Nuclear spin temperature

Fig. 7.31 The steady-state value of the optical nuclear-spin polarisation p = pi n a fluorene-dghio crystal doped with acridine, measured at room temperature, as a function of the orientation of the polarising magnetic field whose value is Bp = S mT. Pi is the equilibrium polarisation, and T5 is the nuclear-spin temperature. From [35].

Figure 5.12 shows the J= — 0 transition of the linear molecule cyanodiacetylene (H—C=C—C=C—C=N) observed in emission in Sagittarius B2 (Figure 5.4 shows part of the absorption spectrum in the laboratory). The three hyperfine components into which the transition is split are due to interaction between the rotational angular momentum and the nuclear spin of the nucleus for which 1= 1 (see Table 1.3). The vertical scale is a measure of the change of the temperature of the antenna due to the received signal. 

As in the case of hydrogen and tritium, deuterium exhibits nuclear spin isomerism (see Magnetic spin resonance) (14). However, the spin of the deuteron [12597-73-8] is 1 instead of S as in the case of hydrogen and tritium. As a consequence, and in contrast to hydrogen, the ortho form of deuterium is more stable than the para form at low temperatures, and at normal temperatures the ratio of ortho- to para-deuterium is 2 1 in contrast to the 3 1 ratio for hydrogen. 

The idea of having two distinct quasi-Fermi levels or chemical potentials within the same volume of material, first emphasized by Shockley (1), has deeper implications than the somewhat similar concept of two distinct effective temperatures in the same block of material. The latter can occur, for example, when nuclear spins are weakly coupled to atomic motion (see Magnetic spin resonance). Quasi-Fermi level separations are often labeled as Im p Fermi s name spelled backwards. 

Similar principles apply to ortho- and para-deuterium except that, as the nuclear spin quantum number of the deuteron is 1 rather than as for the proton, the system is described by Bose-Einstein statistics rather than the more familiar Eermi-Dirac statistics. Eor this reason, the stable low-temperature form is orriio-deuterium and at high temperatures the statistical weights are 6 ortho 3 para leading to an upper equilibrium concentration of 33.3% para-deuterium above about 190K as shown in Eig. 3.1. Tritium (spin 5) resembles H2 rather than D2. 

This equation is called the Curie law and relates the equilibrium magnetization M0 to the strength of the magnetic field B0. The constants have the following meaning I is the nuclear spin quantum number (see below), y is the gyromagnetic ratio specific for a given isotope, h is Planck s constant, kB is Boltzmann s constant, N is the number of nuclei and T is the temperature. 

The rotational temperature of H2O in the molecular beam is quite low, about 10 K. As in the hydrogen molecule, the water molecule has para and ortho rotational levels with nuclear spin-statistics of 1 3 respectively. Since the para and ortho rotational levels have different nuclear wavefunc-tions, the conversion between the para and ortho levels is extremely slow, as in the hydrogen molecule. In H2O, the nuclear spin-statistics for the lowest rotational levels are as follows ... 

TEMPO, and any of its (not too bulky) derivatives, is comparable in molecular mass with the spin trap DMPO, so the tumbling in water at ambient temperature should again average out all anisotropy. The spectrum is even simpler (namely, three identical lines see the high-temperature traces in Figure 10.4) than that of the hydrox-yl-DMPO adduct because only the 14N nuclear spin contributes to the spectrum ... 

In very pure hydrogen, there can be hardly any permanent chemical change produced by irradiation. However, the ion-molecule reaction (5.1) does occur in the mass spectrometer, and it is believed to be important in radiolysis. The H2 molecule can exist in the ortho (nuclear spin parallel) or para (antiparallel) states. At ordinary temperatures, equilibrium should favor the ortho state by 3 1. However, the rate of equilibration is slow in the absence of catalysts but can be affected by irradiation. Initially, an H atom is produced either by the reaction (5.1) or by the dissociation of an excited molecule. This is followed by the chain reaction (H. Eyring et al, 1936)... 

Before reviewing existing examples, a very brief explanation on the mechanisms of decoherence for molecular spin qubits is necessary more details are available elsewhere [67]. Broadly speaking, the three decoherence sources for these systems are spin bath decoherence, oscillator bath decoherence and pairwise dipolar decoherence, and can be regulated by a combination of temperature, magnetic field and chemical design of the system [70]. The spin bath mainly consists of nuclear spins, but in general it also includes any localized excitations that can couple to the... 

---