Internal Vibrational Energy Redistribution Research Articles

Internal vibrational energy redistribution and vibrationally induced nonlinearity of HCN

We explain internal vibrational energy redistribution (IVR) by the existence of concave regions of the potential energy surface (PES). Starting from some reported contradictions in HCN infrared band intensities and in rotational constants, we propose a changed assignment of the two bands at 16640 and 16674 cm −1. We assume a linear-to-bent structure transition of the HCN with high excited stretching modes. Due to the disappearance of the degeneracy of the bending mode of the linear structure, a zero energy part is regained which we add up the known experimental energy transition. The bent states emerge with a second rotational quantum number K at K=1 and K=0, and allow us to estimate an average geometry with an angle of 141° giving a snapshot of the isomerisation pathway up to the saddle point to CNH.

A new quantum isotope effect: Extreme local mode selectivity in unimolecular dissociations imposed by antagonism between dynamic propensities of educts and zero point energies of products

We predict a new quantum isotope effect for unimolecular dissociations of molecules with two equivalent but isotopically substituted bonds l (light isotope) and h (heavy isotope), e.g., HOT where l=HO and h=OT. Consider two near-degenerate local vibrational excitations of bonds l or h, with energies between the gap of product zero point energies. Dynamically, these excitations should induce preferential fissions of bonds l or h, but energetically, these decay channels are open and closed, respectively. Therefore, local excitation of bond h must be followed by extremely slow internal vibrational energy redistribution to bond l before dissociation, whereas local excitation of bond l induces direct, rapid decay. The resulting decay rates differ by many orders of magnitudes. The effect is demonstrated by fast Fourier transform propagation of representative wavepackets for a model system, HOT→H+OT. Extended applications to more excited educts HOT also confirm an effect discovered previously for HOD, i.e., local mode selective control of competing bond fissions H+OT←HOT→HO+T.

An explanation of the direction of the relaxation in the HCN gas microwave laser, and a new assignment of further laser lines along the path of internal vibrational energy redistribution

We assume a direct connection between the shape of the anharmonic potential energy surface of a molecule and its internal vibrational energy redistribution. In the case of HCN there is a transition from convex to concave behaviour of the equipotential lines in the fundamental range. This explains the existence of the known laser transitions in the line systems of 1110–0400 and 1200, 1220–0510, and leads to the assignment of further lines at 35.211, 18.335, and 14.792 cm−1 to 1000–0310, 0330 and the line at 12.928 cm−1 to 2000–0600. For the pure IVR transition we assume symmetry selection ±↔± without a Coriolis resonance.

Interpretation of the IR spectral intensity anomaly of HCN by potential energy surface bifurcation along a normal mode

Stable normal IR modes vibrate in a convex basin of the potential energy surface (PES). The change to locally concave equipotential lines in a higher energy normal mode direction is interpreted as a possible reason for irreversible and rotation controlled internal vibrational energy redistribution (IVR) during excitation of that normal mode. In hydrogen cyanide (HCN) such an IVR process already occurs in the CN fundamental μ 1. The concept is proven by an explanation of the spectroscopic data: The disturbed IR absorption, the very high intensity in the neighbouring Q-branch of the bending overtone 3 v 1 2, the emerging of only higher J lines in the R branch of μ 1, the intensity anomaly in the further state (1, 1 1, 0), and the nearly normal behaviour of the DCN μ 1 band.

Stochastic theory of intramolecular vibrational energy redistribution and dissociation in the presence of radiation

A theory for internal vibrational energy redistribution and dissociation in polyatomic molecules in the presence of a strong radiation field is formulated. The fundamental assumption is that a random phase approximation is valid at specific time intervals. This results in the replacement of the Schrödinger equation by a master-type equation, which is further approximated by a Fokker–Planck diffusionlike equation. Energy transfer is described as a flow of probability among the quantum states, and the dissociation dynamics are embodied in the boundary conditions. By virtue of the continuous character of the Fokker–Planck equation, the computational difficulty of its numerical solution depends only on the number of degrees of freedom and not on the number of states. Due to the high density of levels encountered in a polyatomic molecule, this is of paramount importance in reducing the problem to a manageable size. A multiple time scale stochastic formulation, which allows for a mixed quantum-stochastic approach, is also described. No assumptions regarding the strength of the intramolecular coupling are made, and energy conservation is specifically enforced. The coefficients of the Fokker–Planck equation are shown to be expressible in terms of simple functions of the molecular potential, which involve raising and lowering operators. Finally, the coefficients of the Fokker–Planck equation are calculated using the best available potential information for the case of the ozone molecule in a strong infrared laser field, and their physical significance is discussed.