Rotational cooling of aniline in axis-symmetric and planar pulsed supersonic expansions

Abstract

Laser-induced fluorescence (LIF) has been used to measure rotational temperatures, T R , of the aniline molecule in axissymmetric and planar pulsed supersonic expansions of He, Ne, and Ar. The rotational contours of the 0-0 band of the ' B~ +-' A , transition of aniline were measured by LIF and were fitted by computer-simulated rotational contours, resulting in the determination of T R . For axis-symmetric jets expanded from a 0.06 cm nozzle at stagnation pressures p = 100-2300 Torr, we find 7 '~ to vary from 15 2 2 K to 2.2 + 0.2 K. The degree of rotational cooling increases in the order of He 5 Ne e Ar.The pressure dependence of T R on p is T R a p-077-0.0S for Ar (in the range p = 100-1000 Torr) and T n U 3 i 0 0 5 for He (in the range p = 300-2300 Torr), manifesting the implications of the velocity slip effect. Evidence for extensive formation of van der Waals complexes in the aniline-Ar system a t p 2 1000 Torr was inferred from the saturation of TR with increasing p in this range. In planar supersonic jets of Ar expanded from a 0.2 X 35 mrn nozzle slit at p = 5-200 Torr, the rotational temperature varied in the range TR = 90 + 10 K to 12 C 2 K, exhibiting thedependence TR x~-' .~~~~~~~~' .Additional diagnostic information on planar jets was obtained for the dependence of the rotational temperature on the distance from the nozzle slit.La fluorescence induite par laser a Cte utilisee pour mesurer les tempkratures rotationnelles, TR, de la molkcule d'aniline dans des expansions supersoniques pulskes, a symktrie axiale ou plane, de He, Ne at Ar. On a mesurC les contours rotationnels de la bande 0-0 dc la transition ' B z t ' A , de I'aniline. et ces mesures ont etk ajustees sur des contours rotationnels obtenus par simulation sur ordinateur, afin de dkterminer TR. Dans le cas de I'expansion de jets ? I symetrie axiale sortant d'un orifice de 0,06 crn a des pressions de stagnation p = 100-2300 Torr, nous trouvons que TR varie de 15 t 2 K a 2.2 t 0,2 K. Le degrC de refroidissement rotationnel croit dans I'ordre He 5 Ne e Ar. La variation de TR en fonction de la pression p est donnee par TR a p-".77.0.0s pour Ar (dans I'intervalle p = 100-1000 Tom), et par T R p~~" -8 3 1 -0~0 5 pour He (dans I'intervalle p = 300-2300 Torr), manifestant les implications de I'effet de glissement de vitesse. La saturation de TR avec I'accroissement de la pression au-dessous de 1000 Tom prouve qu'il y a, a des pressions de cet ordre, formation abondante de complexes de Van der Waals dans le systeme aniline-Ar. Dans le cas des jets supersoniques plans sortant d'une fente de 0,2 X 35 mm 5 5-200Tom, la tempkrature rotationnelle varie dans un intervalle allant de TR = 90 + 10 K a TR = 12 + 2 K, avec une dependance de la pression donnee par TR a 1 1 -~.~~~=~~. '~) . Comme information additionnelle pour le diagnostic des jets plans, on a determink I'effet de la distance 6 la fente sur la temperature rotationnelle.[Traduit par le journal]Can.

The structure of a weakly bound ionic trimer: Calculations for the He24H− complex

“…The study of the 4 HeH Ϫ interaction has been recently revived because of the corresponding interest in the isoelectronic 4 He 2 dimer and, as mentioned before, its bound state features. The negative ionic character of 4 HeH Ϫ , however, leads to a very different two-body ͑2B͒ interaction with respect to the neutral dimer and, therefore, to a very different potential-energy curve.…”

“…The negative ionic character of 4 HeH Ϫ , however, leads to a very different two-body ͑2B͒ interaction with respect to the neutral dimer and, therefore, to a very different potential-energy curve. Since the H atom has a sizeable electron affinity ͑0.7542 eV͒ while the He Ϫ system is unstable, the two-particle system will preferably have HeϩH Ϫ as its lower dissociating threshold for the ground electronic state.…”

“…The ability to cool molecules below 1 K indeed facilitates the exploration of ultracold molecular physics and ultracold collisions, as has been shown by recent research. [4][5][6][7] As an example of it, small bosonic helium clusters, 4 He N , have been the subject of many experimental and theoretical studies and the existence of weakly bound 4 He dimers has been suggested over the years by independent experiments. 8,9 The more recent transmission grating observations 10 also provide a promising technique to study 4 He N clusters with N up to about 20-30 atoms, thereby indicating in a quantitative way the relative stabilities of these very weakly molecular species.…”

“…An illuminating review of such methods 11 and some of the results for specific cases 12,13 have been already published in recent years and we, therefore, refer any interested reader directly to them without giving here any further detail. Among the variety of studied systems the clusters of 4 He possess a rather rich and intriguing set of properties related to their very weak interaction forces and to their weak response to external fields induced by additional atomic and molecular ''impurities.'' The latter species, in fact, could be added to the initial helium cluster by taking advantage of the ease with which these aggregates can pick up one or more of such impurities.…”

“…The latter species, in fact, could be added to the initial helium cluster by taking advantage of the ease with which these aggregates can pick up one or more of such impurities. 14 In the present study we specifically intend to analyze in some detail the structural consequences of adding to the simplest 4 He cluster, the helium dimer, one of the ionic impurities which has been already shown to interact with one 4 He atom through a very weak potential, i.e., the H Ϫ impurity. 15 The problem here, in fact, is to analyze the overall spatial shape of this special triatomic ion and to establish the number and nature of the bound states of this system in order to relate the present findings with those already obtained and discussed by us for another impurity that shows similarly weak interactions with 4 He and 4 He 2 : the Li atomic species.…”