Question: From whence do electrons escape?
About the article "Photoelectric effect", section "Three-step model".

Candidate passages:
In a semiconductor it is sometimes useful to think of the current as due to the flow of positive "holes" (the mobile positive charge carriers that are places where the semiconductor crystal is missing a valence electron).
This is the case in a p-type semiconductor.
A semiconductor has electrical conductivity intermediate in magnitude between that of a conductor and an insulator.
This means a conductivity roughly in the range of 10−2 to 104 siemens per centimeter (S⋅cm−1).
In the classic crystalline semiconductors, electrons can have energies only within certain bands (i.e.
ranges of levels of energy).
Energetically, these bands are located between the energy of the ground state, the state in which electrons are tightly bound to the atomic nuclei of the material, and the free electron energy, the latter describing the energy required for an electron to escape entirely from the material.
The energy bands each correspond to a large number of discrete quantum states of the electrons, and most of the states with low energy (closer to the nucleus) are occupied, up to a particular band called the "valence band".
Semiconductors and insulators are distinguished from metals because the valence band in any given metal is nearly filled with electrons under usual operating conditions, while very few (semiconductor) or virtually none (insulator) of them are available in the "conduction band", the band immediately above the valence band.
The ease with which electrons in the semiconductor can be excited from the valence band to the conduction band depends on the band gap between the bands.
The size of this energy bandgap serves as an arbitrary dividing line (roughly 4 eV) between semiconductors and insulators.
With covalent bonds, an electron moves by hopping to a neighboring bond.
The Pauli exclusion principle requires the electron to be lifted into the higher anti-bonding state of that bond.
For delocalized states, for example in one dimension – that is in a nanowire, for every energy there is a state with electrons flowing in one direction and another state with the electrons flowing in the other.
For a net current to flow, more states for one direction than for the other direction must be occupied.
For this to occur, energy is required, as in the semiconductor the next higher states lie above the band gap.
Often this is stated as: full bands do not contribute to the electrical conductivity.
However, as the temperature of a semiconductor rises above absolute zero, there is more energy in the semiconductor to spend on lattice vibration and on exciting electrons into the conduction band.
The current-carrying electrons in the conduction band are known as "free electrons", although they are often simply called "electrons" if context allows this usage to be clear. 
In the RHEED setup, only atoms at the sample surface contribute to the RHEED pattern.
The glancing angle of incident electrons allows them to escape the bulk of the sample and to reach the detector.
Atoms at the sample surface diffract (scatter) the incident electrons due to the wavelike properties of electrons.
The diffracted electrons interfere constructively at specific angles according to the crystal structure and spacing of the atoms at the sample surface and the wavelength of the incident electrons.
Some of the electron waves created by constructive interference collide with the detector, creating specific diffraction patterns according to the surface features of the sample.
Users characterize the crystallography of the sample surface through analysis of the diffraction patterns.
Figure 2 shows a RHEED pattern.
Two types of diffraction contribute to RHEED patterns.
Some incident electrons undergo a single, elastic scattering event at the crystal surface, a process termed kinematic scattering.
Dynamic scattering occurs when electrons undergo multiple diffraction events in the crystal and lose some of their energy due to interactions with the sample.
Users extract non-qualitative data from the kinematically diffracted electrons.
These electrons account for the high intensity spots or rings common to RHEED patterns.
RHEED users also analyze dynamically scattered electrons with complex techniques and models to gather quantitative information from RHEED patterns. 
Photoelectric effect.
The photoemission or photoelectric effect is a quantum electronic phenomenon in which electrons (photoelectrons) are emitted from matter after the absorption of energy from electromagnetic radiation such as UV light or X-ray.
When UV light or X-ray is absorbed by matter, electrons are excited from core levels into unoccupied states, leaving empty core states.
Secondary electrons are generated by the decay of the core hole.
Auger processes and inelastic electron scattering create a cascade of low-energy electrons.
Some electrons penetrate the sample surface and escape into vacuum.
A wide spectrum of electrons is emitted with energies between the energy of the illumination and the work function of the sample.
This wide electron distribution is the principal source of image aberration in the microscope. 
Neutral atoms or condensed matter cannot emit EUV radiation.
Ionization must take place first.
EUV light can only be emitted by electrons which are bound to multicharged positive ions; for example, to remove an electron from a +3 charged carbon ion (three electrons already removed) requires about 65 eV.
Such electrons are more tightly bound than typical valence electrons.
The existence of multicharged positive ions is only possible in a hot dense plasma.
Alternatively, the free electrons and ions may be generated temporarily and instantaneously by the intense electric field of a very-high-harmonic laser beam.
The electrons accelerate as they return to the parent ion, releasing higher energy photons at diminished intensities, which may be in the EUV range.
If the released photons constitute ionizing radiation, they will also ionize the atoms of the harmonic-generating medium, depleting the sources of higher-harmonic generation.
The freed electrons escape since the electric field of the EUV light is not intense enough to drive the electrons to higher harmonics, while the parent ions are no longer as easily ionized as the originally neutral atoms.
Hence, the processes of EUV generation and absorption (ionization) strongly compete against each other.EUV light can also be emitted by free electrons orbiting a synchrotron. 
Electron irradiation uses electrons accelerated in an electric field to a velocity close to the speed of light.
Electrons are particles, and therefore do not penetrate the product beyond a few centimeters, depending on product density.
Electron facilities rely on substantial concrete shields to protect workers and the environment from radiation exposure.