Using Tomograpy To Delineate Density Distribution In Gas Jet

Sometimes there are concepts that are simply too small for humans to comprehend. An attosecond, being 10-18 seconds, is one of these. Imagine taking one second, and dividing that second up into individual attoseconds. If each of those attoseconds was expanded so that they were themselves each a second long you would have 1018 seconds, about 32 billion years. It takes one attosecond for a beam of light to travel across two hydrogen atoms. One of the things we are capable of doing is actually creating bursts of laser with time scales of tens of attoseconds. To do this, a laser of femtosecond period is used to ionise a noble gas. The separated electron is accelerated back towards the atom by the inducing laser beam’s electric field. When recombination happens the excess energy is emitted, normally as an ultraviolet photon. As the laser interacts with multiple atoms simultaneously an ultraviolet laser pulse is emitted on the time scale of the recombination, in other words an attosecond time scale.

This process, called high harmonic generation, relies on both the electric field distributions of the driving laser and also the location distribution of the gas. Understanding the gas distribution and possibly controlling it would lead to greater control over the created attosecond laser pulses. Computer tomography has found uses in medicine, geophysics and material science; it can also be used here to look at the spacial density distribution of the ionised medium. The methodology is relatively simple. A cylindrical jet of gas, the medium in this case, is projected. The laser is focussed through the gas jet and into a spectrometer while an ion detector works on the aftermath of the gas jet. As the laser is moved on its axis throughout the gas jet, reading can be taken, which, based on known equations, can be used to calculate the density as a continuous function of position. This will undoubtedly lead to greater experimental precision in temporally short condensed matter physics. Hopefully this work can transcend beyond just laser generation, as understanding gas jets can be useful in particle physics, nuclear physics and chemistry.

Paper links: Laser induced strong-field ionization gas jet tomography


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