Laser Plasma Section

Time- resolved x-ray diffraction studies using Ultrashort K-α X-rays

To understand the atomic structure of a material K-α line radiation is often used. The atomic structure information is derived from the diffraction pattern recorded from the material under investigation. In order to study the dynamics of atomic lattice structure under external applied pressure, time resolved x-ray diffraction is required. However, the K-α sources based on x-ray tube have long pulse duration and therefore cannot be used for time resolved atomic lattice studies under dynamic condition which occurs over ultrashort time scale (~ps). The plasma produced by focusing femtosecond (fs) duration laser pulses is an efficient source of inner-shell x-ray line (K-α) radiation as well as of very short pulse duration due to short duration of the driving laser pulse. The line radiation is generated by inner shell transitions. These transitions are occurring when the inner shell electrons are knocked out by hot electrons produced during the interaction of the laser pulses with the solid target.

Figure 1: a) The K-α x-ray emission spectra of titanium, and copper, from plasma produced by fs laser pulses. b)  Normalized K-α line radiation profiles at two different laser fluences.
Figure 1: a) The K-α x-ray emission spectra of titanium, and copper, from plasma produced by fs laser pulses. b) Normalized K-α line radiation profiles at two different laser fluences.

Generation and detection of K-α radiation

The interaction of ultrashort laser pulse with copper target generates the Cu K-α radiation. The laser has energy ~200 mJ, pulse duration ~ 45 femtoseconds (10-15 sec), and it was focussed on the target at an intensity of ~1017 W cm-2. The duration of x-ray pulse is a convolution of Interaction time of hot electrons with matter and the inherent level relaxation time. Former is of the order of laser pulse duration and latter is of the order of few femtoseconds. Hence the duration of K-α radiation is few tens of femtoseconds. This laser solid interaction generates a very clean x ray source with high brightness, narrow spectral line width, smaller source size, and the K-α x-ray emission is synchronized with the driving laser pulse. These properties make it a very attractive source for the pump-probe studies to observe the fast dynamical processes relevant to physical, chemical and biological processes.
The K-α x-ray source is characterized for photon flux and spectral profile using x-ray CCD camera based dispersion-less x-ray spectrograph and a high resolution crystal spectrograph respectively. The spectra recorded from them is shown in Fig. 1 a) and 1 b) respectively. K-α line emission energies from Ti and Cu solid targets are 4.5 and 8.05 keV respectively. The fine structure of Cu K-α line recorded with crystal spectrograph clearly shows the two peaks known as K-α1 and K-α2 lines. Effect of various parameters on the spectral properties of K-α line radiation is also studied.

Time resolved x ray diffraction (TXRD) studies using Laser Plasma K-α Source
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One of the most promising applications of the ultra-short K-α x-ray source is in time resolved x-ray diffraction of samples related to material science research. As stated above, the K-α x-ray source is well synchronized with the driving laser pulse and hence can be used in time resolved x-ray diffraction studies using pump probe technique. Here the ultra-short duration K-α source act as a probe beam and the crystal / sample is irradiated by the ultrashort laser pulse acting as a pump beam. Using this technique, we have carried out measurements on shock-wave profiles in a silicon crystal using Ti K-α -ray line radiation as probe pulse.

Figure 2: (a) Experimental lay out (b) variation of the FWHM of the K-α line with the delay between the pump and the probe pulse.
Figure 2: (a) Experimental lay out (b) variation of the FWHM of the K-α line with the delay between the pump and the probe pulse.

Figure 2 (a) shows the experimental setup. A part of the uncompressed laser pulse (800 nm, 200 ps) was used as pump to irradiate a 500 μm thick flat Si (111) crystal sample at an intensity of 6 GW/cm2. The time delay between the pump laser pulse and the probing x-ray pulse was adjusted by a delay line. The diffracted x-ray spectrum was accumulated on an x-ray CCD camera, for each time delay, with an angular resolution better than 0.02 rad. The distortion in lattice spacing due to shock wave contraction or thermal expansion resulted in change in direction of the diffracted x rays which in turn resulted in shifting and broadening of diffracted x-ray line. Since the lattice distortion is not very large hence the broadening in the diffracted x-ray spectrum is more prominent than the respective peak shift. The broadening of the diffracted signals is predominantly towards larger Bragg diffraction angle when shock wave is contracting the lattice, whereas the thermal disordering for time delay > 1 ns give rise to broadening towards lower angle side. Figure 2 (b) shows the broadening of the Ti K-α line as a function of time delay between the pump and the probe pulse.

The observed x-ray rocking curve gives signature of the expansion and contraction through the spread in opposite directions resulting in overall broadening of the x-ray diffraction peak. It is observed that the rocking curve broadens with increasing delay to reach a maximum until the time when the shock wave propagating through the sample reaches the x-ray penetration depth. Thereafter, the K-α1 width decreases and comes close to the pristine (undisturbed) value. The reduction in broadening of the rocking curve is attributed to the reduction in the magnitude of pressure within the penetration depth of probe x-rays in the sample, or due to the passing of the pressure wave beyond the penetration depth of the x-rays in the sample.


For more details contact Dr. H. Singhal (himanshu@rrcat.gov.in)

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