Controlling neutral fragmentation yields of methane by femtosecond laser pulses

We demonstrate the control of neutral fragmentation of methane（CH4） induced by a Ti：sapphire intense laser pulse（800 nm, 40 fs） by using a pump–probe spectroscopy. Enhancement of the fluorescence emission from the neutral radical CH（A2Δ → X2Π） induced by the intense laser field（~1014 W/cm2） is observed when the wavelength of the probe laser pulse is tuned to 400 nm. The phenomena are explained based on excitation enhancement of the super-excited state of the parent molecule resulting in an increase in neutral dissociation of the methane molecules.     

Photoemission mechanisms of methane in intense laser fields

The visible and near-UV emission spectroscopy of methane(CH4) induced by a femtosecond intense laser field(800 nm, 40 fs, 1014W∕cm2) is studied. By measuring the decay profiles of the neutral fragment product CH(A2Δ→ X2Π), two reaction pathways, i.e., the electron-ion recombination through e-t CHt4 and the direct disintegration of CHt4 are found to be responsible for populating the electronic excited states of the neutral fragment product CH, which gives rise to the photoemissions. Our results provide complementary information on previous understanding of the strong-field-induced photoemission mechanism of CH4 through neutral dissociation of superexcited states.     

Interaction of intense laser pulses with hydrogen atomic clusters

The interaction between intense femtosecond laser pulses and hydrogen atomic clusters is studied by a simplified Coulomb explosion model. The dependences of average proton kinetic energy on cluster size, pulse duration, laser intensity and wavelength are studied respectively. The calculated results indicate that the irradiation of a femtosecond laser of longer wavelength on hydrogen atomic clusters may be a simple, economical way to produce highly kinetic hydrogen ions. The phenomenon suggests that the irradiation of femtosecond laser of longer wavelength on deuterium atomic clusters may be easier than that of shorter wavelength to drive nuclear fusion reactions. The product of the laser intensity and the squared laser wavelength needed to make proton energy saturated as a function of the squared cluster radius is also investigated. The proton energy distribution calculated is also shown and compared with the experimental data. Our results are in agreement with the experimental results fairly well.     

Upconversion properties of Er^3＋/Yb^3＋ co-doped TeO2-Nb2O5-Li2O glasses

The Er^3＋/Yb^3＋ co-doped TeO2-Nb2O5-Li2O glass is prepared by conventional melting method, and its upconversion spectra are measured. The intense green upconversion luminescence upon excitation with a 976 nm laser diode is observed with the naked eyes. The dependence of luminescence intensity on the ratio of Yb^3＋/Er^3＋ is discussed in detail, and the relationship between the ratio of green luminescence intensity to red luminescence intensity and the ratio of Yb^3＋/Er^3＋ is also studied, The luminescence intensity increases with the ratio of Yb^3＋/Er^3＋ increasing. The ratio of Yb^3＋/Er^3＋ plays a more important role than the concentration of Er^3＋ in determining the upconversion luminescence intensity. The ratio of green luminescence intensity to red luminescence intensity reaches a maximum when ratio of Yb^3＋/Er^3＋ is 3. Thus the glass could be one of the potential candidates for LD pumping solid-state lasers.     

On the fluorescence enhancement mechanism of Er3+ in germanate glass containing silver particles

The spectral properties of trivalent erbium ions(Er3+) are systematically studied in a melt-quenched germanate glass(60 GeO2-20PbO-10BaO-10K2O-0.1Ag2O) containing silver(Ag) particles.Thermal treatment of the material leads to the precipitation of Ag particles as observed by transmission electron microscopy and confirmed by absorption spectrum for the obvious surface plasmon resonance peak of Ag particles.The fluorescence from Er3+ in the 10-min-annealed sample with Ag particles is found to be 4.2 times enhanced compared with the unannealed sample excited by 488-nm Ar+ laser.A comparison is made between a spectral study performed on the unannealed Er3+-doped sample and the one annealed for 20 min.The data of absorption cross section and Judd-Ofelt intensity parameters show the agreement between the two samples no matter whether there are Ag particles,indicating that the introduction of Ag particles by post-heat treatment has no effect on the crystal field environment of Er3+ ions.The fluorescence enhancement is attributed to the surface plasmon oscillations of Ag particles in germanate glass.     

Evaluation of Optical Properties of Self-Frequency-Doubling Crystal Yb：GdYAl3（BO3）4 for Laser Applications

Yb：GdYAl3 （B03）4 （Yb：GdYAB） is investigated as a new laser crystal for potential applications in self-frequency doubling. The emission and absorption properties of Yb：GdYAB crystal are studied, and the emission decay times of the upper laser level are measured. The emission cross sections are evaluated using the absorption cross section and principle of reciprocity. The other laser performance parameters, such as the minimum inversion fraction βmin, pump saturation intensity Isat and minimum pump intensity 1rain, are also calculated. The results are discussed in the framework of requirements for an effective diode-pumped Yb^3＋ laser system. Yb：GdYAB is expected to exhibit the most useful laser properties and to be superior to Yb：YAB crystal that has been excellent self-frequency-doubling crystal at present in many key spectroscopic parameter values.     

Spectroscopic Properties of Kiton Red in Hybrid TiO2／Ormosil Films

TiO2/ormosil films doped with kiton red have been prepared by the sol-gel method. Spectroscopic properties of the entrapped dye are studied by absorption and emission techniques. The results indicate that the absorption and fluorescence spectra of kiton red depend strongly on the properties of the ormosil matrices. The heattreatment of dye-doped film leads to the increasing fluorescence intensity and the largest intensity is obtained after heat-treatment of 150℃.     

Stereodynamics study of the exchange reaction O(~3P)+CH_4 →H+OCH_3

A new London-Eyring-Polanyi-Sato potential energy surface is employed in this work to study the stereo properties of the O(3P)+CH4 →H+CH3O reaction in its rovibrationally ground state using the quasiclassical trajectory method(QCT).Our calculations are performed at a range of collision energies,Ec=1.5eV～3.5eV,and the excitation function obtained by the QCT method accords well with the experimental data.The product rotational polarization is calculated,and the product shows a strong rotational polarization in the centre-of-mass coordinate system.The orientation of the product rotational angular momenta is sensitive to the increase in collision energy,and the alignment of the product rotational angular momenta shows some of the properties of the heavy heavy-light mass combination reactions.In the isotopic substituted reaction study,when the H atoms in methane are replaced by D atoms,the rotational polarization is obviously reduced.The polarization-dependent differential cross section is also studied by this QCT calculation to provide detailed information about the rotational alignment and orientation of the product.     

Time-Dependent Quantum Dynamics of T＋CH4 → CH3＋HT Reaction

Reaction probability, cross section and rate constant are studied for polyatomic reaction T CH4 → CH3 HT using the semirigid vibrating rotor target (SVRT) model. The numerical calculation for the reaction system is carried out using the time-dependent wavepacket method, and the wavepacket is propagated by the splitoperator method. The calculation exhibits a variety of features that can be used for comparison with future experimental investigations. The reaction probability as a function of the translational energy shows slight oscillatory structures, similar to those observed in H abstraction reactions H H2 and H CH4. The comparisons with the H CH4 reaction are described.     

Stereodynamics study of the H′（^2S） ＋ NH（X^3∑-） 〉 N（4S） ＋ H2 reaction

The stereodynamics and reaction mechanism of the H′（^2S） ＋ NH （X^3∑^-） → N（^4S） ＋ H2 reaction are thoroughly studied at collision energies in the 0.1 eV-1.0 eV range using the quasiclassical trajectory （QCT） on the ground 4A″ potential energy surface （PES）. The distributions of vector correlations between products and reagents P（φr）, P（φr） and P（φr,φr） are presented and discussed. The results indicate that product rotational angular momentum j′ is not only aligned, but also oriented along the direction perpendicular to the scattering plane; further, the product H2 presents different rotational polarization behaviors for different collision energies. Furthermore, four polarization-dependent differential cross sections （PDDCSs） of the product He are also calculated at different collision energies. The reaction mechanism is analyzed based on the stereodynamics properties. It is found that the abstraction mechanism is appropriate for the title reaction.     

Stereodynamics in reaction O（1D） ＋ CH4 〉OH ＋ CH3

A theoretical study of the stereodynamics for reaction O(1D) + CH4→OH + CH3 has been carried out using the quasiclassical trajectory method(QCT) on a potential energy surface structured by Gonzalez et al. The integral cross sections(ICSs), differential cross sections(DCSs) and product rotational angular momentum polarization have been calculated. With the collision energy increasing, the ICS decreases. There is no threshold energy, because no barrier is found on the minimum energy path. The DCS results show that the backward and forward scatterings exist at the same time. With the collision energy increasing, the dominant rotation of the product changes from the right-handed direction to the left-handed direction in planes parallel to the scattering plane. In the isotopic effect study, the decrease of the mass factor weakens the polarization degree of the rotational angular momentum vectors of the products.     

Two-Photon Transitions of ~(85)Rb 5D_(5/2) State by Using an Optical Frequency Comb and a Continuous-Wave Laser

A high-resolution two-photon spectrum of 5S_(1/2) → 5P_(3/2)→ 5D_(5/2) transitions in a thermal ~(85)Rb vapor cell is presented by using an optical frequency comb and a cw laser.The fluorescence of 6P_(3/2) → 5S_(1/2) spontaneous emission is detected when the cw laser frequency is scanned from the 5S_(1/2) ground state to 5P_(3/2) hyperfine levels and the optical frequency comb repetition rate is fixed.The hyperfine splittings(F_f = 2-5) of the 5D_(5/2) excited state are well resolved.The dependences of fluorescence intensities on the cw laser intensity and temperature of~(85)Rb vapor cell are studied,respectively.The experimental results are in good agreement with the theoretical analyses.     

Dynamical Properties of a Single-Mode Laser Driven by Quadratic Pump Noise

We propose a single-mode laser model driven by quadratic pump noise with cross correlation between the real and imaginary parts of the pump noise. The effect of the cross-correlation coefficient λp between the real and imaginary parts of the pump noise on dynamical properties is studied by using of the linear approximation. The theoretical expressions of intensity correlation function λ（t）, normalized intensity fluctuation λ（0） are calculated. It is found that the most conspicuous effects of λp on both the intensity correlation function and the normalized intensity fluctuation appear at ｜λP｜ = /0.5, but not at ｜λP｜ = 1.     

The characteristics of O＋HD （v = 0, j = 0） reaction dynamics

The analytical potential energy function of HDO is constructed at first using the many-body expansion method.The reaction dynamics of O+HD(v = 0,j = 0) in five product channels are all studied by quasi-classical trajectory(QCT) method.The results show that the long-lived complex compound HDO is the dominant product at low collision energy.With increasing collision energy,O+HD → OH+D and O+HD → OD+H exchange reactions will occur with remarkable characteristics,such as near threshold energies,different reaction probabilities,and different reaction cross sections,implying the isotopic effect between H and D.With further increasing collision energy(e.g.,up to 502.08 kJ/mol),O+HD → O+H+D will occur and induce the complete dissociation into single O,H,and D atoms.     

Ellipticity-dependent ionization/dissociation of carbon dioxide in strong laser fields

Ionization and dissociation of linear triatomic molecules, carbon dioxide, are studied in 50-fs 800-nm strong laser fields using time-of-flight mass spectrometer. The yields of double charged ions CO2+2 and various fragment ions(CO+,O n+, and C n+(n = 1, 2)) are measured as a function of ellipticity of laser polarization in the intensity range from 5.0 ×1013W/cm2 to 6.0 × 1014W/cm2. The results demonstrate that non-sequential double ionization, which is induced by laser-driven electron recollision, dominates double ionization of CO2 in the strong IR laser field with intensity lower than2.0 × 1014W/cm2. The electron recollision could also have contribution in strong-field multiple ionization and formation of fragments of CO2 molecules. The present study indicates that the intensity and ellipticity dependence of ions yields can be used to probe the complex dynamics of strong-field ionization/dissociation of polyatomic molecules.     

Dynamics of H2 in Intense Femtosecond Laser Field

Fragmentation of 1t2 in femtosecond laser field is studied by using velocity map ion imaging method and 70 fs laser at 405nm and 810nm from 0.3 to 3.0 × 10^14 W//cm^2. Angular distributions from Coulomb explosion are independent of the laser wavelengths and intensities, which means the explosion starts from the same electronic state. Angular distributions from dissociation channels in the 810 nm laser field broaden with the laser intensity. Lobe structures in angular distributions from Coulomb explosion and above threshold dissociation channels are reported. Fine structures in the distribution of internuclear distances are observed.     

First-Principle Study of H2 Adsorption on Mg3N2（ll0） Surface

The adsorption of H2 on two kinds of Mg3N2（110） crystal surface is studied by first principles. Adsorption sites, adsorption energy, and the electronic structure of the Mg3N2（110）/H2 systems are calculated separately. It is found that H2 is mainly adsorbed as chemical adsorption, on these sites the 1-12 molecules are dissociated and the H atoms tend to the top of two N, respectively, forming two NH, or the H atoms tend to the same N forming one NH2. There are also some physicM adsorption sites. One of the bridge sites of Mg3N2 （110） surface is more favorable than the other sites. On this site, H atoms tend to the top of two N, forming two NH. This process belongs to strong chemical adsorption. The interaction between 1-12 molecule and Mg3N2（110） surface is mainly due to the overlap-hybridization among Hls, N 2s, and N 2p states, covalent bonds are formed between the N and H atoms.     

Optical planar waveguides in yb3＋-doped phosphate glasses produced by He＋ ion implantation

Optical planar waveguides in Yb 3+-doped phosphate glasses are fabricated by implanting triple-energy helium ions. The guiding modes and the near-field intensity distribution are measured by using the prism-coupling method and the end-face coupling setup with a He-Ne laser at 633 nm The intensity calculation method (ICM) is used to reconstruct the refractive index profile of the waveguide. The absorption and the fluorescence investigations reveal that the glass bulk features are well preserved in the active volumes of the waveguides, suggesting the fabricated structures for possible applications as waveguide lasers.     

Improved anti-Stokes energy transfer between rare earth ions in Er（0.5）Yb（9.5）：FOV oxyfluoride vitroceramics explains the strong color reversal

The widely used energy transfer theory is a foundation of luminescence,in which the rates of Stokes and antiStokes processes have the same calculation formula.An improvement on the anti-Stokes energy transfer to explain the fluorescence intensity reversal between the red and green fluorescence of Er(0.5)Yb(9.5):FOV is reported in the present article.The range of the intensity reversal Σ was measured to be 877.Dynamic processes for 16 levels were simulated.A coefficient,the improvement factor of the intensity ratio of Stokes to anti-Stokes processes in quantum Raman theory compared to classical Raman theory,is introduced to successfully describe the anti-Stokes energy transfer.A new method to calculate the distance between the rare earth ions,which is critical for the energy transfer calculation,is proposed.The validity of these important improvements is also proved by experiment.     

Growth and optical properties of Cr~（3＋）-doped CdWO_4 single crystals

A high-quality Cr 3+:CdWO4 single crystal at a size of approximatelyΦ25×80 mm is grown using the Bridgman method with CdO,WO3,and Cr2O3 as raw materials and their molar ratio of 100:100:0.5.The temperature gradient of solid-liquid interface at growth is approximately 50?C/cm and the growth rate is 0.05 mm/h.The X-ray diffraction(XRD),absorption,excitation,and emission spectra of different parts of the as-grown and O2-annealed crystals are investigated.Two strong broad optical absorption bands of about 472 and 708 nm are observed,and they are associated with the transitions 4 A2→ 4 T1 and 4 A2→ 4 T2.The weak 4 T2→ 2 E transition(the R-line)at 632 nm is also observed.The crystal-field parameter Dq and the Racah parameters B and C are estimated to be 1 412.4,776.8,and 3 427.6 cm? 1,respectively,according to the absorption spectra and crystal-splitting theory.A broadband fluorescence at about 1 000 nm due to 4 T2→ 4 A2 transition is produced by exciting the samples at 675 nm.After being annealed in an O2 atmosphere,the crystals become more transparent,while the effective light absorption of Cr 3+ ions is evidently enhanced and the emission intensity is also strengthened due to the reduction of oxygen vacancies in the CdWO4 crystal after annealing.