Environmental Engineering Reference
In-Depth Information
gas as a function of the radiation wavelength. Absorption of radiation by certain
atoms leads to the formation of electrons and ions that can be detected by changes
in the electric current. Calibration of this method makes it possible to measure the
content of some admixtures at very low concentrations.
A more widespread application based on absorption of resonant radiation con-
cerns the generation of a photoresonant plasma. To accomplish this, the energy of
the resonant radiation absorbed by a gas or vapor is transformed into the energy
required to ionize the gas. There is a sequence of processes that determine the
generation of a plasma by this means. Resonantly excited atoms are formed as a
result of the absorption of resonant radiation. Collision of these excited atoms with
each other leads to the formation of more highly excited atoms and subsequently
to their ionization. Electrons released in the ionization process establish an equi-
librium with the excited atoms that leads to plasma formation.
There are various applications of a photoresonant plasma. In particular, because
some of the absorbed energy is transformed to energy of the plasma expansion
at the end of the process, a photoresonant plasma is a convenient way to gener-
ate acoustic signals with adjustable parameters. Another type of application makes
use of the high specific absorbed energy. Then a photoresonant plasma can be a
source of multicharged ions. Other applications of photoresonant plasmas make
use of both of these special properties of the plasma, as well as the possibility of
transforming atoms into ions for ease of their detection.
In considering the formation of a photoresonant plasma, we will be guided by
an alkali metal plasma, where the process of plasma formation proceeds according
to the scheme
2 A !
A 2 C
e
Δ ε
i .
(3.102)
Here collisions of two excited atoms may lead to atom ionization, and
i is the
energy that is required for this process. Table 2.14 gives values of the rate constant
k as for this process for resonantly excited atoms of alkali metals at a temperature of
500 K.
We first consider the equilibrium of incident resonant radiation with the radi-
ation flux inside the plasma. Evidently, the mean free path of incident radiation
is small compared with a dimension of the region occupied by a plasma, and Ta-
ble2.22showsthatforalaboratoryplasmaofsize L
Δ ε
1 cm this condition holds
true even if the wavelength is far from the spectral line center. Below we ignore
quenching of excited atoms by plasma electrons and find the number density of
excited atoms N in this case. Let us analyze the balance of an incident radiative
flux j C and an isotropic radiation flux i ( z ) that is created by radiation of excited
atoms, where z is the coordinate inside the gas region along the photon beam.
For the stationary regime of propagation of resonant radiation through the plasma,
we obtain the equality of incident and reflecting photon fluxes near the boundary,
which gives j C (0)
i (0)/4, that is, under equilibrium between incident resonant
radiation and an absorbed gas we have i (0)
D
4 j C (0) at the plasma boundary.
Let us find the number density of excited atoms N
D
from the balance equation
between the rate of emission events N /
τ
per unit volume, where
τ
is the effective
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