High resolution infrared spectra of a carbon dioxide molecule solvated with helium atoms

Abstract

Infrared spectra of He(N)-CO(2) clusters with N up to about 20 have been studied in the region of the CO(2) nu(3) fundamental band ( approximately 2350 cm(-1)) using a tunable diode laser spectrometer and pulsed supersonic jet source with cooled (>-150 degrees C) pinhole or slit nozzles and high backing pressures (<40 atm). Compared to previous studies of He(N)-OCS and -N(2)O clusters, the higher symmetry of CO(2) 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 He(N)-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 CO(2) 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.

Spectroscopy and dynamics in helium nanodroplets

“…These include OCS [256,257,258], NNO [259], CO 2 [260,261], and CO [246,262]. They have been able to follow how the spectra evolve as helium atoms increasingly solvate the molecules, following the spectra up to N = 19 for the cases of CO and NNO.…”

Infrared spectra of seeded hydrogen clusters: (para-H2)N–N2O and (ortho-H2)N–N2O, N=2–13

“…12 Prominent Q-branch features were present for orthoH 2 and HD, but absent for paraH 2 . The vibrational shifts and rotational parameters continued trends previously observed 16 for the binary complexes (with N = 1), but there was no evidence for decoupling of the rotational motion, as observed in He clusters, [5][6][7] which could be attributed to the onset of superfluid behavior. The present paper reports infrared spectra of (hydrogen) N -N 2 O clusters, as observed in the 2225 cm -1 region of the ν 1 fundamental band of N 2 O.…”

“…So far, helium clusters are more thoroughly studied, with a number of chromophores being utilized (OCS, N 2 O, CO 2 , and CO) [1][2][3][4][5][6][7] , and analogous microwave spectra also being detected in some cases. It is thus possible to extend previous helium nanodroplet isolation spectroscopy (HENDI) results [8][9][10] which involved cluster sizes of N ≈ 10 3 to 10 4 , to the regime of small clusters with N ≈ 2 to 20.…”

“…12 In general, it * Current address: Department of Chemistry, Faculty of Science, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan.…”

Structure of small hydrogen nanoclusters containing ortho-molecules

“…Recall that normal hydrogen n H 2 widely used in industry, power engineering, and scientific research and corresponding to thermody namic equilibrium of the para and ortho components at room temperatures and higher is the mixture of 1/4 of parahydrogen and 3/4 of orthohydrogen whereas normal deuterium n D 2 is the mixture of 2/3 of orth odeuterium and 1/3 of paradeuterium (e.g., see the review [28]). Some experimental works explored nor mal hydrogen clusters [29] and orthohydrogen clusters doped with an impurity molecule, for instance that of OCS [30] or N 2 O [31]. In crossed molecular beam studies of the dynamics of elementary processes involving H 2 and D 2 , beams of normal hydrogen or deuterium are used very often.…”