Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Direct resolution of unoccupied states in solids via two photon
  photoemission   Non-linear effects in photoemission are shown to open a new access to the
band structure of unoccupied states in solids, totally different from hitherto
used photoemission spectroscopy. Despite its second-order nature, strong
resonant transitions occur, obeying exact selection rules of energy, crystal
symmetry, and momentum. Ab-initio calculations are used to demonstrate that
such structures are present in low-energy laser spectroscopy experimental
measurements on Si previously published. Similar resonances are expected in
ultraviolet angle-resolved photoemission spectra, as shown in a model
calculation on Al.

35
The categories of this paper are: mtrl-th
22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
6
The categories of this paper are: astro-ph
28
The categories of this paper are: alg-geom
24
The categories of this paper are: dg-ga
37
The categories of this paper are: supr-con
14
The categories of this paper are: math-ph
18
The categories of this paper are: econ
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
20
The categories of this paper are: acc-phys
3
The categories of this paper are: physics
34
The categories of this paper are: chem-ph
9
The categories of this paper are: nlin
33
The categories of this paper are: plasm-ph