Laser excitation energies

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. 

Since the separation energies between these sharp features in the 1D DOS are strongly dependent on the CNT diameter, a change in the laser excitation energy may bring into optical resonance a CNT with a different diameter. However,... 

Figure 1.14 Raman spectra from a 0.1 wt% Mo03/y-AI203 catalyst obtained by using different (488, 325, and 244 nm) laser excitation energies [108], The UV-Vis absorbance spectrum is reported in the inset to indicate that while the catalyst does not absorb light in the visible region, it does show two UV absorption peaks at 290 and 220 nm. The data clearly illustrate the advantage of using ultraviolet (244 nm) light for Raman excitation, since the spectrum obtained with visible (488 nm) radiation is dominated by the fluorescence of the solid. (Reproduced with permission from Elsevier.)...

Fig. 6.17 Tunnelling and saddle point ionization in Li. (a) Experimental map of the energy levels of Li m = 1 states in a static field. The horizontal peaks arise from ions collected after laser excitation. Energy is measured relative to the one-electron ionization limit. Disappearance of a level with increasing field indicates that the ionization rates exceed 3 x 105 s 1. The dotted line is the classical ionization limit given by Eqs. (6.35) and (6.36). One state has been emphasized by shading, (b) Energy levels for H (n = 18-20, m = 1) according to fourth order perturbation theory. Levels from nearby terms are omitted for clarity. Symbols used to denote the ionization rate are defined in the key. The tick mark indicates the field where the ionization rate equals the spontaneous radiative rate, (c) Experimental map as in (a) except that the collection method is sensitive only to states whose ionization rate exceeds 3 x 105 s-1. At high fields, the levels broaden into the continuum in agreement with tunnelling theory for H (from ref. 32).

Under the assumption of sufficiently high laser excitation energies, the heat loss at peak particle temperature is dominated by sublimation. Hence, the maximal signal is proportional to the particle volume... 

M. 5. Identification of the Constituents of Double-Walled Carbon Nanotubes using Raman Spectra Taken with Different Laser-Excitation Energies. J. Mater. Res. 2003, 8, 1251-1258. 

So-called intermediate frequency modes (IFM) lie in the spectral region between corbm and coq (600-1100 cm ) [3]. Some of IFMs, which have the dispersive dependence on laser excitation energy, are attributed to combination modes described by DR theory. 

Figure 1.21. Dependence of the partial amplitude a2 of the second component from the laser excitation energy for the para-substituted p-C 1PX (p-ClPi [ ], p-ClP4 [A]) and meta-substituted peryleneimide dendrimers m-C 1PX (ra-ClPx [ ]. m-ClP4 [AD-...

Large steady-state population inversions can occur between vibrational modes with minimum laser excitation energy. ... 

Few numerical values can be given for LEI in flames. In a flame at 2500 K, the rate of ionization of an element is enhanced ca. 100-fold for each electron volt (1.6x 10 J) of laser excitation energy. If the energy gap between the excited level and the ionization continuum is less than 1 eV. the probability of ionization within 10 ns is close to unity. If cw laser excitation is used 100% ionization can be achieved for an energy gap of ca. 3 eV (and less). 

To assess this qnestion in the present case, we characterized in detail the laser-induced spectroscopy of PM567 dye both in liquid and solid solutions with and without POSS nanoparticles, pumped rmder the same experimental conditions. The pump pulses were now incident on the samples at a 30° angle and the laser excitation energy was gradually increased from 0.9 to 6 mJ/pulse. The emission from the front-face of the sample was collected with an optical fiber, sent to an spectrometer with 0.1 nm resolution and detected with a CCD camera. 

Fig. 7.10. Emission spectra of phthalocyanine in helium droplets (N = 4 10 ), for four different laser excitation energies ve- The upper traces in each frame are vertically shifted and enlarged by a factor as indicated.The spectra consist of two sets of identical structures with the only difference that the line intensities of the red shifted set increases at the expense of the blue shifted set with increasing excitation energy.

Figure 12.6. Representative high-pressure Raman spectra of Ge QDs with different laser excitation energies, [3].

With increasing pressure, the first-order Si Raman peak at 521 cm shifts to higher frequencies with a pressure coefficient of 0.52 cm kbar which can be used as an internal measure of the pressure [22]. It is noteworthy that the linewidth of the Si-Ge mode, observed at 419 cm at ambient pressure, broadens and the peak blueshifts with pressure. As pressure increases, the spectrum at 304 cm splits into two peaks. As discussed earlier in Section 12.3.1, the Ge-Ge and Si-2TA modes overlap at ambient pressure, making it hard to distinguish them. Under pressure, the Ge-Ge mode shows a blueshift and the Si-2TA mode shows a redshift. As a result, these two peaks are clearly resolved. We have also used different laser excitation energies El for the measurement of Raman spectra at various pressures in the Ge QDs as shown in Figure 12.6. The spectra are normalized to the substrate Si phonon intensity. 

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