| Résumé | Infrared spectra of HeN–CO₂ clusters with N up to about 20 have been studied in the region of the CO₂ v 3 fundamental band (~2350 cm⁻¹) using a tunable diode laser spectrometer and pulsed supersonic jet source with cooled (>-150 °C) pinhole or slit nozzles and high backing pressures (<40 atm). Compared to previous studies of HeN–OCS and –N₂O clusters, the higher symmetry of CO₂ results in simpler spectra but less information content. Discrete rotation–vibration transitions have been assigned for N=3 – 17, and their analysis yields the variation of the vibrational band origin and B rotational constant over this size range. The band origin variation is similar to HeN– OCS, with an initial blueshift up to N=5, followed by a monotonic redshift, consistent with a model where the first five He atoms fill a ring around the equator of the molecule, forcing subsequent He atom density to locate closer to the ends. The B value initially drops as expected for a normal molecule, reaching a minimum for N=5. Its subsequent rise for N=6 to 11 can be interpreted as the transition from a normal (though floppy) molecule to a quantum solvation regime, where the CO₂ molecule starts to rotate separately from the He atoms. For N>13, the B value becomes approximately constant with a value about 17% larger than that measured in much larger helium nanodroplets. |
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