Quadrupole doublet

The quadrupole doublet has an isomer shift corresponding to iron in the ferric or Fe " state. After reduction in H2 at 675 K the catalyst consists mainly of metallic iron, as evidenced by the sextet, along with some unreduced iron, which gives rise to two doublet contributions of Fe " and Fe " in the centre. The overall degree of iron reduction, as reflected by the relative area under the bcc ion sextet, is high. Fischer-Tropsch synthesis at 575 K in CO and FI2 converts the metallic iron into the Flagg carbide, Fe5C2. The unreduced iron is mainly present as Fe ". Exposure of the carburized catalyst to the air at room temperature leaves most of the carbide phase unaltered but oxidizes the ferrous to ferric iron. 

Figure 4. Moessbauer spectrum of Fe enriched FeZSM-5 recorded at 90 K. Solid line is the best fit obtained with the two quadrupole doublets. The sample was calcined at 820 K, excess of inactive iron was removed by oxalate extraction.

This procedure is applicable if the relaxation between the spin states is fast (t<1 X 10 s) and thus the quadrupole doublets of the two spin states collapse into one. It is found that, in the intermediate temperature range, the widths of the two lines are significantly enlarged. This shows that the assumption of fast relaxation strictly does not apply. In spite of this, the areas of the lines ean be well reproduced within the Debye model employing the same Debye temperature for both spin states, p 123 K. 

Fig. 22. The fraction n, of the S = ijl state as a function of il . The values at the lower temperatures ( ) result from a fit of the Mossbauer spectra of Fe(J-mph)NO by two symmetrical quadrupole doublets. The values at the higher temperatures (A) are extrapolated. According to Ref. [164]...

Using the value t = 0.2 for the effective thickness, the amount of resonance nuclei ( Fe) for a good thin absorber can be easily estimated according to the relation = tl(fA-(to)- For a quadrupole doublet with two equal absorption peaks of natural width and a recoil-free fraction of the sample/a = 0.7 one obtains... 

Thus, one may state as a rule of thumb that an absorber should have about 21 pg cm of Fe (0.37 pmol cm ) or ca. 1 mg cm of natural iron (17.9 pmol cm ) for a symmetric quadrupole doublet with natural line width. 

In a conventional Fe Mossbauer experiment with a powder sample, one would observe a so-called quadrupole doublet with two resonance lines of equal intensities. The separation of the lines, as given by (4.36), represents the quadrupole splitting The parameter Afg is of immense importance for chemical applications of the Mossbauer effect. It provides information about bond properties and local symmetry of the iron site. Since the quadrupole interaction does not alter the mean energy of the nuclear ground and excited states, the isomer shift S can also be derived from the spectrum it is given by the shift of the center of the quadrupole spectrum from zero velocity. 

Mossbauer spectroscopy is particularly suitable to study ST since (1) the spectral parameters associated with the HS and LS states of iron(II) clearly differ and (2) the time-scale of the technique ( 10 s) allows the detection of the separate spin states in the course of the transition. Typically, Mossbauer spectra of HS iron(II) show relatively high quadrupole splitting (AEq 2-3 mm s ) and isomer shift (3 1 mm s ), while for LS iron(II), these parameters are generally smaller (AEq < 1 mm s 3 < 0.5 mm s ). Among the early applications of Mossbauer spectroscopy to study ST phenomena in iron(II) complexes is the work of Dezsi et al. [7] on [Fe (phen)2(NCS)2] (phen = 1,10-phenanthroline) as a function of temperature (Fig. 8.2). The transition from the HS ( 12) state (quadrupole doublet of outer two lines with AEq 3 mm s ) to the LS CAi) state (quadrupole... 

At room temperature, the spectmm shows only a HS quadrupole doublet characteristic of the HS T2) state. On lowering the temperature, a new quadrupole doublet (inner two lines with AEq 0.5 mm s characteristic of the LS state) develops. The thermally induced ST occurs abruptly near 178 K (from [7])... 

Fig. 8.3 Fe Mossbauer spectra of [Fe(HB(3,5-(CH3)2pz)3)2] (pz = pyrazolyl) obtained at the indicated temperatures. The spectra were fitted with two symmetric quadrupole doublets the outer two lines refer to the HS state and the inner (unresolved quadrupole doublet) to the LS state (fiom [10])...

Fig. 8.4 Fe Mossbauer spectra of [Fe(2-pic)3]Cl2 C2H50H as a function of temperature source Co/Cu at room temperature). The outer two lines represent the quadrupole doublet of the T2 (HS) state, the inner two lines that of the Aj (LS) state. The solid lines are obtained by a least-squares computer fit of Lorentzian lines to the experimental spectra (from [11])...

Fig. 8.16 Fe Mossbauer spectra of [Fe2 (PMAT)2](BF4)4-DMF at selected temperatures. At 298 K, the only quadrupole doublet is characteristic of iron(II) in the HS state. SCO from HS to LS occurs at one Fe(II) site of the dinuclear complex at ca. 225 K. The second Fe(II) site remains in the HS state, but feels the spin state conversion of the neighboring atom by local distortions communicated through the rigid bridging ligand, giving rise to a new quadrupole doublet (dark gray), i.e., HS in [HS-LS], in the Mossbauer spectrum. The intensity ratio of the resonance signals of HS in [HS-LS] to that of LS (black) in [HS-LS] is close to 1 1 at all temperatures (from [32])...

Fig. 8.18 Fe Mossbauer spectra of the trinuclear complex [Fe3(iptrz)6(H20)6](Trifl)6 recorded at the indicated temperatures. Only the central Fe(Il) site undergoes thermally induced ST, HScfred) LS (blue). The outer two Fe(II) sites, which remain in HS state at all temperatures, feel the ST occurring at the central iron site through a molecular distortion, the latter giving rise to a new quadrupole doublet, HSq HSo ° (from [35])...

Fig. 8.22 Left. Fe Mossbauer absorption spectra of [Fe(phen)2(NCS)2] as a function of temperature versus Co/Rh as source (which was kept at 295 K). Right. Time-integral Fe Mossbauer emission spectra of [ Co/Co(phen)2(NCS)2] as source as a function of temperature versus ILiiFe (CN)g as absorber (which was kept at 295 K). Assignment The quadrupole doublet with the larger quadrupole splitting (ca. 3 mm s ) refers to the Fe(ll)-HS state, and that with the smaller quadrupole splitting (ca. 0.5 mm s ) to the Fe(II)-LS state (from [43])...

NFS spectra recorded at 300 K for -cut and c-cut crystals are shown in Fig. 9.17 [48]. The/factors for the two orientations were derived from the speed-up of the nuclear decay (i.e., from the slope of the time-dependent intensity in Fig. 9.17a and from the slope of the envelope in Fig. 9.17b). The factors obtained f ( P = 0.122 (10) and f = 0.206(10) exhibit significant anisotropic vibrational behavior of iron in GNP. This anisotropy in f is the reason for the observed asymmetry in the line intensity of the quadrupole doublet (in a conventional Mossbauer spectrum in the energy domain) of a powder sample of GNP caused by the Goldanskii-Karyagin effect [49]. 

An orientation-dependent asymmetry in line intensity of a quadrupole doublet is, in contrast to the aforementioned case, caused by the polarization response of... 

Each of the two beamlines are focussed by a single electromagnetic quadrupole doublet, and a beam size of about 1 pm2 is achieved with a beam current of approximately 50 pA. Electrostatic deflectors are mounted to permit beam sweeping on the sample both to produce images and to limit heating effects. 

Figure 3.27 shows the Mossbauer spectrum that results from splitting of the 57Fe excited state, a quadrupole doublet, for a sample containing randomly oriented molecules such as found in polycrystalline solids or frozen solutions. The two doublets are separated in energy by the quadrupole splitting, A Eq, defined by the... 

Fig. 7 The Mossbauer spectra of [Fe(HB(3,5-(CH3)2pz)3)2] obtained at the indicated temperatures and fitted with symmetric quadrupole doublets...

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