Creation of Novel Quasi-Bound States in High-Frequency Intense Laser Fields

Abstract

It has been shown that the photon field of intense infrared lasers works as a static electric field for electrons in atoms and molecules [1,2]. The primary process for the molecules placed in such a field is field-ionization. The photon field is, however, an alternating field. It forces electrons to quiver rather them kick away. It gives electrons a chance to travel around a molecule. The use of high frequency and intense lasers emphasizes the change of motion of electrons bound in atoms and molecules. With increasing laser frequency, the ionization rate is apt to be suppressed [3–5], and one can discuss quasi-bound electronic states of atoms and molecules in intense fields. In high-frequency intense laser fields, electron wavefunctions are deformed, and electronic states of molecules come to have different characters. Energy levels are shifted by the ac Stark effect, and acquire an energy width due to ionization. This kind of change is, in a sense, gradual and continuous. If we gradually increase the laser intensity, the electronic states change their characters continuously, in other words, adiabatically. In addition to such adiabatic changes, it is known that an abrupt change occurs on atomic and molecular electronic states. New electronic states can be created by the effect of intense fields. Those states are called light-induced states (LIS) [6]. In Sect. 2.2, we discuss the origin of LIS and clarify the condition, i.e., the combination of laser intensity and frequency, for the formation of LIS. Due to the changes in the electron wavefunction, atoms acquire different chemical properties. The nature of the chemical bond is modified in intense fields. For instance, bond hardening and softening in the H2 molecular ion have been observed experimentally [7–9, 11]. Deformation in molecular structure has also been reported [12–14]. In a recent study, the creation of a covalent bond between two helium atoms in intense fields is theoretically predicted [15]. We discuss laser-induced chemical bonding in Sect. 2.3.