Jin-Wei Hu’s research while affiliated with Dalian University of Technology and other places

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 condition of arbitrary combination of δ 1 and δ 2 for both the Gaussian-pulse field and the slowly-turned-on and rapidly-turned-off (STRT) field. It is found that the introduce of the detunings in the two pluses can enhance the asymmetry of the time profile and broaden the linewidth of the laser field. The two factors can further enhance the PA probability for both resonant and nonresonant regions. Compared to the Gaussian field, the STRT field which is even more asymmetric in time profile and more broaden in linewidth can significantly enhance the PA probability, with the nonresonant transition dominating the PA process. It can be expected that once the two detunings are both positive, which is a relatively loose restriction for coherent control, a considerable PA probability can be obtained in a two-color STRT field.

The absorption spectrum of the SrLi molecule from the ground X2Σ+ state to the excited (2)2Σ+ state is simulated with molecular rotation and vibration taken into account. By taking 84Sr6Li as an example, the spectra at three different temperatures T = 1, 10 and 100 K are obtained. The absorption spectrum from the initial vibrational level v = 0 of X2Σ+ to the vibrational levels v′= 0–7 of (2)2Σ+ covers the range from 8400 to 10,200 cm−1. The absorption peak corresponds to the transition v = 0 → v′= 1, of which the rotational quantum number J′ of the excited state has been determined in detail for the P and R branches. The band head occurs in R branch for T = 10 and 100 K, corresponding to the excitation to the rotational levels J′ = 5 ~ 8. Additionally, the spectra for six combinations of the isotopes (84Sr/85Sr/88Sr and 6Li/7Li) have been compared. Due to the isotope effect, for transitions to v′≥ 1, the corresponding spectral bands gradually separate into two classes, of which the spectral bands for xSr7Li are energetically lower than those for xSr6Li (x = 84, 85, 88).

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, (1) ¹ Π g , (1) ¹ Π u , (2) ¹ Π u , and ( 2 ) 1 Σ u + , from the initial electronic ground state X 1 Σ g + . It is found that the three-photon couplings between X 1 Σ g + and the three ungerade states [(1) ¹ Π u (2) ¹ Π u , and ( 2 ) 1 Σ u + ], play dominant roles in the population transfer process. By scanning the pulse duration τ from 50 to 200 fs, and varying the delay time δt 0 from 0 to 2 τ fs, we find that the final PA population is strongly dependent on the two parameters. For a given δt 0 , the parameter τ can induce a significant variation (2 ∼ 6.8 times) for the final PA population transfer, and for a given τ , one can also obtain a significant variation (2.7 ∼ 3.5 times) of the final PA population by varying δt 0 . Additionally, the dynamics of the coherently vibrational wavepackets of the four excited states are also influenced by the two parameters.

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 energy, and the effective range of photon energy which can result in the cross section greater than 0.1 Å ² all vary significantly. These differences are ascribed to the variations of the PECs, the vertical excitation energy, the vibrational wavefunction of the X ¹ Σ ⁺ state, and the Franck–Condon factors, etc. These findings provide a deep insight into the spectrum of the NaH molecule and are expected to inspire further relevant investigations in experiment and theory.

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 form several different transition paths. We find that X1Σg+⟶+3ℏω|i〉⟶-ℏω(1)1Πg is the major path and the “V”-type Raman transition path from |i〉 to (1)1Πg takes the second place.

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 suppressed at high temperatures, a distinguishable periodic field-free orientation is still observable at T=1000 K. Secondly, the variation of |〈cosθ〉|max with ω0 is strongly dependent on the temperature. We ascribe this variation to different thermal distributions of the initial rovibrational states and to the consequently different final rotational distributions.

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 A, we use thermal-random-phase wavefunctions with the random phases expanded in both the vibrational and rotational degrees of freedom to model the thermal ensemble of the initial eigenstates. This method is compared with the other two methods (B and C) at different temperatures. Method B is also based on thermal-random-phase wavefunctions, except that the random-phase expansion is merely used for the vibrational degree of freedom. Method C is based on the independent propagation of every initial eigenstate, instead of the thermal-random-phase wavefunctions. Taking the (1)1Πg state as the target state, it is found that although these three methods can present the same population on the (1)1Πg state, the computation efficiency of method A increases dramatically with the increase in temperature. With this efficient method A, we find that the PA process at 1000 K can also induce rotational coherence, i.e., the molecular field-free alignment in the excited electronic states.

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 calculated, respectively. The rising and falling times of the STRT pulse are set to be τr=10 ps and τf=0.2 ps, and the two Gauss-type pulses are set to be τr=τf=10 ps and τr=τf=5.1 ps, respectively. Compared to the calculation considering a single initial continuum state (non-thermal-average calculations), the PA probabilities with the consideration of the thermal average are reduced to different extents according to different laser pulses. The PA process induced by the STRT pulse is more robust than that by the two Gauss-type pulses. For the STRT pulse, the PA probability is reduced by roughly 30%, while for the two Gauss-type pulses, it is reduced by roughly 97% and 64%, respectively. Because of the larger bandwidth and the asymmetric time profile of the STRT pulse, the PA probability induced by this pulse is related to both the off-resonant and the near-resonant transitions in a much broader phase space formed by the initial continuum states and the final vibrational states. Consequently, the thermal average of the initial continuum states has relatively smaller influence on the PA probability induced by the STRT pulse. By changing the pulse shape of the standard Gaussian to the sin2 and the Lorentz ones, the STRT pulse still results in the highest PA probability and is the most robust to the thermal average.