arXiv subjects

N. Moazzen-Ahmadi

Publications and source records attributed to N. Moazzen-Ahmadi.

27 records · Page 2Linked to original sources

arXiv2019

Infrared bands of CS2 dimer and trimer at 4.5 cm-1

We report observation of new infrared bands of (CS2)2 and (CS2)3 in the region of the CS2 ν1+ ν3 combination band (at 4.5 cm-1) using a quantum cascade laser. The complexes are formed in a pulsed supersonic slit-jet expansion of a gas mixture of carbon disulfide in helium. We have previously shown that the most stable isomer of (CS2)2 is a cross-shaped structure with D2d symmetry and that for (CS2)3 is a barrel-shaped structure with D3 symmetry. The dimer has one doubly degenerate infrared-active band in the ν1+ ν3 region of the CS2 monomer. This band is observed to have a rather small vibrational shift of -0.846 cm-1. We expect one parallel and one perpendicular infrared-active band for the trimer but observe two parallel and one perpendicular bands. Much larger vibrational shifts of -8.953 cm-1 for the perpendicular band and -8.845 cm-1 and +16.681 cm-1 for the parallel bands are observed. Vibrational shifts and possible vibrational assignments, in the case of the parallel bands of the trimer, are discussed using group theoretical arguments.

physics.atm-clus↗

arXiv2018

The N2O-CS2 dimer is cross-shaped

The infrared spectrum of the cross-shaped van der Waals complex N2O-CS2 is observed in the region of the N2O nu1 fundamental band (~2220 cm-1) using a tuneable diode laser to probe a pulsed supersonic slit jet expansion. Both 14N- and 15N-substituted species are studied. Analysis of their spectra establishes that this dimer has a cross-shaped structure, similar to its isoelectronic cousin CO2-CS2. This is the first spectroscopic observation of N2O-CS2, and the molecular parameters determined here should be useful for detection of its pure rotational microwave spectrum.

physics.atm-clus↗

arXiv2018

Infrared spectra of Rg1,2 - C6H6 complexes, Rg = He, Ne, Ar

Infrared spectra of Rg1,2 - C6H6 complexes (Rg = He, Ne, Ar) are observed in the region of the nu12 fundamental of C6H6 using a pulsed supersonic jet expansion and a tunable optical parametric oscillator laser source. The mixed trimer He - Ne - C6H6 is also detected. Four bands are analyzed for each complex, namely nu12 itself (~3048 cm-1) and three linked combination bands (~3079, 3100, and 3102 cm-1). The results are consistent with previous ultraviolet and microwave results, with Ne2 - C6H6 and He - Ne - C6H6 being analyzed spectroscopically here for the first time.

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arXiv2018

Characterization of OCS-HCCCCH and N2O-HCCCCH dimers: Theory and experiment

The infrared spectra of the weakly-bound dimers OCS-HCCCCH, in the region of the nu1 fundamental band of OCS (2050 cm-1), and N2O-HCCCCH, in the region of the nu1 fundamental band of N2O (2200 cm-1), were observed in a pulsed supersonic slit jet expansion probed with tunable diode/QCL lasers. Both OCS-HCCCCH and N2O-HCCCCH were found to have planar structure with side-by-side monomer units having nearly parallel axes. These bands have hybrid rotational structure which allows for estimates of the orientation of OCS and N2O in the plane of their respective dimers. Analogous bands for OCS-DCCCCD and N2O-DCCCCD were also observed and found to be consistent with the normal isotopologues. Various levels of ab initio calculations were performed to find stationary points on the potential energy surface, optimized structures and interaction energies. Four stable geometries were found for OCS-HCCCCH and three for N2O-HCCCCH. The rotational parameters at CCSD(T*)-F12c level of theory give results in very good agreement with those obtained from the observed spectra. In both dimers, the experimental structure corresponds to the lowest energy isomer.

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arXiv2018

Infrared spectrum and intermolecular potential energy surface of the CO-O2 dimer

Only a few weakly-bound complexes containing the O2 molecule have been characterized by high resolution spectroscopy, no doubt due to the complications added by the oxygen molecule's unpaired electron spin. Here we report an extensive infrared spectrum of CO-O2, observed in the CO fundamental band region using a tunable quantum cascade laser to probe a pulsed supersonic jet expansion. The rotational energy level pattern derived from the spectrum consists of stacks of levels characterized by the total angular momentum, J, and its projection on the intermolecular axis, K. Five such stacks are observed in the ground vibrational state, and ten in the excited state (v(CO) = 1). They are divided into two groups, with no observed transitions between groups. The groups correspond to different projections of the O2 electron spin, and correlate with the two lowest rotational states of O2, (N, J) = (1, 0) and (1, 2). The rotational constant of the lowest K = 0 stack implies an effective intermolecular separation of 3.82 Angstroms, but this should be interpreted with caution since it ignores possible effects of electron spin. A new high-level 4-dimensional potential energy surface is developed for CO-O2, and rotational energy levels are calculated for this surface, ignoring electron spin. By comparing calculated and observed levels, it is possible to assign detailed quantum labels to the observed level stacks.

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arXiv2018

Infrared spectra of C2H4 dimer and trimer

Spectra of ethylene dimers and trimers are studied in the nu11 and (for the dimer) nu9 fundamental band regions of C2H4 (~2990 and 3100 cm-1) using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. The deuterated trimer has been observed previously, but this represents the first rotationally resolved spectrum of (C2H4)3. The results support the previously determined cross-shaped (D2d) dimer and barrel-shaped (C3h or C3) trimer structures. However, the dimer spectrum in the nu9 fundamental region of C2H4 is apparently very perturbed and a previous rotational analysis is not well verified.

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arXiv2017

The He2 - OCS complex: comparison between theory and experiment

OCS is an ideal probe for quantum solvation effects in cold helium clusters. He2-OCS is the "second step" in going from a single OCS molecule to a large doped superfluid helium cluster. Here assignment of the spectrum of He2-OCS is significantly extended to higher values of J, K, and vt (the low frequency torsional vibration). The observation of a new infrared band, OCS nu1 + nu3, taken together with the known nu1 band, enables assignments to be verified by comparing ground state combination differences. Relatively straightforward scaling of previously calculated theoretical energy levels gives a remarkably good fit to experiment

physics.atm-clus↗

arXiv2017

Three new infrared bands of the He-OCS complex

Three new infrared bands of the weakly-bound He-OCS complex are studied, using tunable lasers to probe a pulsed supersonic slit jet expansion. They correspond to the (0400) <-- (0000), (1001)<-- (0000), and (0401) <-- (0000) transitions of OCS at 2105, 2918, and 2937 cm-1, respectively. The latter band is about 7900 times weaker than the previously studied OCS nu1 fundamental. Vibrational shifts relative to the free OCS monomer are found to be additive. Since carbonyl sulfide has previously been shown to be a valuable probe of superfluid quantum solvation effects in helium clusters and droplets, the present results could be useful for future studies of vibrational effects in such systems.

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arXiv2002

Instability Heating of Sympathetically-Cooled Ions in a Linear Paul Trap

Sympathetic laser cooling of ions stored within a linear-geometry, radio frequency, electric-quadrupole trap has been investigated using computational and theoretical techniques. The simulation, which allows 5 sample ions to interact with 35 laser-cooled atomic ions, revealed an instability heating mechanism, which can prevent ions below a certain critical mass from being sympathetically cooled. This critical mass can however be varied by changing the trapping field parameters thus allowing ions with a very large range of masses to be sympathetically cooled using a single ion species. A theoretical explanation of this instability heating mechanism is presented which predicts that the cooling-heating boundary in trapping parameter space is a line of constant $q_u$ (ion trap stability coefficient), a result supported by the computational results. The threshold value of $q_u$ depends on the masses of the interacting ions. A functional form of this dependence is given.

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