1 Femtosecond Laser Filamentation Induced Phenomena and Applications
5
The emission from the N 2 (C
3
u ) state to the empty N 2 (C
3
g ) state (hence,
population inversion) gives rise to fluorescence at 337 nm, etc. with gain.
Population inversion between an excited state and the ground state of the N 2
+
ions had been intriguing in recent years. In principle, both the ground and excited
states of N 2
+ were prepared through tunnel ionization of neutral N 2 from the ground
state. Normal wisdom would predict that the population in the excited state should
be lower than that in the ground state [18]. However, experiment showed that there
was population inversion [15]. The myth was recently clarified [17]. The population
inversion was realized by the ionization of N 2 into the ground state X
2
+
g and the
excited state B
2
+
u of N 2
+ [18] together with the simultaneous coupling among these
two states and the intermediate state A
2
u . The intermediate A
2
u state was essential
in the coupling; it served as a population reservoir in the process of transferring
the population from the ground X
2
+
g state into the intermediate A
2
u state thus
depleting the population from the ground X
2
+
g state; population inversion was thus
established between the B
2
+
u state and the ground X
2
+
g state.
1.4 Population Inversion in the Air Filament Containing
Other Molecules
Lasing action (or population inversion) inside the filament in air does not pertain only
to nitrogen. Oxygen also exhibited population inversion in the fs intense field [19,
20]. In fact, we have observed that dissociation of H 2 O molecules in the fs Ti-sapphire
laser filament in a humid air gave rise to OH radicals which were excited into a state of
population inversion. Gain was observed in the u.v. fluorescence (around 308.9 nm)
from OH [21]. Another observation was that inside a filament of the second harmonic
(400 nm) of a fs Ti-sapphire laser in air in the laboratory with some humidity, after
the primary interaction (ionization and dissociation), subsequent collision would
induce more chemical reactions. We observed that there was gain in the fluorescence
(336.0 nm) from NH from inside the filament [22]. The excited NH obviously came
from both ionization and dissociation of H 2 O and N 2 as well as subsequent collision
events.
The above observations have not been analyzed thoroughly yet. However, one
could speculate a little. For example, to create a free radical such as NH after ionization and dissociation of N 2 and H 2 O, H and N or their ions would have to collide
to form the final product NH. For two ground state atoms, the collision would have
to be sufficiently strong; i.e. the relative collision speed would have to be high so
that a sufficient amount of kinetic energy would be absorbed by the colliding particles to overcome the potential barrier between two ground state atoms. Presumably,
there is not much kinetic energy transferred from the laser field to the dissociated
atoms during tunnel ionization and dissociation of the molecules inside the filament
core. One possible scenario is Coulomb explosion. However, the probability is too
low. This is because Coulomb explosion requires an intensity of the order of 10
15
5
The emission from the N 2 (C
3
u ) state to the empty N 2 (C
3
g ) state (hence,
population inversion) gives rise to fluorescence at 337 nm, etc. with gain.
Population inversion between an excited state and the ground state of the N 2
+
ions had been intriguing in recent years. In principle, both the ground and excited
states of N 2
+ were prepared through tunnel ionization of neutral N 2 from the ground
state. Normal wisdom would predict that the population in the excited state should
be lower than that in the ground state [18]. However, experiment showed that there
was population inversion [15]. The myth was recently clarified [17]. The population
inversion was realized by the ionization of N 2 into the ground state X
2
+
g and the
excited state B
2
+
u of N 2
+ [18] together with the simultaneous coupling among these
two states and the intermediate state A
2
u . The intermediate A
2
u state was essential
in the coupling; it served as a population reservoir in the process of transferring
the population from the ground X
2
+
g state into the intermediate A
2
u state thus
depleting the population from the ground X
2
+
g state; population inversion was thus
established between the B
2
+
u state and the ground X
2
+
g state.
1.4 Population Inversion in the Air Filament Containing
Other Molecules
Lasing action (or population inversion) inside the filament in air does not pertain only
to nitrogen. Oxygen also exhibited population inversion in the fs intense field [19,
20]. In fact, we have observed that dissociation of H 2 O molecules in the fs Ti-sapphire
laser filament in a humid air gave rise to OH radicals which were excited into a state of
population inversion. Gain was observed in the u.v. fluorescence (around 308.9 nm)
from OH [21]. Another observation was that inside a filament of the second harmonic
(400 nm) of a fs Ti-sapphire laser in air in the laboratory with some humidity, after
the primary interaction (ionization and dissociation), subsequent collision would
induce more chemical reactions. We observed that there was gain in the fluorescence
(336.0 nm) from NH from inside the filament [22]. The excited NH obviously came
from both ionization and dissociation of H 2 O and N 2 as well as subsequent collision
events.
The above observations have not been analyzed thoroughly yet. However, one
could speculate a little. For example, to create a free radical such as NH after ionization and dissociation of N 2 and H 2 O, H and N or their ions would have to collide
to form the final product NH. For two ground state atoms, the collision would have
to be sufficiently strong; i.e. the relative collision speed would have to be high so
that a sufficient amount of kinetic energy would be absorbed by the colliding particles to overcome the potential barrier between two ground state atoms. Presumably,
there is not much kinetic energy transferred from the laser field to the dissociated
atoms during tunnel ionization and dissociation of the molecules inside the filament
core. One possible scenario is Coulomb explosion. However, the probability is too
low. This is because Coulomb explosion requires an intensity of the order of 10
15