We present analytic results for population transfer between two bound states using two lasers. Consideration of continuum-continuum transitions leads to new concepts that replace pulse energy, exponential transition law, and Lorentzian form of photoelectron energy distribution.
Transfer of molecules or atoms from an initial state to a certain final state is often wanted, and use of a single laser may be impractical. Transfer via an intermediate state of higher energy has been demonstrated, using laser pulses that overlap in time. We have proposed using the continuum in place of the intermediate state, and encouraging results have been computed for a simple model. Our latest calculations show the effects of operating this scheme near the bottom of the continuum or near a resonance having the Fano shape, and the effects of laser-driven continuum-continuum transitions.
We study the ir excitation of HeH+ using the counterintuitive arrangement of two pulsed lasers with the continuum as the intermediate state. The time-dependent Schr\"odinger equation describing the system is integrated numerically using the split-operator technique. We examine the maximum population transfer as a function of the pulse separation, pulse width, relative pulse strength, two-photon detuning, and relative position of the intermediate continuum state. The largest population transfer found is 63%.
We show that efficient transfer of molecules or atoms from one bound state to another is possible via the continuum in some cases, using two overlapping laser pulses. The structure of the continuum determines the pulse delay, laser frequencies, and laser intensities that should be used. We present simple formulas that relate the optimum values of pulse intensities, pulse separation, and detuning; these estimates can be used as a guide. More detailed calculations give quantitative results for two specific potentials. \textcopyright{} 1996 The American Physical Society.
Two laser pulses that overlap in time have been successfully used for coherent excitation of a molecule or atom to a desired state. An intermediate state that is at or near resonance with the laser frequencies has been used in past experiments and calculations. Here we replace this intermediate state by a continuum of intermediate states. An analytic solution of a simple model suggests that the continuum can be used in such an excitation scheme, provided that the laser pulses are arranged in the so-called counterintuitive order. We use a special case of this solution to relate this work to ionization suppression and coherent population trapping.
Laser induced continuum structures (LICS) can be produced by a strong laser embedding an excited atomic bound state into a flat atomic continuum, leading to a tunable resonance of adjustable width that can be probed by a second laser. If this second laser is of similar intensity to the embedding laser then the distinction between the two becomes artificial and saturation effects become important. In this paper the effect on LICS of transitions caused by both lasers between the original atomic continuum and a second atomic continuum are studied in the Markoff approximation by means of Laplace transform methods and allowing for the ionization of each atomic state by both laser fields. Formal results correct to all orders are also given in terms of a T-matrix approach. Numerical calculations are presented showing the effects on the LICS resonant structures of continuum-continuum coupling processes. Significant changes to the Fano profiles (second laser weak) and to coherence holes (second laser strong) occur. Analytical results are also given.