Coherence and Transit Narrowing

For transitions between two atomic levels a) and b) with the lifetimes a combination of level-crossing spectroscopy with saturation ef- 

Another technique of subnatural linewidth spectroscopy is based on transient effects during the interaction of a two-level system with the monochromatic wave of a cw laser. Assume that the system is irradiated by the cw monochromatic wave E = Eq cos cot, which can be tuned into resonance with the energy separation [coab = (E - E y)/h] of the two levels a) and b) with decay constants ya, Kb (Fig-14.55). 

If the level b) is populated at the time / = 0 by a short laser pulse one can calculate the probability /a(, 0 to find the system at time t in the level a) as a function of the detuning A=o) coab of the cw laser. Using time-dependent perturbation theory one gets [14.142,14.143]  

If the fluorescence emitted from a) is observed only for times t T, one has to integrate (14.71). This yields the signal 

Measuring the intensity of the laser-induced fluorescence Ipi(B) as a function of the magnetic field B, one obtains a Hanle signal which depends nonlinearly on the laser intensity and which has a halfwidth 7b(II) 7b xvT+S, S = Il/Is being the saturation parameter (Sect.7.2). 

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The forward spectrum of sodium at density of 1.8 10 cm irradiated by laser with frequency loi to the blue of the D2 transition is shown in figure 1. This spectrum contains, in addition to the laser radiation, a spectrally broad off-axis conical emission attributed by us to a Cherenkov-type emission, and a narrow coherent peak to the blue of the Di transition - the "blue peak". This spectrally narrow emission is on-axis and has a threshold on the laser intensity. Thus, this emission is a stimulated one. Within the resolution of our spectrometer, its frequency does not depend on the laser detuning from the D2 transition. 

Considering the resolution of the nuclear frequency spectrum, this two-pulse echo experiment is not optimal. The nuclear frequencies are here measured as differences of frequencies of the ESR transitions, so that the line widths correspond to those of ESR transitions. The nuclear transitions have longer transverse relaxation times Tin and thus smaller line widths. In fact, if the second mw pulse is changed from a n pulse to a Ji/2 pulse, coherence is transferred to nuclear transitions instead of forbidden electron transitions. This coherence then evolves for a variable time T and thus acquires phase v r or vpT. Nuclear coherence cannot be detected directly, but can be transferred back to allowed and forbidden electron coherence by another nil pulse. The sequence (jt/2)-x-(Jt/2)-r-(jt/2)-x generates a stimulated echo, whose envelope as a function of T is modulated with the two nuclear frequencies v and vp. The combination frequencies v+ and v are not observed. The modulation depth is also 8 211. The lack of combination lines simplifies the spectrum and the narrower lines lead to better resolution. There is also, however, a disadvantage of this three-pnlse ESEEM experiment. Depending on interpulse delay x the experiment features blind spots. Thus it needs to be repeated at several x values. 

The electronic absorption spectra of complex molecules at elevated temperatures in condensed matter are generally very broad and virtually featureless. In contrast, vibrational spectra of complex molecules, even in room-temperature liquids, can display sharp, well-defined peaks, many of which can be assigned to specific vibrational modes. The inverse of the line width sets a time scale for the dynamics associated with a transition. The relatively narrow line widths associated with many vibrational transitions make it possible to use pulse durations with correspondingly narrow bandwidths to extract information. For a vibration with sufficiently large anharmonicity or a sufficiently narrow absorption line, the system behaves as a two-level transition coupled to its environment. In this respect, time domain vibrational spectroscopy of internal molecular modes is more akin to NMR than to electronic spectroscopy. The potential has already been demonstrated, as described in some of the chapters in this book, to perform pulse sequences that are, in many respects, analogous to those used in NMR. Commercial equipment is available that can produce the necessary infrared (IR) pulses for such experiments, and the equipment is rapidly becoming less expensive, more compact, and more reliable. It is possible, even likely, that coherent IR pulse-sequence vibrational spectrometers will... 

Vibrational echo experiments permit the use of optical coherence methods to study the dynamics of the mechanical degrees of freedom of condensed phase systems. Because vibrational transitions are relatively narrow, it is possible to perform vibrational echo experiments on well-defined transitions and from very low temperature to room temperature or higher. Further, vibrational echoes probe dynamics on the ground state potential surface. Therefore, the excitation of the mode causes a minimal perturbation of the solvent. 

In 1995 Frydman et al. realized that the line narrowing of the central transition can be achieved by changing the coherence state of the spins instead of spatial reorientation of the spinning axis [57,58], and proposed a third method of avoiding second-order quadrupolar broadening, known as Multiple-Quantum MAS (MQ-MAS). This two-dimensional method uses fast MAS and cor-... 

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