2021 Vol. 34, No. 1

2021, 34(1): i-ii.
2021, 34(1): iii-iv.
In this review, we present a brief overview on the recent advances in Ångström-resolved tip-enhanced Raman spectromicroscopy. We first introduce the theoretical understanding of the confinement of light at the atomistic scale, and explain how the Raman scattering from a single molecule happens under the "illumination" of such an atomically confined light. Then we describe the latest developments on Ångström-resolved tip-enhanced Raman spectromicroscopy, particularly on a new methodology called "scanning Raman picoscopy" for visually constructing the chemical structure of a single molecule in real space. Finally, we give a perspective of this technique in various applications where identifying the chemical structures of materials at the chemical bond level is required.
Recent progress in ultrafast lasers, ultrafast X-rays and ultrafast electron beams has made it possible to watch the motion of atoms in real time through pump-probe technique. In this review, we focus on how the molecular dynamics can be studied with ultrafast electron diffraction where the dynamics is initiated by a pumping laser and then probed by pulsed electron beams. This technique allows one to track the molecular dynamics with femtosecond time resolution and Ångström spatial resolution. We present the basic physics and latest development of this technique. Representative applications of ultrafast electron diffraction in studies of laser-induced molecular dynamics are also discussed. This table-top technique is complementary to X-ray free-electron laser and we expect it to have a strong impact in studies of chemical dynamics.
Two-dimensional electronic spectroscopy (2DES) is a powerful method to probe the coherent electron dynamics in complicated systems. Stabilizing the phase difference of the incident ultrashort pulses is the most challenging part for experimental demonstration of 2DES. Here, we present a tutorial review on the 2DES protocols based on active phase managements which are originally developed for quantum optics experiments. We introduce the 2DES techniques in box and pump-probe geometries with phase stabilization realized by interferometry, and outline the fully collinear 2DES approach with the frequency tagging by acoustic optical modulators and frequency combs. The combination of active phase managements, ultrashort pulses and other spectroscopic methods may open new opportunities to tackle essential challenges related to excited states.
Many physical, chemical, and biological processes happen in liquid-vapor interface and are profoundly influenced with the local microstructures. In contrast to the liquid bulk, molecular orientation is the remarkable one of asymmetric structural features of the interface. Here we report an experimental method, namely, electron-impact time-delayed mass spectrometry and give a brief review about our recent progresses. This brand-new method not only enables us to have more insights into the interfacial structures, as done with small-angle X-ray and neutron scatterings and vibrational sum frequency generation spectroscopy, but also provides opportunity to explore the electron-driven chemical reactions therein.
Spectroscopic characterization of clusters is crucial to understanding the structures and reaction mechanisms at the microscopic level, but it has been proven to be a grand challenge for neutral clusters because the absence of a charge makes it difficult for the size selection and detection. Infrared (IR) spectroscopy based on threshold photoionization using a tunable vacuum ultraviolet free electron laser (VUV-FEL) has recently been developed in the lab. The IR-VUV depletion and IR+VUV enhancement spectroscopic techniques open new avenues for size-selected IR spectroscopies of a large variety of neutral clusters without confinement (i.e., an ultraviolet chromophore, a messenger tag, or a host matrix). The spectroscopic principles have been demonstrated by investigations of some neutral water clusters and some metal carbonyls. Here, the spectroscopic principles and their applications for neutral clusters are reviewed.
One of the themes of modern molecular reaction dynamics is to characterize elementary chemical reactions from "quantum state to quantum state", and the study of molecular reaction dynamics in excited states can help test the validity of modern chemical theories and provide methods to control chemical reactions. The subject of this review is to describe the recent experimental techniques used to study the reaction dynamics of metal atoms in the gas phase. Through these techniques, information such as the internal energy distribution and angular distribution of the nascent products or the three-dimensional stereodynamic reactivity can be obtained. In addition, by preparing metal atoms with specific excited electronic states or orbital arrangements, information about the reactivity of the electronic states enriches the relevant understanding of the electron transfer mechanism in metal reaction dynamics.
In this study, we report the design and simulation of an electrostatic ion lens system consisting of 22 round metal plates. The opening of the extractor plate is covered with metal mesh, which is for shielding the interaction region of the lens system from the high DC voltages applied to all other plates than the repeller and extractor plates. The Simion simulation shows that both velocity-mapping and time focusing can be achieved simultaneously when appropriate voltages are applied to each of the plates. This makes the ion lens system be able to focus large ionic volumes in all three dimensions, which is an essential requirement for crossed ion-molecule scattering studies. A three-dimensional ion velocity measurement system with multi-hit and potential multi-mass capability is built, which consists of a microchannel plate (MCP), a P47 phosphor screen, a CMOS camera, a fast photomultiplier tube (PMT), and a high-speed digitizer. The two velocity components perpendicular to the flight axis are measured by the CMOS camera, and the time-of-flight, from which the velocity component along the flight axis can be deduced, is measured by the PMT. A Labview program is written to combine the two measurements for building the full three-dimensional ion velocity in real time on a frame-by-frame basis. The multi-hit capability comes from the fact that multiple ions from the camera and PMT in the same frame can be correlated with each other based on their various intensities. We demonstrate this by using the photodissociation of CH3I at 304 nm.
The recently constructed cryogenic cylindrical ion trap velocity map imaging spectrometer (CIT-VMI) has been upgraded for coincidence imaging of both ionic and neutral photofragments from photodissociation of ionic species. The prepared ions are cooled down in a home-made cryogenic cylindrical ion trap and then extracted for photodissociation experiments. With the newly designed electric fields for extraction and acceleration, the ion beam can be accelerated to more than 4500 eV, which is necessary for velocity imaging of the neutral photofragments by using the position-sensitive imaging detector. The setup has been tested by the 355 nm photodissociation dynamics of the argon dimer cation (Ar$_2$$^+$). From the recorded experimental images of both neutral Ar and ionic Ar$^+$ fragments, we interpret velocity resolutions of $\Delta v/v$$\approx$4.6% for neutral fragments, and $\Delta v/v$$\approx$1.5% for ionic fragments, respectively.
By using scanning tunneling microscope induced luminescence (STML) technique, we investigate systematically the bias-polarity dependent electroluminescence behavior of a single platinum phthalocyanine (PtPc) molecule and the electron excitation mechanisms behind. The molecule is found to emit light at both bias polarities but with different emission energies. At negative excitation bias, only the fluorescence at 637 nm is observed, which originates from the LUMO→HOMO transition of the neutral PtPc molecule and exhibits stepwise-like increase in emission intensities over three different excitation-voltage regions. Strong fluorescence in region (Ⅰ) is excited by the carrier injection mechanism with holes injected into the HOMO state first; moderate fluorescence in region (Ⅱ) is excited by the inelastic electron scattering mechanism; and weak fluorescence in region (Ⅲ) is associated with an up-conversion process and excited by a combined carrier injection and inelastic electron scattering mechanism involving a spin-triplet relay state. At positive excitation bias, more-than-one emission peaks are observed and the excitation and emission mechanisms become complicated. The sharp molecule-specific emission peak at ~911 nm is attributed to the anionic emission of PtPc$^-$ originated from the LUMO+1→LUMO transition, whose excitation is dominated by a carrier injection mechanism with electrons first injected into the LUMO+1 or higher-lying empty orbitals.
We study the photodissociation dynamics of CS$_2$ in the ultraviolet region using the time-sliced velocity map ion imaging technique. The S($^3$P$_J$)+CS($X^1\Sigma^+$) product channels were observed and identified at four wavelengths of 201.36, 203.10, 204.85 and 206.61 nm. In the measured images of S($^3$P$_{J=2, 1, 0}$), the vibrational states of the CS($X^1\Sigma^+$) co-products were partially resolved and the vibrational state distributions were determined. Moreover, the product total kinetic energy releases and the anisotropic parameters were derived. The relatively small anisotropic parameter values indicate that the S($^3$P$_{J=2, 1, 0}$)+CS($X^1\Sigma^+$) channels are very likely formed via the indirect predissociation process of CS$_2$. The study of the S($^3$P$_{J=2, 1, 0}$)+CS($X^1\Sigma^+$) channels, which come from the spin-orbit coupling dissociation process of CS$_2$, shows that nonadiabatic process plays a role in the ultraviolet photodissociation of CS$_2$.
To obtain insight into the catalytic reaction mechanism of biodiesels over ZSM-5 zeolites, the pyrolysis and catalytic pyrolysis of methyl butanoate, a biodiesel surrogate, with H-type ZSM-5 (HZSM-5) were performed in a flow reactor under atmospheric pressure. The pyrolysis products were identified and quantified using gas chromatography-mass spectrometry (GC-MS). Kinetic modelling and experimental results revealed that H-atom abstraction in the gas phase was the primary pathway for methyl butanoate decomposition during pyrolysis, but dissociating to ketene and methanol over HZSM-5 was the primary pathway for methyl butanoate consumption during catalytic pyrolysis. The initial decomposition temperature of methyl butanoate was reduced by approximately 300 K over HZSM-5 compared to that for the uncatalyzed reaction. In addition, the apparent activation energies of methyl butanoate under catalytic pyrolysis and homogeneous pyrolysis conditions were obtained using the Arrhenius equation. The significantly reduced apparent activation energy confirmed the catalytic performance of HZSM-5 for methyl butanoate pyrolysis. The activation temperature may also affect some catalytic properties of HZSM-5. Overall, this study can be used to guide subsequent catalytic combustion for practical biodiesel fuels.
The interaction energy of two molecules system plays a critical role in analyzing the interacting effect in molecular dynamic simulation. Since the limitation of quantum mechanics calculating resources, the interaction energy based on quantum mechanics can not be merged into molecular dynamic simulation for a long time scale. A deep learning framework, deep tensor neural network, is applied to predict the interaction energy of three organic related systems within the quantum mechanics level of accuracy. The geometric structure and atomic types of molecular conformation, as the data descriptors, are applied as the network inputs to predict the interaction energy in the system. The neural network is trained with the hierarchically generated conformations data set. The complex tensor hidden layers are simplified and trained in the optimization process. The predicted results of different molecular systems indicate that deep tensor neural network is capable to predict the interaction energy with 1 kcal/mol of the mean absolute error in a relatively short time. The prediction highly improves the efficiency of interaction energy calculation. The whole proposed framework provides new insights to introducing deep learning technology into the interaction energy calculation.