Spectroscopic signatures of bond-breaking internal rotation. II. Rotation-vibration level structure and quantum monodromy in HCP

Jacobson, Matthew P.; Child, Mark S.

doi:doi:10.1063/1.1330746

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Spectroscopic signatures of bond-breaking internal rotation. I. Saddle point induced polyad breakdown

With the input of order 10–20 000 cm−1 of vibrational energy, the hydrogen atoms in small gas-phase molecules such as HCN, HCP, and acetylene can undergo internal rotation about the heavy-atom core (CN–CP–CC), breaking and reforming covalent bonds in the process. This article investigates the quantum and classical dynamics of covalent bond-breaking internal rotation, particularly the vibrational energy flow between the hindered internal rotor mode and a stretch mode. The aim is to relate polyad ...

Theory of sub-Doppler Autler–Townes splitting in molecules: Alignment and orientation of the angular momentum in nonpolar molecules

Within a density matrix framework, an analytical expression for the optical–optical double resonance (OODR) line shape for a molecule in the presence of a strong coupling laser is derived. The coupling laser is tuned to a transition between the upper level in the OODR sequence and another excited level which is not part of the OODR sequence. The pump and probe lasers are treated perturbatively, while the coupling laser is treated exactly. The line shape reveals sub-Doppler Autler–Townes (AT) spl...

The rotation-vibration level structure of ground electronic state HCP is investigated at vibrational energies approaching and exceeding that of the linear CPH saddle point. With respect to energies above the saddle point, we investigate possible spectroscopic manifestations of strong Coriolis interactions between the hindered, bond-breaking internal rotation of the hydrogen about the CP core and the rotation of the molecule in the space-fixed axis system. With respect to energies below the saddle point, we provide new interpretations, from quantum and semiclassical points of view, of previously observed anomalously large B (rotational) and g₂₂ (energy dependence on the vibrational angular momentum) constants for the large-amplitude pure bending states of HCP (referred to elsewhere as “isomerization” or saddle node states). We also predict similar anomalies in other spectroscopic constants, including the “centrifugal distortion” constant D and the “rotational l-resonance parameter q₂. These changes in the effective spectroscopic rotation-vibration constants are shown to be a direct consequence of the spherical pendulum topology of the HCP bend/internal rotor system, which is associated with a phenomenon called quantum monodromy, defined as the absence of a smoothly valid set of quantum numbers for all states. Our semiempirical model for the HCP bend/internal rotor mode is derived using principles of semiclassical inversion and provides new insights into the breakdown in the ability of rovibrational effective Hamiltonians to model highly vibrationally excited states of HCP. © 2001 American Institute of Physics.

EDITORIALLY RELATED

Spectroscopic signatures of bond-breaking internal rotation. I. Saddle point induced polyad breakdown
Matthew P. Jacobson et al.
J. Chem. Phys. 114, 250 (2001)JCPSA6000114000001000250000001

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KEYWORDS and PACS

Keywords

hydrogen compounds, rotational isomerism, rotational-vibrational states, dissociation, topology

PACS

82.30.Lp

Decomposition reactions (pyrolysis, dissociation, and fragmentation)
33.15.Hp

Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.20.Vq

Vibration-rotation analysis
02.40.Pc

General topology