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The rotational spectrum of the weak complex between argon and pyrrole has been observed by use of a Fourier transform microwave spectrometer with a pulsed supersonic nozzle molecular beam source. The complex is a nearly symmetric prolate top with the Ar atom located above the plane of pyrrole, 3.553 (10) A from the pyrrole center of mass. The line between the Ar atom and the pyrrole center of mass makes an angle of 5 . 5 (20)' with the normal to the pyrrole plane and is displaced toward the N atom. The dipole moment of the complex, 1.707 (3) D, is smaller than that of free pyrrole, 1.767 (2) D, and the negative end of the complex's dipole moment is rotated from the pyrrole plane toward the Ar atom. The I4N quadrupole coupling constants have been determined. Based on the centrifugal distortion constant 0, and a pseudcdiatomic model, the binding energy of the complex is estimated to be 300 cm-I. The dipole moment and I4N quadrupole coupling constants of pyrrole were redetermined.The laser-induced fluorescence (LIF) spectra of trans-stilbene in a supersonic free jet have been investigated to discuss the vibrational level structure in the SI state. Since the rotational temperature is reduced to as low as 1.5 K, the congested vibronic bands can be resolved to uncover the level structure in the SI state. The rotational band contours have been measured to determine structural parameters of the SI state. It is concluded that in the SI state the C=C bond length, r c e , is larger by 0.05 A and the C=C-C angle, t9c,c--c, is smaller by 10' than those in the So state. Most of the transition lines to the SI vibrational levels above 800 cm-' exhibit splittings caused by Fermi coupling. The number of the observed split bands is larger in a transition to a higher Si vibrational level. Above 1200 cm-' from the band origin, the line broadening is found for respective lines, and this is attributed to a congestion caused by Fermi coupling with dense background levels. The observed change in the level structure caused by 13C or D isotope substitution is ascribed to differences between the level coupling schemes in the normal and isotopic species.

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As part of our continuing spectroscopic program of measuring the rotational spectra of large gas-phase molecules that may exist in observable quantities in the interstellar medium, we have studied the millimeter-and submillimeterwave spectrum of the alcohol n-propanol (n-CH 3 CH 2 CH 2 OH ), a straight-chain molecule with one more carbon atom than ethanol, which has already been detected in hot molecular cores. Although the molecule is thought to exist in five different conformers, we have only studied what is most likely to be the Gauche-trans (Gt) conformer, and analyzed over 2800 lines through 375 GHz in frequency. These lines have been added to both previously and newly measured rotational lines at lower frequencies. The global data set has been fitted to experimental accuracy, allowing us to determine accurate spectral parameters so as to predict useful frequencies for many more transitions of the conformer.