Ultrafast Dynamics Through Conical Intersections in 2,6-dimethylpyridine Studied with Time-resolved Photoelectron Imaging

The ultrafast dynamics through conical intersections in 2,6-dimethylpyridine has been studied by femtosecond time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. Upon absorption of 266 nm pump laser, 2,6-dimethylpyridine is excited to the S2 state with a ππ character from So state. The time evolution of the parent ion signals consists of two exponential decays. One is a fast component on a timescale of 635 fs and the other is a slow component with a timescale of 4.37 ps. Time-dependent photo- electron angular distributions and energy-resolved photoelectron spectroscopy are extracted from time-resolved photoelectron imaging and provide the evolutive information of S2 state. In brief, the ultrafast component is a population transfer from S2 to S1 through the S2/S1 conical intersections, the slow component is attributed to simultaneous IC from the S2 state and the higher vibrational levels of S1 state to So state, which involves the coupling of S2/S0 and S1/So conical intersections. Additionally, the observed ultrafast S2--＋S1 transition occurs only with an 18% branching ratio.

Photoelectron Imaging of AgOCH3- and Ag-（CH3OH）x （x=l, 2）

The AgOCH3- and Ag-（CH3OH）x（x=l, 2） anions are studied by photoelectron imaging as well as ab initio calculations. The adiabatic and vertical detachment energies （ADE and VDE） of AgOCH3- are determined as 1.29（2） and 1.34（2） eV, respectively, from the vibrational resolved photoelectron spectrum. The Ag-（CH3OH）l,2 anionic complexes are characterized as metal atomic anion solvated by the CH3OH molecules with the electron mainly localized on the metal. The photoelectron spectra of Ag-（CH3OH）x （x=O, 1, 2） show a gradual increase in VDE with increasing x, due to the solvent stabilization. Evidence for the methanol-methanol hydrogen bonding interactions appears when the Ag- is solvated by two methanol molecules.

Femtosecond Two-Photon Detachment of Cu^- Studied By Photoelectron Imaging

The wavelength dependence of photoelectron angular distributions （PADs） of two-photon detachment of Cu^- has been directly studied by using the photoelectron map imaging. Results show that for the laser field intensity of 6.0×10^10W/cm^2, PADs exhibit dramatic change with the external field wavelength. Comparison between the experimental observation and the lowest-order perturbation theory prediction indicates that the pattern of PADs can be explained by the interference of the s and d partial waves in the final state. Relative contri- butions of s and d partial waves in the two-photon detachment at different laser wavelengths are obtained.

Unraveling Vibrational Wavepacket Dynamics using Femtosecond Ion Yield Spectroscopy and Photoelectron Imaging

Time-resolved photoionization is a powerful experimental approach to unravel the excited state dynamics in isolated polyatomic molecules. Depending on species of the collected signals, different methods can be performed: time-resolved ion yield spectroscopy (TR-IYS) and time-resolved photoelectron imaging (TR-PEI). In this review, the essential concepts linking photoionization measurement with electronic structure are presented, together with several important breakthroughs in experimentally distinguishing the oscillating wavepacket motion between different geometries. We illustrate how femtosecond TR-IYS and TR-PEI are employed to visualize the evolution of a coherent vibrational wavepacket on the excited state surface.

Design Near-threshold Photoelectron Imaging Spectrometer Based on UV Laser Induced Photoelectron Emission Anion Source

We have developed a compact photoelectron imaging facility, including an anion source with dissociative photoelectron attachment to molecules, a linear time-of-flight mass spec-trometry （TOFMS）, and an orthogonal high-resolution threshold photoelectron velocity map imaging spectrometer （VMI）. Intense and cold cluster anions were prepared in photoelectron- attachment processes upon pulsed UV laser ablation of metal target. Combining this anion source with TOFMS-VMI, the achieved mass resolution is about 200, and the electron ki- netic energy resolution is better than 3%, i.e., 30 meV for 1 eV electrons. More importantly, low-energy photoelectron imaging spectra for CH3S- and S2- at 611.46 nm are obtained. In both cases, the refined electron affinities are determined to be 1.86264-0.0020 eV for CH3S and 1.67444-0.0035 eV for S2, respectively. Preliminary results suggest that the apparatus is a powerful tool for estimating precise electron affinities values from threshold photoelectron imaging spectroscopy.

Photoelectron imaging on vibrational excitation and Rydberg intermediate states in multi-photon ionization process of NH3 molecule

The ionization processes of NH3 molecule are studied by photoelectron velocity map imaging technique in a linearly polarized 400-nm femtosecond laser field. The two-dimensional photoelectron images from ammonia molecules under different laser intensities are obtained. In the slow electron region, the values of kinetic energy of photoelectrons corresponding to peaks 1, 2, 3, and 4 are 0.27, 0.86, 1.16, and 1.6 eV, respectively. With both the kinetic energy and angular distribution of photoelectrons from NH3 molecules, we can confirm that the two-photon excited intermediate Rydberg state is A^1 A2" (v2'=3) state for photoelectron peaks 2, 3, 4, and the three peaks are marked as 1223 (2 + 2), 1123 (2 + 2), and 1023 (2 + 2) multi-photon processes, respectively. Then, peak 1 is found by adding a hexapole between the source chamber and the detection chamber to realize the rotational state selection and beam focusing. Peak 1 is labeled as the 1323 (3 + 1) multi-photon process through the intermediate Rydberg state E^1A1'. The phenomena of channel switching are found in the slow electron kinetic energy distributions. Our calculations and experimental results indicate that the stretching vibrational mode of ammonia molecules varies with channels, while the umbrella vibration does not. In addition, we consider and discuss the ac-Stark effect in a strong laser field. Peaks 5 and 6 are marked as (2 + 2 + 1) and (2 + 2 + 2) above threshold ionization processes in the fast electron region.

Photoelectron imaging spectroscopic signatures of CO activation by the heterotrinuclear titanium-nickel clusters

A series of heterotrinuclear Ti_(2)Ni(CO)_(n)^(-)(n=6-9)carbonyls have been generated via a laser vaporization supersonic cluster source and characterized by mass-selected photoelectron velocity-map imaging spectroscopy.Quantum chemical calculations have been carried out to identify the structures and understand the experimental spectral features.The results indicate that a building block of Ti-Ti-Ni-C four-membered ring with the C atom bonded to Ti,Ti,and Ni is dominated in the n=6-8 complexes,whereas a structural motif of Ti-Ti-Ni triangle core is preferred in n=9.These complexes are found to be capable of simultaneously accommodating all the main modes of metal-CO coordination(i.e.,terminal,bridging,and side-on modes),where the corresponding mode points to the weak,moderate,high C-O bond activation,respectively.The number of CO ligands for a specific bonding mode varies with the cluster size.These findings have important implications for molecular-level understanding of the interaction of CO with alloy surfaces/interfaces and tuning the appropriate CO activation via the selection of different metals.

Cryogenic Photodetachment Spectroscopy and High-Resolution Resonant Photoelectron Imaging of Cold para-Ethylphenolate Anions

Valence-bound molecular anions with polar neutral cores(μ>2.5 D)can support highly diffuse dipole-bound states(DBSs)as electronically excited states just below the detachment threshold.Such weakly bound nonvalence excited states have little influence on the structure of the neutral core,and they usually have the same vibrational frequencies.DBSs can be systematically searched using photodetachment spectroscopy(PDS),which can yield the binding energies of the DBSs,the electron detachment threshold of the anion,and above-threshold vibrational levels of the DBSs(Feshbach resonances).We have shown that the combination of PDS and resonant photoelectron spectroscopy(rPES)at the Feshbach resonances is a powerful approach to obtain rich vibrational information for complex molecular radicals.A prerequisite for this technique is to produce vibrationally cold anions,made possible by a cryogenically controlled Paul trap.In this article,we report a PDS and rPES study of cold para-ethylphenolate anions(p-EP^(-)).The electron affinity of the p-EP radical is measured to be 17425±3 cm^(-1)(2.1604±0.0004 eV),and a DBS is found at 145 cm^(-1) below the detachment threshold of p-EP^(-).Thirty-four vibrational levels are observed for the DBS,including two bound levels and 32 Feshbach resonances.Frequencies for 17 vibrational modes of the p-EP radical are measured from the combination of PDS and rPES,including six symmetry-forbidden modes with A″symmetry.The current study confirms again the power of combining cryogenic ion cooling with PDS and highresolution rPES to obtain spectroscopic information on complex molecular radicals.

CO activation by the heterobinuclear transition metal-iron clusters: A photoelectron spectroscopic and theoretical study

Spectroscopic characterization of CO activation on multiple metal-containing catalysts remains an important and challenging goal for identifying the structure and nature of active site in many industrial processes such as Fischer-Tropsch chemistry and alcohol synthesis.Here,we use mass-selected photoelectron velocity-map imaging spectroscopy and quantum chemical calculations to study the reactions of CO molecules with several heterobinuclear transition metal-iron clusters M-Fe(M=Ti,V,Cr).The mass spectra reveal the favorable formation of MFe(CO)_(4)^(-)with relatively high thermodynamic stability.The MFe(CO)_(4)^(-)(M=Ti,V,Cr) complexes are established to have a metal-Fe bonded M-Fe(CO)_(4) structure with C_(3 v) geometry.While the positive charge and unpaired electrons are mainly located on the M atom,the natural charge of Fe(CO)_(4) is about-2 e.The MFe(CO)_(4)^(-)(M=Ti,V,Cr) can be seen as being formed via the interactions between the M^(+)fragment and the [Fe(CO)_(4)]^(2-)core,which satisfies the 18-electron rule.The CO molecules are remarkably activated in these MFe(CO)_(4)^(-).These results shed insight into the structure-reactivity relationship of heterobinuclear transition metal carbonyls and would have important implications for understanding of CO activation on alloy surfaces.

Spectroscopic characterization of chain-to-ring structural evolution in platinum carbide clusters

Metal carbides play an important role in catalysis and functional materials.However,the structural characterization of metal carbide clusters has been proven to be a challenging experimental target due to the difficulty in size selection.Here we use the size-specific photoelectron velocity-map imaging spectroscopy to study the structures and properties of platinum carbide clusters.Quantum chemical calculations are carried out to identify the structures and to assign the experimental spectra.The results indicate that the cluster size of the chain-to-ring structural evolution for the PtC_(n)^(-)anions occurs at n=14,whereas that for the PtC_(n) neutrals at n=10,revealing a significant effect of charge on the structures of metal carbides.The greatest importance of these building blocks is the strong preference of the Pt atom to expose in the outer side of the chain or ring,exhibiting the active sites for catalyzing potential reactions.These findings provide unique spectroscopic snapshots for the formation and growth of platinum carbide clusters and have important implications in the development of related single-atom catalysts with isolated metal atoms dispersed on supports.

Electron Affinities of the Early Lanthanide Monoxide Molecules

The photoelectron imagings of LaO-, CeO-, PRO-, and NdO- at 1064 nm are reported. The well resolved photoelectron spectra allow the electron affinities to be determined as 0.99（1） eV for LaO, 1.00（1） eV for CeO, 1.00（1） eV for PrO, and 1.01（1） eV for NdO, respectively. Density functional calculations and natural atomic orbital analyses show that the 4f electrons tend to be localized and suffer little from the charge states of the molecules. The photodetached electron mainly originates from the 6s orbital of the metals. The ligand field theory with the δ=2 assumption is still an effective method to analyze the ground states of the neutral and anionic lanthanide monoxides.

CO oxidation on the heterodinuclear tantalum-nickel monoxide carbonyl complex anions

The series of heterodinuclear metal oxide carbonyls in the form of TaNiO(CO)_(n)(n=5-8) are generated in the pulsed-laser vaporization source and characterized by mass-selected photoelectron velocity-map spectroscopy.During the consecutive CO adsorption,the μ^(2)-O-bent structure initially is the most favorable for TaNiO(CO)_(5),and subsequently both μ^(2)-O-bent and μ^(2)-O-linear structures are degenerate for TaNiO(CO)_(6),then the μ^(2)-O-linear structure is most preferential for TaNiO(CO)_(7),and finally the η^(2)-CO_(2)-tagged structure is the most ene rgetically competitive one for TaNiO(CO)_(8),i.e., the CO oxidation occurs at n=8.ln contrast to the literature reported CO oxidation on heteronuclear metal oxide complexes generally proceeding via Langmuir-Hinshelwood-like mechanism,complementary theo retical calculations suggest that both Langmuir-Hinshelwood-like and Eley-Rideal-like mechanisms prevail for the CO oxidation reaction on TaNiO(CO)_(8) complex.Our findings provide new insight into the composition-selective mechanism of CO oxidation on heteronuclear metal complexes,of which the composition be tailored to fulfill the desired chemical behaviors.