Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Direct observation of optically induced transient structures in graphite
  using ultrafast electron crystallography   We use ultrafast electron crystallography to study structural changes induced
in graphite by a femtosecond laser pulse. At moderate fluences of ~< 21mJ/cm^2,
lattice vibrations are observed to thermalize on a time scale of ~8ps. At
higher fluences approaching the damage threshold, lattice vibration amplitudes
saturate. Following a marked initial contraction, graphite is driven
nonthermally into a transient state with sp^3-like character, forming
interlayer bonds. Using ab initio density functional calculations, we trace the
governing mechanism back to electronic structure changes following the
photo-excitation.

23
The categories of this paper are: solv-int
29
The categories of this paper are: funct-an
21
The categories of this paper are: adap-org
9
The categories of this paper are: nlin
34
The categories of this paper are: chem-ph
5
The categories of this paper are: gr-qc
33
The categories of this paper are: plasm-ph
7
The categories of this paper are: hep-th
18
The categories of this paper are: econ
28
The categories of this paper are: alg-geom
12
The categories of this paper are: nucl-th
35
The categories of this paper are: mtrl-th
37
The categories of this paper are: supr-con
10
The categories of this paper are: q-bio
27
The categories of this paper are: comp-gas
24
The categories of this paper are: dg-ga