Angular Distributions Of Fragment Ions Research Articles

Dissociative Ionization of Molecular CF2Br2 under 800 and 400 nm Intense Femtosecond Laser Fields

The interaction between the CF2Br2 molecule and 800/400 nm intense femtosecond laser fields is investigated by direct current (dc) sliced velocity mapping imaging implementation. By analyzing the kinetic energy release distribution and angular distribution of fragment ions, the dissociation channels along C-Br bond cleavage have been determined. The isotropic structure of the angular distribution for CF2Br+ ions is attributed to the coupling between the excited states. Additionally, a unique elimination channel of CF2Br2+ → CF2 + Br2+ has been observed and identified in the case of 400 nm laser field, in which the two C-Br bonds break asynchronously.

Ionic Angular Distributions Induced by Strong-Field Ionization of Tri-Atomic Molecules* *Supported by the National Natural Science Foundation of China (Grant Nos. 11534004, 11874179, and 11704149), and the Natural Science Foundation of Jilin Province (Grant Nos. 20180101289JC and 20190103045JH).

Angular distributions of fragment ions from ionization of several tri-atomic molecules (CO 2, OCS, N 2 O and NO 2 ) by strong 800-nm laser fields are investigated via a time-of-flight mass spectrometer. Anisotropic angular distributions of fragment ions, especially those of atomic ions, are observed for all of the molecules studied. These anisotropic angular distributions are mainly due to the geometric alignment of molecules in the strong field ionization. Distinct different patterns in ionic angular distributions for different molecules are observed. It is indicated that both molecular geometric structure and ionization channels have effects on the angular distributions of strong field ionization/fragmentation.

Dissociation dynamics of energy-selected ions using threshold photoelectron-photoion coincidence velocity imaging

Threshold photoelectron-photoion coincidence (TPEPICO) is a powerful method to prepare and analyze internal energy- or state-selected ions. Here, we review the state-of-the-art TPEPICO imaging technique combining with tunable vacuum ultraviolet (VUV) synchrotron radiation and its recent applications at Hefei Light Source (HLS), especially on the fundamental data measurement and the dissociation dynamics of ions. By applying the double velocity map imaging for both electrons and ions in coincidence, the collection efficiency of the charged particles, the electron energy resolution and the resolving power of the released kinetic energy in dissociation have been greatly improved. The kinetic energy and the angular distributions of fragment ions dissociated from parent ions with definitive internal energy or state have been acquired directly from TPEPICO images. Some dissociation mechanisms involving non-adiabatic quantum effects, like conical intersection and internal conversion, have been revealed. Moreover, the mass-selected threshold photoelectron spectroscopy (MS-TPES) shows tremendous advantages in isomer-specific analysis of complex systems.

Multiorbital effects in strong-field ionization and dissociation of aligned polar molecules CH3I and CH3Br

Controlling the molecular axis offers additional ways to study molecular ionization and dissociation in strong laser fields. We measure the ionization and dissociation yields of aligned polar ${\mathrm{CH}}_{3}X$ ($X=\mathrm{I}$, Br) molecules in a linearly polarized femtosecond laser field. The current data show that maximum ionization occurs when the laser polarization is perpendicular to the molecular $\mathrm{C}\ensuremath{-}X$ axis, and dissociation prefers to occur at the laser polarization parallel to the $\mathrm{C}\ensuremath{-}X$ axis. The observed angular distributions suggest that the parent ions are generated by ionization from the HOMO. The angular distribution of fragment ions indicates that dissociation occurs mainly from an ionic excited state produced by ionization from the HOMO-1.

Angular distributions in two body breakup of OCSq+ ions

SynopsisSeveral two-body dissociation channels of multiply ionized OCS molecules generated by Xe9+ impact are studied for the angular distributions of fragment ions with respect to the incident projectile direction. We have used the technique of recoil ion momentum spectroscopy (RIMS) for measuring momenta of the fragment ions in coincidence.

High resolution Coulomb explosion spectra and angular distributions of fragment ions of N 2 in a femtosecond laser field

Femtosecond laser field-induced ionization and Coulomb explosion are systematically investigated using high-resolution time-of-flight mass spectroscopy. Meanwhile a good alignment of the \(\text{N}_{2}\) is achieved geometrically. Based on the energy and momentum conservation laws, the events from different Coulomb explosion channels are identified accurately and further used to obtain the Kinetic Energy Release (KER) by the created molecular ion pairs and the angular distributions of the fragment ions. The KERs measured at laser intensities varying from \(\text{4 }\times {10}^{14}\,\text{W}/\mathrm {cm}^{2}\) to \(\text{2 }\times {10}^{15}\,\text{W}/\mathrm {cm}^{2}\) are found to stay constant. The angular distributions are measured at laser intensity of \(\text{9 }\times {10}^{14}\,\text{W}/\mathrm {cm}^{2}\). The atomic ions \(\text{N}^{+}\), \(\text{N}^{2+}\) and \(\text{N}^{3+}\) exhibit highly anisotropic distributions and for higher charge state, the angular distributions become narrower. With good exclusion of channel N(1,0), the non-zeroes normal to the laser polarization vector in channel N(1,1) still exist, which indicates the presence of geometric alignments (GA). The elusive shrink structure at \(\uptheta\)=0\(^\circ\) for channels N(1,1), N(1,2) and N(2,3) is observed, which implies that the non-sequential process exists, and the electron rescattering plays role in the ionization process.

Ionization and dissociation of linear triatomic molecules in strong laser fields

Ionization of molecules in strong femtosecond laser fields is a key process for understanding a variety of strong-field molecular physical phenomena. In this study, we performed an experimental study on strong-field ionization and dissociation of three linear triatomic molecules, CO2, CS2, and OCS, using time-of-flight mass spectrometer. We measured the yield of different parent ions and various fragment ions as a function of laser intensity in the range of 2.0×1013W/cm2 to 8.0×1014W/cm2, in both 800-nm and 400-nm strong laser fields. The mechanism of strong-field single and double ionization, as well as fragmentation, was discussed based on the experimental results. In addition, the angular distributions of fragment ions were obtained. The results show the anisotropic distributions in the formation of highly charged atomic fragment ions, which may be attributed to the alignment of the molecules in strong laser fields before dissociation.

Signatures of Interatomic-Coulombic-Decay in electron-impact ionization of argon dimers

SynopsisWe study electron-impact induced dissociation of small argon clusters at a projectile energy of 120 eV. Kinetic-energy-release (KER) spectra for the final charge states 2Ar+, Ar+ + Ar2+ and Ar2+ + Ar+ of the Ar2 and Ar3 parent species and electron energies have been measured together with angular distributions of fragment ions. They are used to identify dissociation mechanisms such as Interatomic-Coulombic-Decay (ICD).

Electron-impact-induced dissociation of small argon clusters

We study electron-impact-induced dissociation of small van der Waals--bound argon complexes at a projectile energy of 120 eV. Kinetic-energy-release (KER) spectra of the ${\text{Ar}}_{2}$ and ${\text{Ar}}_{3}$ parent species for the final charge states $2{\text{Ar}}^{+}, {\text{Ar}}^{+}+{\text{Ar}}^{2+}$, and ${\text{Ar}}_{2}^{+}+{\text{Ar}}^{+}$ and electron energies have been measured together with angular distributions of fragment ions. They are used to identify dissociation mechanisms such as interatomic Coulombic decay (ICD).

Adaptive strong-field control of chemical dynamics guided by three-dimensional momentum imaging

Shaping ultrafast laser pulses using adaptive feedback can manipulate dynamics in molecular systems, but extracting information from the optimized pulse remains difficult. Experimental time constraints often limit feedback to a single observable, complicating efforts to decipher the underlying mechanisms and parameterize the search process. Here we show, using two strong-field examples, that by rapidly inverting velocity map images of ions to recover the three-dimensional photofragment momentum distribution and incorporating that feedback into the control loop, the specificity of the control objective is markedly increased. First, the complex angular distribution of fragment ions from the nω+C2D4→C2D3++D interaction is manipulated. Second, isomerization of acetylene (nω+C2H2→C2H2(2+)→CH2++C+) is controlled via a barrier-suppression mechanism, a result that is validated by model calculations. Collectively, these experiments comprise a significant advance towards the fundamental goal of actively guiding population to a specified quantum state of a molecule.

Fragmentation of N2 in 410 nm Intense Femtosecond Laser Field

Fragmentations of N2 in linearly polarized femtosecond 410 and 820 nm intense laser fields were studied by using the velocity mapping technique. Different behaviors of N2 at 410 and 820 nm were observed. Both the kinetic energy distributions and angular distributions of fragment ions in 410 nm field show weak dependency on laser intensities in the non-saturation regime, in contrast to the case in 820 nm. Different excited electronic states, i.e., non-Coulombic potentials populated via vertical excitation, are suggested to play crucial roles in fragmentations at short wavelength.

Angular distributions of fragment ions ofN2in a femtosecond laser field

We experimentally measure the kinetic energy and angular distributions of fragment ions of ${\mathrm{N}}_{2}$ as a function of $820\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ femtosecond laser intensity. The angular dependence of fragments directly maps to the orbital symmetry of transient molecular ions. According to the observed energy and angular distribution of different charged fragment ions, reasonable fragmentation channels are proposed. Both the asymmetric channel (3, 1) and the symmetric channel (2, 2) are supposed to be generated directly from (2, 1) by different ionization mechanisms.

Anisotropic Angular Distribution of Fragment Ions Formed in Single Photoionization of Nitrous Oxide in the Range of 15 ~ 36 eV

Anisotropic Angular Distribution of Fragment Ions Formed in Single Photoionization of Nitrous Oxide in the Range of 15 ~ 36 eV

Anisotropy of the angular distribution of fragment ions in dissociative double photoionization of N2O molecules in the 30–50eV energy range

The double photoionization of N2O molecules by linearly polarized light in the 30-50 eV energy range has been studied by coupling ion imaging technique and electron-ion-ion coincidence. For the two possible dissociative processes, leading to N++NO+ and O++N2+, angular distributions of ionic fragments have been measured, finding an evident anisotropy. This indicates that the molecules ionize when their axis is parallel to the light polarization vector and the fragments are separating in a time shorter than the dication rotational period. The analysis of results provides, in addition to the total kinetic energy of ionic fragments, crucial information about the double photoionization dynamics.

Anomalous angular distribution of fragment ions from rare-gas diatomic molecules with intense, femtosecond, near-infrared laser pulses

Almost isotropic angular distributions are observed for fragment ions from ${\mathrm{Xe}}_{2}$ produced with intense femtosecond laser pulses ($800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $4\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$). The results observed for ${\mathrm{Xe}}_{2}$ present a great contrast to those for ${\mathrm{I}}_{2}$ molecules used as reference molecules, which show typical anisotropic angular distributions resulting from enhanced ionization. The kinetic energy spectra of the fragment ions from ${\mathrm{Xe}}_{2}$ reveal that the internuclear distance remains near the equilibrium distance for various pulse widths from 45 to $710\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, indicating that in the rising edge of the pulses, the internuclear distance of ${\mathrm{Xe}}_{2}^{\phantom{\rule{0.4em}{0ex}}n+}$ ($n=0$, 1, 2) does not reach the favorable region for enhanced ionization even though enhancement can potentially work for ${\mathrm{Xe}}_{2}^{\phantom{\rule{0.4em}{0ex}}n+}$. The dynamic alignment of neutral ${\mathrm{Xe}}_{2}$ during the pulses is also discussed on the basis of numerical simulations.

Fragmentation of H2O molecules following the interaction with slow, highly charged Ne ions

We measured the energy and angular distribution of fragment ions from collisions of 2–90 keV Neq+ ions (q = 1, 3, 5, 7 and 9) with H2O molecules. A strong dependence on the charge state of the incident ion was observed. The results are interpreted within the framework of a Coulomb explosion model. Deviations from this model in the energy and angular distribution of recoil ions are attributed to post-collision effects by the electric field of the projectile. Absolute cross sections for ion production were measured and found to be consistent with the cross sections calculated using the classical over-barrier model.

The angular distributions of fragment ions from labelledand unlabelled N2O in intense laser fields

The mass spectra and angular distributions of fragment ionsarising from a Coulomb explosion of highly charged parent nitrousoxide ions, obtained in the femtosecond regime (~1016 W cm-2), are investigated. The N ion angulardistributions from 14N2O show maxima when the laserpolarization is parallel and orthogonal to the time-of-flightaxis. Measurements with labelled molecules(15N-14N = 16O) indicate that the maxima arisefrom the peripheral and central N atoms in the molecularstructure. The anisotropic distributions may be explained byassuming increased ionization and fragmentation when themolecular axis is parallel to the laser field. The bond angleprior to explosion is determined to be ~140°,irrespective of the charge state of the precursor, andcalculations of the kinetic energies imparted to the fragment ionssuggest that dissociation occurs at the equilibriuminternuclear distance of the neutral molecule.

An investigation of the angular distributions of fragment ions arising from the linear CS2 and CO2 molecules

The nonlinear interaction of the triatomic molecules CS2 and CO2 with the intense field of a linearly polarized laser beam of femtosecond (fs) pulse duration, was used to study the ionization and dissociation of the parent molecule. The fragment ion angular distributions arising from the Coulomb explosion of the parent ions were also measured. For CS2, the angular distributions of CS2+, CS22+, CS23+, CS+, CS2+, Sn+ (n6) and Cm+ (m4) ions are presented for a laser intensity of 1 × 1016 W cm-2 at a wavelength of 790 nm and pulse duration of 50 fs. The angular distributions of the parent molecular ions are all isotropic. The Sn+ fragments are peaked along the time-of-flight (TOF) axis, whereas the Cm+ fragments explode perpendicularly to this. Similar results for CO2 are also presented for comparison. The S ion distributions do not narrow as their ionic charge increases, and it is argued that the angular distributions for CS2 are due mainly to the angular dependence of the ionization probability. On the other hand, the distributions from the lighter CO2 molecule are thought to be at least partly due to alignment via dipole moments induced by the laser, as in this case the On+ angular distributions are seen to narrow as their charge increases. The conclusion of these results is that the laser pulse may be too short for the CS2 molecule to align in the pulse. Angular distributions are also presented for varying laser pulse durations, in the range of 50 fs to 300 ps. The dynamics of the ionization/dissociation mechanism are discussed in the context of the TOF mass spectra and angular distributions recorded for CS2.

Dynamic and geometric alignment ofCS2in intense laser fields of picosecond and femtosecond duration

CS$_2$ is identified as a molecule for which distinction can be made between dynamic and geometric alignment induced by intense laser fields. Measured anisotropic angular distributions of fragment ions arise from (i) dynamic alignment of the S-C-S axes along the laser polarization vector for 35-ps laser pulses and (ii) geometric alignment due to an angle-dependent ionization rate in the case of 100-fs pulses. Results of classical calculations of the alignment dynamics support our observations. By comparing mass spectra obtained with linearly- and circularly-polarized light it is not possible to distinguish between dynamic and geometric alignment. 33.80.Rv, 33.90.+h, 42.50.Vk

Variable angle reflectron time-of-flight mass analyzer (VARTMAN) for studying synchrotron radiation photochemistry of polyatomic molecules

Abstract A new experimental apparatus, variable angle reflectron time-of-flight mass analyzer (VARTMAN), for studying synchrotron radiation photochemistry of the inner core excited organic molecules has been designed and constructed. A new type of reflectron time-of-flight (RTOF) mass analyzer, having a mesh-less reflector and tandem ion lens system, was designed to obtain moderate mass-resolution, even for fragment ions with kinetic energies of several eV while preserving kinetic energy information of the ions. In order to observe the angular distribution of fragment ions, the detection angle of the RTOF axis is variable with respect to the linear polarization axis of the synchrotron radiation. The new type of TOF can be operated in; (1) all solid angle (4π) collection mode for ions with kinetic energy up to 10 eV, (2) energy and angle resolve mode with a limited solid angle and (3) linear TOF mode. The mass resolution of the RTOF was measured to be 80 for fragment ions with kinetic energy up to 5 eV.