The experiments with 'liquid droplet clusters' is a novel area of research which is fascinating in its own right since microdroplets are macroclusters having bulk-like packing densities, and yet exhibit certain properties of large molecules/small clusters due to their spatial confinement. In otherwords, microdroplets combine the properties of size-confined molecular/cluster systems and the high-density targets. We have initiated radically new experiments to understand the interaction dynamics (charge-transfer, ionization and fragmentation) of droplets in their collisions with highly charged ions (HCI) of low energy (in the KeV-MeV range). These experiments are of interest not only from the fundamental point of view of understanding ion-droplet collisions, but also from a technological point of view since they are expected to bring forth new advancements in techniques to produce highly-excited molecules/ions and pulsed x-ray sources.
The microdroplets (of low viscosity liquids under high pressure)are generated by uniform break up of a high-velocity microjet that emerges from a piezocrystal-driven microcapillary tube of 10 microns diameter. On applying a disturbance of wavelength /lambda = 2.pi.rj (rj is the radius of the jet) symmetric about the jet axis, the microjet breaks up into a train of uniformly-sized microdroplets, with the interdroplet spacing equal to the disturbance wavelength. The size of the droplets (in air), as determined by Mie scattering, is 17 microns.
The infrastructure developed by us for such experiments,constitutes a beam line (15 degrees) for transporting the highly-charged ions from the ECR source to the experimental chamber, and an UHV experimental chamber with a large number of ports for mounting various detectors like x-ray and particle detectors. The 15 degree beam line that is dedicated for such novel experiments has become operational of late. It consists of indigenously fabricated electrostatic quadrupoles (doublet and triplet) and electrostatic steerer. The photograph on the right shows the 15 degree beam line at LEIBF. The high voltage feedthroughs for the quadrupoles and steerers have been fabricated in-house.
The photograph on the left shows an indigenously fabricated electrostatic quadrupole triplet. The commercially available beam diagnostic elements like the beam profile monitor (BPM) and the Faraday cup have been installed at suitable positions along the beam line. All the high voltage power supplies (for quadrupoles) are remote controlled through RS232 interface. A manually-operated double slit in the beam line allows for croping the peripheral beam. The unique combination of the dipole magnet and the electrostatic quadrupole doublet, together with the electrostatic quadrupole triplet allow for obtaining a m/q selected and doubly focussed low energy highly charged ion beam with unit magnification at the center of the experimental chamber.
The UHV experimental chamber for HCI-droplet interaction studies is shown in the photograph on the left. It houses a cryopumping system and a differential pumping system apart from the conventional high-capacity pumps (rotary-backed diffusion pump), for achieving and sustaining high vacuum when the liquid droplets are introduced into it. A special arrangement for collecting the droplet train and pumping it out, has also been made. The chamber has a number of ports around the interaction region so as to accomodate photon and particle detectors. A flexible bellow arrangement for mounting the piezocrystal driven microcapillary tube allows for aligning the droplet train in vacuum. The microdroplets interact with low energy highly charged ion beam in crossed-beams geometry. Such an interaction results in multiple ionization of the target droplets, leaving them in highly excited states, followed by fragmentation of the same. The dynamics of the collisional interaction can be understood by monitoring x-ray emissions and detecting the product ions formed in the coulomb explosion of the droplet, by time-of-flight (TOF)spectrometry.