

Dissociation dynamics of temporary negative ion NF3(-) formed in the low-energy (0.5 to 4.5 eV) electron attachment is investigated by the anion velocity slice imaging spectroscopy. The kinetic and angular distributions of the F(-) fragment indicate that this fragment is produced via two distinctly different dissociation channels, namely, two-body and three-body fragmentations, at the higher electron attachment energies. The anisotropic distributions of the fast F(-) ions are interpreted as the two-body dissociations relevant to the (2)E resonant state of NF3(-), whereas the slow F(-) can be produced via various three-body dissociations with the products of NF(X (3)Σ(-)) + F + F(-), NF(b (1)Σ(+)) + F + F(-), or N + F2 + F(-), depending on the electron attachment energy.