Coulomb Explosion Imaging Method Research Articles

Probing Conical Intersection in the Multipathway Isomerization of CH3Cl Using Coulomb Explosion.

Ubiquitous ultrafast isomerization is paramount in photoexcited molecules, in which non-adiabatic coupling among multiple electronic states can occur. We use the pump-probe Coulomb explosion imaging method to study the isomerization of CH3Cl molecules. We find that the isomerization under our strong field pump-probe scheme proceeds along multiple pathways, which are encoded in several distinct branches of the time-resolved kinetic energy release spectra for the CH2++HCl+ Coulomb explosion channel. Apart from the isomerized dissociative pathway in neutral and cationic excited states, the pump laser can also induce coherent vibrational dynamics in two coupled intermediate states and set up the initial conditions for the two concurrently proceeding isomerization pathways. The isomerization of CH3Cl provides an intriguing example of a chemical reaction consisting of multiple pathways and non-adiabatic dynamics.

Effect of structure and orientation of incident carbon-cluster ions Cn+(n≤4) on secondary-ion emission induced by electronic excitation

We present experimental results that show the secondary-ion yield depends on the structure and orientation of incident cluster ions. A beam of carbon-cluster ions ${{\mathrm{C}}_{n}}^{+}$ ($n\ensuremath{\le}4$) with an energy of 0.9 MeV per atom, which were obtained from a tandem electrostatic accelerator, was incident on a glycine target deposited on a carbon foil. The structure and orientation of the cluster ions after passing through the target were determined by the Coulomb explosion imaging method, and the positive secondary ions were simultaneously measured by time-of-flight mass spectrometry. When looking at the orientation dependence of linear structured ${{\mathrm{C}}_{2}}^{+}, {{\mathrm{C}}_{3}}^{+}$, and ${{\mathrm{C}}_{4}}^{+}$ projectiles, parallel orientations with respect to the beam direction were found to enhance the secondary-ion yield compared with perpendicular orientations. Additionally, comparison of the structure dependence of ${{\mathrm{C}}_{3}}^{+}$ and ${{\mathrm{C}}_{4}}^{+}$ projectiles with linear and ring structures showed that the secondary-ion yield was larger for linear structures. Interestingly, there was a clear correlation between the secondary-ion yield and the deposited energy density in the cluster-ion track. The energy density was calculated with electronic stopping power for incident cluster ions using linear-response dielectric theory and the cross section of the track of the incident cluster considering its geometrical structure and orientation. These results suggest that the deposited energy density in the cluster-ion track is a key parameter in the secondary-ion emission process induced by swift cluster-ion impacts.

Studying Molecular Structure and Dynamics via Coulomb Explosion Imaging with X-rays and Ultrafast Laser Pulses

SynopsisCoincidence momentum imaging is a powerful method to study molecular structure and dynamics. Here, we used the Coulomb explosion imaging method together with synchrotron radiation and ultrafast laser pulses in order to distinguish between geometric isomers of C2H2Br2, C2H2Cl2, and difluoroiodobenzene and to investigate their fragmentation dynamics following both strong-field and inner-shell photoionization.

Wake effects in the evolution of fast molecular ions through thin foils

In order to improve the geometrical characterization of molecular ions using the Coulomb Explosion Imaging method, we discuss the need to include wake effects in a full description of the interactions felt by the swift dissociated atomic ions moving through thin foils. Comparison of simulations and experimental results for the structure of diatomic (HD+) and triatomic (CH2+) molecular ions is significantly improved when including the polarization effects induced in the target by the passage of swift ions.

Vibrational relaxation and dissociative recombination ofH2+induced by slow electrons

We present calculations of cross sections and rate coefficients for the dissociative recombination of ${\mathrm{H}}_{2}^{+}$ ions initially in $v=0--6$ vibrational levels, together with rate coefficients for the competing electron-induced vibrational deexcitation. We used the multichannel quantum defect theory with a second-order treatment of the K matrix, and show that electronic interactions dominate not only the dissociative recombination but also the vibrational relaxation induced by slow electrons. Most of our rate coefficients for dissociative recombination are in good agreement with the measurements at the TSR storage ring [S. Krohn et al., Phys. Rev. A 62, 032713 (2000)]. On the contrary, our rates for vibrational deexcitation, close to former results obtained by R-matrix calculations [B. K. Sarpal and J. Tennyson, Mon. Not. R. Astron. Soc. 263, 909 (1993)], are smaller by up to one order of magnitude than the experimental values which are deduced from the time evolution of the vibrational populations, measured by the Coulomb explosion imaging method.

Mapping of coherent and decohering nuclear wave-packet dynamics inD2+with ultrashort laser pulses

Fast ionization of ${\mathrm{D}}_{2}$ leads to the coherent population of many vibrational states of ${\mathrm{D}}_{2}^{+}.$ Usually, only the squared absolute values of the vibrational state amplitudes, known as Franck-Condon factors, are observed since insufficient experimental time resolution averages out all coherence effects. We propose a Coulomb explosion imaging method to visualize the coherent motion of bound wave packets using ultrashort (5 fs), intense pump-probe laser pulses. With this type of experiment decoherence times in the fs to ps range may become directly observable and provide essential information for coherent control.

Electron-induced vibrational deexcitation ofH2+

Using the Coulomb explosion imaging method, the change of the relative population for the first six vibrational states of ${\mathrm{H}}_{2}^{+}$ during the interaction with low-kinetic-energy electrons has been measured. A model based on rate coefficients for dissociative recombination and superelastic collision processes is developed to explain the time dependence of the relative vibrational populations. Using this model, we demonstrate that superelastic collisions with rate coefficients of $(1\ensuremath{-}4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}{\mathrm{cm}}^{3}{\mathrm{s}}^{\ensuremath{-}1}$ (about an order of magnitude higher than available theoretical predictions) can explain the observed electron-induced vibrational deexcitation of ${\mathrm{H}}_{2}^{+}.$

Application of ultrathin diamond-like-carbon targets to Coulomb explosion imaging

Thin diamond-like carbon (DLC) foils were tested as stripping targets for molecular structure studies using the Coulomb explosion imaging method. The multiple scattering of MeV atomic and molecular ion beams penetrating DLC foils was measured and compared to Formvar foils which have been used in CEI setups so far. The DLC targets were found to be of similar thickness (0.7–0.9 μg/cm 2) as the Formvar foils but of higher efficiency for CEI measurements as they exhibit less pinholes. Other advantages of DLC foils are: smaller linear thickness due to their higher density, higher homogeneity and better control of the production process. The production of targets with even smaller thicknesses is anticipated.

Excited state double proton transfer in the 7-azaindole dimer revisited

A review of the Coulomb explosion imaging method is presented along with a discussion regarding its use in determining the mechanism of the excited state double proton transfer (ESDPT) reaction of 7-azaindole dimer. A comprehensive discussion of the tautomerization process is outlined to fully explain the stepwise mechanism exhibited in our studies. Finally, recent comments are addressed relating to our technique and the interpretation of our results obtained by the method. Our findings are shown to be in excellent agreement with those of Zewail and co-workers who employed a different method of following the evolution of the stepwise ESDPT.

Coulomb explosion imaging: the CH 2+, H 2O + and NH 2+ ions as benchmarks

In a previous paper we calculated the average distribution of bending angles for CH 2 + ions at 300 K using ab initio surfaces for the pair of Renner interacting ground state potentials in order to test the results of Coulomb explosion imaging (CEI) experiments. Here we calculate state selected bending angle distributions, and Boltzmann-averaged bond length distributions for CH 2 + in order to inspire more CEI experiments. Using published potential energy curves for H 2O + and NH 2 +, we predict their bending angle distributions (and that of ND 2 +) to provide further benchmarks to test the CEI method.

Ground state ofCH2+:Experimental aspects and theoretical implications

Hot ${\mathrm{CH}}_{2}^{+}$ molecular ion ensembles were prepared, accelerated, and stored for radiative cooling to room temperature. The structure of the species was measured by the Coulomb explosion imaging method at different stages of cooling. The bending angle distributions were extracted and compared with recent theories as well as a previous Coulomb explosion imaging measurement. The comparison reveals an apparent large nonadiabatic contribution to the low-lying ${\mathrm{CH}}_{2}^{+}$ wave functions.

Direct measurement of bending conformations in triatomic dihydride ions

The distributions of bond angles in the triatomic dihydride ion series ${\mathrm{CH}}_{2}^{+}$, ${\mathrm{NH}}_{2}^{+}$, and ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$ have been studied using the Coulomb explosion imaging method. These distributions were measured as a function of the cooling of the internal degrees of freedom of these ions. The distribution for the coldest sample of ${\mathrm{CH}}_{2}^{+}$ molecules shows the most probable structure to be bent with substantial tunneling through the linear conformation. The most probable geometry for ${\mathrm{NH}}_{2}^{+}$ was found to be linear, though the angular distribution is significantly different from a shape of a harmonic-oscillator ground-state prediction. In the case of ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$, we find a bent structure as expected from theory. Evidence for a linear excited state in ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$ is seen in the hotter distributions. Comparison to the adiabatic theoretical predictions shows good agreement with the most probable geometries. However, the measured distributions are systematically wider than the squared vibronic wave functions derived from the corresponding potential-energy surfaces.

Experimental evidence for anomalous nuclear delocalization in C2H3+

The structure of protonated acetylene (C 2 H 3 + ), including correlations, is measured and analyzed by an advanced Coulomb explosion imaging method. In the data analysis, it is essential to include large-amplitude motions of the nuclei within the molecule and many-body correlation features for revealing the correct structure. We find that the nuclear conformations in this molecule differ markedly from theoretical predictions and previous experimental findings

Observation of laser excitation of rhombic C4 using the coulomb explosion method

The combination of laser photodetachment of C 4 − and the Coulomb Explosion Imaging method was applied for the investigation of the structure of several C4 isomers and was correlated with their measured electron affinities.

The structure of small carbon clusters

Abstract New isomers of Cn for n = 4, 5, 6 have been observed using the combination of laser photodetachment technique and the Coulomb Explosion Imaging method. Electron affinities of these isomers were found to be lower than the corresponding known linear isomers. The structure of low electron affinity C 4 isomer was found to be rhombic in accordance with recent theory. Indications for the non linearity of the low electron affinity isomers of C 5 and C 6 are also presented.

The structures of C2H+ and C2H+2 as measured by Coulomb explosion imaging

The structures of ethynyl and acetylene cations were investigated utilizing the Coulomb explosion imaging method. The geometrical probability density functions for the bending modes were obtained for the protonated and deuterated forms of each species. Detailed analyses of the data reveal that both the measured linear equilibrium geometries and the observed extent of the correlated bending vibrations agree with ab initio calculations for C2H+2 and C2D+2. C2H+ and C2D+ are found to be linear, but the extent of the bending amplitudes are by far larger than the corresponding theoretical predictions.

The structure of small carbon clusters

New non linear isomers of C n forn=4, 5, 6 have been observed using the combination of the laser photodetachment technique and the Coulomb Explosion Imaging method. Electron affinities of these isomers were found to be lower than the corresponding known linear isomers. The structure of low electron affinity C4 isomer was found to be rhombic in accordance with recent theory. Indications for the non linearity of the low electron affinity isomers of C5 and C6 are also presented.

Structure and Photochemistry of C4 as Studied by the Coulomb Explosion Imaging Method

AbstractThe Coulomb Explosion Imaging (CEI) method was used to determine the structure and photochemistry of carbon clusters. The cyclic structure of C4 was found using a combination of laser photodetachment techniques and the CEI method. The cyclic C4 electron affinity was found to be 2.1 ± 0.1 eV, more than 1 eV lower than the electron affinity of the linear isomer.

The structure of carbon clusters as studied by the coulomb explosion method

The Coulomb explosion imaging (CEI) method has been used to determine the structure and photochemistry of carbon clusters. The cyclic isomer of Cn for 4 ⩽n⩽ 6 was observed, using a combination of laser photo-detachment techniques and the CEI method. The cyclic C4 electron affinity was found to be 2.1 ± 0.1 eV, more than one eV lower than the electron affinity of the linear isomer. Upper limits for the electron affinities of the cyclic C5, and C6 were also established.