Coherent multiple excitations

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

In the RPA, the linear-response contrihution to the stopping power of equation (15), which is proportional to Z, is fully originated in the creation of single e-h pairs and plasmons. Furthermore, the contrihution to the actual (beyond RPA) Zj stopping power due to coherent multiple excitations such as double plasmons is expected to be relatively small. Nevertheless, accurate measurements of electron energy-loss spectra showed evidence for the existence of coherent double-plasmon excitations [17,18]. 

This argument motivates the idea that the way to control photodissociation is to eii e more than one initial state, or in greater generality, to use multiple excitation pathways. In this chapter we demonstrate that such a strategy allows us to actively /rtipijence and control which photodissociation product is formed. These ideas, which firbduce the notion of coherent control, will be later shown to hold true for any lical process, not just for photodissociation. 

Fixed spatial phase in the grating pattern also facilitates experiments with multiple excitation pulses (20). A second, delayed pulse incident on the diffractive optic is split in the same manner as the first and results in a second excitation pattern with the same peak and null positions. Thus, multiple excitation gratings, delayed temporally and shifted spatially if desired, can be used for excitation of phonon-polaritons whose coherent superposition is well controlled. A preliminary experiment of this type has been reported (21). 

Here we extend the simple three-level EIT system to mote complicated and versatile configurations in a multi-level atomic system coupled by multiple laser fields. We show that with multiple excitation paths provided by different laser fields, phase-dependent quantum interference is induced either constractive or destractive interfereiKe can be realized by varying the relative phases among the laser fields. Two specific examples are discussed. One is a three-level system coupled by bichromatic coupling and probe fields, in which the phase dependent interference between the resonant two-photon Raman transitions can be initiated and controlled. Another is a four-level system coupled by two coupling fields and two probe fields, in which a double-EIT confignration is created by the phase-dependent interference between three-photon and one-photon excitation processes. We analyze the coherently coupled multi-level atomic system and discuss the control parameters for the onset of constructive or destructive quantum interference. We describe two experiments performed with cold Rb atoms that can be approximately treated as the coherently coupled three-level and four-level atomic systems respectively. The experimental results show the phase-dependent quantum coherence and interference in the multi-level Rb atomic system, and agree with the theoretical calculations based on the coherently coupled three-level or four-level model system. 

Next we will examine possible origins of quantum interference in 2PP via the Cs a-resonance on the Ag(lll) surface. Quantum interference occurs when a transition from the same initial and final states can occur coherently via multiple excitation pathways. When taking the coherent sum over all pathways, phase differences cause constructive and destructive interferences between different amplitudes. Accounting only for the resonant interactions, the photoemitted intensity of the 2PP process depends on one-photon transition amplitudes promoting an electron first from an initial state to two possible, intermediate... 

Another approach to obtain spatially selective chemical shift information is, instead of obtaining the entire image, to select only the voxel of interest of the sample and record a spectrum. This method called Volume Selective spectroscopY (VOSY) is a ID NMR method and is accordingly fast compared with a 3D sequence such as the CSI method displayed in Figure 1.25(a). In Figure 1.25(b), a VOSY sequence based on a stimulated echo sequence is displayed, where three slice selective pulses excite coherences only inside the voxel of interest. The offset frequency of the slice selective pulse defines the location of the voxel. Along the receiver axis (rx) all echoes created by a stimulated echo sequence are displayed. The echoes V2, VI, L2 and L3 can be utilized, where such multiple echoes can be employed for signal accumulation. 

Excitation of Single- and Multiple-Quantum Coherences Using rf-Pulses... 

The essential principle of coherent control in the continuum is to create a linear superposition of degenerate continuum eigenstates out of which the desired process (e.g., dissociation) occurs. If one can alter the coefficients a of the superposition at will, then the probabilities of processes, which derive from squares of amplitudes, will display an interference term whose magnitude depends upon the a,. Thus, varying the coefficients a, allows control over the product properties via quantum interference. This strategy forms the basis for coherent control scenarios in which multiple optical excitation routes are used to dissociate a molecule. It is important to emphasize that interference effects relevant for control over product distributions arise only from energetically degenerate states [7], a feature that is central to the discussion below. 

Figure 14.8 (a) General scheme of two-dimensional multiple-quantum (MQ) spectroscopy. During both intervals of free precession (MQ coherences during tx and SQ coherences during t2) dipolar decoupling can be achieved by MAS. Possible pulse sequences used for excitation/reconversion of MQ coherences are (b) broadband... 

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