Momentum Imaging Technique Research Articles

Low kinetic energy release upon dissociation of benchmark molecular ions

We explore very slow laser-induced dissociation processes of molecular ion beams employing an upgraded coincidence 3D momentum imaging technique. A selection of our studies, focused on zero-photon dissociation of H2+, two-body breakup of D3+, and vibrational structure in O2+ dissociation, will be presented.

Intense laser-induced dissociation pathways of CD+

The dissociation of CD+ induced by intense 30 fs pulses of central wavelengths 385 nm and 790 nm, over a range of peak intensities (5 × 1013 − 1 × 1016 W/cm2), was measured using a coincidence 3D momentum imaging technique. The observed kinetic energy release and angular distributions of the different products are presented and the possible dissociation pathways are discussed.

Observation of the dynamics leading to a conical intersection in dissociative electron attachment to water

Following prior work on the lower-energy resonances, we apply techniques of momentum imaging and ab initio scattering calculations to the process of dissociative electron attachment to water via the highest-energy ${}^{2}{B}_{2}$ resonance. We focus on the H${}^{\ensuremath{-}}$ anion fragment, which is produced via dynamics passing through and avoiding the conical intersection with the lower ${A}_{1}$ state, leading to OH (${}^{2}\ensuremath{\Pi}$) and OH (${}^{2}\ensuremath{\Sigma}$), respectively. The momentum imaging technique, when combined with theoretical calculations on the attachment amplitude and dissociation dynamics, demonstrates that the angular distributions provide a signature of the location of the conical intersection in the space of nuclear configurations.

Triple F+ ejection from SF6 bombarded by swift ions

Many-body explosion of SF3 +6 is investigated by the coincidence momentum imaging technique. Investigation is focused on the F+:F+:F+ dissociative channel. The momentum map of these channels indicates that the loss of neutral F from SF3 +6 occurs in the first step of the break-up followed by Coulomb explosion of the residue. The angular correlation between each F+ ion is used to figure out the nature of explosion. The scalar product of three momentum vectors is determined for estimating the co-planarity of all three ejected F+ ions. From the analysis of angular correlation for coplanar and orthogonal ejection of F+ ions, we conclude that there are two dominant conformations of SF3 +3.

Exploring few-photon, few-electron reactions at FLASH: from ion yield and momentum measurements to time-resolved and kinematically complete experiments

In this paper, we briefly review a series of recent pioneering experiments on few-photon-induced multiple fragmentation of atoms and molecules performed at the free electron laser (FEL) at Hamburg (FLASH) using a ‘reaction microscope’, i.e. a coincident momentum imaging technique. For atoms we consider the most basic nonlinear process, namely direct or sequential two-photon double ionization (TPDI). In particular, benchmark data for theory are presented, such as recoil–ion momentum distributions for direct TPDI of He and Ne, and fully differential data for sequential TPDI of Ne. For molecules we show how one can identify contributions from different reaction channels by inspecting the measured kinetic energy and angular distributions of ionic fragments and even infer the time delay between the two-photon absorption events in TPDI by detailed comparison with theory. Finally, we describe a novel split mirror arrangement for EUV-pump–EUV-probe experiments and present the very first time-resolved data on the dynamics of N2 molecules irradiated by the FLASH light.

Suppressed Dissociation ofH2+Vibrational States by Reduced Dipole Coupling

The suppression of H(2)(+) strong-field dissociation has intrigued experimentalists and theorists since the early days of laser-molecular science. We unravel a vibrational suppression effect due to weak dipole-matrix element coupling strengths of certain vibrational states, dependent on the laser frequency-a form of Cooper minima. This effect is demonstrated by our full-dimensional calculations on H(2)(+) dissociation and persists for a broad range of laser conditions including both weak and strong-field dissociation. Using a crossed-beams coincidence, three-dimensional momentum-imaging technique, the vibrational suppression effect is clearly observed for H(2)(+) and HD(+) at 790 and 395 nm, in good agreement with our theory.

Dissociation and ionization of anHD+beam induced by intense 395-nm ultrashort laser pulses

The majority of research on intense $(\ensuremath{\sim}{10}^{14}\text{ }\text{W}/{\text{cm}}^{2})$ ultrashort $(<100\text{ }\text{fs})$ laser-molecule interactions has been focused on studies of ${\text{H}}_{2}^{+}$ fragmentation, mainly due to its elemental structure. So far the bulk of this work has been conducted using near infrared light while studies at shorter wavelengths are comparatively scarce. We report a detailed investigation of the interaction of 395 nm, 40 fs pulses with an ${\text{HD}}^{+}$ molecular-ion beam, measured using a coincidence three-dimensional momentum imaging technique. This allows us to clearly discriminate dissociation and ionization channels. From the kinetic energy and angular distributions, insight is gained into the intensity dependence of the main breakup processes. We observe the onset of above-threshold dissociation above ${10}^{14}\text{ }\text{W}/{\text{cm}}^{2}$, a higher intensity than required for 790 nm due to a smaller transition dipole moment for the low vibrational states probed at 395 nm. Ionization spectra display structure consistent with the above-threshold Coulomb explosion mechanism that we have proposed [B. D. Esry et al., Phys. Rev. Lett. 97, 013003 (2006)].

Experimental characterization of the metastableD2−ion by photofragment imaging

The rovibrational states of the metastable anionic hydrogen molecule $\text{D}_{2}{}^{\ensuremath{-}}$ were studied in photofragmentation experiments at 532 nm employing a fast (keV) ion beam. By a three-dimensional fragment momentum imaging technique, the photodetachment and the dissociation of the system into neutral atoms were investigated at different times after production of the molecular ions. The present data are sensitive to both the rotational states and the vibrational wave functions of the anions and suggest that the level of rotational excitation, while still exceptionally high, is somewhat lower than estimated in recent theoretical predictions.

Multiple Explosion Pathways of the Deuterated Benzene Trication in 9-fs Intense Laser Fields

The fragmentation of deuterated benzene (C6D6) in ultrashort intense laser fields (9 fs, 1 x 10(15) W/cm2) is studied by the ion-coincidence momentum imaging technique. Five two-body and eight three-body Coulomb explosion pathways from the trication (C6D6(3+)), associated with the deprotonation and ring-opening reactions, are identified. It is found from the fragment momentum correlation that all the observed three-body explosion processes proceed sequentially via the two-body Coulomb explosion forming molecular dications, C(m)D(n)(2+), with (m,n) = (6,5), (5,5), (5,4), (4,4), (4,3), and (3,3), which further dissociate into pairs of monocations. The branching ratio of the fragmentation pathways estimated from the number of the observed coincidence events indicates that the fragmentation is nonstatistical.

Three-body fragmentation ofCO22+uponK-shell photoionization

The fragmentation dynamics of $\mathrm{C}{\mathrm{O}}_{2}$ molecules subsequent to $K$-shell photoexcitation and ionization was studied using a multicoincidence ion momentum imaging technique. The detailed analysis via fragment momentum correlation plots (Newton diagrams) clearly reveals concurrent fragmentation mechanisms for the three-body dissociation of $\mathrm{C}\mathrm{O}_{2}{}^{2+}$ into ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}+\mathrm{O}$, for both linear and bent geometry states of $\mathrm{C}\mathrm{O}_{2}{}^{2+}$. The experimental results are supported by a classical trajectory simulation based on a Coulomb explosion model, which elucidates energy and angular correlations between fragments for different fragmentation processes.

Higher-order contributions observed in three-dimensional(e,2e)cross-section measurements at 1-keV impact energy

We present experimental and theoretical fully differential cross sections for single ionization by fast, 1-keV $(v=8.6\phantom{\rule{0.3em}{0ex}}\text{a.u.})$ electron impact. The cross sections were measured using a momentum imaging technique for electrons and ions (reaction microscope), which covers a large fraction of the emission angles for emitted low-energy electrons $(E<15\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and a wide range of scattering angles. Therefore comprehensive data sets are obtained for ionizing collisions at small relative momentum and energy transfer from the projectile to the target system. The experimental data are compared with predictions from several state-of-the-art theoretical calculations. At this high impact energy the calculated cross section for electron emission out of the scattering plane appears to be particularly sensitive to the treatment of higher orders in the projectile-target interaction within perturbative models.

Controlling molecular fragmentation using low energy electrons

We show that functional group dependence exists in dissociative attachment of electrons to molecules and this leads to site and bond selectivity in fragmenting organic molecules at the N-H, O-H and C-H sites using electron energy as a control parameter. This phenomenon is investigated further by measuring the momentum distribution of hydride ions using the newly developed ion momentum imaging technique. We find that while the electron attachment at the O-site leads to a two-body fragmentation, attachment at the C-site leads to few-body fragmentation. Several new phenomena like 'bond orientation dependent electron attachment' and direct screening of one part of the molecule by another part to the incoming electron are unravelled in the very rich momentum distribution data of the hydride ions that we have obtained at various resonances.

EICO measurements of inner-core excited mixed rare-gas clusters

The spectra of deep inner-core excited mixed rare-gas clusters were recorded by using electron ion coincidence (EICO) and multi-hit momentum imaging (MHMI) techniques. The EICO spectra for Ar99Kr1 clusters reveal that singly charged ions are emitted from the inner-core excited clusters in addition to the multiple charged ions. The dependence of the EICO spectra on photon energy and cluster size suggests that the holes created through vacancy cascade on the krypton atoms are transferred to the surrounding atoms, and that the singly charged ions are the primary product of the krypton photoabsorption. Charge localization is suggested for the inner-core excited mixed rare-gas clusters from the analysis of the EICO peak width. The MHMI measurements give us direct evidence for the strong charge migration from X-ray absorbing atoms to surrounding atoms. The photon energy dependence of the PSD image for fragment ions suggests that the momentum of the fragment ions depends on the number of charges generated by the vacancy cascade.

Ejection of triatomic hydrogen molecular ion from methanol in intense laser fields

The ejection of a triatomic or diatomic hydrogen molecular ion from methanol (CH 3OH and CD 3OH) in intense laser fields (86 fs, 800 nm, ≈10 14 W/cm 2) is investigated based on the momentum vector distributions of the fragment ions measured by the mass-resolved momentum imaging technique. From the relative yield of D 3 + , HD 2 + , D 2 + and HD + ejected from CD 3OH 2+ and the anisotropic distributions of their momentum vectors, the rates of the formation of triatomic ( D 3 + and HD 2 + ) and diatomic ( D 2 + and HD +) fragment ions, k t = 0.15 ps −1 and k d = 0.31 ps −1, respectively, and the H/D exchange rate, k ex = 0.13 ps −1, are derived.

Dynamics of core-ionization and excitation of molecules probed by multiple coincidence momentum imaging spectroscopy

We have been developing a multiple coincidence momentum imaging technique on BL27SU at SPring-8 in Japan. The apparatus consists of electron and ion time-of-flight analyzers with multi-hit two-dimensional position-sensitive detectors, and a supersonic jet. Recent studies with this technique on the molecular deformation and dissociation pathway of core-excited HCCH, the symmetry of the continuum states of the bent molecule of NO 2, and 2p photoelectron angular distributions in the molecular frame for Ar dimers are reviewed.

Photon–ion collisions and molecular clocks

The timing of molecular rearrangements can be followed in the time domain on a femtosecond scale by using momentum imaging techniques. Three examples are discussed in this paper: first, the diffraction of electrons ejected from the K-shell of one of the atomic constituents of the molecule takes a ‘picture’ of the molecule, and the correlation between the momentum vector of the photoelectron and the subsequent fragmentation pattern is used to estimate the time delay which accompanies the latter process. Second, the kinetic energy release of proton pairs from the double ionization of hydrogen by fast laser pulses is timed using the optical cycle as a clock. The mechanisms of rescattering, sequential and enhanced ionization are clearly identified in the momentum spectra. Third, the operation of rescattering double ionization in the case of nitrogen and oxygen molecules is discussed.

High resolution kinetic energy release spectra and angular distributions from double ionization of nitrogen and oxygen by short laser pulses

We have used momentum imaging techniques to measure in high resolution the kinetic energy release spectra and angular distributions of coincident O+ and N+ ion pairs produced by short laser pulses (8–35 fs) on targets of N2 and O2 at peak intensities between 1 and 12 × 1014 W cm−2. We record the full momentum vectors of both members of each pair and achieve a kinetic energy release resolution of less than 0.3 eV. We find that the process proceeds through well-defined electronic states of the excited molecular dications. Using linear and circularly polarized light, we identify two mechanisms for the production of these states, rescattering and sequential ionization. By using 8 fs pulses, we observe that the internuclear distance can be frozen during the pulse. For low intensities and 8 fs pulses, emission from N2 is strongly directed along the polarization vector, while that for O2 is not, a result we interpret as being due to the different symmetries of the outer orbitals of these molecules. For high intensities and longer pulses, the distributions increasingly fold towards the polarization vector, ultimately peaking at zero degrees for both molecules. For oxygen, a local peaking for molecules aligned at right angles to the polarization vector is seen. A discussion and interpretation of the results are presented.

Probe of bending motion following the 1s(-1)pi* excitation of N2O.

The doubly degenerate core-excited Pi state of N2O splits into two due to the static Renner-Teller effect. The lower state, A1, has a bent stable geometry and the molecule excited to this state starts to deform itself toward this bent geometry. To probe the effect of the potential energy surfaces of the core-excited A1 states on the nuclear motion, we measure the momenta of the three atomic ions in coincidence by means of the ion momentum imaging technique. We find that the potential energy surface affects the molecular deformation significantly. N2O in the terminal N 1s(-1)3piA1 excited state is observed to be bent more than that in the central N 1s(-1)3piA1 excited state. This means that N2O in the terminal N 1s(-1)3piA1 excited state bends faster than that in the central N 1s(-1)3piA1 excited state. When the excitation energy is decreased within the 1s(-1)3pi resonances, the nuclear motion in the A1 states becomes faster. This is interpreted by the notion that the excitation occurs onto the steeper slope part of the potential energy surface of the excited state for the lower excitation energy. The branching ratio of the A1 excitation increases with the decrease in the excitation energy.

Asymmetric nuclear motion of theF1s–ionized state inBF3probed by quadruple-ion-coincidence momentum imaging

Using the quadruple-ion-coincidence momentum imaging technique, we find that the momentum correlation of the four atomic ions departed from one ${\mathrm{BF}}_{3}^{4+}$ parent molecular ion produced via multiple Auger decay after $\mathrm{F}1s$ ionization exhibits asymmetric fragmentation in which the ${\mathrm{B}}^{+}$ ion is ejected in the direction opposite to one of the ${\mathrm{F}}^{+}$ ions. This observation provides evidence of symmetry lowering, from ${D}_{3h}$ to ${C}_{3v}$ in the $\mathrm{F}1s$--ionized state.

Nuclear Dynamics of Core-Excited and Ionized Small Polyatomic Molecules Probed by Multiple Coincidence Momentum Imaging Technique

Our recent studies on nuclear dynamics of core-excited and core-ionized molecules probed by a multiple coincidence momentum imaging technique are reviewed. This technique consists of a time-of-flight spectrometer with a multi-hit two-dimensional position sensitive detector and a supersonic jet. One can probe the symmetries and the nuclear motions of polyatomic molecules using this technique. As a specific example, we illustrate how to resolve the symmetries of the CO2 C 1s−12πu core-excited states, which split into two states, A1 and B1, due to D∞h → C2v symmetry lowering by the static Renner-Teller effect, and how to probe the nuclear motion caused in these states. We discuss also symmetry lowering D3h → C3v and D3h → C2v initiated by the B 1s → 4a2'' excitation and F 1s ionization, respectively, in the BF3 molecule and nuclear motions in these core-excited and ionized states.