Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Hard X-ray Transient Grating Spectroscopy on Bismuth Germanate   Optical-domain Transient Grating (TG) spectroscopy is a versatile
background-free four-wave-mixing technique used to probe vibrational, magnetic
and electronic degrees of freedom in the time domain. The newly developed
coherent X-ray Free Electron Laser sources allow its extension to the X-ray
regime. Xrays offer multiple advantages for TG: their large penetration depth
allows probing the bulk properties of materials, their element-specificity can
address core-excited states, and their short wavelengths create excitation
gratings with unprecedented momentum transfer and spatial resolution. We
demonstrate for the first time TG excitation in the hard X-ray range at 7.1
keV. In Bismuth Germanate (BGO), the nonresonant TG excitation generates
coherent optical phonons detected as a function of time by diffraction of an
optical probe pulse. This experiment demonstrates the ability to probe bulk
properties of materials and paves the way for ultrafast coherent
four-wave-mixing techniques using X-ray probes and involving nanoscale TG
spatial periods.

31
The categories of this paper are: ao-sci
14
The categories of this paper are: math-ph
5
The categories of this paper are: gr-qc
33
The categories of this paper are: plasm-ph
9
The categories of this paper are: nlin
35
The categories of this paper are: mtrl-th
36
The categories of this paper are: cmp-lg
20
The categories of this paper are: acc-phys
8
The categories of this paper are: hep-ex
12
The categories of this paper are: nucl-th
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The categories of this paper are: bayes-an
11
The categories of this paper are: hep-lat
23
The categories of this paper are: solv-int
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The categories of this paper are: patt-sol
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The categories of this paper are: chao-dyn
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The categories of this paper are: dg-ga