Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ultrafast exciton formation at the ZnO(10${\overline{\textbf{1}}}$0)
  surface   We study the ultrafast quasiparticle dynamics in and below the ZnO conduction
band using femtosecond time-resolved two-photon photoelectron spectroscopy.
Above band gap excitation causes hot electron relaxation by electron-phonon
scattering down to the Fermi level $E_\text{F}$ followed by ultrafast (200 fs)
formation of a surface exciton (SX). Transient screening of the Coulomb
interaction reduces the SX formation probability at high excitation densities
near the Mott limit. Located just below the surface, the SX are stable with
regard to hydrogen-induced work function modifications and thus the ideal
prerequisite for resonant energy transfer applications.

30
The categories of this paper are: q-alg
15
The categories of this paper are: nucl-ex
26
The categories of this paper are: bayes-an
8
The categories of this paper are: hep-ex
14
The categories of this paper are: math-ph
21
The categories of this paper are: adap-org
13
The categories of this paper are: quant-ph
28
The categories of this paper are: alg-geom
31
The categories of this paper are: ao-sci
0
The categories of this paper are: hep-ph
27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
35
The categories of this paper are: mtrl-th
23
The categories of this paper are: solv-int
24
The categories of this paper are: dg-ga
6
The categories of this paper are: astro-ph