Excitation profiles

Because of the two frequencies, Wj and Wg, that enter into the Raman spectrum, Raman spectroscopy may be thought of as a two-dimensional fomi of spectroscopy. Nomially, one fixes oij and looks at the intensity as a frmction of tOj, however, one may vary tOj and probe the intensity as a frmction of tOj - tOg. This is called a Raman excitation profile. 

RRS has also introduced the concept of a Raman excitation profile (REPy for thefth mode) [46, 4lZ, 48, 49, 50 and M]. An REP. is obtained by measuring the resonance Raman scattering strength of thefth mode as a fiinction of the excitation frequency [, 53]. Flow does the scattering intensity for a given (thefth) Raman active vibration vary with excitation frequency within an electronic absorption band In turn, this has led to transfomi theories that try to predict... 

Champion P M and Albrecht A C 1981 On the modeling of absorption band shapes and resonance Raman excitation profiles Chem. Phys. Lett. 82 410-13... 

Remade F and Levine R D 1993 Time domain information from resonant Raman excitation profiles a direct inversion by maximum entropy J. Chem. Phys. 99 4908-25... 

Lee S-Y 1998 Forward and inverse transforms between the absorption lineshape and Raman excitation profiles XVith int. Conf on Raman Spectroscopy ed A M Heyns (New York Wiley) pp 48-51... 

Mantini A R, Marzocchi M P and Smulevich G 1989 Raman excitation profiles and second-derivative absorption spectra of beta-carotene J. Chem. Phys. 91 85-91... 

Kung C Y, Chang B-Y, Kittrell C, Johnson B R and Kinsey J L 1993 Continuously scanned resonant Raman excitation profiles for iodobenzene excited in the B continuum J. Phys. Chem. 97 2228-35... 

Okamoto H, Nakabayashi T and Tasumi M 1997 Analysis of anti-Stokes RRS excitation profiles as a method for studying vibrationally excited molecules J. Phys. Chem. 101 3488-93... 

Kneipp K, Kneipp H and Seifert F 1994 Near-infrared excitation profile study of surface-enhanced hyper-Raman scattering and surface-enhanced Raman scattering by means of tunable mode-locked... 

The integral describes the spatial amplitude modulation of the excited magnetization. It represents the excitation or slice profile, g(z), of the pulse in real space. As drops to zero for t outside the pulse, the integration limits can be extended to infinity whereupon it is seen that the excitation profile is the Fourier transfonn of the pulse shape envelope ... 

A sine-shape has side lobes which impair the excitation of a distinct slice. Other pulse envelopes are therefore more commonly used. Ideally, one would like a rectangular excitation profile which results from a sine-shaped pulse with an infinite number of side lobes. In practice, a finite pulse duration is required and therefore the pulse has to be truncated, which causes oscillations in the excitation profile. Another frequently used pulse envelope is a Gaussian frmction ... 

Yoshino and co-workers also reported the optical response of a heterojunclion device comprising a P30T and C60 bilayer [90]. The photoresponse of these devices shows a broad excitation profile ranging from 750 nm into the UV. 

As illustrated by the spectra of P. furiosus 3Fe Fd shown in Fig. 8, the relative intensities of the Raman bands for [Fe3S4l clusters vary considerably with excitation wavelength. However, because of the extensive mixing of Fe-S and Fe-S modes, excitation profiles in the region 400-650 nm appear to be of little use in effecting electronic... 

Figure 1.15 Time domain representation of a hard rectangular pulse and its frequency domain excitation function. The excitation profile of a hard pulse displays almost the same amplitude over the entire spectral range.

Gaussian pulses are frequently applied as soft pulses in modern ID, 2D, and 3D NMR experiments. The power in such pulses is adjusted in milliwatts. Hard" pulses, on the other hand, are short-duration pulses (duration in microseconds), with their power adjusted in the 1-100 W range. Figures 1.15 and 1.16 illustrate schematically the excitation profiles of hard and soft pulses, respectively. Readers wishing to know more about the use of shaped pulses for frequency-selective excitation in modern NMR experiments are referred to an excellent review on the subject (Kessler et ai, 1991). 

Shoute, L. C. T, Bartholomew, G. P., Bazan, G. C. and Kelley, A. M. (2005) Resonance hyper-Raman excitation profiles of a donor-acceptor substituted distyrylbenzene one-photon and two-photon states./. Chem. Phys., 122,184508. 

The shape of any rf pulse can be chosen in such a way that the excitation profile is a rectangular slice. In the light of experimental restrictions, which often require pulses as short as possible, the slice shape will never be perfect. For instance, the commonly used 900 pulse is still acceptable, while a 1800 pulse produces a good profile only if it is used as a refocusing pulse. Sometimes pulses of even smaller flip angles are used which provide a better slice selection (for a discussion of imaging with small flip angles, see Section 1.7). 

Soft-pulse multiple irradiation In this method, pre-saturation is done using shaped pulses having a broader excitation profile. Therefore, it is a more suitable method for the suppression of multiplets. This technique is very effective, easy to apply and easy to implement within most NMR experiments. In aqueous solutions, however, slowly exchanging protons would be detectable due to the occurrence of transfer of saturation. In addition, the spins with resonances close to the solvent frequency will also be saturated. 

The analysis of carotenoid identity, conformation, and binding in vivo should allow further progress to be made in understanding of the functions of these pigments in the photosynthetic machinery. One of the obvious steps toward improvement could be the use of continuously tuneable laser systems in order to obtain more detailed resonance Raman excitation profiles (Sashima et al 2000). This technique will be suitable for the investigation of in vivo systems with more complex carotenoid composition. In addition, this method may be applied for the determination of the energy of forbidden Sj or 2 Ag transition. This is an important parameter, since it allows an assessment of the energy transfer relationship between the carotenoids and chlorophylls within the antenna complex. 

Sashima, T., Koyama, Y., Yamada, T., and Hashimoto, H. 2000. The lBu+, lBu", and 2Ag" energies of crystalline lycopene, P-carotene, and mini-9-P-carotene as determined by resonance-Raman excitation profiles Dependence of the I IUi state energy on the conjugation length../. Phys. Chem. B 104 5011-5019. 

Sashima, T., M. Shiba, H. Hashimoto, H. Nagae, and Y Koyama. 1999. The 2A energy of crystalline aR-trans-spheroidene as determined by resonance-Raman excitation profiles. Chem. Phys. Lett. 290 36 42. 

The resonance Raman excitation profiles are shown in Figures 8, 9 and 10. The radical modes clearly peak with the electronic transition of Ru(bpy) at ca. 360 nm (Figure 8), demonstrating a bpy 7T 7T assignment for this absorption. The neutral bpy... 

Figure 10, RR excitation profiles of the three bpy modes of Ru(bpy)s2 that exhibit long-wavelength resonance enhancement (see text). Key M> 1016 cm 1 A, 1368 cm 1 and , 1496 cm 1.

The magnitudes of geometric changes in molecules on electronic excitation can be determined from the excitation profiles of resonance-enhanced Raman bands, most accurately where both the resonant absorption band and the profiles show vibronic structure. 

Unfortunately, the above analysis can never be widely applicable to the determination of excited-state geometries since so few molecules and ions exhibit vibronically structured absorption bands and excitation profiles, even at low temperatures. Moreover, some questions arise as to the possible breakdown of the Condon approximation. Other types of molecule for which similar analyses have been carried out include 3-carotene, carotenoids (9) and certain carotenoproteins such as ovorubin (10). In these cases the excitation profiles of three skeletal a bands are monitored, and estimates for the change in C-C and C=C bonds lengths ( 0.02 A) have been made. 

Figure L Absorption spectrum of K[MnOJ/K[CIOJ mixed crystal at room temperature (a), and experimental (O) and best-fit calculated (—) excitation profile of the vt (at) band of [MnOJ" in K[MnOj/K[ClOJ mixed crystal for T/hc = 300 cm1 and 8 = 0.092 A (b).

Frequency-selective REDOR (fsREDOR) is a very powerful technique developed for the study of 13C and 15N uniformly labeled peptides or proteins [92]. The basic idea of this technique is to combine REDOR and soft n pulses to recouple a selected 13C-15N dipole-dipole interaction in a multiple-spin system. Usually one could use Gaussian shaped pulses to achieve the required selective n inversions. Other band selective shaped pulses have been developed for a more uniform excitation profile [93]. In its original implementation, fsREDOR was used to extract the intemuclear distances of several model crystalline compounds [92], In the past few years, this technique has proven to be very useful for the study of amyloid fibrils as well. For the Ure2p10 39 fibril samples containing 13C and 15N uniformly... 

VC=C n the yellow solution. In both cases the experimental data are quite well represented considering that there is only one parameter in the fit. The model also adequately describes the Raman excitation profile. The good agreement between the model and the experimental results confirms the basic premise of the model that the individual polymer chains in solution contain a distribution of chain lengths which determine the absorption characteristics of the solutions. 

In the case of polyenes, the term I scattering is dominant in the Raman spectra resonant with the 1 flu excited states22,26 the contribution of the term m scattering is important in the Raman spectra resonant with the 2 Ag excited states27. In addition, an interference effect between the 1 flu and 2 kg states in a Raman-intensity vs excitation-photon-energy plot (which is called an excitation profile) of the in-phase C=C stretch has been suggested28. 

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