  We show that if the laser is intense enough, it may always ionize an atom or
induce transitions between discrete energy levels of the atom, no matter what
is its frequency. It means in the quantum transition of an atom interacting
with an intense laser of circular frequency $\omega$, the energy difference
between the initial and the final states of the atom is not necessarily being
an integer multiple of the quantum energy $\hbar\omega$. The absorption spectra
become continuous. The Bohr condition is violated. The energy of photoelectrons
becomes light intensity dependent in the intense laser photoelectric effect.
The transition probabilities and cross sections of photo-excitations and
photo-ionizations are laser intensity dependent, showing that these processes
cannot be reduced to the results of interactions between the atom and separate
individual photons, they are rather the processes of the atom interacting with
the laser as a whole. The interaction of photons on atoms are not simply
additive. The effects are non-perturbative and non-linear. Some numerical
results for processes between hydrogen atom and intense circularly polarized
laser, illustrating the non-perturbative and non-linear character of the
atom-laser interaction, are given.

Quantum Physics