A pedagogical overview of intramolecular vibrational redistribution (IVR) phenomena in vibrationally excited molecules is presented. In the interest of focus and simplicity, the topics covered deal primarily with IVR in the ground electronic state, relying on examples from the literature to illustrate key points. The experimental topics discussed attempt to sample systematically three different energy regimes on the full potential surface corresponding to (i) “low”, e.g., moderate- to high-resolution vibrational spectroscopies, (ii) “intermediate”, e.g., stimulated emission pumping and high overtone spectroscopies, and (iii) “high”, e.g., photofragment/predissociation dynamical spectroscopies. The interplay between experiment and theory is highlighted here because it has facilitated enormous advances in the field over the past decade.

Vibrational Energy Flow in Highly Excited Molecules: Role of Intramolecular Vibrational Redistribution

Intramolecular vibrational redistribution is a fundamental phenomenon observed in polyatomic molecules when sufficiently excited vibrationally. In this paper, results mostly from the last two decades of research on this subject are summarized, obtained either from infrared spectroscopy with a resolution of as high as cm or, in a different approach, by using various pump-probe schemes with a temporal resolution from dozens of picoseconds to subpicoseconds.

https://iopscience.iop.org/article/10.3367/UFNe.0182.201210e.1047/pdf

Intramolecular vibrational redistribution: from high-resolution spectra to real-time dynamics

Theories of intramolecular vibrational energy transfer

We used rotational cooling of molecules to ∼5 K by supersonic expansion and state‐selective, multilevel saturation spectroscopy to obtain high‐resolution spectra of the fundamental and first and second overtone transitions of C–H stretching modes in ground‐electronic‐state benzene and its dimer. Greatly reduced linewidths (<3 cm−1 FWHM) in the rich spectra show that previously reported spectra have suffered from inhomogeneous congestion. Our observed spectral widths indicate that the vibrational lifetimes of the C–H stretches are at least a few ps, even at the energy of the second overtone (8800 cm−1). The ‘‘local mode’’ picture appears to apply when at least three quanta of C–H stretching motion are present. Spectra of the dimer are similar to those of the monomer but show a red shift of a few cm−1, the appearance of combination bands involving van der Waals vibrational modes, some intensity changes, and a broadening of spectral features that increases with the vibrational energy. The dimer’s predissociation lifetime at ∼3000 cm−1 vibrational energy exceeds ∼3 ps.

/pdf/local-modes-of-benzene-and-benzene-dimer-studied-by-infrared-3m1g0ckjfd.pdf

Local modes of benzene and benzene dimer, studied by infrared–ultraviolet double resonance in a supersonic beam

The structure of microsolvated benzene derivatives and the role of aromatic substituents.

The absorption spectrum of trifluoromethane has been recorded between 900 and 14 000 cm−1 with resolutions between 0.004 and 0.5 cm−1 (pressure broadened). 22 bands were assigned as arising from the interacting CH stretching and bending manifolds, which account for most of the absorption in the overtone region. The results can be understood quantitatively with an effective, tridiagonal many‐level Fermi resonance Hamiltonian. The experimental and theoretical results are summarized in Table II. The Hamiltonian is given in Table III and shows a very large stretching–bending interaction constant ‖ksbb‖=106 cm−1, which is even larger than the diagonal anharmonic constant for the stretching vibration ‖x′ss‖=62 cm−1. This leads to extensive vibrational redistribution between stretching and bending motions at high levels of excitation. The time dependent redistribution is calculated with the spectroscopic Hamiltonian. A rotational analysis is presented for some of the bands involved in the Fermi resonance. The ef...

Tridiagonal Fermi resonance structure in the IR spectrum of the excited CH chromophore in CF3H

The near IR and visible vibrational absorption spectra of CHF3 were recorded up to wave numbers of 17 500 cm−1 providing complete frequency coverage, together with paper I, from the low frequency fundamentals to the N=6  CH stretching–bending overtone multiplet. All strong bands in the high overtone spectra could be predicted and assigned by means of the tridiagonal Fermi resonance Hamiltonian, including a few combinations with intense CF3 stretching vibrations already observed for the low overtones. Improved vibrational Fermi resonance constants are presented on the basis of a fit to 35 assigned bands. An analysis of the rotational fine structure of the 2ν4 (E) overtone component and several Fermi resonance component bands result in values for αb and αs, which allow us to determine Be. In the high overtone bands no rotational fine structure is observed. The bands can be understood by introducing additional homogeneous rovibrational structures of phenomenological widths Γ≊1 to 10 cm−1. The results are dis...

Tridiagonal Fermi resonance structure in the vibrational spectrum of the CH chromophore in CHF3. II. Visible spectra

The time-dependent processes occurring in polyatomic molecules during and after intense infrared irradiation, for example in infrared photochemistry, are discussed in terms of underlying structures in the absorption spectra. Origins of homogeneous and inhomogeneous spectral structures are identified in high-resolution spectra of small molecules (CF3H and CF3CCH). Temperature-dependent bandshapes of large polyatomic molecules, (CF3)3CH and (CF3)3C—CCH are evaluated in terms of the most relevant parameters of the vibrational structures, using a new method. The bandshape parameters can be interpreted in terms of intramolecular vibrational processes on the picosecond and subpicosecond time-scales. Characteristic differences in the dynamic coupling behaviour of the acetylenic CH stretching and the saturated CH stretching vibrations are identified in the fundamental and the overtone spectra.

Time-dependent processes in polyatomic molecules during and after intense infrared irradiation

The Fermi‐resonance overtone spectra of the CH chromophore up to about 18 000 cm−1 are evaluated by variational vibrational calculations for the CHX3 molecules trideuteromethane (CHD3), trifluoromethane (CHF3), chloroform (CHCl3) and 1,1,1,3,3,3‐hexafluoro‐2‐trifluoromethylpropane [(CF3)3CH]. Using appropriate model potential functions in a normal coordinate subspace, one can derive parameters for the CH chromophore potential and empirical dipole moment functions. For CHD3 and CHF3 ab initio (SCF‐CI and vibrational variational) calculations are presented, the results of which compare well with the experiments and for CHD3 also with previous (MRD‐CI) ab initio results. For all cases an accurate similarity transformation to the equivalent tridiagonal form of the effective hamiltonian can be made and the corresponding spectroscopic parameters agree with previous results. Comparison is also made with results from an internal coordinate model Hamiltonian.

Vibrational spectrum, dipole moment function, and potential energy surface of the CH chromophore in CHX3 molecules

The vibrational spectra of chlorodifluoromethane (Freon 22) have been measured at intermediate and high resolutions between 800 and 12 000 cm-1 by interferometric (FTIR) techniques. The CH overtone spectra exhibit a pronounced multiple-resonance structure which can be understood in terms of the ‘universal’ anharmonic CH stretching-bending hamiltonian. Quantitative predictions from this hamiltonian agree well (without adjustments) with previous experimental data on the visible spectra, which could not be correctly assigned before.

Hans-Rolf Dübal

Papers

Tridiagonal Fermi resonance structure in the IR spectrum of the excited CH chromophore in CF3H

Tridiagonal Fermi resonance structure in the vibrational spectrum of the CH chromophore in CHF3. II. Visible spectra

Time-dependent processes in polyatomic molecules during and after intense infrared irradiation

Vibrational spectrum, dipole moment function, and potential energy surface of the CH chromophore in CHX3 molecules

Multiple anharmonic resonances in the vibrational overtone spectra of CHClF2