We report the first study of coherent transients excited by ultrafast pulses of THz radiation. Using a newly developed optoelectronic source of well-collimated beams of subpicosecond pulses of THz radiation to excite N20 vapor, we have observed the subsequent emission from the vapor of coherent THz pulse trains extending to as long as 1 nsec. The origin of these subpicosecond THz pulses (echoes) is a periodic rephasing, during the free-induction decay, of the more than fifty coherently excited rotational lines with commensurate transition frequencies.From the decay and reshaping of the echoes the coherent relaxation time T2 and the anharmonicity factor for the N20 molecule are evaluated.PACS nombers: 42.50.Md, 42.50.Kb, 42.50. gg Coherent transients resulting from the interaction of electromagnetic radiation with resonant systems were first studied for nuclear and paramagnetic spin systems at radio and microwave frequencies. ' Fig. 1(a) and has been 1991 The American Physical Society

Via terahertz (THz) coherent transients and THz time-domain spectroscopy, we have measured the far-wing absorption line profile of the ensembles of collision-broadened ground state rotational lines of methyl bromide, methyl chloride, and methyl fluoride vapors out to more than 200 line widths from resonance, corresponding to frequency offsets as much as 5 times the resonant frequency. On these far wings the measured absorption is approximately an order of magnitude less than that predicted by the van Vleck−Weisskopf theory. Our observations show that as the offset frequency is increased, a transition occurs from the regime of the van Vleck−Weisskopf theory to the regime of the Lorentz theory. These measurements are fit to a new molecular response theory which explicitly includes the molecular orientation time during a collision. Due to the broad bandwidth of the THz pulses, we demonstrate the validity of this molecular response theory for the far-wing absorption of methyl fluoride, chloride, and bromide. The excellent theoretical fit to our measurements encompassing the frequency range over which this transition occurs indicates a molecular response time on the order of 200 fs. These measurements also permit determination of the line-width dependence on the rotational quantum number J.

In order to evaluate how much Total Solar Irradiance (TSI) has influenced Northern Hemisphere surface air temperature trends, it is important to have reliable estimates of both quantities. Sixteen different estimates of the changes in TSI since at least the 19th century were compiled from the literature. Half of these estimates are “low variability” and half are “high variability”. Meanwhile, five largely-independent methods for estimating Northern Hemisphere temperature trends were evaluated using: 1) only rural weather stations; 2) all available stations whether urban or rural (the standard approach); 3) only sea surface temperatures; 4) tree-ring widths as temperature proxies; 5) glacier length records as temperature proxies. The standard estimates which use urban as well as rural stations were somewhat anomalous as they implied a much greater warming in recent decades than the other estimates, suggesting that urbanization bias might still be a problem in current global temperature datasets – despite the conclusions of some earlier studies. Nonetheless, all five estimates confirm that it is currently warmer than the late 19th century, i.e., there has been some “global warming” since the 19th century. For each of the five estimates of Northern Hemisphere temperatures, the contribution from direct solar forcing for all sixteen estimates of TSI was evaluated using simple linear least-squares fitting. The role of human activity on recent warming was then calculated by fitting the residuals to the UN IPCC’s recommended “anthropogenic forcings” time series. For all five Northern Hemisphere temperature series, different TSI estimates suggest everything from no role for the Sun in recent decades (implying that recent global warming is mostly human-caused) to most of the recent global warming being due to changes in solar activity (that is, that recent global warming is mostly natural). It appears that previous studies (including the most recent IPCC reports) which had prematurely concluded the former, had done so because they failed to adequately consider all the relevant estimates of TSI and/or to satisfactorily address the uncertainties still associated with Northern Hemisphere temperature trend estimates. Therefore, several recommendations on how the scientific community can more satisfactorily resolve these issues are provided.

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