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<jats:p>Low temperature (77 K) photoluminescence measurements have been performed on different GaAs substrates to evaluate the GaAs crystal quality. Several defect-related luminescence peaks have been observed, including 1.452 eV, 1.476 eV, 1.326 eV peaks deriving from 78 meV Ga<jats:sub>As</jats:sub> antisite defects, and 1.372 eV, 1.289 eV peaks resulting from As vacancy related defects. Changes in photoluminescence emission intensity and emission energy as a function of temperature and excitation power lead to the identification of the defect states. The luminescence mechanisms of the defect states were studied by photoluminescence spectroscopy and the growth quality of GaAs crystal was evaluated.</jats:p>

<jats:p>N<jats:sub>2</jats:sub>O is a significant atmospheric greenhouse gas that contributes to global warming and climate change. In this work, the high sensitivity detection of atmospheric N<jats:sub>2</jats:sub>O is achieved using wavelength modulation spectroscopy (WMS) with an inter-band cascade laser operating around 3.939 µm. A LabVIEW-based software signal generator and software lock-in amplifiers are designed to simplify the system. In order to eliminate the interference from water vapor, the detection was performed at a pressure of 0.1 atm (1 atm = 1.01325×10<jats:sup>5</jats:sup> Pa) and a drying tube was added to the system. To improve the system performance for long term detection, a novel frequency locking method and 2 <jats:italic>f/</jats:italic>1 <jats:italic>f</jats:italic> calibration-free method were employed to lock the laser frequency and calibrate the power fluctuations, respectively. The Allan deviation analysis of the results indicates a detection limit of <jats:italic>∼</jats:italic> 20 ppb (1 ppb = 1.81205 µg/m<jats:sup>3</jats:sup>) for a 1 s integration time, and the optimal detection limit is <jats:italic>∼</jats:italic> 5 ppb for a 40-s integration time.</jats:p>

<jats:p>Highly excited vibrational dynamics of phosphaethyne (HCP) integrable system are investigated based on its dynamic potentials. Taking into consideration the 2:1 Fermi resonance between H–C–P bending vibrational mode and C–P stretching vibrational mode, it is found that the effects of H–C stretching vibrational mode on vibrational dynamic features of the HCP integrable system are significant and regularly vary with Polyad numbers (<jats:italic>P</jats:italic> number). The geometrical profiles of the dynamic potentials and the corresponding fixed points are sensitive to the variation of H–C stretching vibrational strength when <jats:italic>P</jats:italic> numbers are small, but are not sensitive when <jats:italic>P</jats:italic> numbers become larger and the corresponding threshold values become lower. The phase space trajectories of different energy levels in a designated dynamic potential (<jats:italic>P</jats:italic> = 28) were studied and the results indicated that the dynamic potentials govern the various dynamic environments in which the vibrational states lie. Furthermore, action integrals of the energy levels contained in dynamic potential (<jats:italic>P</jats:italic> = 28) were quantitatively analyzed and elucidated. It was determined that the dynamic environments could be identified by the numerical values of the action integrals of trajectories of phase space, which is equivalent with dynamic potentials.</jats:p>

<jats:p>We develop a benchmark system for van der Waals interactions obtained with MP2+ΔCCSD(T) method at complete basis set limit. With this benchmark, we examine the widely used PBE+D3 method and recently developed SCAN+rVV10 method for density functional theory calculations. Our benchmark is based on two molecules: glycine (or Gly, an amino acid) and uracil (or U, an RNA base). We consider six dimer configurations of the two monomers and their potential energy surfaces as a function of relative distance and rotation angle. The Gly-Gly, Gly-U, and U-U pairs represent London dispersion, hydrogen bonding, and <jats:italic>π</jats:italic>–<jats:italic>π</jats:italic> stacking interactions, respectively. Our results show that both PBE+D3 and SCAN+rVV10 methods can yield accuracy better than 1 kcal/mol, except for the cases when the distance between the two monomers is significantly smaller than the equilibrium distance. In such a case, neither of these methods can yield uniformly accurate results for all the configurations. In addition, it is found that the SCAN and SCAN+rVV10 methods can reproduce some subtle features in a rotational potential energy curve, while the PBE, PBE+D3, and the local density approximation fail.</jats:p>

<jats:p>Fluorescence loss spectrum for detecting cold Rydberg atoms with high sensitivity has been obtained based on lock-in detection of fluorescence of 6P<jats:sub>3/2</jats:sub> state when cooling lasers of the magneto-optical trap are modulated. The experiment results show that the signal to noise ratio has been improved by 32.64 dB when the modulation depth (converted to laser frequency) and frequency are optimized to 4 MHz and 6 kHz, respectively. This technique enables us to perform a highly sensitive non-destructive detection of Rydberg atoms.</jats:p>

<jats:p>The wave packet evolution of an atom irradiated by an intense laser pulse is systematically investigated by using the numerical solution of the time-dependent Schrödinger equation. There are two types of spatial interference structures in the time-dependent evolution of the atomic wave packet. With the increasing of the evolution time, the interference fringe spacing for type I (type II) becomes larger (smaller). As the wavelength of the incident laser increases, the interference of the wave packet is changed from type II to type I, and the shift of interference type can be attributed to the contribution of excited states by using the energy analysis of the time-dependent wave function.</jats:p>

<jats:p>We investigated the nonsequential double ionization (NSDI) in Ar by two-color elliptically polarized laser field with a three-dimensional (3D) classical ensemble method. We study the relative phase effect of NSDI and distinguish two particular recollision channels in NSDI, which are recollision–impact ionization (RII) and recollision-induced excitation with subsequent ionization (RESI), according to the delay-time between the recollision and the final double ionization. The numerical results indicate that the ion momentum distribution is changed and the triangle structure is more obvious with the decrease of the relative phase. We also demonstrate that the RESI process always dominates in the whole double ionization process and the ratio of RESI and RII channels can be influenced by the relative phase.</jats:p>

<jats:p>We study the multistability of fixed points for a dimerized system of Rydberg atoms driven by two laser fields and trapped in a two-dimensional (2D) square lattice. For identical driving fields, the fixed points of this system exhibit stable uniform, unstable uniform, stable nonuniform, or oscillating nonuniform phases in the presence of a bistable region. For different driving fields, however, all (stable, unstable, or oscillating) phases become nonuniform instead, which then results in two islets isolated from the mainland of Rydberg excitation. We also show that a tristable region may stretch out from the (nonuniform) bistable region near the islets, indicating that a richer phase diagram can be attained by tuning the Rabi frequencies and/or detunings of the driving fields. Last but not least, the fixed points can adiabatically evolve from the islets to the mainland but can not inversely evolve from the mainland to the islets.</jats:p>

<jats:p>The interaction of an atom with an intense laser field provides an important approach to explore the ultrafast electron dynamics and extract the information of the atomic and molecular structures with unprecedented attosecond temporal and angstrom spatial resolution. To well understand the strong field atomic processes, numerous theoretical methods have been developed, including solving the time-dependent Schrödinger equation (TDSE), classical and semiclassical trajectory method, quantum <jats:italic>S</jats:italic>-matrix theory within the strong-field approximation, <jats:italic>etc</jats:italic>. Recently, an alternative and complementary quantum approach, called Bohmian trajectory theory, has been successfully used in the strong-field atomic physics and an exciting progress has been achieved in the study of strong-field phenomena. In this paper, we provide an overview of the Bohmian trajectory method and its perspective on two strong field atomic processes, <jats:italic>i.e.</jats:italic>, atomic and molecular ionization and high-order harmonic generation, respectively.</jats:p>

<jats:p>We utilized a set of fused silica thin plates to broaden the spectrum of 1 kHz, 30 fs Ti:sapphire amplified laser pulses to an octave. Following the compression by chirped mirror pairs, the generated few-cycle pulses were focused onto an argon filled gas cell. We detected high order harmonics corresponding to a train of 209 as pulses, characterized by the reconstruction of attosecond beating by interference of two-photon transition (RABITT) technique. Compared with the conventional attosecond pulse trains, the broad harmonics in such pulse trains cover more energy range, so it is more efficient in studying some typical cases, such as resonances, with frequency resolved RABITT. As the solid thin plates can support high power supercontinuum generation, it is feasible to tailor the spectrum to have different central wavelength and spectral width, which will make the RABITT source work in different applications.</jats:p>