Helium clusters seeded with CO molecules: new results for HeN-¹³C¹⁸O and the approach to the nanodroplet limit

Infrared spectra of helium clusters seeded with doubly substituted carbon monoxide molecules, ¹³C¹⁸O, have been studied in order to complement recent helium nanocluster results and to determine whether additional isotopic data would help to separate vibrational and rotational contributions to the observed transitions. The experiments were made by direct infrared absorption in pulsed supersonic jet expansions using a tunable diode laser probe in the region of the fundamental band (~2045 cm⁻¹ for ¹³C¹⁸O). Even with data on the R(0) transitions from four CO isotopomers, it was found that a clear and consistent separation of vibration and rotation could not be achieved for HeN–CO clusters in the size range N≈10–20. Isotope shifts observed for clusters with ¹³C¹⁸O (relative to ¹²C¹⁶O) were found to be close to the sums of the shifts previously determined for ¹³C¹⁶O and ¹²C¹⁸O. The new measurements generally supported previous assignments of cluster size, but some modifications for the range N=14–16 are suggested here. New measurements for HeN–¹²C¹⁶O under conditions favoring larger clusters (high backing pressure and low jet temperature) showed that individual transitions could be resolved even at N ≈ 50. For larger clusters, a partly resolved "lump" of transitions was observed to approach the nanodroplet limit.