Between any molecules one can find a weak attractive interaction. This so called van der Waals interaction is due to electrostatic forces, induction and dispersion, the first two depending strongly on the relative position of the partners. If this attraction is stronger than the repulsion between the two molecules, stable complexes, which are also known as "dimers", can form. These bonds are usually weak compared to chemical bonds.
Emission from dimers has been detected in planetary atmospheres (Frommhold et al. 1984). It is unknown how much of the mass of the interstellar medium (ISM) is tied up in such dimers. It has even been suggested that some of the dark matter in the outer part of disk galaxies may consist of H2 dimers (see the discussion in Combes & Pfenniger 1997). Dimers have been proposed to be a major constituent of cometary matter (Krasnopolsky et al. 1988). Interstellar grain mantles, which are thought to be similar in composition to cometary ices may be a reservoir for dimers.
The few previous searches (e.g. Liszt 1978, Vanden Bout et al. 1979, CO-CO; Schenewerk et al. 1985, HCN-HCN) were unsuccessful, partly because no reliable laboratory measurements were available. Within the last years there has been significant progress in this field. New experimental techniques were developed which give access to precise transition frequencies.
After H2, the next most abundant molecule is believed to be CO, which has a dipole moment of only 0.1 Debye. The H2-CO dimer is only weakly bound. Although the large abundances of CO and H2 may be favorable for the production of H2-CO, the bond of this complex is easily destroyed by shock waves. The limits recently obtained for this dimer (Allen et al. 1997) show that this complex is not very abundant in the ISM.
We have searched for the (H2O)2 complex toward Galactic sources and toward comet C/1995 O1 (Hale-Bopp). The dimer is composed of two rare molecules, and therefore requires very high densities to be produced in the gas phase. Our decision to search for (H2O)2 was based on four reasons: First, the transition frequencies are well known. Second, water is rather abundant in ices in interstellar cores and cometary nuclei. Therefore dimers have a chance to form. Third, from the observations of deuterated water, grain mantle evaporation is a common phenomenon in hot molecular cores (Gensheimer et al. 1996). Finally, the relatively strong bond suggests that water dimer might survive the evaporation process and has a comparably long lifetime in the interstellar and interplanetary gas.
© European Southern Observatory (ESO) 1998
Online publication: June 26, 1998