Linewidths vibrational spectroscopy

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

Time-resolved spectroscopy is performed using a pump-probe method in which a short-pulsed laser is used to initiate a T-jump and a mid-IR probe laser is used to monitor the transient IR absorbance in the sample. A schematic of the entire instrument is shown in Fig. 17.4. For clarity, only key components are shown. In the description that follows, only those components will be described. A continuous-wave (CW) lead-salt (PbSe) diode laser (output power <1 mW) tuned to a specific vibrational mode of the RNA molecule probes the transient absorbance of the sample. The linewidth of the probe laser is quite narrow (<0.5 cm-1) and sets the spectral resolution of the time-resolved experiments. The divergent output of the diode laser is collected and collimated by a gold coated off-axis... 

Vibrational structures of several metallofullerenes have been studied by IR and Raman spectroscopy (Dunsch et al., 1998a Hulman et al., 1997 Kikuchi et al., 1993 Lebedkin et al., 1998a,b Pichler et al., 1996). Some of the vibrational absorption lines of Sc2C2 Cg2 (HI) are strongly enhanced if compared with the spectrum of the empty cage (Hulman et al., 1997 Krause et al., 1999 Pichler et al., 1996). With decreasing temperature, a dramatic narrowing of the lines was observed. The linewidth shows an... 

Low-frequetjy dyamics. An NMR timescale is defined by the frequency shifts and linewidths which span the range 10 to 10 Hz, depending on the primary broadening interaction. The effects of dynamical processes on NMR spectra depends on the ratio of the rate of the process to the frequency shift caused by the motion. In absolute terms, these frequency shifts and linewidths are much smaller than other commonly used spectroscopies (IR, Raman, Mossbauer, X-ray), atomic vibrations, and even relatively low-frequency vibrations such as rigid-unit modes. 

For a nonradiative lifetime of 10 13s, the linewidth will be 50 cm-1, causing the rotational structure to disappear and the band to become diffuse. For a shorter nonradiative lifetime, 10 15s, the vibrational structure disappears and the spectrum becomes similar to a continuous spectrum. As spectral lines become broader and broader, their intensity is spread out and becomes lost in the background. However, low-resolution techniques, such as photoelectron spectroscopy or electron impact spectroscopy, can enable detection of strongly predissociated bands (see Table 7.2). 

It is well known, however, that the width of a spectral line, at least in principle, yields information on the dephasing dynamics of the optical transition. Spectral lineshapes of purely electronic transitions in solids unfortunately are seldom determined by dynamic interactions, but, at least at low temperature, quite often by the effects of strain. The observed, named inhomogeneous linewidth is therefore of little interest. In case of vibronic transitions, however, the effect of vibrational relaxation on the lineshape may exceed the inhomogeneous linebroadening. Even so, classical spectroscopy quite often fails to elucidate the nature and strength of the perturbing forces on the optical (homogeneous) lineshape. 

The accuracy of stabilizing the two lasers onto molecular transitions increases with decreasing linewidth. Therefore, the narrow Lamb dips of Doppler-broadened molecular transitions measured with saturation spectroscopy (Sect. 2.2) are well suited [921]. This was proved by Bridges and Chang [922] who stabilized two CO2 lasers onto the Lamb dips of different rotational lines within the vibrational transitions (00°1) (10°0) at 10.4 pm and (00°1) (02°0) at 9.4 pm. The superimposed beams of the two lasers were focused into a GaAs crystal, where the difference frequency was generated. 

Due to the weak coupling between external and internal degrees of freedom, the internal (rotational and vibrational) temperature of the HD" " ions (see Section 18.7.2) is at 300 K, in thermal equilibrium with the vacuum chamber, with a significant (>5%) population for rotational levels up to / = 6. Indeed, 12 transitions between 1391 and 1471 nm, from lower rotational levels 7 = 0 to 7 = 6 were observed using diode laser spectroscopy. A telecom-type diode laser with a linewidth of 5 MHz on... 

Since the Mossbauer effect is intimately related to any motion of the emitting or absorbing nucleus on either a microscopic or macroscopic scale, Mossbauer spectroscopy provides a potential means by which information on nuclear dynamics, and hence on the dynamics of a system in which the Mossbauer nucleus acts as a probe, can be obtained. Any motion of the Mossbauer nucleus can influence the Mossbauer spectrum in two ways. Firstly, because this motion may be related to the vibrational properties of the system it can influence the recoil-free fraction and hence the absorption intensity of the spectrum itself. Since the absolute absorption intensity is dependent on a large number of other factors, which may be diflicult to determine accurately, any change in recoil-free fraction is most usefully followed as a function of temperature in order to obtain information on the vibrational properties of the system. The second way in which the effects of any motion of the Mossbauer nucleus in the source or absorber are manifested is in the Mossbauer spectroscopic linewidths, as this motion can be thought of as an additional Doppler motion which may partially smear out the resonant absorption. Since the linewidths are also... 

As an example of a spectrum obtained with a 10 m instrument (grating radius 10m) a recording of the carbon K emission line from the CO2 molecule is shown in Fig.5.7. As can be seen, the high resolution reveals a clear structure due to molecular vibration. Through careful analysis of a spectrum of this kind it is possible to evaluate the C-O bond length very accurately in the core-ionized molecule. It turns out that the bond length is shortened by about 2% when the Is core vacancy has been formed in the carbon atom. From the linewidth it is also possible to evaluate the natural lifetime of the C Is state (Sect.9.4.5). The lifetime is of the order 10" s. Atomic structure research using X-ray emission spectroscopy has been discussed in [5.9-13]. 

An alternative spectroscopic approach would be to determine the rotational temperature from the intensities in a rotationally resolved vibrational or electronic spectrum. In this case, even if one cannot resolve individual rovihrational transitions, one can still estimate the temperature by simulating the rotational contour of an individual vibronic band [120]. For this, one needs to know the rotational constants of the molecule and the direction of the transition moment however, even rough estimates of these quantities can lead to a reasonable temperature estimate. In measuring either Doppler widths or rotational band contours, the linewidth one obtains may contain a contribution from the finite lifetime of the molecule, determined by its intramolecular vibrational energy redistribution and/or dissociation rate if some type of photofragment spectroscopy is used, and this can make the temperature appear to be higher than it really is. 

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