In this letter, we discuss the increase in the average cluster size by lowering the stagnation temperature of the methane (CH4) gas. The Coulomb explosion experiments are conducted to estimate the cluster size and the size distribution. The average CH4 cluster sizes Nav of 6 230 and 6 580 are acquired with the source conditions of 30 bars at 240 K and 60 bars at 296 K, respectively. Empirical estimation suggests a five-fold increase in the average size of the CH4 clusters at 240 K compared with that at room temperature under a backing pressure of 30 bars. A strong nonlinear Hagena parameter relation (\Gamma *\propto T^{-3.3}) for the CH4 clusters is revealed. The results may be favorable for the production of large-sized clusters by using gases at low temperature and high back pressures.

We use an electrostatic model to study the average kinetic energy of ions ejected from the pure Coulomb explosions of methane clusters (CA4)n (light atom A=H and D). It is found that the ratio of the average kinetic energy of the ions to their initial average electrostatic potential energy is irrelevant to the cluster size. This finding implies that as long as the ratio is given, the average kinetic energies of the ions can be simply estimated from their initial average electrostatic potential energies, rather than from the time-consuming simulations. The ratios for the different charge states of carbon ions are presented.

A simplified Coulomb explosion model is presented for the analysis of the explosion dynamics of hydrogen clusters driven by an ultrashort intense laser pulse. The scaling of the proton kinetic energy with cluster size has been studied in detail based on this model. It is found that the maximum kinetic energy the protons acquire in the laser-cluster interaction rises to a peak and then decreases slightly as the cluster size increases, which can be explained very well by investigating the temporal evolution of outer ionization rate of different-size clusters. It is also indicated that there exists an optimum cluster size to maximize the proton energy for given laser parameters. Moreover, taking the cluster-size distribution as a log-normal function distribution into account, the maximum proton energy increases sharply with the cluster size and then levels off before beginning to fall slowly. The inclusion of a cluster-size distribution into the simulations considerably improves the fit with experimental data. These discussions are useful for the optimum-match determination of laser-cluster parameters to obtain the maximum proton energy in experiments.

An investigation of the cluster size dependence of the maximum energy of protons ejected from explosion of methane clusters in an intense femtosecond laser field has been conducted on the basis of the cluster size estimation by Rayleigh scattering measurements. The interaction of a 2*10^(16)-W/cm2 intense laser pulse (790 nm, 60 fs) with the methane clusters revealed that the clusters were Coulomb exploded and the maximum energy (E_(max)) of the protons produced was linearly proportional to the square of the cluster radius (r_(c)^(2)). In a cluster size range, with the methane cluster radii up to about 3 nm, the established relation of E_(max) and r_(c)^(2) was found to be E_(max) (keV) = 3.3+0.75r_(c)^(2) (nm2), in good agreement with the simulation results. This demonstrated that Coulomb explosion of ionic clusters (C^(+4)H^(+)_(4))_(n) took place following the cluster vertical ionization in the laser-cluster interaction.

A deuterium cluster jet produced in the supersonic expansion into vacuum of deuterium gas at liquid nitrogen temperature and moderate backing pressures is studied by Rayleigh scattering techniques. The experimental results show that deuterium clusters can be created at moderate gas backing pressures ranging from 8 to 23 bar, and a maximum average cluster size of 350 atoms per cluster is estimated. The temporal evolution of the cluster jet generated at the backing pressure of 20 bar demonstrates a two-plateau structure. The possible mechanism responsible for this structure is discussed. The former plateau with higher average atom and cluster densities is more suitable for the general laser-cluster interaction experiments.

The interaction of intense femtosecond laser pulses with rare gas clusters was studied experimentally, the time-of-flight spectra of ions from exploding clusters at different gas densities have been measured. It is found that while the relative components of ions in low and high energy of the ion energy spectrum decrease with the increase of the gas density, the average ion energies are the same for different gas densities, which indicates that the effect of gas density on laser-cluster interaction is not important under our experimental conditions.

Noble gases (argon, krypton, and xenon) are puffed into vacuum through a nozzle to produce clusters for studying laser-cluster interactions. Good estimates of the average size of the argon, krypton and xenon clusters are made by carrying out a series of Rayleigh scattering experiments. In the experiments, we have found that the scattered signal intensity varied greatly with the opening area of the pulsed valve. A new method is put forward to choose the appropriate scattered signal and measure the size of Kr cluster.

本文详细讨论了一种在空间直接显示原子的超精细结构的方法,并观察到Na原子3~2P(1/2)态的两个超精细能级结构。发散的片状原子束和发散的片状激光束在空间相互作用,当多普勒频移等于原子跃迁频率和激光器频率之差时,原子被激发并在空间发光。计算和实验都已证实发光轨迹是一圆弧。荧光带的数目表明起精细能级的数目,根据空间位置可定出能级间隔,荧光带的宽度反映原子束的速度分布,实验研究了钠原子的3~2S(1/2)—3~2P(1/2)的荧光轨迹,照片显示了两个清楚分开的荧光带,带的间距和计算结果相符。