Suk Kyoung Lee

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.

A Reaction Accelerator: Mid-infrared Strong Field Dissociation Yields Mode-Selective Chemistry

Mode-selective chemistry has been a dream of chemists since the advent of the laser in the 1970s. Despite intense effort, this goal has remained elusive due to efficient energy randomization in polyatomic molecules. Using ab initio molecular dynamics calculations, we show that the interaction of molecules with intense, ultrashort mid-infrared laser pulses can accelerate and promote reactions that are energetically and entropically disfavored, owing to efficient kinetic energy pumping into the corresponding vibrational mode(s) by the laser field. In a test case of formyl chloride ion photodissociation, the reactions are ultimately complete under field-free conditions within 500 fs after the laser pulse, which effectively overcomes competition from intramolecular vibrational energy redistribution (IVR). The approach is quite general and experimentally accessible using currently available technology.

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.

Bond-selective dissociation of polyatomic cations in mid-infrared strong fields.

Strong field-induced dissociation by intense mid-infrared pulses was investigated in bromofluoroform monocation (CF3Br(+)) and iodobenzene dication (C6H5I(2+)) using ab initio molecular dynamics calculations. In both systems, bond -selective dissociation was achieved using appropriate laser polarizations and wavelengths. For CF3Br(+), energetically disfavored fluorine elimination was strongly enhanced at wavelengths of 7 to 8 μm with polarization along a C-F bond. This is the result of two effects: the deposition of high enough kinetic energy into the molecule by the laser field and the near-resonant excitation of the C-F stretching mode. At shorter and off-resonant wavelengths, bromine elimination becomes significant due to rapid intramolecular vibrational energy redistribution (IVR). For C6H5I(2+), the branching ratios for the dissociation of the ortho-, meta-, and para-hydrogens can be controlled simply by changing the laser polarization. These results show the general applicability of bond selective dissociation of cations by intense mid-infrared laser fields.

Novel molecular elimination mechanism in formaldehyde photodissociation: the roaming H atom pathway

We present a state-correlated experimental investigation of formaldehyde (H2CO) dissociation to H2 and CO following excitation to a series of vibrational bands in the first electronically excited state, S1. The CO was detected by resonance-enhanced multiphoton ionization at various rotational states of CO (J = 5–45) and the CO velocity distributions were measured using state-resolved DC Slice Imaging. These high-resolution measurements reveal the internal state distribution of the correlated H2 cofragments. The results show that the rotationally hot CO (JCO = 40) is produced in conjunction with vibrationally cold H2 fragments (ν = 0–3), consistent with dissociation through the celebrated skewed transition state. After excitation of formaldehyde at energies near and above the threshold for dissociation to radical products (H2CO → H+HCO), a second molecular elimination channel appears which is characterized by rotationally cold CO (J 5–15) correlated with highly vibrationally excited H2 (ν = 5–7). These products are formed through a novel roaming H-atom mechanism that involves intramolecular H abstraction and avoids the region of the transition state to molecular elimination entirely. The current measurements give insight into the energy dependence of the branching of these different reaction mechanisms.

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.

Orbital alignment in photodissociation probed using strong field ionization.

The photodissociation of molecules often produces atomic fragments with polarized electronic angular momentum, and the atomic alignment, for example, can provide valuable information on the dynamical pathways of chemical reactions unavailable by other means. In this work, we demonstrate for the first time that orbital polarization in chemical reactions can be measured with great sensitivity using strong field ionization by exploiting its extreme nonlinearity.

Laser-induced low energy electron diffraction in aligned molecules

We measured the photoelectron spectra and angular distributions of partially aligned N(2), O(2), and CO(2) in the rescattering plateau of above threshold ionization (ATI). The measured ATI electrons have relatively low collision energies (<15 eV). The photoelectron angular distributions (PAD) show clearly species and energy dependence. A simple two-center interference model was not able to consistently retrieve structural properties. We conclude that due to the interplay between the electrons and rescattering potential, the molecular structural information is obscured and cannot be extracted conveniently. However, the sensitivity of the PAD to the scattering potential in laser-induced electron diffraction promises a practical tool for studying electron-ion scattering dynamics.

A new electron-ion coincidence 3D momentum-imaging method and its application in probing strong field dynamics of 2-phenylethyl-N, N-dimethylamine

We report the development of a new three-dimensional (3D) momentum-imaging setup based on conventional velocity map imaging to achieve the coincidence measurement of photoelectrons and photo-ions. This setup uses only one imaging detector (microchannel plates (MCP)/phosphor screen) but the voltages on electrodes are pulsed to push both electrons and ions toward the same detector. The ion-electron coincidence is achieved using two cameras to capture images of ions and electrons separately. The time-of-flight of ions and electrons are read out from MCP using a digitizer. We demonstrate this new system by studying the dissociative single and double ionization of PENNA (2-phenylethyl-N,N-dimethylamine). We further show that the camera-based 3D imaging system can operate at 10 kHz repetition rate.

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.