Laser-cluster Interaction Research Articles

The radial dimension of a supersonic jet expansion from conical nozzle

In a laser-cluster interaction experiment, the radial dimension of a supersonic gas jet is an important parameter for the characterization of interaction volume. It is noted that due to the lateral gas expansion, the diameter of a supersonic gas jet is larger than the idealized diameter of a gas jet from a conical nozzle. In this work the effect of the lateral expansion on the radial dimension of gas jet was investigated by simulations. Based on the simulation results, the diameter of gas jet l was compared in detail with the corresponding diameter lT in the idealized straight streamline model and the diameter lH at a half of maximum atom density of gas jet. The results reveal how the deviation of l from lT (lH) changes with respect to the opening angles of conical nozzles, the heights above the nozzle, the nozzle lengths and the gas backing pressures. It is found that the diameter of gas jet l is close to the idealized diameter lT and lH in the case where a long conical nozzle with a large opening angle is used under a low gas backing pressure. In this case, the effect of the lateral expansion is so weak that the edge of gas jet becomes sharp and the radial distribution of atom density in gas jet tends to be uniform. The results could be useful for the characterization of a supersonic gas jet.

Thermal and log-normal distributions of plasma in laser driven Coulomb explosions of deuterium clusters

In this work, we explore the possibility that the motion of the deuterium ions emitted from Coulomb cluster explosions is highly disordered enough to resemble thermalization. We analyze the process of nuclear fusion reactions driven by laser–cluster interactions in experiments conducted at the Texas Petawatt laser facility using a mixture of D2+3He and CD4+3He cluster targets. When clusters explode by Coulomb repulsion, the emission of the energetic ions is “nearly” isotropic. In the framework of cluster Coulomb explosions, we analyze the energy distributions of the ions using a Maxwell–Boltzmann (MB) distribution, a shifted MB distribution (sMB), and the energy distribution derived from a log-normal (LN) size distribution of clusters. We show that the first two distributions reproduce well the experimentally measured ion energy distributions and the number of fusions from d–d and d-3He reactions. The LN distribution is a good representation of the ion kinetic energy distribution well up to high momenta where the noise becomes dominant, but overestimates both the neutron and the proton yields. If the parameters of the LN distributions are chosen to reproduce the fusion yields correctly, the experimentally measured high energy ion spectrum is not well represented. We conclude that the ion kinetic energy distribution is highly disordered and practically not distinguishable from a thermalized one.

Terahertz emission during interaction of ultrashort laser pulses with gas cluster beam

We present the results of experimental study of terahertz (THz) generation in gas cluster beam excited by intense femtosecond laser pulses. Cluster beam was produced by partial condensation of pure Ar and mixtures CF2Cl2+He, Ar+He during their expansion through a conical nozzle into vacuum. There were used two excitation schemes in our experiments: single color and two color (fundamental frequency mixed with its second harmonic). We have studied how THz signal scales with various control parameters such as laser pulse duration, gas backing pressure and laser pulse energy. Simultaneously we measured intensity of X-Ray emission which originates from laser-cluster interaction. We found that in a single color scheme energy of THz pulses from Ar cluster beam strongly decreases in the region of minimum laser pulse duration while X-Ray power is maximal under these conditions. Both in single- and two color excitation regimes THz signal demonstrated growth without saturation with increasing of optical pulse energy up to its peak value of 25 mJ.

Ultraintense laser-cluster interactions: Effects of the cluster shape

The disintegration of nonicosahedral rare-gas clusters in ultraintense extreme ultraviolet pulses is studied. The clusters quickly form a nanoplasma and evolve only according to the nanoplasma's dynamics, which are determined predominately by the cluster's initial shape. It is found that the cluster's disintegration follows a simple model that is well predicted using only the initial structure. The main finding is that the ions disintegrate tangentially from the surface of the cluster's overall shape. In ellipsoidal clusters, the work done on the ions near the semiminor axis by the other particles (ions and electrons) is larger than the work done on the ions near the semimajor axis. This leads to an inversion of the ellipsoidal axes due to the different axes expanding at different rates.

Simultaneous streak and frame interferometry for electron density measurements of laser produced plasmas.

A system of two collinear probe beams with different wavelengths and pulse durations was used to capture simultaneously snapshot interferograms and streaked interferograms of laser produced plasmas. The snapshots measured the two dimensional, path-integrated, electron density on a charge-coupled device while the radial temporal evolution of a one dimensional plasma slice was recorded by a streak camera. This dual-probe combination allowed us to select plasmas that were uniform and axisymmetric along the laser direction suitable for retrieving the continuous evolution of the radial electron density of homogeneous plasmas. Demonstration of this double probe system was done by measuring rapidly evolving plasmas on time scales less than 1 ns produced by the interaction of femtosecond, high intensity, laser pulses with argon gas clusters. Experiments aimed at studying homogeneous plasmas from high intensity laser-gas or laser-cluster interaction could benefit from the use of this probing scheme.

Investigation of the on-axis atom number density in the supersonic gas jet under high gas backing pressure by simulation

The supersonic gas jets from conical nozzles are simulated using 2D model. The on-axis atom number density in gas jet is investigated in detail by comparing the simulated densities with the idealized densities of straight streamline model in scaling laws. It is found that the density is generally lower than the idealized one and the deviation between them is mainly dependent on the opening angle of conical nozzle, the nozzle length and the gas backing pressure. The density deviation is then used to discuss the deviation of the equivalent diameter of a conical nozzle from the idealized deq in scaling laws. The investigation on the lateral expansion of gas jet indicates the lateral expansion could be responsible for the behavior of the density deviation. These results could be useful for the estimation of cluster size and the understanding of experimental results in laser-cluster interaction experiments.

Mass spectrometric and charge density studies of organometallic clusters photoionized by gigawatt laser pulses.

Clusters on exposure to nanosecond laser pulses of gigawatt intensity exhibit a variety of photo-chemical processes such as fragmentation, intracluster reaction, ionization, Coulomb explosion, etc. Present article summarizes the experimental results obtained in our laboratory utilizing time-of-flight mass spectrometer which deal with one such aspect of cluster photochemistry related to generation of multiply charged atomic ions upon excessive ionization of cluster constituents (Coulomb explosion) at low intensity laser field (∼109 W/cm2 ). To understand the mechanism of laser-cluster interaction, laser as well as cluster parameters were varied. Mass spectrometric studies were carried out at different laser wavelength as well as varying the nature of cluster constituents, backup pressure, nozzle diameter, etc. In addition, charge density measurements were also preformed to get information about the total number of ions generated upon laser-cluster interaction as a function of laser wavelength. In case of pure molecular clusters, the charge state of atomic ions as well as charge density was observed to enhance with increasing laser wavelength, signifying efficient coupling of the cluster medium with nanosecond laser pulse at longer wavelength. While in case of clusters doped with species having comparatively lower ionization energy, the efficiency of laser-cluster interaction was less, in contrast to studies carried out using femtosecond lasers. Results obtained in the present work have been rationalized on the basis of proposed three-stage cluster ionization mechanism, that is, multiphoton ionization ignited-inverse Bremsstrahlung heating and electron ionization. © 2015 Wiley Periodicals, Inc. Mass Spec Rev. 36:188-212, 2017.

Absorption-Ablation-Excitation Mechanism of Laser-Cluster Interactions in a Nanoaerosol System

The absorption-ablation-excitation mechanism in laser-cluster interactions is investigated by measuring Rayleigh scattering of aerosol clusters along with atomic emission from phase-selective laser-induced breakdown spectroscopy. For 532nm excitation, as the laser intensity increases beyond 0.16 GW/cm^{2}, the scattering cross section of TiO_{2} clusters begins to decrease, concurrent with the onset of atomic emission of Ti, indicating a scattering-to-ablation transition and the formation of nanoplasmas. With 1064nm laser excitation, the atomic emissions are more than one order of magnitude weaker than that at 532nm, indicating that the thermal effect is not the main mechanism. To better clarify the process, time-resolved measurements of scattering signals are examined for different excitation laser intensities. For increasing laser intensity, the cross section of clusters decreases during a single pulse, evincing the shorter ablation delay time and larger ratios of ablation clusters. Assessment of the electron energy distribution during the ablation process is conducted by nondimensionalizing the Fokker-Planck equation, with analogous Strouhal Sl_{E}, Peclet Pe_{E}, and Damköhler Da_{E} numbers defined to characterize the laser-induced aerothermochemical environment. For conditions where Sl_{E}≫1, Pe_{E}≫1, and Da_{E}≪1, the electrons are excited to the conduction band by two-photon absorption, then relax to the bottom of the conduction band by electron energy loss to the lattice, and finally serve as the energy transfer media between laser field and lattice. The relationship between delay time and excitation intensity is well correlated by this simplified model with quasisteady assumption.

A validation of a simple model for the calculation of the ionization energies in X-ray laser-cluster interactions

The outer-ionization of an electron from a cluster is an unambiguous quantity, while the inner-ionization threshold is not, resulting in different microscopic quantum-classical hybrid models used in laser-cluster interactions. A simple local ionization threshold model for the change in the ionization energy is proposed and examined, for atoms and ions, at distances in between the initial configuration of the cluster to well into the cluster's disintegration. This model is compared with a full Hartree-Fock energy calculation which accounts for the electron correlation effects using the coupled cluster method with single and double excitations with perturbative triples (CCSD(T)). Good agreement is found between the two lending a strong theoretical support to works which rely on such models for the final and transient properties of the laser-cluster interaction.

Ionisation of methyl iodide clusters using nanosecond laser pulses: detection of multiply charged positive ions, negative ions and energetic electrons

Methyl iodide clusters have been ionised using laser pulses of intensity ∼109 W cm−2 at 266, 355, 532 and 1064 nm and the ions produced in the laser–cluster interaction were analysed with a time-of-flight mass spectrometer.

Efficient coupling of nanosecond laser pulses with the cluster medium: Generation of hydrogen-like [C](5+) atomic ions.

Clusters exhibit diverse photochemical behavior as a function of laser parameters, i.e. wavelength, pulse duration and intensity. One such aspect of cluster photochemistry is the generation of energetic multiply charged atomic ions, upon efficient interaction of clusters with intense laser pulses. In the present work, mass spectrometric investigations have been carried out on clusters of tetrahydrofuran (THF, C4 H8 O) - a saturated cyclic ether - subjected to nanosecond laser pulse (spanning from UV to IR wavelength range) with the aim of shedding light on the complex mechanism of laser-cluster interactions, which is still ambiguous. THF clusters, generated via supersonic expansion of room-temperature THF vapours seeded in argon, were subjected to gigawatt intensity laser pulses (355, 532 and 1064 nm) obtained from a nanosecond Nd:YAG laser. The ions generated upon laser-cluster interaction were characterized using a time-of-flight mass spectrometer. At 355 nm, THF clusters exhibit the usual multiphoton dissociation/ionization behavior while, at 532 nm, observation of multiply charged atomic ions of carbon (up to [C](4+) ) and oxygen (up to [O](3+) ) was ascribed to Coulomb explosion of THF clusters. For studies carried out at 1064 nm, multiply charged atomic ions of carbon up to [C](5+) having an ionization energy of ~392 eV were observed, at a laser intensity of 10(10) W/cm(2) . The observation of [C](5+) atomic ions signifies efficient coupling of the laser energy with the cluster medium, using a nanosecond laser pulse. The results have been rationalized on the basis of a three-stage cluster ionization mechanism, suggesting the crucial role of the threshold laser intensity for initiating ionization within the cluster and generation of optimum charge centers for efficient extraction of energy from the laser pulse.

Interaction of high intensity laser with non-uniform clusters and enhanced X-ray emission

Laser irradiated clusters with non-uniform density variation are shown to broaden surface plasmon resonance very significantly. As the clusters get heated and expand hydro-dynamically, the Bremsstrahlung X-ray emission yield passes through a maximum in time. The maximum yield decreases with increase in non-uniformity in the electron density inside the clusters. At higher laser intensity, the nonlinearity in laser cluster interaction may arise even prior to electron heating, via the relativistic mass variation and the nonlinear restoration force on electrons. For clusters with radius less than one tenth of the laser wavelength, the restoration force nonlinearity dominates.

KeV electron heating in laser-cluster interaction probed by X-ray and electron spectroscopy

The interaction of intense laser pulses with nanoscopic rare-gas clusters provides a testing ground for laser-atom interaction at solid-state densities. We investigate the driven electronic dynamics on the femtosecond time scale both experimentally and theoretically using two complementary observables: the laser intensity dependence of characteristic X-ray emission and of high-energy (keV) electron spectra.

Evidence for charge-induced dipole reaction in laser ionized van der Waals clusters: a case of Fe2+ reacting with argon atoms inside a cluster

Fe(CO)5-doped argon clusters have been photo-ionized by 532 nm, nanosecond laser pulses and the ions generated as a result of laser–cluster interaction were detected using time-of-flight mass spectrometry.

Generation of multiply charged tin and carbon ions in low intensity Coulomb explosion of tetramethyl tin clusters: Role of screening effects

Present work reports multiple ionization and subsequent Coulomb explosion of tetramethyl tin clusters induced by gigawatt intensity laser pulses. The time-of-flight mass spectra and charge density measurements revealed that the efficiency of laser–cluster interaction depends on ionization wavelength. Extent of energy absorbed from optical pulses by the cluster medium was found to increase with laser wavelength. Experimental results obtained in the present study have been rationalized on the basis of three stage cluster ionization model, i.e. multiphoton ionization ignited – inverse bremsstrahlung heating (IBS) – electron ionization. In addition to the experiments, theoretical calculations have been performed to account for screening effects in clusters. Our calculations suggest that the charged particles, generated upon initial multiphoton ionization of cluster constituents, significantly lower the Coulombic barrier of atoms/molecules that are present in its vicinity. This, in turn, decreases the ionization energy of cluster constituents, during subsequent steps of ionization, ultimately increasing the ionization level of clusters via the process of enhanced ionization due to ion shielding. Thus, above calculations predict dominant role of screening effects in evolution of higher charge state atomic ions during laser–cluster interaction.

Parallel implementation of three-dimensional molecular dynamic simulation for laser-cluster interaction

The objective of this article is to report the parallel implementation of the 3D molecular dynamic simulation code for laser-cluster interactions. The benchmarking of the code has been done by comparing the simulation results with some of the experiments reported in the literature. Scaling laws for the computational time is established by varying the number of processor cores and number of macroparticles used. The capabilities of the code are highlighted by implementing various diagnostic tools. To study the dynamics of the laser-cluster interactions, the executable version of the code is available from the author.

Xe K-shell X-ray generation using conical nozzle and 25 TW laser

AbstractTo increase X-ray photon number generated by laser-cluster interaction, it is important to understand the dependence of X-ray generation on cluster size. We carried out Xe K-shell X-ray generation using a conical nozzle with Xe clusters, the radius of which was controllable by adjusting the backing pressure. The experiment clarifies the result that the Xe K-shell X-ray photon number increases with increasing cluster radius from 8 to 12 nm, and saturates at the radius between 12 and 17 nm. We also investigated the Xe K-shell X-ray photon number dependence on laser intensity, and found that the threshold laser intensity of the Xe K-shell X-ray generation exists between 2 × 1017 and 5 × 1018 W/cm2.

Photoionization of atomic and molecular clusters doped with low ionization energy molecules: Effect of laser wavelength, intensity and cluster composition

Photoionization of (Ar)l, (Xe)m, (SF6)n and (Xe)m (SF6)n cluster systems, each doped with Fe(CO)5, has been studied at selected laser wavelength ranging from UV to IR region using time-of-flight mass spectrometer (TOFMS). In addition to TOFMS studies, charge density measurements were carried out to quantify the total ion yield generated during the laser–cluster interaction studies. In the first set of experiments, doped clusters of identical composition were irradiated with laser pulses of different wavelength. In this case the charge state of atomic ions was observed to increase with increasing laser wavelength while the total ion yield was found to decrease. Results obtained in the present study have been rationalized on the basis of the expansion dynamics of charged clusters which depends on the number of charge centers generated during initial multiphoton ionization step as well as on the binding energy of cluster constituents. In the second set of experiments, clusters of different composition were irradiated with laser pulses of identical wavelength and intensity. In this case doped molecular clusters were observed to interact more efficiently with optical pulses as compared to doped atomic clusters. This was attributed to higher binding energy of the constituents within the doped molecular clusters which reduces the rate of expansion of charged clusters thereby enhancing the efficiency of laser–cluster interaction.

Optimization of the neutron yield in fusion plasmas produced by Coulomb explosions of deuterium clusters irradiated by a petawatt laser

The kinetic energy of hot (multi-keV) ions from the laser-driven Coulomb explosion of deuterium clusters and the resulting fusion yield in plasmas formed from these exploding clusters has been investigated under a variety of conditions using the Texas Petawatt laser. An optimum laser intensity was found for producing neutrons in these cluster fusion plasmas with corresponding average ion energies of 14 keV. The substantial volume (1-10 mm(3)) of the laser-cluster interaction produced by the petawatt peak power laser pulse led to a fusion yield of 1.6×10(7) neutrons in a single shot with a 120 J, 170 fs laser pulse. Possible effects of prepulses are discussed.

Correlation between X-ray yield and electron spectra in laser-cluster interaction

Measurements of electron energy distribution obtained for several experimental conditions show evidence for a strong correlation between X-ray yields and the high energy part of the electron spectra in laser-cluster interaction.