Quadrupole modulation functions

Small corrections due to the somewhat different target temperatures employed in the various runs can be made unambiguously, since the temperature dependence of the electric field gradient Vzz for Pb in Hg has been experimentally determined [MAH79]. Fig.2 PAD quadrupole modulation functions... 

The physical origins of the effect can be illustrated from Figure 2, which shows the energy level scheme for an I = 1 nucleus, such as coupled to an S = 1/2 electron spin. In the case considered here (i.e., the electron-nuclear interaction, the nuclear Zeeman interaction, and the nuclear quadrupole interaction, all of the same order), microwaves can induce both allowed and semi-forbidden transitions between states in the Mj = 1/2 manifold (a) and the Ms = - 1/2 manifold (/ ). Simultaneous excitation of both kinds of transitions by the echo generating microwave pulses gives rise to interference effects, which manifest themselves as variations in the echo amplitude and thus cause the modulation of the echo envelope. Where a number of nuclei are coupled to the same electron spin, the level scheme becomes more complicated, but it is possible to factor out contributions due to coupling with each nucleus in the overall modulation pattern. If v(U l2>" n) is the modulation function due due to coupling with n nuclei, then... 

ESEEM is a pulsed EPR technique which is complementary to both conventional EPR and ENDOR spectroscopy(74.75). In the ESEEM experiment, one selects a field (effective g value) in the EPR spectrum and through a sequence of microwave pulses generates a spin echo whose intensity is monitored as a function of the delay time between the pulses. This resulting echo envelope decay pattern is amplitude modulated due to the magnetic interaction of nuclear spins that are coupled to the electron spin. Cosine Fourier transformation of this envelope yields an ENDOR-like spectrum from which nuclear hyperfine and quadrupole splittings can be determined. 

The polymer sample (35 mg) was pyrolyzed in a quartz cell which was directly attached to the inlet flange of a quadrupole mass spectrometer. Gases evolved from the pol3raier compound were dynamically sampled via a 1.0-mm diameter orifice, formed into a modulated molecular beam, and mass analyzed. Information was obtained on the total yield of volatile products, product composition, and individual product yields as a function of temperature. 

Spin relaxation in a nucleus is induced by random fluctuations of local magnetic fields. These result from time-dependent modulation of the coupling energy of the resonating nuclear spin with nearby nuclear spins, electron spins, quadrupole moments, etc. Any time-dependent phenomenon able to modulate these couplings can contribute to nuclear relaxation. The distribution of the frequencies contained in these time-dependent phenomena is described by a correlation function, characterized by a parameter Tc, the correlation time. Its reciprocal can be considered as the maximum frequency produced by the fluctuations in the vicinity of the nuclear spin. If more than one process modulates the coupling between the nuclear spin and its surroundings, the reciprocal of the effective correlation time is the sum of the reciprocals of the various contributions... 

AGC [12] was a big leap forward, which allowed variable ionization times as a function of ion population. This feature controlled the space-charge effect allowing the user to generate quadrupole-like mass spectra for good matches against known databases. Axial modulation was the next big step in allowing the chemist to tune the instrument with standard compounds that were readily available, namely bromofluorobenzene (BFB) and decafluorotriphenylphosphine (DFTPP). The use of... 

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