Atmospheric and astrophysical investigations: the role of rotational spectroscopy

The physico-chemistry of the Earth's atmosphere has been one of the main subjects of studies over last years. In particular, the composition of the atmosphere is indeed very important to understand chemical processes linked to depletion of stratospheric ozone and greenhouse effect. The vertical concentration profiles of atmospheric gases can be provided by remote sensing measurements, but they require the accurate knowledge of the parameters involved: line positions, transition intensities, pressure-broadened half-widths, pressure-induced frequency shifts and their temperature dependence. In particular, the collisional broadening parameters have a crucial influence on the accuracy of spectra calculations and on reduction of remote sensing data. As far as astrophysics is concerned, it is well-known that the knowledge of very accurate rest frequencies is important for astrophysical purposes: precise transition frequencies are essential, for instance, in studies of molecular excitation, radiative transfer, systematic velocity gradients, ambipolar diffusion in star-forming regions, and also for the identification of new species. On a general ground, the investigation of the phenomena that allow to understand the chemistry of the interstellar medium requires laboratory investigations as in turn astronomical observations require the knowledge of the spectroscopic parameters involved. Rotational spectroscopy, thanks to its intrinsic high resolution, is a powerful tool for providing most of the information mentioned above: accurate or even very accurate rotational transition frequencies, accurate spectroscopic as well as hyperfine parameters, accurate pressure-broadening coefficients and their temperature dependence. For instance, by exploiting the Lamb-dip technique it is possible to further increase the high resolution power of rotational spectroscopy and then resolve hyperfine structures and/or measure very accurate rest frequencies. With respect to collisional phenomena and line shape analysis studies, by applying the source frequency modulation technique it has been found that rotational spectroscopy may provide very good results: not only this technique does not produce uncontrollable instrumental distortions or broadenings, but also, having an high sensitivity, it is particularly suitable for this kind of investigations. A number of examples will be presented to illustrate the role of rotational spectroscopy in the of atmospheric and astrophysical investigations with particular emphasis on the work carried out at the Laboratory of Millimeter-wave Spectroscopy of Bologna. As measurements in the THz frequency range have recently opened up, the THz spectrometer working at the University of Bologna and its applications in the frame of atmospheric and astrophysical investigations will be also presented. The usefulness of the interplay of experiment and theory for such investigations will be finally mentioned.