Explosion, ion acceleration, and molecular fragmentation of methane clusters in the pulsed beam of a free-electron laser

Abstract

X-ray lasers offer new possibilities for creating and probing extreme states of matter. We used intense and short X-ray pulses from the FLASH soft X-ray laser to trigger the explosions of CH4 and CD4 molecules and their clusters. The results show that the explosion dynamics depends on cluster size, and indicate a transition from Coulomb explosion to hydrodynamic expansion in larger clusters. The explosion of CH4 and CD4 clusters shows a strong isotope effect: the heavier deuterons acquire higher kinetic energies than the lighter protons. This may be due to an extended inertial confinement of deuterons vs. protons near a rapidly charging cluster core during exposure. [5,6] and solid samples [7][8][9]. In this paper we follow the X-ray-induced explosion of methane (CH 4 ) and heavy methane (CD 4 ) clusters and compare the results to the explosion of the individual molecules. Clusters are eminently suitable to study size-dependent phenomena in the explosion dynamics, and we employed clusters, containing 1000 to 250000 molecules and corresponding to sizes of 4 -32 nm diameter. Molecular clusters bridge the gap between isolated molecules and bulk condensed material. An understanding of ultrafast explosion dynamics in clusters of various dimensions has relevance to imaging by "diffraction before destruction" [10], plasma physics, and fusion research [11,12].The target response to an intense X-ray pulse depends on the wavelength, the intensity of the field, and the size of the sample. Photoionization liberates electrons, which can escape from small samples, and these positively charged objects undergo a Coulomb explosion. The probability of electrons escaping from a cluster depends on its size [13][14][15]. In larger objects, photoelectrons elicit secondary electron cascades and these create additional ionizations. As the overall positive charge of the sample increases during the pulse, photoelectrons will not be able to escape the growing positive potential [13,16]. Trapped photoelectrons create more cascades and the electrons move inwards to neutralize the core of the cluster, leaving behind a positively charged outer layer, which then peels off [17][18][19][20][21]. As a result, large samples burn from the outside inwards. The core of such large objects expands hydrodynamically due to the growing electron pressure within [17,18]. Theoretical studies predict a * Electronic address: nicusor@xray.bmc.uu.se transition from Coulomb explosion to hydrodynamic expansion with increasing sample size [13].The study of Coulomb explosion of molecular clusters with intense infrared lasers has shown the advantage of using heteronuclear molecules, such as CH 4 or CD 4 , to analyze the dynamics of the explosion [22,23]. In a heteronuclear cluster, the light ions can gain additional energy during the explosion by interacting with multiply charged heavy ions (C n+ ). This kinematic effect would be indicative of a Coulomb process, while in a hydrodynamics expansion all the cluster constituents are expected to gain the same energy. We ...

Explosion Dynamics of Methane Clusters Irradiated by 38 nm XUV Laser Pulses

“…The transmitted XUV pulses were then focused into another gas jet (the cluster target) by a spherical Si/Sc mirror, designed to reflect preferentially the 21 st harmonic. size of N = 70, 000 molecules, in a good agreement with previous experiments [13,14].…”

“…Previous experimental investigations into intense, high-frequency laser-cluster interactions have relied on free-electron lasers as the photon source. Several experiments at the free-electron lasers FLASH (DESY facility) [13,14] and LCLS (SLAC facility) [15] used different intense soft X-ray energies (92 eV at FLASH, and 850 eV at LCLS) to study the explosion dynamics of CH 4 and CD 4 as well as xenon clusters. The results of the FLASH experiment showed that the explosion dynamics depend on the cluster size, and indicated a transition from Coulomb to a hydrodynamic explosion as the cluster size increased, while the SLAC experiment demonstrated the formation of a xenon nanoplasma that exploded hydrodynamically.…”

Time-resolved x-ray imaging of a laser-induced nanoplasma and its neutral residuals

“…Gas phase clusters are ideal systems to investigate the interaction between intensive light pulses and matter and in particular to follow the underlying processes of the formation and control of highly excited plasma states [1,2]. It is of fundamental interest to understand those many-particle dynamics [3][4][5][6][7][8][9][10] but it has also interesting applications ranging from surgery [11] and controlled material processing [12] over strategies for supressing radiation damage in single-shot diffractive x-ray imaging of biomolecules [13][14][15] to the generation of neutron pulses by the ignition of nuclear fusion in deuterium clusters [16]. Hence for example in time-resolved measurements and calculations on clusters in intense IR pulses the significance of collective heating processes was found [17][18][19], which are the underlying principle for the highly efficient energy absorption of clusters in this wavelength regime [20].…”

CO2exploding cluster dynamics probed by XUV fluorescence

“…However, impressive advances in laser science are quickly driving the investigation of laser-cluster interaction towards new frontiers. On the one hand, studies concerning clusters exposed to intense XUV and x-ray pulses are nowadays made possible by the availability of free electron lasers [14][15][16][17] and laser-driven high order harmonic sources [18,19]. On the other hand, the recent development of intense and ultrafast laser sources based on optical parametric amplifiers [20] has opened the way to the study of strong-field laser-matter interaction in the mid-IR spectral region [21].…”

“…On the other hand, the recent development of intense and ultrafast laser sources based on optical parametric amplifiers [20] has opened the way to the study of strong-field laser-matter interaction in the mid-IR spectral region [21]. In spite of the large number of experimental investigations, the mentioned studies mostly concentrate on atomic clusters made up of noble gases, whereas the evolution of molecular clusters excited by intense laser pulses is much less explored [2,16].…”