Effect of laser pulse width on ionization efficiency in supersonic beam-multiphoton ionization-mass spectrometry

The efficiency in multiphoton ionization is calculated for various molecules with different lifetimes, based on a kinetic model using rate equations. The results are strongly dependent on the types of resonance ionization, i.e., ionization through congested intermediate levels or ionization through a discrete intermediate level. In the former case, a laser pulse shorter than the lifetime is always preferred for efficient ionization. In the latter case, a 10-ps laser pulse provides the maximum ionization efficiency for a molecule with a lifetime in excess of 10 ps. For a molecule having a lifetime shorter than 10 ps, the use of a laser whose pulse width is adjusted to the same value as the lifetime is suggested. The results obtained by a computer simulation are in reasonably good agreement with the experimental data obtained for monohalogenated benzenes.     

Mode-locked ring laser with output pulse width of 0.4 ps

The output pulse width of a mode-locked ring laser composed of an erbium-doped fiber amplifier, Mach-Zehnder optical modulator, and optical band-pass filter depends largely on the repetition frequency and the wavelength characteristics of these optical circuit elements. In previous experiments, the output pulse width was in the order of 5 ps at a repetition frequency of 5 GHz. The principal reason was that the narrow passage band of the optical circuit elements made it extremely difficult to generate an ultra-short optical pulse. Consequently, we examined how to narrow the optical pulse width by flattening the wavelength characteristics of these optical circuit elements. Furthermore, we drove the optical modulator in the cavity using a frequency multiplier to operate at an effectively higher frequency By widening the wavelength passage band of all the devices in the optical circuit, we achieved an output pulse width of 0.4 ps at a repetition frequency of 5 GHz; the pulse peak power was more than +23 dBm, and the time-bandwidth product was 0.34. We successfully tested an ultra-short optical pulse source with an output pulse width of 0.4 ps with no external pulse compression using a mode-locked ring laser     

Dependence of gold nanoparticle production on pulse duration by laser ablation in liquid media

The dependence on laser fluence and laser pulse duration of size, size distribution and concentration of gold nanoparticles synthesized by laser ablation in liquid media was investigated. It was demonstrated that increasing laser energy from 1 to 5 mJ/pulse enhances the ablation rate by a factor of 100. The behavior of the ablation rate, hence of the nanoparticle concentration, as a function of pulse duration (varied from 40 fs to 200 ps) was found to strongly differ from that in air, which can be explained by photoionization and important losses of laser energy in the femtosecond regime. The optimal pulse duration for maximum ablation rate in liquid media was found to be equal to 2 ps.     

Laser two-photon ionization of pyrene on contaminated soils

We report a first attempt to use the laser multiphoton ionization method for analysis of trace aromatic compounds on the surface of environmental (soils) and artificial (silica) samples. The measurement setup is composed of a N(2) pulsed laser and a fast conductivity detection system. The technique has been tested for detection of pyrene deposited on moist fine-powdered samples. The observed photoionization signals have indicated a gradual increase in the photoionization current and charge as a function of increasing concentration of pyrene/hexane solutions used for sample contamination. Contaminants have been analyzed in several (organic and inorganic) environmental samples, and a method to compensate for matrix effects is suggested. A contamination model is assumed and applied in order to renormalize all signals and provide an useful calibration plot. This calibration plot provides an upper estimate of pyrene LOD as 35 ng/g.     

Dependence of the breakdown of water drops on the parameters of a CO2 laser pulse

Results of a theoretical study are presented pertaining to vaporization and explosion of water drops by a pulsed CO₂ laser, with the nonuniformity of internal heat generation taken into account.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 38, No. 6, pp. 994–998, June, 1980.     

Dynamic target ionization using an ultrashort pulse of a laser field

Ionization processes under the interaction of an ultrashort pulse of an electromagnetic field with atoms in nonstationary states are considered. As an example, the ionization probability of the hydrogen-like atom upon the decay of quasi-stationary state is calculated. The method developed can be applied to complex systems, including targets in collisional states and various chemical reactions.     

Investigation of the ionization of atom driven by a few-cycle laser pulse in phase space

We perform a comprehensive study of the ionization of hydrogen atom by a few-cycle laser pulse with a fixed carrier-envelop in phase space through calculating the Wigner functions. The results show that the electrons always locate around the nucleus before the laser electric field force reaches a certain intensity. While as the force of the laser electric field increases, the electrons are gradually deviated away from the nucleus. The ionized electrons are driven back to the core region and make a wiggling motion because of the variation of the laser electric field force with time. Moreover Wigner functions enable us to clearly show the classical and quantum behaviours during ionization process are presented in phase space, and explicitly attribute the arc fringes to the interference effect between the electronic wavepackets born at different ionization times.     

激光辐照丙酮溶液中固体靶制备纳米碳粉

用激光液相法制备出纳米碳粉．脉冲激光辐射靶材表面生成碳的高能等离子体团，等离子体团与丙酮溶液中的液相体系发生能量交换形成了纳米碳颗粒．用团簇成核的经典理论分析了形成纳米碳颗粒的机理，制备出的纳米碳粉大部分呈球状，少部分呈枝状形貌，为非晶或多晶态组织结构，粒径分布为20--80nm；纳米碳的颗粒大小和微观结构主要与反应温度和激光工艺参数有关．     

Nonlinear dynamics of a plasma torch generated by a laser pulse of large width

Experimental data on the nonlinear dynamics of a plasma torch generated by a laser pulse of large width acting upon a graphite target are presented and discussed. The mushroom shape of the luminous region and the duration of emission observed in experiment are explained by the development of the Richtmyer-Meshkov instability at the carbon plasma-air interface and by the formation of nanoparticles in the plasma expanding into the buffer gas.     

Dynamic measurements of temperature in the process of pulse laser treatment of materials

We propose a method for measurement of temperature and the heating and cooling rates of the surface of metal alloys in the process of pulse laser treatment and estimate the dynamic error of measurements. We obtain experimental dependences of heating and cooling of the nonirradiated surface of a Fe₈₃B₁₇ specimen and demonstrate the possibility of registering thermal processes with heating and cooling rates of up tol0⁶K/sec. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 2, pp. 103–107, March-April, 2000.     

Unburned carbon measurement in fly ash using laser-induced breakdown spectroscopy with short nanosecond pulse width laser

The unburned carbon in fly ash is one of the important factors for the boiler combustion condition. Controlling the unburned carbon in fly ash is beneficial for fly ash recycle and to improve the combustion efficiency of the coal. Laser-induced breakdown spectroscopy (LIBS) technology has been applied to measure the fly ash contents due to its merits of non-contact, fast response, high sensitivity, and real-time measurement. In this study, experimental measurements have been adopted for fly ash flows with the surrounding gases of N₂ and CO₂, while the CO₂ concentration varified to evaluate the CO₂ effect on the unburned carbon signal from fly ash powder. Two kinds of pulse width lasers, 6?ns and 1?ns, were separately adopted to compare the influence of laser pulse width. Results showed that compared with that using 6?ns pulse width laser, plasma temperature was lower and had less dependence on delay time when using 1?ns pulse width laser, and spectra had more stable background. By using 1?ns pulse width laser, the emission signal from surrounding CO₂ also decreased because of the less surrounding gas breakdown. The solid powder breakdown signals also became more stable when using 1?ns pulse width laser. It is demonstrated that 1?ns pulse width laser has the merits for fly ash flow measurement using LIBS.     

Influence of terahertz pulse width on two-dimensional terahertz spectroscopy

The influence of the width of terahertz (THz) pulses on two-dimensional THz spectroscopy (2DTS) has been studied theoretically via a classical method in which the expressions for the second-order nonlinear polarizations with different nonlinear sources are derived by using a perturbation approach. Compared to the common disposal method in which the THz pulse is treated as a delta function, some terms that were of unknown physical meaning or vanished will come into force when the width is considered. Three types of nonlinear sources, i.e. anharmonicity, nonlinear damping and nonlinear coupling, are considered for a single mode system. The simulation results demonstrate that the width of the incident THz pulse can markedly affect the properties of 2DTS and that different sources have different influences. This study reveals a more practical insight for 2DTS and could provide much information, such as the optimal width and interval of THz pulses, to guide possible future experiments.     

Dependence on the sample width of signals from a near-field optical microscope

We numerically investigated signals from a scanning near-field optical microscope (NOM) for samples of various sizes by using the finite-difference time-domain method. Under the usual conditions that apply to detection, the signal intensity depends on the width of the sample, even though the sample will be wider than the wavelength, which is much larger than the aperture, i.e., the lateral resolution of the NOM. This is an obstacle to measuring the local optical constant of samples by means of obtaining the signal intensity. When waves propagating in all directions are collected, this dependence on the sample width is reduced. The whole angle detection is important for observing the distribution of the optical constants.     

The pulse width effect on the shock response of the hard disk drive

A finite element model (FEM) of the ST drive from Seagate is developed in ANSYS to investigate the shock response of the hard disk drive (HDD). The FEM includes the pivot bearing, the head stack assembly (HSA) and the disk. The free state of the HSA is determined by an iterative procedure to produce the prescribed preloading force at the head–disk interface. The FE model is then verified by conducting a modal analysis over the HSA. The obtained mode shapes and resonant frequencies are compared with the modal testing results. An acceleration pulse is applied to the shaft and the whole disk surface at the same time to study the shock response of the HDD. The head slap behavior is examined at the slider–disk interface. The effect of the pulse width on the head slap behavior is studied. The duration of the acceleration pulse varies from 0.3 to 1 ms. It is found that the lift-up height of the slider reaches a peak value at different pulse widths when the amplitude of acceleration pulse changes. This is due to the nonlinear behavior introduced by the contact surfaces.     

基于FPGA的纳秒量级脉冲宽度精确测量研究

为使测量光电开关发出的数字脉冲宽度的精度达到1 ns和掌握光电开关的实时工作状态。该文通过基于FPGA精确测量法的直接计数法和移相计数法进行测量,得出移相计数法的测量误差在1 ns左右,并验证其设计的正确性和有效性。设计通信系统和上位机软件进行实时监控,通信系统采用成熟的串口RS232通信方式,用PFGA设计串口通信模块,实现将数据传输到工控机;再用LabVIEW编写上位机软件,便于工作人员直观观察数据并存储数据到Excel,实验结果证明测量系统可靠、有效。