Raman-type transition

In this section we present a brief historical account of LICS without going into too many details, as our main concern is aspects of quantum interferences. LICS was initially suggested by Heller and Popov [4] and by Armstrong et al. [27], who termed the effect "pseudo-autoionization." Early theoretical investigations on LICS focused on coherent population trapping [28, 29], multichannel effects [30-33], and Raman-type transitions [2, 34 0]. 

Figure 3.15 The STIRAP via a continuum experiment. Left panel The coupling scheme in He. The pump and Stokes lasers couple the initially populated state 2s So to the target state 4s So via the ionization continuum (solid lines). Population transferred to the target state can be ionized by the pump laser (dotted line), too. An off-resonant Raman-type transition between the initial and target states is also indicated (dash-dotted lines). Right panel The measured electron signals versus laser tuning of the pump laser in the two-photon resonance range for the counter-intuitive pulse ordering. The circles and the triangles indicate slow and fast electrons, respectively. The solid lines show numerical simulations. Taken from Ref. [237].

In the A-type OODR scheme (Fig. 5.32) the probe laser induces downward transitions from the upper level = 2) of the pump transition to lower levels / = m). This process, which is called stimulated emission pumping (SEP), may be regarded as a resonantly induced Raman-type transition. In case of monochromatic pump and probe lasers tuned to the frequencies co and co2, respectively, the resonance condition for a molecule moving with velocity v is... 

Two-photon induced level crossing [865], which relies on the OODR scheme of Raman-type transitions (Fig. 7.8), has been performed with the two neon transitions... 

Fig. 7.8 Level scheme for two-photon stimulated Hanle effect. Only the sublevels M = 1 of the lower state 11) are pumped by Raman-type transitions...

Two-photon-induced level crossing [12.30], which relies on the OODR scheme of Raman-type transitions (Fig. 12.7) has been performed with the two neon transitions at = 632.8 nm and 3-39 nm, which have the common upper 3Sj level. A HeNe laser is simultaneously oscillating on both transitions. The Hanle signal S(B) is monitored via the fluorescence from the 2P4 level at A = 667.8 nm. 

Three-photon excitation may also be used for a Raman-type process, depicted in Fig. 2.42b, which proceeds via two virtual levels. This Doppler-free technique was demonstrated for the 3 5 i/2 3 Pi/2 transition of the Na atom, where the photons with the momentum hk and hk[ are absorbed while the photon with the momentum hk2 is emitted. The hyperfine structure of the upper and lower states could readily be resolved [266]. 

Three-photon excitation may also be used for a Raman-type process, depicted in Fig. 7.37b, which proceeds via two virtual levels. This Doppler-free technique was demonstrated for the 3 81/2 transition of the... 

The experimental widths in Fig. 6 are clearly still due to incomplete averaging of the Doppler broadening. The degenerate component can be fit to a superposition of a narrow Lorentzian and a wider one. The former is ascribed to a Raman-type resonance between two Zeeman sublevels with m = m 1. Such transitions can be induced if the linear polarization of the beam at (O is orthogonal to that of the beam at 0)2 Thus the narrow component is ascribed a width (AcOjj ) T, proportional to 0 . The Lorentzian fit to this component is indicated by the dashed line in Fig. 6. It is tempting to attribute the broader component to a process involving the diagonal density matrix element with a width P T. Such processes... 

Another related issue is the computation of the intensities of the peaks in the spectrum. Peak intensities depend on the probability that a particular wavelength photon will be absorbed or Raman-scattered. These probabilities can be computed from the wave function by computing the transition dipole moments. This gives relative peak intensities since the calculation does not include the density of the substance. Some types of transitions turn out to have a zero probability due to the molecules symmetry or the spin of the electrons. This is where spectroscopic selection rules come from. Ah initio methods are the preferred way of computing intensities. Although intensities can be computed using semiempirical methods, they tend to give rather poor accuracy results for many chemical systems. 

One effect of mechanical anharmonicity is to modify the Au = t infrared and Raman selection rule to Au = 1, 2, 3,. .., but the overtone transitions with Au = 2, 3,... are usually weak compared with those with Au = t. Since electrical anharmonicity also has this effect both types of anharmonicity may contribute to overtone intensities. 

Quantum effects are observed in the Raman spectra of SWCNTs through the resonant Raman enhancement process, which is seen experimentally by measuring the Raman spectra at a number of laser excitation energies. Resonant enhancement in the Raman scattering intensity from CNTs occurs when the laser excitation energy corresponds to an electronic transition between the sharp features (i.e., (E - ,)" type singularities at energy ,) in the ID electronic DOS of the valence and conduction bands of the carbon CNT. 

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

IR and Raman spectroscopic studies on films and powders of PDHS indicate that the hexyl side chains are crystallizing into a hydrocarbon type matrix (40). This is indicated by the presence of a number of sharp characteristic alkane bands which become dramatically broadened above the transition temperature. Similar changes are observed for n-hexane below and above the melting point. CPMAS 29Si NMR studies on PDHS also show that the rotational freedom of the side chains increases markedly above the transition temperature (41,42). All of the spectral evidence... 

Pendent arm 1,4,7-triazacyclononane macrocycles (91) and (92) have been used to stabilize the zinc-to-phenoxyl bond allowing characterization of these compounds.477 The interest in the zinc complexes comes from the wide potential range in which it is redox stable allowing observation of the ligand-based redox processes, this allows study of the radical by EPR and the electronic spectra is unperturbed by d-d transitions. Macrocycles of the type l,4,7-tris(2-hydroxybenzyl)-1,4,7-triazacylononane form a bound phenoxyl radical in a reversible one-electron oxidation of the ligand. The EPR, resonance Raman, electronic spectra, and crystal structure of the phenoxide complexes were reported. This compound can be compared to a zinc complex with a non-coordinated phenoxyl radical as a pendent from the ligand.735... 

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