The rotational relaxation of molecular nitrogen has been investigated down to temperatures of about 5 K with a combination of resonance-enhanced multiphoton ionization and supersonic beam time-of-flight techniques. The average rotational relaxation cross section obtained shows a maximum value of 50−60 Å2 at 20−30 K. For lower temperatures this cross section decreases and reaches a value smaller than 30 Å2 at T ≈ 5 K. For temperatures above 30 K, the cross section decreases slowly as the temperature grows and converges approximately to the determinations from other non-jet techniques and theoretical estimates available for T > 80 K. The results are compared to previous measurements from other groups using different methods, and general good agreement is found. However, we observe a significant discrepancy with some of the data from electron-beam-induced fluorescence that yield much larger cross sections for temperatures lower than 30 K.

No abstract

No abstract

No abstract

IntroductionThe measurement of microwave and IR high resolution spectra in freely expanding jets has become almost a routine. However, all attempts to apply Raman technique in similar purposes did not manifest similar spectacular progress up to now. First estimates demonstrating sufficient sensitivity of the nonlinear CARS technique for the studies of molecular association in a jet have been performed by Byer and co-workers [1 ] . Later Nibler et al. [2] observed CARS spectra of the associating carbon dioxide, enabling them to draw preliminary conclusions on the structure of (C02)2 dimers. In a few years a number of coherent Raman spectra have been obtained for H20 [3] , HC1 [4] , C02 [5] , NH3[6] , C2H4 [7] , and HCN [8] complexes. The assignment of the new features to dimers or polymers was based on the analysis of the shifts of the band centers with respect to the monomer transitions. It is well known that in the spectra of hydrogen bonded species these shifts amount to more than several tens of wavenumbers. Therefore, in the spectra of HCN polymers, for example, the attribution of newly appearing bands can be performed even without detailed analysis of their vibro-rotational structure. Significantly weaker intermolecular interaction in C02 dimers results in very small frequency shifts. Accordingjto [9, 10] the shifts of JR active transitions amounts to +1 .628884 cm1 for V3 C02 band, -1 .29 cm for 2v2 +V3 and -0.85 cm1 for vi + V3 bands.An assignment of a new band at 1 28 1 cm1 in the CARS spectrum of an expanding C02 +He mixture to the low frequency Fermi dyad member of (C02)2 has been suggested first in [2] . Recently Nibler et.al. [11] have revised this attribution. They have shown that the first evidences of molecular association appear on the wing of monomeric Q branch as a group of closely and irregularly spaced lines concentrated within ca. 1 cm from the Q branch center vi = 1285.4 cm. The increase in stagnation pressure up to = 14 atm causes the growth of a very complicated structure concentrated in the vicinity of 1281 cm , which has been refered earlier to dimeric band. Further changes of the CARS spectrum with increasing stagnation pressure relate with an appearance of a new band at 1275.5 cm1, surely belonging to large clusters or condensed phase in bulk. In this work we communicate new results of our CARS observations and their theoretical modeling aimed to the detection of dimeric O-8194-7504-9/94/$6.oo SPIE Vol. 2205 High-Resolution Molecular Spectroscopy (1993) / 95 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/26/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspxcarbon dioxide associates. To perform preliminary analysis we have developed a kinetic scheme of dimer formation in expansions of pure carbon dioxide or its mixture with rare gas through the slit or round orifice. The on-axis distributions of (C02)2 dimeric concentrations calculated so far enabled us to choose optimum focusing of the laser beams and to restrict the stagnation pressures to the val...