Excitation schemes

The sinc fiinction describes the best possible case, with often a much stronger frequency dependence of power output delivered at the probe-head. (It should be noted here that other excitation schemes are possible such as adiabatic passage [9] and stochastic excitation [fO] but these are only infrequently applied.) The excitation/recording of the NMR signal is further complicated as the pulse is then fed into the probe circuit which itself has a frequency response. As a result, a broad line will not only experience non-unifonn irradiation but also the intensity detected per spin at different frequency offsets will depend on this probe response, which depends on the quality factor (0. The quality factor is a measure of the sharpness of the resonance of the probe circuit and one definition is the resonance frequency/haltwidth of the resonance response of the circuit (also = a L/R where L is the inductance and R is the probe resistance). Flence, the width of the frequency response decreases as Q increases so that, typically, for a 2 of 100, the haltwidth of the frequency response at 100 MFIz is about 1 MFIz. Flence, direct FT-piilse observation of broad spectral lines becomes impractical with pulse teclmiques for linewidths greater than 200 kFIz. For a great majority of... 

Henrich and WolP have studied the formation of Mo(CO)5 by catching Mo and Mo recoils in Cr(CO)g. The Molybdenum isotopes were produced in several different reactions, so that the recoil energy varied over a wide range. It was found that the yields of Mo(CO)g with the two isotopes differed from each other, but varied only slightly as a function of initial recoil energy. These authors were also able to show that the isotope effect of about 8% is nearly insensitive to radiation received by the sample (and catcher) during the bombardment. They argued that there remains only one possible cause of this isotope effect, that is, differences in the de-excitation schemes of the product nuclei. 

The present excitation scheme of quadratic chirping can be applied to higher dimensional systems easily. As an example, we consider the bond-selective... 

In order to relate the formal expression (41) to the experiment of Section III, we now specialize the discussion to the case of one- versus three-photon excitation, limiting attention for simplicity to fragmentation of diatomic systems, relevant to the experiments of Refs. 30, 32, 33, 35, and 39. We note later, however, that our conclusions are general and equally applicable to other excitation schemes and to ionization processes. 

The generalization to the case of a thermally averaged parent state describes an interesting modulation curve that reflects in position and width the rotational eigenvalue spectrum of the resonant intermediate [31]. This structure has been observed in studies of HI ionization in Ref. 33. A schematic cartoon depicting the excitation scheme and the form of the channel phase for the case of a thermally averaged initial state is shown in Fig. 5g. 

The excitation of the ground state H-atom product (n = 1) is made by the following two-step excitation scheme ... 

Just as above, we can derive expressions for any fluorescence lifetime for any number of pathways. In this chapter we limit our discussion to cases where the excited molecules have relaxed to their lowest excited-state vibrational level by internal conversion (ic) before pursuing any other de-excitation pathway (see the Perrin-Jablonski diagram in Fig. 1.4). This means we do not consider coherent effects whereby the molecule decays, or transfers energy, from a higher excited state, or from a non-Boltzmann distribution of vibrational levels, before coming to steady-state equilibrium in its ground electronic state (see Section 1.2.2). Internal conversion only takes a few picoseconds, or less [82-84, 106]. In the case of incoherent decay, the method of excitation does not play a role in the decay by any of the pathways from the excited state the excitation scheme is only peculiar to the method we choose to measure the fluorescence (Sections 1.7-1.11). 

Fig. 1.14. Multiphoton excitation scheme in Hj showing the laser-induced fluorescence detection method...

Several other chemical lasers with similar excitation schemes soon followed vibrationally excited CO was formed during flash photolysis of a CSj-Oj mixture at 1 torr with 150 torr He buffer gas Investigations of the line spectrum (270 new laser lines have been found 00) gain and power output of induced emission under various helium pressures and with the addition of CO, established that CO was being selectively excited by different mechanisms into different vibrational levels. The experimental results enabled a few reaction schemes to be selected out of several other possibilities, which could be excluded. 

Figure 2.4 displays (a) room temperature PARS spectrum, (b) jet-cooled action spectrum, (c) REMPI spectrum of CH3NH2, and, on the right side of each panel, the respective excitation schemes. The spectra are characterized by a multiple peak structure, related to the (7-branches of different bands, and the peaks of the action spectrum show up whenever the difference frequency of the SRS laser beams matches that of a specific vibrational transition. However, whereas in the PARS spectrum two of the peaks, of the degenerate CH3 stretch, V2 (2961 cm ) and the CH3 symmetric stretch, V3 (2820 cm ), are dominant and the others quite weak, in the action spectrum all peaks carry significant intensity and, in particular, the PARS low-intensity peaks become prominent. An additional dominant peak, in both the PARS and action spectra, due to the NH2 symmetric stretch, Vj (3361 cm ), is beyond the wave number span of the figure. 

Figure 19. Excitation scheme for probing bound-free transitions. Initially the photoion of the mother molecule appears. After dissociation, however, the relating fragment ion is observed.

L. Woste Infrared multiphoton excitation schemes have theoretically and experimentally well been studied over the past 15 years. If now we repeat this at femtosecond time scales and intensities, should not potential energy deformations, as proposed by Prof. Manz, be considered, or are they inherently incorporated into the consideration ... 

R. W. Field I must apologize for not being sufficiently clear about the excitation scheme we use for our acetylene experiments. Although the initial and final states are both on the acetylene X1 g surface, the final state we prepare is the result of two electronic transitions (A X followed by A —X) rather than one vibrational-rotational infrared or Raman transition. There is a profound difference between the knowledge of the excitation function needed to describe electronic versus vibrational processes. 

Here Wn is the average energy of the two fine structure levels. The real numerical coefficients a and b, where a2+b2=1, depend on the polarizations used in the excitation scheme, but are constant in time. Thus the relative amounts of d5/2 and d3/2 states do not change with time but simply decay together at the radiative decay rate T. However, the relative amounts of m character oscillate at the fine structure frequency, and this oscillation is manifested in any property which depends upon m, such as the fluorescence polarized in a particular direction, or the field ionization signal due to a particular value of m. This fact becomes more apparent... 

Fig. 22.8 The two different excitation schemes A and B used to observe the Ba 5d7d perturber as a forced autoionization resonance. Path A leads to a q parameter near zero, while B leads to a large q parameter (from ref. 21).

Common to all narrow-bandwidth excitation schemes is sequential scanning of an experimental parameter in order to adjust the Raman shift in CRS detection. In order to obtain an entire CRS spectrum, this is not only time consuming but also prone to sources of noise induced by fluctuations in laser pulse parameters. As a consequence, dynamical changes in a CRS spectrum are difficult to follow. This problem can be circumvented by use of multiplex CRS spectroscopies [48, 49], which will be discussed in combination with CARS and SRS microscopy in Sects. 6.3 and 6.4, respectively. 

Cutting, B., et al., Sensitivity enhancement in saturation transfer difference (STD) experiments through optimized excitation schemes. Magn Reson Chem, 2007, 45, 720-724. 

Using the two-laser excitation scheme illustrated in Figure 11.7 Vander Wal, Scott, and Crim (1991) measured part of the absorption spectrum for excitation of the 40 ) state. Figure 1.9 shows the comparison with the result of a two-dimensional calculation. The corresponding bound-state wavefunction, depicted in Figure 13.4, qualitatively resembles the 130 ) wavefunction with one additional node along the reaction path. Accordingly, the absorption spectrum has three instead of two pronounced... 

FIGURE 7 Potential energy curves for Na2, showing the excitation scheme for incoherent phase control of the photodissociation of the molecule. [Reproduced with permission from Chen, Z., Shapiro, M., and Brumer, P. (1993). J. Chem. Phys. 98, 6843. Copyright American Institute of Physics.]... 

FIGURE 22 Three-level excitation scheme used for STIRAP. [Reproduced with permission from Bergmann, K., Theuer, H., and Shore, B. W. (1998). Rev. Mod. Phys. 70, 1003.]... 

Figure 3-20. (a) Schematic of sequential two-photon HF excitation scheme and (b) HF absorption spectrum. Note that HF absorption above 150 nm (66, 700 cm" ) is very weak from Lee (1985). 

Figure 4-18. Energy diagram for phenol(NH3) for a cluster size of n s 5. The excitation scheme shows how delayed ionization by X2 can produce cluster ions in the outer potential well for reactive PhOH B clusters excited by ).y (from Steadman and Syage 1991).

In this section, we will discuss signal excitation methods. We start with the commonly used pulse sequences before moving on to new and hybrid excitation schemes. 

The excitation schemes, or pulse sequences, that optimize the SNR depend on the relaxation times and frequencies inherent to the quadrupolar nuclei under observation and on the characteristics of the excitation/detection hardware. Focusing first on relaxation times, we define two broad categories (i) 7 , 1 and (ii) 7 2 Tl. Most materials fall in the former category, and the pulse sequences discussed in Sector 4.1.1. all involve the creation and detection of multiple spin echoes, which generally requires that r2e be at least several tens of milliseconds. The most notable exception is RDX, which is in the second category under ambient conditions. For materials with very short 7 ,s such as RDX at elevated temperatures, a single echo pulse sequence, or even a single-pulse FID acquisition, may be required. 

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