15min:

A. R. W. MCKELLAR, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada.

It is now possible to probe cold (<0.5 K) helium clusters in the size range N 2 to 70 by means of the vibration-rotation spectrum of an embedded infrared chromophore molecule such as CO₂, often with atom-by-atom resolution. To some extent, hydrogen clusters can also be studied in this way, as shown by our previous work in which CO, OCS, and N₂O were the chromophores.

Here we extend the study of hydrogen clusters to the case of CO₂ as the probe. The symmetry of CO₂ provides an important difference compared to the other probe molecules. This has the effect of eliminating half of the rotational levels (for the normal C¹⁶O₂ or C¹⁸O₂ isotopomers) and of accentuating the differences between paraH₂ and orthoH₂ clusters. As in the case of (H₂)_N-OCS and (H₂)_N-N₂O, we find that (H₂)_N-CO₂ cluster transitions are relatively easy to identify up to about N = 7, but difficult to follow above this point. However, in contrast to the previous work there is intriguing evidence for a series of weak but regularly-spaced transitions which may extend to N 15 or beyond.