Gold chloride cluster ions generated by vacuum laser ablation

In this work, a simple way for study the possibility of formation a vapor cluster species of tetrachloroauric acid (HAuCl₄), using the laser ablation in the absence of a buffer or reactive atmosphere, and without a postablation supersonic expansion on a commercial matrix assisted laser desorption/ionization time-of-flight mass spectrometer, is reported. Tetrachloroauric acid is known as precursor for the synthesis of gold nanostructures and the complex salts; therefore it is an important task to discover and quantify the species arising from HAuCl₄, in order to understand their role in the gold assisted reactions. Mass spectrum of HAuCl₄ in a reflector negative-ion mode contains the hydrated mono- and dinuclear gold clusters in the m/z range 286–436, and gold chloride clusters in the m/z range 447–795. In the first part of spectrum, m/z range 286–436, the hydrated gold cluster species of type Au _n ^? (H₂O)_m (n?=?1–2; m?=?1, 2, 5, 7, 8) and [Au_n(OH)_k]^?(H₂O)_m (n?=?1–2; k?=?1–2; m?=?1, 4–8) were found. Besides that, there are gold chloride clusters with general formula [AuH_r(HCl)₂]^?(H₂O)_m (m?=?1–5; 8–9; r?=?0–2) in this part of spectrum. In the second part of spectrum, the m/z range 447–795, only gold chloride clusters were obtained. Their general formulae can be written as [AuCl_t(HCl)_v]^?(H₂O)_m (t?=?1–4; v?=?5–8; m?=?2–4, 6–8) and [Au_n(HCl)_v]^?(H₂O)_m (n?=?1–2, v?=?4–5, m?=?1–2, 5, 7). The analysis of concentration effects on the LDI mass spectra of gold clusters reveals that the relative intensities of signals for the mono- and dinuclear Au clusters increase with decreasing the concentration of water HAuCl₄ solutions.     

Photon-assisted synthesis of C60 polymers by laser irradiation

The harmonics of a free electron laser (FEL) were irradiated in vacuum to surfaces of compressed C₆₀ and a mixture of C₆₀ and I₂. The power and frequency of the fundamental FEL macro-pulse were ca. 0.5 mJ/pulse and 2 Hz, respectively. The irradiation time was 120-180 min. After irradiation of FEL with a typical wavelength of 450 or 345 nm, the Raman peak of Ag(2)-derived vibration mode of C₆₀ shifted to the lower-energy side. The Raman peak shift of the mixture powder sample was greater than that of pure C₆₀. Furthermore, changes of the crystalline structure indicated that various intermolecular combinations occurred by irradiation. These results strongly suggest that three-dimensional polymerization of C₆₀ was promoted by laser irradiation and the effect of photon-assisted hole-doping from iodine atoms to C₆₀ molecules.     

Enhancement and modulation of terahertz radiation by multi-color laser pulses

We study theoretically intense terahertz radiation from multi-color laser pulse with uncommon frequency ratios. Comparing the two-color laser scheme, of which the uncommon frequency ratio should be set to be a specific value, we show that by using multi-color harmonic laser pulses as the first pump component, the lasers as the second pump component can be adjusted in a continuous frequency range. Moreover, these multi-color laser pulses can effectively modulate and enhance the terahertz radiation, and the terahertz yield increases with the increase of the wavelength of the uncommon pump component and is stable to the laser relative phase. Finally, we utilize the electron densities and velocities of ionization events to illustrate the physical mechanism of the intense terahertz generation.     

Contribution of vaporization and boiling to thermal-spike sputtering by ions or laser pulses

Here we consider what, in our terminology, we designate as normal vaporization, normal boiling, and phase explosion. In the case of vaporization, one is dealing with the emission of particles (atoms or molecules) from the extreme outer surface of either a solid or liquid for any temperature exceeding 0 K. In the case of boiling, one is (at least ideally) dealing with heterogeneously nucleated bubbles which diffuse to the outer surface of a liquid or solid and then escape, the latter being possible for temperatures equal to or exceeding the boiling temperature (T(b)). In the case of phase explosion one is dealing with the consequences of what happens when a liquid approaches the thermodynamic critical temperature (T(tc) or T(c)), and massive homogeneous nucleation takes place. Although these three mechanisms have been reviewed in reasonable detail in recent work, we will here present evidence, apparently not previously considered, that boiling, whether the distance scale is atomically small (5-15 nm, as for laser-pulse impact on a metal in the absence of thermal diffusion) or much larger, has a prohibitive kinetic obstacle because it requires bubble diffusion if the bubbles are formed other than at the outer surface. That is to say, boiling will never be a significant process whether with ion or laser-pulse impact. This leaves vaporization and phase explosion as the only possible thermal-spike processes capable of expelling material from an ion- or laser-pulse bombarded surface in a significant quantity. But even with vaporization it can be shown that a kinetic obstacle, although not as severe as for boiling, will enter. The final result is that only phase explosion will normally be relevant for sufficiently short time scales.     

Active manipulation of the selective alignment by two laser pulses

This paper solves numerically the full time-dependent Schrdinger equation based on the rigid rotor model, and proposes a novel strategy to determine the optimal time delay of the two laser pulses to manipulate the molecular selective alignment. The results illustrate that the molecular alignment generated by the first pulse can be suppressed or enhanced selectively, the relative populations of even and odd rotational states in the final rotational wave packet can be manipulated selectively by precisely inserting the peak of the second laser pulse at the time when the slope for the alignment parameter by the first laser locates a local maximum for the even rotational states and a local minimum for the odds, and vice versa. The selective alignment can be further optimised by selecting the intensity ratio of the two laser pulses on the condition that the total laser intensity and pulse duration are kept constant.     

Enhancement of temporal contrast of high-power femtosecond laser pulses using two saturable absorbers in the picosecond regime

We characterized the transmission properties of a color-glass-filter (RG850) saturable absorber (SA) in a wide range of pulse durations (from 25 fs to 5 ps). The transmission properties were strongly related to the energy fluence, pulse duration, and chirp parameter. On the basis of these properties, the input pulse duration, chirp parameter, and energy fluence were optimized to maintain the width of the transmitted laser spectrum as much as possible with minimal energy loss. We demonstrated that, by transmitting a positively chirped 2.8-ps laser pulse to two identical SAs at an energy fluence of 15 mJ/cm², the temporal contrast ratio of the main pulse to the amplified spontaneous emission was enhanced by 4 orders of magnitude without any significant energy loss or strong spectral narrowing in a 10-Hz, 100-TW femtosecond laser system.     

双色高频激光作用下原子低阶次谐波的理论研究

通过数值求解含时薛定谔方程,研究了高频双色激光脉冲与原子相互作用产生的光辐射.研究表明,光辐射谱中既有基频光的谐波,又可观测到谐波能量附近的多个频率的光辐射产生,且辐射的峰值强度随着入射激光强度的提高呈指数增强,相邻辐射频率差值为入射的两束激光脉冲频率差.     

Self-organization of thin metal films by irradiation with nanosecond laser pulses

Modifications in thin metal films under intensive laser irradiation were studied. Gold, silver, copper, chromium and aluminum films with the thickness of 100 nm were deposited on the glass substrate. Back-side irradiation through the substrate with a burst of nanosecond pulses tightly focused to a line was applied. The film removal threshold with a single pulse F_th was estimated for every material and laser fluence was kept above it in the range of 1.5-3 F_th during experiments. Diverse behavior of the films depending on the metal, the shift between pulses and laser fluence was observed. In chromium, the regular structures were developed in a quite wide range of processing parameters. In gold, three kinds of ripples were observed: transverse (similar to ripples in chromium), longitudinal and a structure of ripples oriented at 60° to each other. The combination of physical properties facilitated the regular assembly of the molten metal in chromium and to some extent in gold.     

On the interaction of femtosecond laser pulses with cluster targets

The heating of clusters by femtosecond laser pulses is studied theoretically and experimentally. Both the formation of a cluster target and the results of experimental studies of the cluster plasma by the methods of X-ray emission spectroscopy are considered. A numerical model of cluster formation in a supersonic gas jet is proposed. It is shown that detailed studies of two-phase gas-dynamic processes in a nozzle forming the jet give the spatial distributions of all parameters required for the correct calculation of the cluster heating by short laser pulses. Calculations of nozzles of different configurations show that in a number of cases an almost homogeneous cluster target can be formed, whereas in other cases the distributions of parameters prove to be not only inhomogeneous but also even nonmonotonic. A simple physical model of the plasma production by a femtosecond laser pulse and a picosecond prepulse is proposed. It is shown that a comparison of X-ray spectra with detailed calculations of the ion kinetics makes it possible to determine the main parameters of the plasma being produced.     

Dependence of the efficiency of formation of Ba2+ ions by the two-electron mechanism in two laser fields on the laser radiation intensity

The mechanism of formation of doubly charged ions upon nonlinear ionization of Ba atoms as a result of the simultaneous action of yttrium-aluminum garnet laser radiation and its second harmonic is investigated. It is shown that this mechanism is two-electron. The experimental data are compared with the results of the calculation carried out for Gaussian laser beams.     

X-ray production by irradiation of solid targets with sub-picosecond excimer laser pulses

A table-top excimer laser system generating sub-ps pulses was used to irradiate solid targets at intensities of up to 4×10¹⁵ W/cm². Soft X-ray spectra of various materials were measured. The X-ray conversion efficiencies were between 1–5%. Streak camera measurements showed instrument-limited X-ray pulse duration of a few ps.Partially based on the plenary talk X-Ray Generation by Sub-Picosecond UV Laser by F. P. Schäfer, G. Kühnle, S. Szatmári, and M. Steyer, presented at the XVI Int. Quant. Electron. Conf., Tokio, July 18–21, 1988, Technical Digest (The Japan Society of Applied Physics) p. 2     

Distributions of ions in a cluster plasma created by a laser pulse

Energy and charge distributions of ions are calculated for a cluster beam irradiated by a high-power ultrashort laser pulse. It is shown that the self-consistent field of a cluster ionized by the laser beam strongly affects the characteristics of the ion distributions obtained after the cluster explodes. The mean concentration of atoms bound into clusters in a beam, the cluster size distribution, and the focal-spot diameter are found to have a weak effect on both energy and charge distributions of the ions, whereas the energy spectrum of the produced ions is determined by the mean cluster size.     

Observation of enhanced field-free molecular alignment by two laser pulses

We show experimentally that field-free alignment of iodobenzene molecules, induced by a single, intense, linearly polarized 1.4-ps-long laser pulse, can be strongly enhanced by dividing the pulse into two optimally synchronized pulses of the same duration. For a given total energy of the two-pulse sequence the degree of alignment is maximized with an intensity ratio of 1:3 and by sending the second pulse near the time where the alignment created by the first pulse peaks.     

Generation of multiply charged silicon and germanium ions by excimer laser pulses

Plasma bursts were produced by focusing excimer-laser (XeCl, 308 nm) pulses on Ge and Si targets. At moderate laser fluences (30 MW/cm²) high-intensity Ge³⁺ and Si³⁺ ion pulses were extracted from the laser-produced plasma. A peculiar electrical circuit allows a self-bunching of the beam. By time-of-flight method, the currents produced by the ions of different charge number were measured. Peak currents of 620 mA and 800 mA were recorded for Ge³⁺ and Si³⁺ ions, respectively, with an extraction voltage of only 400 V.     

Enhancement of proton acceleration by hot-electron recirculation in thin foils irradiated by ultraintense laser pulses

MeV-proton production from solid targets irradiated by 100-fs laser pulses at intensities above 1x10(20) W cm(-2) has been studied as a function of initial target thickness. For foils 100 microm thick the proton beam was characterized by an energy spectrum of temperature 1.4 MeV with a cutoff at 6.5 MeV. When the target thickness was reduced to 3 microm the temperature was 3.2+/-0.3 MeV with a cutoff at 24 MeV. These observations are consistent with modeling showing an enhanced density of MeV electrons at the rear surface for the thinnest targets, which predicts an increased acceleration and higher proton energies.     

利用两束同色激光场和半周期脉冲驱动原子产生单个阿秒脉冲

利用分裂算符方法数值求解一维氦原子的含时薛定谔方程,研究了氦原子在两束同色激光场与半周期脉冲(Hcps)形成的组合场驱动下所发射高次谐波的特点.研究结果表明,氦原子在这种组合场驱动下,高次谐波谱的平台区域能得到很大的扩展,其截止位置可延伸到I_P+9.6U_P,通过构造截止位置附近的高次谐波谱能够得到脉宽为63 as的单个阿秒脉冲.经过分析后发现,半周期脉冲的加入不仅使高次谐波谱平台能够得到扩展,同时还抑制了电子长路径对高次谐波的贡献. 关键词： 两束同色激光场 半周期脉冲 高次谐波 阿秒脉冲     

Optical limiting of high-repetition-rate laser pulses by carbon nanofibers suspended in polydimethylsiloxane

The mechanism of optical unidirectional (OUD) transmission in parallel subwavelength dual-metal gratings was investigated. It was found that this kind of OUD phenomenon originates from the coupling of the surface plasmon polaritons (SPPs) between the front grating and a layer of metal film which replaces the rear grating. The higher the intensity of the coupled SPPs at the entrances of the rear grating, the higher the transmittance can be achieved. Basing on this property, an effective OUD example was achieved by exploring the intensity difference at the entrances of the rear gratings between the two incidences of opposite directions. In this kind of OUD, the positive transmittance can exceed 80 % and the difference between the transmittances of the two opposite directions can be as large as 63 %. The detailed design process was also presented.     

Enhancement of laser ablation yield by two color excitation

This work studied the implementation of multi-wavelength laser diodes on a single substrate through the position-dependent bandgap modification of the In_0.2Ga_0.8As/ GaAs multiple quantum wells (MQWs) by impurity-free vacancy disordering (IFVD). The position-dependent bandgap modification is achieved by performing the IFVD with SiO_x capping layers with different stoichiometries. The lasing wavelength difference of about 31 nm was obtained between the ridge-waveguide laser diodes fabricated with the MQWs that had undergone the same thermal treatments using the SiO_x film provided with SiH₄ flow rates of 20 and 300 sccm. The device performance was not appreciably degraded in the laser diodes fabricated with the MQWs that had undergone the IFVD process . PACS 68.65.Fg; 78.55.-m; 42.55.Px