4
S. L. Chin
The first ‘instantaneous’ interaction is between the strong EM flux and the air
molecules inside the slab. In each slab of EM flux, some known physical events
would occur simultaneously, most of them being uni-molecular interaction (UMI)
while a few are collective interactions (CI) related to the index of refraction of the
gas, hence, the gas density. The following are some examples [7]: tunnel ionization
and dissociation of molecules (UMI), nonlinear pumping of molecular excited states
(UMI), nonlinear excitation of rotational wave packets and molecular alignment
(UMI), odd harmonics generation (CI), self-phase modulation in the neutrals and in
the plasma (super-continuum generation or white light laser) (CI), etc. Many of these
events are measured through measuring optical signals along the filament axis; i.e.
the summation of the signals from successive slabs of light along the filament. This
means that the signal intensity is high. This is one big advantage of having a filament.
In what follows, we shall discuss a few of the above mentioned phenomena. More
detailed discussion can be found in [2–7, 10].
1.3 Air Lasing
Some of the primary events could not be detected directly. One has to measure the
post-interaction signature and infer back to the primary events. For example, in air,
after tunnel ionization of N 2 and O 2 [8], normally, the experimentalist will have to
wait till the electrons or ions are collected and measured. In a high vacuum system,
the pressure is low. An experimentalist could take his/her time (from nanosecond
to microsecond) to make measurement of the electrons or ions using time-of-flight
mass spectrometers without having any fear of collisions among these new particles
or without having to worry about the collective effect of a plasma. In atmospheric
pressure air, counting electrons and ions is too difficult because the mean free time
of collision is too short (ps for electrons in one atmospheric air). One has to rely
upon the measurement of the fluorescence emitted from the system and infer it to
ionization. Moreover, by making the measurement along the propagation axis, the
fluorescence signal would be enhanced. Interestingly, in measuring the fluorescence
from nitrogen molecules after strong interaction inside the filament, some unexpected
additional-uni-molecular interaction was observed.
The unexpected phenomena are explained in the following. Tunnel ionization
of the nitrogen molecules results in population inversion in N 2 and N 2
+ ; this phenomenon is called air lasing [15]. This so-called air lasing comes from the fact that
amplified spontaneous emission along the filament length could be detected easily
[15]. However, the mechanism of air lasing is rather involved [16, 17]. Population
inversion in the N 2 neutral fluorescence comes from collision in the plasma. The
interaction is the following [16].
N 2
+
+ N 2 → N 4
+
N 4
+
+ e → N 2
C
3
u
+ N 2
S. L. Chin
The first ‘instantaneous’ interaction is between the strong EM flux and the air
molecules inside the slab. In each slab of EM flux, some known physical events
would occur simultaneously, most of them being uni-molecular interaction (UMI)
while a few are collective interactions (CI) related to the index of refraction of the
gas, hence, the gas density. The following are some examples [7]: tunnel ionization
and dissociation of molecules (UMI), nonlinear pumping of molecular excited states
(UMI), nonlinear excitation of rotational wave packets and molecular alignment
(UMI), odd harmonics generation (CI), self-phase modulation in the neutrals and in
the plasma (super-continuum generation or white light laser) (CI), etc. Many of these
events are measured through measuring optical signals along the filament axis; i.e.
the summation of the signals from successive slabs of light along the filament. This
means that the signal intensity is high. This is one big advantage of having a filament.
In what follows, we shall discuss a few of the above mentioned phenomena. More
detailed discussion can be found in [2–7, 10].
1.3 Air Lasing
Some of the primary events could not be detected directly. One has to measure the
post-interaction signature and infer back to the primary events. For example, in air,
after tunnel ionization of N 2 and O 2 [8], normally, the experimentalist will have to
wait till the electrons or ions are collected and measured. In a high vacuum system,
the pressure is low. An experimentalist could take his/her time (from nanosecond
to microsecond) to make measurement of the electrons or ions using time-of-flight
mass spectrometers without having any fear of collisions among these new particles
or without having to worry about the collective effect of a plasma. In atmospheric
pressure air, counting electrons and ions is too difficult because the mean free time
of collision is too short (ps for electrons in one atmospheric air). One has to rely
upon the measurement of the fluorescence emitted from the system and infer it to
ionization. Moreover, by making the measurement along the propagation axis, the
fluorescence signal would be enhanced. Interestingly, in measuring the fluorescence
from nitrogen molecules after strong interaction inside the filament, some unexpected
additional-uni-molecular interaction was observed.
The unexpected phenomena are explained in the following. Tunnel ionization
of the nitrogen molecules results in population inversion in N 2 and N 2
+ ; this phenomenon is called air lasing [15]. This so-called air lasing comes from the fact that
amplified spontaneous emission along the filament length could be detected easily
[15]. However, the mechanism of air lasing is rather involved [16, 17]. Population
inversion in the N 2 neutral fluorescence comes from collision in the plasma. The
interaction is the following [16].
N 2
+
+ N 2 → N 4
+
N 4
+
+ e → N 2
C
3
u
+ N 2