Astrophysical investigations: the role of rotational spectroscopy

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 astronomical observations require the knowledge of either the spectroscopic parameters or the transition frequencies 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 [1,2]. 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 a high sensitivity, it is particularly suitable for this kind of investigations [3,4]. A number of examples will be presented to illustrate the role of rotational spectroscopy in the field of astrophysical investigations with particular emphasis on the work carried out at the Laboratory of Millimeter-wave Spectroscopy of Bologna. The frutiful support by quantum-chemical calculations will be also pointed out [2,5]. [1] G. Cazzoli, C. Puzzarini and A. V. Lapinov, Astrophys. J., 592, L95 (2003). [2] G. Cazzoli, C. Puzzarini, S. Stopkowicz and J. Gauss, Astron. Astrophys., 520, A64 (2010). [3] C. Puzzarini, G. Cazzoli and L. Dore, J. Mol. Spectrosc., 216, 428 (2002). [4] G. Cazzoli, C. Puzzarini, J. Quant. Spectrosc. Radiat. Transfer., 113, 1015 (2012). [5] C. Puzzarini, J. F. Stanton and J. Gauss, Int. Rev. Phys. Chem., 29, 273 (2010).