Spectroscopy and molecule opacity investigation on excited states of SiS

The SiS molecule,which plays a significant role in space,has attracted a great deal of attention for many years.Due to complex interactions among its low-lying electronic states,precise information regarding the molecular structure of SiS is limited.To obtain accurate information about the structure of its excited states,the high-precision multireference configuration interaction(MRCI)method has been utilized.This method is used to calculate the potential energy curves(PECs)of the 18Λ–S states corresponding to the lowest dissociation limit of SiS.The core–valence correlation effect,Davidson’s correction and the scalar relativistic effect are also included to guarantee the precision of the MRCI calculation.Based on the calculated PECs,the spectroscopic constants of quasi-bound and bound electronic states are calculated and they are in accordance with previous experimental results.The transition dipole moments(TDMs)and dipole moments(DMs)are determined by the MRCI method.In addition,the abrupt variations of the DMs for the 1^(5)Σ^(+)and 2^(5)Σ^(+)states at the avoided crossing point are attributed to the variation of the electronic configuration.The opacity of SiS at a pressure of 100 atms is presented across a series of temperatures.With increasing temperature,the expanding population of excited states blurs the band boundaries.

Two-proton radioactivity from excited states of proton-rich nuclei within Coulomb and Proximity Potential Model

In the present work,we extend the Coulomb and Proximity Potential Model(CPPM)to study two-proton(2p)radioactivity from excited states while the proximity potential is chosen as AW95 proposed by Aage Withner in 1995.Demonstration reveals that the theoretical results acquired by CPPM exhibit a high level of consistency with prior theoretical models such as the unified fission model(UFM),generalized liquid-drop model(GLDM)and effective liquid-drop model(ELDM).Furthermore,within the CPPM,we predicted the half-lives of potential 2p radioactive nuclei for which experimental data are currently unavailable.The predicted results were then assessed,compared with UFM,ELDM and GLDM models,and examined in detail.

Role of excited states in helium-like ions on high-order harmonic generation

We theoretically investigate high-order harmonic generation(HHG) of helium(He), lithium cation(Li+), and beryllium dication(Be2+) using the time-dependent Hartree–Fock method to solve the three-dimensional time-dependent Schr ¨odinger equation. It is found that the intensity of the HHG increases significantly from a certain harmonic order below the ionization threshold, and the initial position of the enhancement does not depend on the intensity or the wavelength of the driving laser field. Further analysis shows that excited states play an important role on this enhancement,consistent with the excited-state tunneling mechanism [Phys. Rev. Lett. 116 123901(2016)]. Our results unambiguously show that excited-state tunneling is essential for understanding the enhancement of HHG. Accordingly, a four-step model is herein proposed to illustrate the multiphoton excitation effect in helium-like ions, which enriches the physics of HHG enhancement.

A TD-DFT Study for the Excited State Calculations of Microhydration of N-Acetyl-Phenylalaninylamide (NAPA)

Investigating the impact of microhydration on the excited-states and electronic excitation properties of biomolecules has remained one of the important yet challenging aspects of science because of the complexity of developing models. However, with the advent of computational chemistry methods such as TD-DFT, many useful insights about the electronic excitation energy and excited-state nature of biomolecules can be explored. Accordingly, in our study, we have incorporated the TD-DFT/wB97XD/cc-pVTZ method to study the excited state properties of N-acetyl phenylalanine amide (NAPA-A(H2O) n) (n = 1 to 4) clusters from ground to the tenth lowest gaseous singlet excited state. We found that the C=O bond length gradually increases both in N-terminal amide and C-terminal amide after the sequential addition of water molecules because of intermolecular H-bonding and this intermolecular H-bonding becomes weaker after the sequential addition of H2O molecules. The UV absorption maxima of NAPA-A (H2O)n (n = 1 - 4) clusters consisted of two peaks that are S5←S0 (1st absorption) and S6←S0 (2nd absorption) excitations. The first absorption maxima were blue-shifted with the increase in oscillator strength. This means that strong H-bonds reduce the charge transfer and make clusters more rigid. On the other hand, the second absorption maxima were red-shifted with the decrease in oscillator strength. In the ECD spectra, the negative bands indicate the presence of an amide bond and L-configuration of micro hydrated NAPA-A clusters. Finally, our calculated absorption and fluorescence energy confirm that all the NAPA-A (H2O) n (n = 0 - 4) clusters revert to the ground state from the fluorescent state by emitting around 5.490 eV of light.

Broadband Visible Light Harvesting BODIPY-Perylene Dyad and Triad:Synthesis,Photophysical Properties,and Photooxidation Applications

In this study,diodo boron dipyrromethene(BODIPY)is employed a8 the energy donor and 3,4,9,10-perylene tetracarboxylic dianhydride(PDA)as the energy acceptor,enabling the synthesis of two new compounds:a BODIPY-perylene dyad named P1,and a triad named P2.To investigate the impact of the energy donor on the photophysical processes of the system,P1 comprises one diodo-BODIPY unit and one PDA unit,whereas P2 contains two diodo-BODIPY moieties and one PDA unit.Due to the good spectral complementarity between diiodo-BODIPY and PDA,these two compounds exhibit excellent light-harvesting capabilities in the 400-620 nm range.Steady-state fluorescence spectra demonstrate that when preferentially exciting the diodo-BODIPY moiety,it can effectively transfer energy to PDA;when selectively exciting the PDA moiety,quenching of PDA fluorescence is observed in both P1 and P2.Nanosecond transient absorption results show that both compounds can efficiently generate triplet excited states,which are located on the PDA part.The lifetimes of the triplet states for these two compounds are 103 and 89μs,respectively,significantly longer than that of diiodo-BODIPY.The results from the photooxidation experiments reveal that both P1 and P2 demonstrate good photostability and photooxidation capabilities,with P2 showing superior photooxidative efficiency.The photooxidation rate constant for P2 is 1.3 times that of P1,and its singlet oxygen quantum yield is 1.6 times that of P1.The results obtained here offer valuable insights for designing new photosensitizers.

Development and Application of Ultrafast Circular Dichroism Spectroscopy Techniques

Chirality hold broad applications in life sciences,quantum devices,and various other areas.Traditionally,molecular chirality can be characterized by using steady-state circular dichroism spectroscopy.However,the techniques that can characterize excited state chirality are progressively capturing the public interest as it can provide the dynamic information for chirality generation and transfer.In this review,we focus on the theoretical background and the developmental history of femtosecond time-resolved circular dichroism spectroscopy(TRCD)techniques around the world.Additionally,we provide examples to showcase the utility of these techniques in the analysis of the dynamical molecular chemical structures,the investigation of molecular chirality generation,and the detection of electron spin dynamics in semiconductor quantum dots.

Ultrafast photoemission electron microscopy:A multidimensional probe of nonequilibrium physics

Exploring the realms of physics that extend beyond thermal equilibrium has emerged as a crucial branch of condensed matter physics research.It aims to unravel the intricate processes involving the excitations,interactions,and annihilations of quasi-and many-body particles,and ultimately to achieve the manipulation and engineering of exotic non-equilibrium quantum phases on the ultrasmall and ultrafast spatiotemporal scales.Given the inherent complexities arising from many-body dynamics,it therefore seeks a technique that has efficient and diverse detection degrees of freedom to study the underlying physics.By combining high-power femtosecond lasers with real-or momentum-space photoemission electron microscopy(PEEM),imaging excited state phenomena from multiple perspectives,including time,real space,energy,momentum,and spin,can be conveniently achieved,making it a unique technique in studying physics out of equilibrium.In this context,we overview the working principle and technical advances of the PEEM apparatus and the related laser systems,and survey key excited-state phenomena probed through this surface-sensitive methodology,including the ultrafast dynamics of electrons,excitons,plasmons,spins,etc.,in materials ranging from bulk and nano-structured metals and semiconductors to low-dimensional quantum materials.Through this review,one can further envision that time-resolved PEEM will open new avenues for investigating a variety of classical and quantum phenomena in a multidimensional parameter space,offering unprecedented and comprehensive insights into important questions in the field of condensed matter physics.

Excitation and ionization of OCS molecules in strong UV and NIR laser fields

Rydberg state excitation(RSE) is a highly non-linear physical phenomenon that is induced by the ionization of atoms or molecules in strong femtosecond laser fields. Here we observe that both parent and fragments(S, C, OC) of the triatomic molecule carbonyl sulfide(OCS) can survive strong 800 nm or 400 nm laser fields in high Rydberg states. The dependence of parent and fragment RSE yields on laser intensity and ellipticity is investigated in both laser fields, and the results are compared with those for strong-field ionization. Distinctly different tendencies for laser intensity and ellipticity are observed for fragment RSE compared with the corresponding ions. The mechanisms of RSE and strong-field ionization of OCS molecules in different laser fields are discussed based on the experimental results. Our study sheds some light on the strong-field excitation and ionization of molecules irradiated by femtosecond NIR and UV laser fields.

Characterization of the Excited State on Methanol/Ti02（110） Interface

The electronic structure of methanol/TiO2（ll0） interface has been studied by photoemis- sion spectroscopy. The pronounced resonance which appears at 5.5 eV above the Fermi level in two-photon photoemission spectroscopy （2PPE） is associated with the photocatalyzed dissociation of methanol at fivefold coordinated Ti sites （Ti5c） on TiO2 （110） surface [Chem- ical Science 1, 575 （2010）]. To check whether this resonance signal arises from initial or intermediate states, photon energy dependent 2PPE and comparison between one-photon photoemission spectroscopy and 2PPE have been performed. Both results consistently sug- gest the resonance signal originates from the initially unoccupied intermediate states, i.e., excited states. Dispersion measurements suggest the excited state is localized. Time-resolved studies show the lifetime of the excited state is 24 fs. This work presents comprehensive char- acterization of the excited states on methanol/TiO2（110） interface, and provides elaborate experimental data for the development of theoretical methods in reproducing the excited states on TiO2 surfaces and interfaces.

Experimental and Theoretical Investigation on Excited State Intramolecular Proton Transfer Coupled Charge Transfer Reaction of Baicalein

The excited state intramolecular proton transfer （ESIPT） coupled charge transfer of baicalein has been investigated using steady-state spectroscopic experiment and quantum chemistry calculations. The absence of the absorption peak from S1 excited state both in the experi-mental and calculated absorption spectra indicates that S1 is a dark state. The dark excited state S1 results in the very weak fluorescence of solid baicalein in the experiment. The fron- tier molecular orbital and the charge difference densities of baicalein show clearly that the S1 state is a charge-transfer state whereas the S2 state is a locally excited state. The only one stationary point on the potential energy profile of excited state suggests that the ESIPT reaction of baicalein is a barrierless process.

Excited-State Proton Transfer and Decay in Hydrogen-Bonded Oxazole System： MS-CASPT2//CASSCF Study

Herein we have employed high-level multi-reference CASSCF and MS-CASPT2 electronic structure methods to systematically study the photochemical mechanism of intramolecularly hydrogen-bonded 2-（2＇-hydroxyphenyl）-4-methyloxazole. At the CASSCF level, we have optimized minima, conical intersections, minimum-energy reaction paths relevant to the excited-state intramolecular proton transfer （ESIPT）, rotation, photoisomerization, and the excited-state deactivation pathways. The energies of all structures and paths are refined by the MS-CASPT2 method. On the basis of the present results, we found that the ESIPT process in a conformer with the OH... N hydrogen bond is essentially barrierless process; whereas, the ESIPT process is inhibited in the other conformer with the OH... O hydrogen bond. The central single-bond rotation of the S1 enol species is energetically unfavorable due to a large barrier. In addition, the excited-state deactivation of the S1 keto species, as a result of the ultrafast ESIPT, is very efficient because of the existence of two easily-approached keto S1/S0 conical intersections. In stark contrast to the S1 keto species, the decay of the S1 enol species is almostly blocked. The present theoretical study contributes valuable knowledge to the understanding of photochemistry of similar intramolecularly hydrogen-bonded molecular and biological systems.

Structural Dynamics of 3-Dimethylamino-2-methyl-propenal in S2（ππ^＊） State

The photophysics of 3-dimethylamino-2-methyl-propenal （DMAMP） after excitation to the S2 （ππ^＊） electronic state was studied using the resonance Raman spectroscopy and complete active space self-consistent field method calculations. The transition barriers of the ground state tautomerization reactions between DMAMP and its three isomers were determined at B3LYP/6-311＋＋G（d,p） level of theory. The vibrational spectra were assigned. The A- band resonance Raman spectra were obtained in acetonitrile with excitation wavelengths in resonance with the first intense absorption band to probe the structural dynamics of DMAMP. The B3LYP-TD computation was carried out to determine the relative A-band resonance Raman intensities of the fundamental modes, and the result indicated that the vibronic-coupling existed in Franck-Condon region. Complete active space self-consistent field （CASSCF） calculations were carried out to determine the excitation energies of the lower-lying singlet and triplet excited states, the conical intersection points and the intersystem crossing points. The A-band short-time structural dynamics and the corresponding decay dynamics of DMAMP were obtained by analysis of the resonance Raman intensity pattern and CASSCF computations. It was found that a sudden de-conjugation between C1=O6 and C2=C3 occurred at the Franck-Condon region of the S2（ππ^＊） state, while the enhancement of the conjugation interaction between C3 and N（CH3）2, and between C1 and C2 evolutions shortly after the wavepacket leaves away the Pranck-Condon region via the excited state charge redistribution. The de-conjugation interaction between C1=O6 and C2=C3 made the rotation of C3=N（CH3）2 group around the C2-C3 bond much easier, while the enhanced conjugation between C1 and C2, and between C3 and N（CH3）2 made the rotation around the C1-C2 bond and C3-N5 more difficult. It was revealed that the initial structural dynamics of DMAMP was predominantly towards the CI-I（S2/S0） point, while the opportunities towards either CI-2（S2/S0） or CI-3（S2/S0） point were negligible. Two decay channels of DMAMP from S2,FC（ππ^＊） to So or Tl,min via various CIs and ISCs were proposed.

Resonance Raman Spectroscopic and Theoretical Study of Geometry Distortion of Thiourea in 2^1A State

The A-band resonance Raman spectra of thiourea were obtained in water and acetonitrile solution. B3LYP/6-311＋＋G（3df,3pd） and RCIS/6-311＋＋G（3df,3pd） calculations were done to elucidate the ultraviolet electronic transitions, the distorted geometry structure and the saddle point of thiourea in 21A excited state, respectively. The resonance Raman spectra were assigned. The absorption spectrum and resonance Raman intensities were modeled using Heller＇s time-dependent wavepacket approach to resonance Raman scattering. The results indicate that largest change in the displacement takes place with the C--S stretch mode u6 （｜△｜=0.95） and noticeable changes appear in the H5N3H6＋H8N4H7 wag v5 （｜△｜=0.19）, NCN symmetric stretch^-C--S stretch＋N3H6＋H8N4 wag v4 （｜△｜=0.18）, while the moderate intensities of 2-15 and 4-15 are mostly due to the large excited state frequency changes of v15, but not due to its significant change in the normal mode displacement. The mechanism of the appearance of even overtones of the S-CN2 out of plane deformation is explored. The results indicate that a Franck-Condon region saddle point is the driving force for the quadric phonon mechanism within the standard A-term of resonance Raman scattering, which leads to the pyramidalization of the carbon center and the geometry distortion of thiourea molecule in 21A excited state.

Behaviours of the excited states 1s^2 np along lithium isoelectronic sequence from Z=11 to 20

Based on the obtained energy values of 1s^2np （n≤ 9） states for lithium-like systems from Z=11 to 20 （by the authors of this paper： Hu M H and Wang Z W 2004 Chin. Phys. 13 662）, this paper determines the quantum defects, as slowly varying function of energy, of this Rydberg series. Using them as input, it can predict the energies of any highly excited states below the ionization threshold for this series a^cording to the quantum defect theory. The regularities of variation for quantum defects of the series along this isoelectronic sequence are physically analysed and discussed. The screening parameters, which are equal to the effective screening charge of the core-electrons, are also obtained.

Energy levels of 1s^2n d(n≤9)states for lithium-like ions

The full-core plus correlation method with multi-configuration interaction wave functions is extended to the calcu- lation of the non-relativistic energies of ls2nd （n ≤9） states for the lithium isoelectronic sequence from Z = 11 to 20. Relativistic and mass-polarization effects on the energy are calculated as the first-order perturbation correction. The quantum-electrodynamics correction is also included. The fine structure splittings are determined from the expectation values of spin-orbit and spin-other-orbit interaction operators in the Pauli-Breit approximation. Combining the term energies of lowly excited states obtained with the quantum defects calculated by the single channel quantum defect theory, each of which is a smooth function of energy and approximated by a weakly varying function of energy, the ion potentials of highly excited states （n ≥ 6） are obtained with the semi-empirical iteration method. The results are compared with experimental data in the literature and found to be closely consistent with the regularity.

Measurement of Photoionization Cross Sections of the Excited States of Titanium

Resonance-enhanced multiphoton ionization of the titanium atoms has been investigated in the 293 321 nm wavelength. We couple a laser-ablated metal target into a molecular beam to produce free atoms. Ions produced from photoionization of the neutral atoms are monitored by a home-built time-of-flight mass spectrometer. Photoionization cross sections of the excited states of Ti I were deduced from the dependence of the ion signal intensity on the laser intensity for photon energies close to the ionization threshold. The values obtained range from 0.2 Mb to 6.0 Mb. No significant isotope-dependence was found from measurements of the photoionization cross sections of ^46Ti, ^47Ti, and ^48Ti.

Two-proton radioactivity of the excited state within the Gamow-like and modified Gamow-like models

In this study,we systematically investigated the two-proton(2p)radioactivity half-lives from the excited state of nuclei near the proton drip line within the Gamowlike model(GLM)and modified Gamow-like model(MGLM).The calculated results were highly consistent with the theoretical values obtained using the unified fission model[Chin.Phys.C 45,124105(2021)],effective liquid drop model,and generalized liquid drop model[Acta Phys.Sin 71,062301(2022)].Furthermore,utilizing the GLM and MGLM,we predicted the 2p radioactivity halflives from the excited state for some nuclei that are not yet available experimentally.Simultaneously,by analyzing the calculated results from these theoretical models,it was found that the half-lives are strongly dependent on Qand l.

Ab initio calculation on accurate analytic potential energy functions and harmonic frequencies of c^3∑g^＋ and B^1-Пu states of dimer 7Li2

The comparison between single-point energy scanning （SPES） and geometry optimization （OPT） in determining the equilibrium geometries of c^3∑g^＋ and B^1-Пu states of dimer 7Li2 is made at numerous basis sets by using a symmetryadapted-cluster configuration-interaztion （SAC-CI） method in the Gaussian 03 program package. In this paper the difference of the equilibrium geometries obtained by SPES and by OPT is reported. The results obtained by SPES are found to be more reasonable than those obtained by OPT in full active space at the present SAC-CI level of theory. And the conclusion is attained that the cc-PVTZ is a most suitable basis set for these states. The calculated dissociation energies and equilibrium geometries are 0.8818 eV and 0.3090 nm for c^3∑g^＋ state, and 0.3668 eV and 0.2932 nm for B^1-Пu state respectively. The potential energy curves are calculated over a wide internuclear distance range from about 2.5α0 to 37α0 and have a least-squares fit into the Murrell-Sorbie function. According to the calculated analytic potential energy functions, the harmonic frequencies （We） and other spectroscopic data （ωeXe, Be and αe） are calculated. Comparison of the theoretical determinations at present work with the experiments and other theories clearly shows that the present work is the most complete effort and thus represents an improvement over previous theoretical results.

Time-dependent Density Functional Theory Study on the Electronically Excited States of N-Methylformamide in Aqueous Solution

Excited-state hydrogen-bonding dynamics of N-methylformamide （NMF） in water has been investigated by time-dependent density functional theory （TDDFT） method. The ground-state geometry optimizations were calculated by density functional theory （DFT） method, while the electronic transition energies and corresponding oscillation strengths of the low-lying electronically excited states of isolated NMF, water monomers and the hydrogen-bonded NMF-H 2 O were calculated by TDDFT method. According to Zhao＇s rule on the excited-state hydrogen bonding dynamics, our results demonstrate that the intermolecular hydrogen bond C=O···O-H is strengthened and weakened in different electronically excited states. The hydrogen bond strengthening and weakening in the electronically excited state plays an important role in the photophysics of NMF in solutions.

Shape of the geometrically active atomic states of carbon

We have developed a computer code for {／em ab initio} the variational configuration interaction calculation of the electronic structure of atoms via variationally optimized Lagurre type orbitals, treating the orbital effective charges as variational parameters. Excited states of the same symmetry, in order to avoid the inherent restrictions of the standard method of Hylleraas--Unheim and MacDonald, are computed variationally by minimizing the recently developed minimization functionals for excited states. By computing, at the minimum, the one-electron density and the probability distribution of the two-electron angle, and the most probable two-electron angle, we investigate the atomic states of the carbon atom. We show that, without resorting to the （admittedly unproven） concept of hybridization, as an intrinsic property of the atomic wave function, the most probable value of the two-electron angle is around the known angles of carbon bonding, i.e. either 109^／circ or 120^／circ or 180^／circ, depending on each low-lying state of the bare carbon atom.