No abstract

No abstract

We have performed terahertz time-domain spectroscopy measurements on three types of polystyrene foam. We find that between 0.2 and 4 THz, the extinction of this material is low and that it has a remarkably low refractive index of 1.017 to 1.022 in this range, with little variation as a function of frequency. In foam produced with HCFC 142b gas (1-chloro-1,1-difluoroethane) as the blowing agent, we find an additional absorption band at 0.5 THz caused by rotational transitions in this gas. The low extinction and refractive index make polystyrene foam a very suitable material to be used as a dichroic filter that blocks the near-IR and transmits THz radiation with small losses of less than 1.5 cm Ϫ1 for frequencies of Ͻ4 THz, and as a substrate for THz imaging.

No abstract

Several mechanisms have been identified for the polarisation transfer from the electron spins to the nuclear spins in DNP. Unfortunately, these were designated variously, sometimes even confusingly. Different processes were sometimes given the same name and different names were occasionally given to the same process. Distinguishing the various mechanisms experimentally proved to be notoriously difficult, adding to the confusion. Trying to bring some order in the chaos, this paper presents a historical account of the theoretical developments, including a rough description of the various mechanisms and some proposals for unequivocal names for the various mechanisms and processes.