Linewidth spectroscopy

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 = (Ea — Eb)/h] of the two levels a) and b) with decay constants ya, yt (Fig. 9.82). 

F. Shimizu, K. Shimuiu, Y. Gomi, H. Ihkuma Direct observation of hyperfine splittings of the Li P3/2 state by subnatural linewidth spectroscopy. Phys. Rev. A 35, 3149 (1987)... 

High-resolution spectroscopy used to observe hyperfme structure in the spectra of atoms or rotational stnicture in electronic spectra of gaseous molecules connnonly must contend with the widths of the spectral lines and how that compares with the separations between lines. Tln-ee contributions to the linewidth will be mentioned here tlie natural line width due to tlie finite lifetime of the excited state, collisional broadening of lines, and the Doppler effect. 

Physical background. MAS will narrow the inliomogeneously broadened satellite transitions to give a series of sharp sidebands whose intensity envelopes closely follow the static powder pattern so that the quadnipole interaction can be deduced. The work of Samoson [25] gave real impetus to satellite transition spectroscopy by showing that both the second-order quadnipolar linewidths and isotropic shifts are fiinctions of / and Some combinations of / and produce smaller second-order quadnipolar effects on the satellite lines than... 

The natural linewidth comes from the lifetime, r, of the upper state of a spontaneous transition, which is related to the Einstein A coefficient so that r = A l faster transitions have shorter lifetimes and vice versa, and similarly an allowed transition will have a short lifetime for the upper state whereas forbidden transitions will have a long lifetime. The lifetime consideration is very important in the laboratory where transitions have to occur on the timescale of the experiment, otherwise they are not observed. Hence in the laboratory allowed transitions are observed and in general (but not specifically) forbidden transitions are not seen. For astronomy this does not matter. So what if a forbidden transition has a lifetime of 30 million years - the Universe is 15 billion years old - if you wait long enough it will happen. The rules of spectroscopy need to be understood but in space anything goes ... 

Reynolds S, Oxley DP (1996) Measurement and modeling of vibrational-mode lineshape and linewidth in inelastic electron tunneling spectroscopy. Surf Sci 368 324-329... 

The nature of the adsorbed species can be inferred from the usual chemical parameters, i.e. chemical shifts, linewidths and relaxation times. These latter allow the study of the mobility on the surfaces. As an analytical tool, C-NMR spectroscopy can also be used to determine the concentration of reactants or products as a function of time and hence kinetic constants can easily be determined. As a conclusion, a rather complete kinetic study can be carried out involving the nature of interaction between the admolecule and the surface and eventually the nature of the surface active centers. One can finally arrive at the proposition of a reaction mechanism. 

Dye lasers have been one of the most widely used types of tunable laser. In pulsed conditions, typical peak powers are in the range of 10 -10 W. In the cw regime, reported powers are in the order of watts, with linewidths of around 1 MHz. Due to their flexibility in design and performance, dye lasers have been commonly used in a great variety of spectroscopic techniques, including high-resolution spectroscopy. 

With a laser linewidth of less than IKHz spectroscopy has been carried out on water vapor lines around 1885 cm ( v- 5,3jum) with a resolution never obtained before. Application of the spin-flip laser to P- and Q-Branch absorption of NO with 0.08 cm resolution has been reported by Wood et al... 

In the case of a common upper level the fluorescence from this level due to excitation by the monochromatic wave will reflect this selective population with molecules having only velocity components inside the range Auz. Thus, observing the fluorescence in z direction on any transition from this upper level yields linewidths much smaller than the normal doppler width 322). In this way lines which are not resolved in normal fluorescence spectroscopy can be separated even if their frequency difference is much less than their doppler width 323). 

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