  Study on ionization process of molecules in an intense infrared laser field
is of paramount interest in strong-field physics and constitutes the foundation
of imaging of molecular valence orbitals and attosecond science. We show
measurement of alignment-dependent ionization probabilities of the lower-lying
orbitals of the molecules by experimentally detecting alignment-dependence of
fluorescence emission from tunnel ionized carbon dioxide molecules. The
experimental measurements are compared with the theoretical calculations of
strong field approximation (SFA) and molecular ADK models. Our results
demonstrate the feasibility of an all-optical approach for probing the
ionization dynamics of lower-lying orbitals of molecules, which is still
difficult to achieve until now by other techniques. Moreover, the deviation
between the experimental and theoretical results indicates the incompleteness
of current theoretical models for describing strong field ionization of
molecules.
