Yong-Chang Han

• Professor (Associate) at Dalian University of Technology

The conventional understanding of bimolecular reactions, which either proceed directly via well-defined transition states or pass through potential energy wells, is well-established. However, increasing attention and interest have been drawn to nontraditional reaction pathways, such as roaming mechanisms. Here, full-dimensional dynamics simulations...

The ladder-type population transfer of the HF molecule steered by four-color harmonic laser pulses (HLPs) is investigated using the time-dependent quantum wave packet method. It is found that although there exist large background excitations and many (resonant) transition pathways during the driving, nearly 100% of the population could be transferr...

We adopt the genetic algorithm to fit the potential energy curve for B ¹ Π state of ⁸⁵ Rb ¹³³ Cs molecule based on the data of rovibrational energy levels, which were observed previously by Fourier-transform spectroscopy and photoassociation spectroscopy. We explore the effect of different hyperparameter settings on the evolutionary process and fin...

Alkaline earth metal monohalides are currently considered the most promising candidates for direct laser cooling due to the unique electronic structure of their lower states. Among the alkaline earth monohalides family, the radium compounds are of the highest interest because of the largest mass of the radium atom compared to other alkaline earth m...

Generally, the three-body recombination (TBR) of the ultracold helium atoms in the zero-collision-energy limit could be described by the original zero-range (Efimov) universal function [E. Braaten, D. Kang, and L. Platter, Phys. Rev. A 75, 052714 (2007)] using the scattering length a of the He-He interaction and a three-body parameter a* as input,...

Photoassociation (PA) of ⁸⁵ Rb atoms from the ground X ¹ Σ ⁺ g state to the excited A ¹ Σ ⁺ g and b ³ Π u states by two-color laser pulses at 100 µK×k B is investigated by using quantum wavepacket method. Two pulses with detunings δ 1 and δ 2 varying from -1.0 to 1.0 cm ⁻¹ are taken into account. The specific case δ 1 = -δ 2 is generalized to the co...

The increase of the number of the two-bodyrecombination channels strongly challenges the numerical calculation of the accurate rates for the three-body recombination (TBR) process and itsreverse process, collision-induced dissociation(CID), at ultracold temperatures. By taking the $^{4}$He-$^4$He-$^{20}$Ne collision system as anexample, we have obt...

By solving the full-dimensional time-dependent Schrödinger equation with the thermal-random-phase wavepacket method, we investigate the photoassociation (PA) process of hot (1000 K) magnesium atoms induced by two time-delayed femtosecond laser pulses. Driven by the 840 nm fs laser pulses, the Mg 2 molecules can be formed on the four excited states,...

So far, numerous researches on the He2H− system have still been restricted to several analytical forms of the potential energy interaction, where the calculations in the three‐body interaction may be not precise. With that in mind, we have provided an accurate, global, full‐dimensional potential energy surface (PES) for the ground state of the He2H...

The collisions transferring large portions of the energy are often called supercollisions. By taking the D + C2H2 and D + C2H4 collision systems as examples, we investigate the difference between the two systems in the novel supercollision process at the collision energy of 70 kcal/mol. Based on the quasi-classical trajectory calculations, various...

The photodissociation cross section of the NaH molecule from the ground state X ¹ Σ ⁺ to the excited state B ¹ Π is calculated by solving the time-dependent Schrödinger equation with the consideration of two sets of potential energy curves (PECs). It is found that by using PECs, the maximum photodissociation cross section, the corresponding photon...

We investigate the thermal photoassociation dynamics of the Mg atoms, from the X1Σg+ state to the (1)1Πg state, or to the other three higher excited states |i〉=(1)1Πu,(2)1Πu,(2)1Σu+. The weak two-photon coupling between X1Σg+ and (1)1Πg, the strong three-photon couplings between X1Σg+ and |i〉, and the one-photon couplings between (1)1Πg and |i〉 can...

The influence of the thermal average of initial rovibrational states on the orientation of the NaI molecule induced by a single-cycle terahertz pulse is investigated using the random-phase wavefunctions method. |〈cosθ〉|max as a function of the laser central frequency ω0 is obtained at different temperatures. Firstly, although the orientation is sup...

By taking the femtosecond two-photon photoassociation (PA) of magnesium atoms as an example, we propose a method to calculate the thermally averaged population, which is transferred from the ground X1Σg + state to the target (1)1Πg state, based on the solution of full-dimensional time-dependent Schrödinger equation. In this method, named as method...

We investigate theoretically the photoassociation (PA) of two Cs atoms using a slowly-turned-on and rapidly-turned-off (STRT) laser pulse at the temperature of 54μK by taking the thermal average of the initial continuum states into account. For comparison, the PA probabilities under the action of the STRT and typical Gauss-type pulses are calculate...

The population transfer controlled by fundamental, 2nd-harmonic and 3rd-harmonic pulses is investigated by using the time-dependent quantum wave packet method. By varying the phase φ1 of the fundamental pulse, it can be coherent with the 2nd-harmonic and 3rd-harmonic pulses. Compared with two harmonic pulses, the phase φ1in three harmonic pulses is...

Atom-atom-ion three-body recombination (TBR) of mixed spin-polarized ⁴He and X⁻ (X= H or D) systems is investigated, by solving the Schrödinger equation using the adiabatic hyperspherical representation method. It is shown that the dominant products after a TBR in the ultracold limit (E ≤ 0.1 mK × kB) are different for the two systems. For the ⁴He⁴...

The role of sharp avoided crossings (SACs) in a short hyper-radial range R ≤ 50 a.u. in the calculation of recombination for a cold ⁴He3 system is investigated in the adiabatic hyperspherical representation by “turning off and on” the relevant nonadiabatic couplings. The influence of SACs on the recombination is related with the channels of the sys...

We construct a simple model to calculate the trimer bound state energy ET(n) and three-body recombination rate K30 of three identical bosons with van der Waals interaction without using any two- or three-body fitting parameter. Using this simple model, we investigate the influence of the van der Waals finite-range effect on ET(n) and K30. Our calcu...

The influence of the carrier envelope phase (CEP) on the field-free orientation of theNaI molecule is investigated theoretically. The molecular orientation steered by asingle-cycle terahertz (THz) pulse is studied by solving exactly the time-dependentSchrödinger equation including the rovibrational degrees of freedom. We find that thefield-free ori...

We present a simple theoretical model for describing ultracold-atomic collision dynamics and Feshbach resonances (FRs) without onerous numerical computation. An effective quantum-defect phase shift δYeff is derived and the reference potentials are shown acting as an assistant tool in the calculation. With the help of singlet and triplet scattering...

The photoassociation (PA) reactions of H + F → HF and D + F → DF are investigated by using the time-dependent quantum wave packet method. For each system, we consider four cases, where the target state is set to be ν = 16, 17, 18, 19〉, respectively. The effects of the initial collision momentum and laser parameters, including intensity, pulse durat...

A time-dependent quantum wave-packet method is employed to investigate the rovibrational dynamics of population transfer in the ground electronic state controlled by two-color (omega + 2 omega) pulses analogous to coherently control the processes of ionization and dissociation. The population can be transferred to three target states, vertical bar...

The population transfer process between the vibrational levels of the HF molecule is investigated by solving the time-dependent Schrödinger equation. The two vibrational levels of υ = 0 and 2 are set to be the initial and target states (an overtone transition). The transition processes under the one- and two-photon resonance conditions are consider...

The orientation and predissociation dynamics of the NaI molecule are studied by using a time-dependent wavepacket method. The NaI molecule is first pre-oriented by a single-cycle pulse (SCP) in terahertz (THz) region and then predissociated by a femtosecond pump pulse. The influence of the molecular field-free orientation on the predissociation dyn...

Can a molecule be efficiently activated with a large amount of energy in a single collision with a fast atom? If so, this type of collision will greatly affect molecular reactivity and equilibrium in systems including combustion where abundant hot atoms exist. Conventional expectation of energy transfer in an encounter between an atom and molecule...

An efficient three-step excitation scheme is proposed to achieve the strong molecular orientation. By taking the CO molecule for example, via the Raman excitation by the femtosecond pulse L 1 and subsequently resonant excitations by a single-cycle THz pulse L 2, the high degree of the field-free orientation of |⟨ cosθ ⟩ |max = 0.8921 can be obtaine...

The field-free orientation induced by a single-cycle terahertz (THz) laser pulse is studied for two "heavy" molecules, NaI and IBr. Two methods are used and compared in the calculations: One is to solve the exact time-dependent Schrödinger equation (ETDSE) considering the full-rovibrational degrees of freedom, and the other is to invoke the rigid-r...

Aminopyrazine (AP) and AP-methanol complexes have been theoretically studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The excited-state hydrogen bonds are discussed in detail. In the ground state the intermolecular multiple hydrogen bonds can be formed between AP molecule and protic solvents. Th...

The interference minimum in the high-order harmonic spectrum of H2+ is studied by solving the full three-dimensional time-dependent Schrödinger equation for the electronic motion keeping the nuclei fixed. The two-center interference model works well when the internuclear distance is around its equilibrium value where also recombination to the 2Σg+...

We report classical (CT) and quasiclassical (QCT) trajectory calculations of the association reaction H + C2D2 → C2D2H∗, using an ab initio global potential energy surface. The QCT threshold is roughly 4 kcal/mol, in very good agreement with the vibrationally adiabatic ground-state barrier. In contrast, the 0 and 300 K CT threshold energies are rou...

The reaction of O((3)P) with C(2)H(4), of importance in combustion and atmospheric chemistry, stands out as paradigm reaction involving not only the indicated triplet state potential energy surface (PES) but also an interleaved singlet PES that is coupled to the triplet surface. This reaction poses great challenges for theory and experiment, owing...

Translationally hot H collisions with the acetylene are investigated using quasiclassical trajectory calculations, on a recent full-dimensional ab initio-based potential energy surface. Three outcomes are focused on: non-reactive energy transfer via prompt collisions, non-reactive energy transfer via the formation of the vinyl complex, and reactive...

The O((3)P) + C(2)H(4) reaction, of importance in combustion and atmospheric chemistry, stands out as a paradigm reaction involving triplet- and singlet-state potential energy surfaces (PESs) interconnected by intersystem crossing (ISC). This reaction poses challenges for theory and experiments owing to the ruggedness and high dimensionality of the...

We present a quasiclassical trajectory study of the photodissociation of CH₃CHO, using an updated global ab initio-based potential energy surface. Calculations are performed at a total energy corresponding to a photolysis wavelength of 308 nm, and with all trajectories initiated from the acetaldehyde global minimum. By examining the bond...

The effect of the coupling between the valence state B2Π and the Rydberg state C2Π on the absorption spectrum of the NO molecule is studied by using the quantum wave packet dynamics method. The results show that the coupling between the valence state B2Π and the Rydberg state C2Π affects the C2Π ← X2Π absorption spectrum both in the intensity and o...

We present a quasiclassical trajectory study of the photodissociation of CH3CHO using a global ab initio-based potential energy surface. Calculations are performed at a total energy of 160 kcal/mol, corresponding to a photolysis wavelength of 230 nm, and with all trajectories initiated from the acetaldehyde global minimum. Many product channels are...

We model the process of high-order harmonic generation by solving the time-dependent Schrödinger equation for H+2 in the fixed nuclei approximation including full 3D electron motion for nonvanishing angles between the nuclear axis and the linear polarization of the driving pulse. We show that the coherent laser coupling induced between the 2Σ+g(1sσ...

We solve the time-dependent Schrödinger equation for atomic hydrogen in an intense field using spherical coordinates with a radial grid and a spherical harmonic basis for the angular part. We present the high-order harmonic spectra based on three different forms, the dipole, dipole velocity, and acceleration forms, and two gauges, the length and ve...

The interference between two dissociating wave packets of the I2 molecule driven by femtosecond laser pulses is theoretically studied by using the time-dependent quantum wave packet method. Both the internuclear distance- and velocity-dependent density functions are calculated and discussed. It is demonstrated that the interference pattern is deter...

We present a theoretical scenario used for enhancing the degree of field-free orientation for the HF molecule with a moderate permanent dipole moment by using a combination of terahertz few-cycle pulses and rovibrational pre-excitation. An efficient field-free orientation and an obvious enhancement can be achieved by using this scenario even at roo...

Three-peak Autler-Townes (A-T) splitting in the resonant multiphoton ionization photoelectron spectrum for a rotating Li2 molecular system in femtosecond pulse laser fields is studied by using two-dimension time-dependent quantum wave packet method. The A-T splitting results from rapid Rabi oscillation caused by intense femtosecond laser pulses. Be...

We have demonstrated that field-free molecular orientation induced by a terahertz half-cycle pulse can be considerably enhanced by rovibrational selective excitation. The excitation process is controlled by infrared laser pulses. The calculations are performed by solving numerically the full time-dependent Schrödinger equation for the HF molecule i...

The diabatic photodissociation and photoionization processes of the NaI molecule are studied theoretically using the quantum wave packet method. A pump laser pulse is used to prepare a dissociation wave packet that propagates through both the ionic channel (NaI-->Na(+)+I(-)) and the covalent channel (NaI-->Na+I). A Stark pulse is used to control th...

The interference of dissociating wave packets for the Br2 molecule in femtosecond laser field is studied theoretically using time-dependent quantum wave packet method. The interference of dissociating wave packets can be determined by the spectrum of laser field. By shaping laser pulses in frequency domain, the corresponding R- and v-dependent dens...

The above-threshold dissociation (ATD) of the HD+ molecular ion in femtosecond laser field is investigated theoretically The energy-dependent distribution of the dissociated fragments is calculated using an asymptotic-flow expression in the momentum space. The calculations show that the ATD of HD+ is sensitive to the initial vibrational level of gr...

The population transfer via continuum for the Li2 molecule is theoretically investigated using time-dependent quantum wave packet method. With proper laser pulses, a high efficient population transfer can be achieved, and the selective ro-vibrational population transfer via continuum can be realized. The population transfer process is qualitatively...

The dissociation dynamics of Br2 molecules induced by two femtosecond pump pulses are studied based on the calculation of time-dependent quantum wave packet. Perpendicular transition from X 1Sigma g+ to A 3Pi 1u+ and 1Pi 1u+ and parallel transition from X 1Sigma g+ to B 3Pi 0u+, involving two product channels Br (2P3/2)+Br (2P3/2) and Br (2P3/2)+Br...

Steering the population transfer of a five-level polar diatomic NaK molecule with two ultrashort transform-limited laser pulses is theoretically investigated using the time-dependent wave packet method. The dynamic Stark shifts resulted from the interaction of polar molecule with laser pulses can be utilized to modulate the transition frequency and...

The Autler–Townes (AT) splitting in femtosecond photoelectron spectrum of three-level Li2 molecules is theoretically investigated using time-dependent quantum wave packet method. With proper femtosecond laser pulses, three peaks of the AT splitting can be observed in the photoelectron spectrum. The AT splitting stems from rapid Rabi oscillation cau...

The population transfer between the different electronic states through the ionization continuum state is studied for the Na2 molecule using the quantum wave packet method. The population evolutions of the initial state, the target 21Πg state and the ionization continuum state with time are discussed in detail. The time-dependent photoelectron ener...

A time-dependent quantum wave packet method is used to calculate the branching ratio of photodissociation products of the HI molecule in two-color intense few-cycle laser pulses with varying relative phase. It is demonstrated that the relative phase between two intense few-cycle pulses (ω and 2ω) can be used for controlling the branching ratio of t...