Ellipticity controlled dissociative double ionization of ethane by strong fields.

The yields of all dissociation channels of ethane dications produced by strong field double ionization were measured. It was found that the branching ratios can be controlled by varying the ellipticity of laser pulses. The CH3+ formation and H+ formation channels show a clear competition, producing the highest and lowest branching ratios at ellipticity of ∼0.6, respectively. With the help of theoretical calculations, such a control was attributed to the ellipticity dependent yields of different sequential ionization pathways.

Disentangling Strong-Field Multielectron Dynamics with Angular Streaking.

The study into the interaction between a strong laser field and atoms/molecules has led to significant advances in developing spectroscopic tools in the attosecond time-domain and methods for controlling chemical reactions. There has been great interest in understanding the complex electronic and nuclear dynamics of molecules in strong laser fields. However, it is still a formidable challenge to fully model such dynamics. Conventional experimental tools such as photoelectron spectroscopy encounter difficulties in revealing the involved states because the electron spectra are largely dictated by the property of the laser field. Here, with strong field angular streaking technique, we measure the angle-dependent ionization yields that directly reflect the symmetry of the ionizing orbitals of methyl iodide and thus reveal the ionization/dissociation dynamics. Moreover, kinematically complete measurements of momentum vectors of all fragments in dissociative double ionization processes allow access to electron-momentum correlations that reveal correlated multielectron dynamics.

Attosecond Electron Correlation Dynamics in Double Ionization of Benzene Probed with Two-Electron Angular Streaking.

With a novel three-dimensional electron-electron coincidence imaging technique and two-electron angular streaking method, we show that the emission time delay between two electrons can be measured from tens of attoseconds to more than 1 fs. Surprisingly, in benzene, the double ionization rate decays as the time delay between the first and second electron emission increases during the first 500 as. This is further supported by the decay of the Coulomb repulsion in the direction perpendicular to the laser polarization. This result reveals that laser-induced electron correlation plays a major role in strong field double ionization of benzene driven by a nearly circularly polarized field.

Note: An improved 3D imaging system for electron-electron coincidence measurements.

We demonstrate an improved imaging system that can achieve highly efficient 3D detection of two electrons in coincidence. The imaging system is based on a fast frame complementary metal-oxide semiconductor camera and a high-speed waveform digitizer. We have shown previously that this detection system is capable of 3D detection of ions and electrons with good temporal and spatial resolution. Here, we show that with a new timing analysis algorithm, this system can achieve an unprecedented dead-time (<0.7 ns) and dead-space (<1 mm) when detecting two electrons. A true zero dead-time detection is also demonstrated.

Communication: time- and space-sliced velocity map electron imaging.

We develop a new method to achieve slice electron imaging using a conventional velocity map imaging apparatus with two additional components: a fast frame complementary metal-oxide semiconductor camera and a high-speed digitizer. The setup was previously shown to be capable of 3D detection and coincidence measurements of ions. Here, we show that when this method is applied to electron imaging, a time slice of 32 ps and a spatial slice of less than 1 mm thick can be achieved. Each slice directly extracts 3D velocity distributions of electrons and provides electron velocity distributions that are impossible or difficult to obtain with a standard 2D imaging electron detector.

Coincidence ion imaging with a fast frame camera.

A new time- and position-sensitive particle detection system based on a fast frame CMOS (complementary metal-oxide semiconductors) camera is developed for coincidence ion imaging. The system is composed of four major components: a conventional microchannel plate/phosphor screen ion imager, a fast frame CMOS camera, a single anode photomultiplier tube (PMT), and a high-speed digitizer. The system collects the positional information of ions from a fast frame camera through real-time centroiding while the arrival times are obtained from the timing signal of a PMT processed by a high-speed digitizer. Multi-hit capability is achieved by correlating the intensity of ion spots on each camera frame with the peak heights on the corresponding time-of-flight spectrum of a PMT. Efficient computer algorithms are developed to process camera frames and digitizer traces in real-time at 1 kHz laser repetition rate. We demonstrate the capability of this system by detecting a momentum-matched co-fragments pair (methyl and iodine cations) produced from strong field dissociative double ionization of methyl iodide.

Sympathetic cooling of molecular ions in selected rotational and vibrational states produced by threshold photoionization.

We present a new method for the generation of rotationally and vibrationally state-selected, translationally cold molecular ions in ion traps. Our technique is based on the state-selective threshold photoionization of neutral molecules followed by sympathetic cooling of the resulting ions with laser-cooled calcium ions. Using N2(+) ions as a test system, we achieve >90% selectivity in the preparation of the ground rovibrational level and state lifetimes on the order of 15 minutes limited by collisions with background-gas molecules. The technique can be employed to produce a wide range of apolar and polar molecular ions in the ground and excited rovibrational states. Our approach opens up new perspectives for cold quantum-controlled ion-molecule-collision studies, frequency-metrology experiments with state-selected molecular ions and molecular-ion qubits.

Photodissociation of heptane isomers and relative ionization efficiencies of butyl and propyl radicals at 157 nm.

We report an ion imaging and time-of-flight mass spectroscopy study of the photodissociation of a variety of heptane isomers using 157 nm dissociation and ionization. Time-of-flight mass spectra show that C(3)H(7) + C(4)H(9) is the dominant detected product channel following one-color 157 nm dissociation/ionization of heptanes. The results further allow determination of the relative ionization efficiencies of 1- and 2-butyl and propyl radicals at 157 nm. Momentum matching for the two radical products indicates that, for the C3-C4 products, neutral dissociation followed by ionization is the main source of the detected signals. The images show isotropic angular distributions and the translational energy distributions peak at very low energy, with only approximately 0.3 eV or 8% of the available energy appearing in translation. This is consistent with dissociation from the ground state or low-lying triplet states following non-radiative electronic relaxation.