In the idealized isolated-carbon chemical atom interpretation, both beams carry approximately

2.68 eV

per incident particle, close to the

^{1} S_{0}

threshold. However equal incident energies do not imply equal excitation channels.

A single optical photon cannot strongly excite any of the listed even-parity

2 p^{2}

energy levels from the even-parity

^{3} P_{0}

ground state through an electric dipole transition. In particular

^{3} P_{0} \to^{1} S_{0}

also violates

Δ S = 0

and the

J = 0 \leftrightarrow 0

restriction. The lowest strong opposite-parity excitations lie well above

2.68 eV

. Thus one expects no strong resonant absorption or corresponding strong atomic fluorescence from these ground-state chemical atoms at this optical wavelength; weak higher-order processes and scattering are separate effects.

Incident electrons can undergo inelastic scattering and collisional excitation. Their orbital angular momentum and exchange with bound electrons allow transfers excluded for a single electric dipole photon. The

^{1} D_{2}

energy level at

1.264 eV

and the low fine structure excitations are energetically accessible. One therefore expects scattered electrons with discrete energy losses, possible weak forbidden spectral lines, and collisional or surface relaxation. For example

^{1} D_{2} \to^{3} P_{2}

has wavelength approximately

1 0^{7} / (10192 - 43.4) = 985 nm

; spin mixing supplies the weak spin-forbidden radiative channel.

There is a numerical threshold caveat. Literally,

E (^{1} S_{0}) = 21648 h c c m^{- 1} ≃ 2.68401 eV, E_{γ} (462.6 nm) ≃ 2.68016 eV, E_{e} = 2.68000 eV .

(1)

Neither quoted beam reaches

^{1} S_{0}

from the exact ground state if these are taken as exact monoenergetic values. The electron value rounded to two decimal places could represent an actual energy above threshold; if so,

^{1} S_{0}

is collisionally accessible and its decay to

^{1} D_{2}

produces the weak electric quadrupole transition at

1 0^{7} /11456 ≃ 873 nm

, followed by lower-level relaxation. These emissions require populations and sufficiently low collisional quenching of a metastable line.

An actual solid carbon foil has bands, collective excitations and surface losses, so it need not reproduce the isolated-chemical atom spectrum or selection rules. Its optical and electron-induced emission cannot be predicted quantitatively from the five atomic energy levels alone. This is the limitation of the foil wording in the source.