Coherent atomic excitation

Shore B. W. The Theory of Coherent Atomic Excitation, Vol. 2 (John Wiley, New York) (1990). 

Shore BW (1990) The theory of coherent atomic excitation, p. 194, New York, John Wiley Sons... 

Shore, B. W The Theory of Coherent Atomic Excitation Simple Atoms and Fields Wilev Sons New York, 1990 Vol. 1. 

The photochemical excitation delivered by a narrowly defined pump laser pulse achieves three indispensable things it sets time = 0, energizes the reactant molecules, and localizes them in space. It induces molecular coherence as excitation of each of the individual molecules involved leads to a coherent superposition of separate wave packets, a highly locahzed, geometrically well-defined and moving packet—analogous to a classical system, one that can be described using classical concepts of atomic positions and momentum. 

Probably coherent effects in atomic excitation with light were studied first by Wilhelm Hanlc. The very first publication about the effect, which is now called Hanle effect, appeared in Zeitschrift fur Physik as early as in 1924 [1]. In the paper it was shown that the resonance fluorescence of Hg excited with linear polarized light is depolarized by an external magnetic field. That publication docs... 

Possibility of coherent multiple excitation in atom-transfer with a scanning tunneling microscope. Phys. Rev. B, 49, 10655-10662. 

The previous few examples show that, under coherent illumination, one can no longer consider observed spectral intensities as purely characteristic of the atom, since the properties of the Autler-Townes doublet depend on the laser intensity, and the profile of a laser-excited autoionising line will in general possess a shape unrelated to that of an atom excited by a weak source. 

The first example is a three-level A-type system coupled by bichromatic coupling and probe fields, which opens two Raman transition channels [60]. The phase dependent interference between the resonant two-photon Raman transitions depends on the relative phases of the laser fields either constructive interference or destructive interference between the two Raman channels can be obtained by controlling the laser phases. The second example is a four-level system coupled by two coupling fields and two probe fields, in which a double-ElT configuration is created by the phase-dependent interference between the three-photon and one-photon excitation processes, or equivalently two independent Raman transition channels [58,62]. We will provide theoretical analyses of the phase dependent quantum interference in the two multi-level atomic systems and present experimental results obtained with cold Rb atoms. The two systems provide basic platforms to study coherent atom-photon interactions and quantum state manipulations, and to explore useful applications of the phase-dependent interference in the multi-level atomic systems. 

Interaction of an excited-state atom (A ) with a photon stimulates the emission of another photon so that two coherent photons leave the interaction site. Each of these two photons interacts with two other excited-state molecules and stimulates emission of two more photons, giving four photons in ail. A cascade builds, amplifying the first event. Within a few nanoseconds, a laser beam develops. Note that the cascade is unusual in that all of the photons travel coherently in the same direction consequently, very small divergence from parallelism is found in laser beams. 

Radiative Saturation. Higher levels of radiation create a larger population in the excited state, allowing stimulated emission to become a competing process. In this process, atoms in the excited state absorb photons, which re-emit coherently that is with the same frequency, phase and direction as the incident photon. Thus stimulated emission does not produce backscat-tered photons. As the incident energy increases, a greater proportion of the excited atoms absorb a photon and produce stimulated emission before they decay naturally. The net result is that the population of atoms available to produce backscatter decreases, i.e., the medium saturates. 

We use a7r/2 — vr — vr/2 pulse sequence to coherently divide, deflect and finally recombine an atomic wavepacket. The first vr/2 pulse excites an atom initially in the l,p) state into a coherent superposition of states l,p) and 2,p + hkeff). If state 2) is stable against spontaneous decay, the two parts of the wavepacket will drift apart by a distance hkT/m in time T. Each partial wavepacket is redirected by a vr pulse which induces the transitions... 

From a theoretical perspective, the object that is initially created in the excited state is a coherent superposition of all the wavefunctions encompassed by the broad frequency spread of the laser. Because the laser pulse is so short in comparison with the characteristic nuclear dynamical time scales of the motion, each excited wavefunction is prepared with a definite phase relation with respect to all the others in the superposition. It is this initial coherence and its rate of dissipation which determine all spectroscopic and collisional properties of the molecule as it evolves over a femtosecond time scale. For IBr, the nascent superposition state, or wavepacket, spreads and executes either periodic vibrational motion as it oscillates between the inner and outer turning points of the bound potential, or dissociates to form separated atoms, as indicated by the trajectories shown in Figure 1.3. 

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