Geoscience Reference
In-Depth Information
There are also papers that demonstrate conservation of total electron spin for the
optically oriented metastable helium atoms in the reaction He (2
3
S
1
)
C
A, where A
are the atoms of Cd, Ca, Zn, Sr, Ba, Mg, Pb, Yb, and Eu (for instance, Klyucharev
(
1993
)).
The optical orientation of metastable helium atoms can be used to study the PI
reactions with unexcited molecules M:
He
2
3
S
1
C
M
!
He
1
1
S
0
C
M
C
C
e
(1.51)
It is found that the value of polarization of the electrons in experiments on ionization
of CO, CO
2
,andN
2
O molecules by the optically oriented 2
1
S
0
helium atoms is
close to the initial polarization of metastable atoms (Klyucharev
1993
). The effect
of polarization transfer (with conservation of the total electron spin) was also noted
in the case of collisions of metastable helium atoms in the 2
3
S
1
states with the
nitrogen molecules (Klyucharev
1993
). There are also papers that demonstrate the
conservation of total electron spin for the optically oriented metastable helium atoms
in the reaction He(2
3
S
1
)
C
A, where A are the atoms of Cd, Ca, Zn, Sr, Ba, Mg, Pb,
Yb, and Eu (Klyucharev and Bezuglov
1983
).
The efficiency of the atomic and molecular processes involving rare gas
metastable atoms is different for the singlet and triplet states, and at a collision
velocity ¤>>105 cm s
1
their ratio asymptotically tends to 1 (Klyucharev and
Vujnovic
1990
). The molecular ions in the reactions involving the atoms Ne(
3
P
2
)
and the molecules N
2
,CO,andCO
2
are more than 90% of the product yield, and an
isotope effect plays a significant role here (Klyucharev
1993
). A drop in the values
of the PI reaction rate constants in the thermal collisions of the metastable inert
atoms with CO, CO
2
,H
2
,O
2
,N
2
, and nitrogen-containing molecules is common
for all these pairs with a energy decrease in the range 0.03-0.1 eV. The rate
constants are of the order of 10
10
to 10
9
cm
3
s
1
(
E
0.03 eV), which indicates
that intermediate quasi-molecule weakly bound states with a potential depth less
than 0.03 eV are formed during the reaction.
Note that, in general, the total electron spin S of the interacting particles must
not be conserved, as it takes place in the case of the excitation transfer from the
resonant excited xenon atom
3
P
1
to the CO molecule. The case of LS coupling is an
exception to the rule only when the spin of the system is a “good” quantum number.
This relationship breaks down if the atomic weight of the interacting particles is
increasing. If the reaction occurs at the internuclear distances where a relativistic
interaction is of the order of (or greater than) the electrostatic one, the projection
of the total electron angular momentum on the internuclear axis is the motion
integral. At the same time, the width of the quasi-stationary term should decrease
with increase of a projected , because the ionizing electron acquires a large orbital
momentum and interacts weakly with the ion core. The simultaneous use of the
results of the optical measurements and electron spectroscopy techniques allows us
to evaluate the efficiency of the forming of the excited charged particles in the PI
reaction.
