MeV energy plays an important role in the interaction with relativistic laser after the
ionization.
The higher harmonic generation (HHG) from an electron oscillating in the
binding force of atom is also interesting. If (2.1.6) is solved in Coulomb potential
by the atomic nucleus, the electron wave function oscillates with laser frequency, but
it is not the harmonic oscillation. The time evolution of the current by the electron in
(1.3.2) has many higher harmonic Laplace components, as imagined with the
classical electron motion. With the increase of laser intensity, the bound electron
starts to emit HH light, and it can be used for many applications. Even the electron is
abruptly ionized by the field, some electrons recombine to the atom emit the excess
energy as photons in X-ray range. Since this phenomenon happens in a very short
time like laser oscillation period, the generated X-ray pulse will be of the pulse
duration of atto-second. Atto-second X-ray pulse is attractive photon source to study
the fundamental science in many fields. Details are given in [2], and this book stops
this topics here.
2.1.4 Experimental Data
Intense laser is irradiated into the rarefied argon gas to observe the fraction of
ionization of argon atom from Ar
+ to Ar
7+ [5]. The first ionization energy of argon
atom is 15.76 eV. In order to get precise data, short pulse laser of 600 fs pulse length
is used, and laser intensity is varied from 10
14 to 10
17 W/cm
2 as seen in Fig. 2.5,
where each mark represents a single laser short result. The laser wavelength is
1.053 mm, and it almost corresponds to the photon energy of 1 eV. It is clear that
above the intensity of 10
14 W/cm
2 . The sharp increase of the number of ions shows
that the phenomena are nonlinear, and each ionization has a threshold laser intensity
above which the ionization starts. The threshold intensity of the first ionization is
about 2 Â 10
14 W/cm
2 in Fig. 2.5. The rough evaluation based on the uncertain
principle in (2.1.12) is lower estimate of this value, but it is useful for rough
evaluation of the threshold.
There are several models to predict such fraction of ionization over one pulse with
typically Gaussian pulse shape. In Ref. [5], many data for different kinds of gases are
also shown, and all data are compared to six different theoretical works. After the
comparison of the experimental data and the theoretical models, the authors
concluded that a much more primitive model based on Coulomb-barrier suppression
has best fit to the data. This theory ignores the tunneling and other quantum
mechanical effects. This is called barrier-suppression ionization (BSI)
mechanism, and it is given only by the threshold intensity in the form [5]:
2.1 Plasma Generation by Lasers
37
ionization.
The higher harmonic generation (HHG) from an electron oscillating in the
binding force of atom is also interesting. If (2.1.6) is solved in Coulomb potential
by the atomic nucleus, the electron wave function oscillates with laser frequency, but
it is not the harmonic oscillation. The time evolution of the current by the electron in
(1.3.2) has many higher harmonic Laplace components, as imagined with the
classical electron motion. With the increase of laser intensity, the bound electron
starts to emit HH light, and it can be used for many applications. Even the electron is
abruptly ionized by the field, some electrons recombine to the atom emit the excess
energy as photons in X-ray range. Since this phenomenon happens in a very short
time like laser oscillation period, the generated X-ray pulse will be of the pulse
duration of atto-second. Atto-second X-ray pulse is attractive photon source to study
the fundamental science in many fields. Details are given in [2], and this book stops
this topics here.
2.1.4 Experimental Data
Intense laser is irradiated into the rarefied argon gas to observe the fraction of
ionization of argon atom from Ar
+ to Ar
7+ [5]. The first ionization energy of argon
atom is 15.76 eV. In order to get precise data, short pulse laser of 600 fs pulse length
is used, and laser intensity is varied from 10
14 to 10
17 W/cm
2 as seen in Fig. 2.5,
where each mark represents a single laser short result. The laser wavelength is
1.053 mm, and it almost corresponds to the photon energy of 1 eV. It is clear that
above the intensity of 10
14 W/cm
2 . The sharp increase of the number of ions shows
that the phenomena are nonlinear, and each ionization has a threshold laser intensity
above which the ionization starts. The threshold intensity of the first ionization is
about 2 Â 10
14 W/cm
2 in Fig. 2.5. The rough evaluation based on the uncertain
principle in (2.1.12) is lower estimate of this value, but it is useful for rough
evaluation of the threshold.
There are several models to predict such fraction of ionization over one pulse with
typically Gaussian pulse shape. In Ref. [5], many data for different kinds of gases are
also shown, and all data are compared to six different theoretical works. After the
comparison of the experimental data and the theoretical models, the authors
concluded that a much more primitive model based on Coulomb-barrier suppression
has best fit to the data. This theory ignores the tunneling and other quantum
mechanical effects. This is called barrier-suppression ionization (BSI)
mechanism, and it is given only by the threshold intensity in the form [5]:
2.1 Plasma Generation by Lasers
37