Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Direct observation of non-fully-symmetric coherent optical phonons by
  femtosecond x-ray diffraction   We directly measure by femtosecond time-resolved x-ray diffraction the E$_g$
symmetry coherent phonon excited in bismuth by strong optical excitation. The
magnitude of the E$_g$ mode observed is 0.2 pm peak-to-peak, compared against
the 2.7 pm initial displacement of the fully-symmetric A$_{1g}$ mode. The much
smaller motion of the E$_g$ mode is a consequence of the short lifetime of the
electronic states that drive the atomic motion. The experimentally measured
magnitude of the E$_g$ motion allows us to rule out a previously suggested
scenario for explaining the dynamics in bismuth that relies on strong coupling
between these modes.

10
The categories of this paper are: q-bio
24
The categories of this paper are: dg-ga
16
The categories of this paper are: stat
32
The categories of this paper are: atom-ph
0
The categories of this paper are: hep-ph
20
The categories of this paper are: acc-phys
31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
30
The categories of this paper are: q-alg
26
The categories of this paper are: bayes-an
19
The categories of this paper are: eess
36
The categories of this paper are: cmp-lg
3
The categories of this paper are: physics
22
The categories of this paper are: chao-dyn
11
The categories of this paper are: hep-lat
27
The categories of this paper are: comp-gas