Intramolecular Vibrational Dynamics Research Articles

The semiclassical regime of intramolecular vibrational dynamics

We report on a study of intramolecular dynamics after an ultrashort excitation. We show that a semiclassical regime of the vibrational dynamics exists on an intermediate time scale between the initial dephasing on the ultrashort time of the excitation pulse and the Heisenberg time scale associated with the mean level spacing. In this semiclassical regime, intramolecular dynamics presents time recurrences at the periods of the classical orbits. The amplitudes of these semiclassical time recurrences are evaluated for classically integrable and chaotic systems using Gutzwiller and the Berry–Tabor trace formulas.

Jet spectroscopy of p-methoxyaniline and intramolecular vibrational relaxation dynamics in the p-alkoxyaniline series

The vibronic spectroscopy of p-methoxyaniline (p-anisidine) in the S1 and S0 states has been studied using laser induced fluorescence in a supersonic jet apparatus. The band origin is found at 31 581 cm−1. Vibrational modes 6a, 6b, 1, I, and 10b are found to be active in the excitation and emission spectrum and their frequencies are 395, 565, 821, 668(v=2), and 499(v=2) cm−1 in the excited state and 426, 642, 845, 472(v=2), and 525(v=2) cm−1 in the ground state, respectively. The intramolecular vibrational relaxation (IVR) dynamics discerned from the single vibronic level fluorescence (SVLF) spectra is compared with p-aminophenol and p-ethoxyaniline, the other two members of the p-alkoxyaniline series. It is postulated that the IVR dynamics in this series is dependent on the changes in the electronic structure in the excited state rather than the increase in the density of states alone.

A three-dimensional wave-packet method for the CH overtone spectroscopy and intramolecular vibrational relaxation dynamics of the fluoroform molecule

The overtone spectroscopy and intramolecular vibrational relaxation dynamics of CH chromophore in the fluoroform molecule is studied by a three-dimensional (3D) time-dependent wave-packet method, and the results are compared with the experiment and with those of a 2D (stretch–bend) wave-packet method. A third mode (CF symmetrical stretch) is included in order to explain the long time dynamics and the combination bands between the CF stretch fundamental and the Fermi polyad frequencies. The comparison with the 2D study is carried out by the use of a full set of dynamical and spectroscopic variables, based on the autocorrelation function of the bright states of each polyad. The spectroscopic variables then follow by Fourier transforming the autocorrelation function, while the dynamical ones emerge via survival probability in the frame of the dynamical statistical ensemble. These include several relaxation times and the number of cells and rates of phase–space exploration. The specific effect of the third mode is monitored by following the reduced dynamics of the system irrespective of the polyad stretch–bend dynamics, through population evolution. Dynamical results clearly reveal the third mode effects at very short and long times. In the last regime, we can observe a great span of different behaviors, depending on how the third mode excited states are involved. This richer variety of dynamical patterns cannot be observed in a two-mode model and justifies the present work. The spectroscopic results of both models are in good agreement with the experimental results.

Intramolecular vibrational dynamics of diacetylene and diacetylene-d1 via eigenstate-resolved overtone spectroscopy

Abstract The high resolution spectra of several CH overtone bands in diacetylene and diacetylene-d1 were measured using optothermally detected excitation of a collimated molecular beam. The first overtone of the acetylenic CH stretches in these two molecules were recorded in a single resonance scheme using a 1.5 μm color center laser. The second overtone spectra were taken using sequential infrared/infrared double resonance with a 3.0 and a 1.5 μm color center lasers. The perturbations in the spectra have been analyzed to obtain information about the nature and timescales of the underlying intramolecular vibrational redistribution processes. The uncovered dynamical features appear to be dominated by anharmonic couplings and exhibit regular, not chaotic, behavior. The first and second overtone spectra of diacetylene-d1 are consistent with a coupling model which involves coupling through a doorway state and then subsequent coupling to the bath. In diacetylene, a combination band was also recorded which, in the local mode picture, is equivalent to putting two quanta in one acetylenic CH stretch and one quanta at the other end of the molecule. Comparison of this spectrum with the spectrum obtained by putting three quanta in the same CH stretch, is consistent with earlier observations that delocalized combination bands are less perturbed than nearly isoenergetic pure overtone states.

Rotation–vibration state resolved unimolecular dynamics of highly excited CH3O (X̃ 2E). II. Intramolecular vibrational dynamics of excited ‘‘C–O stretch’’ states

The stimulated emission pumping (SEP) spectra of highly excited CH3O (X̃ 2E) reported in the preceding paper [A. Geers, J. Kappert, F. Temps, and J. Wiebrecht, J. Chem. Phys. 101, 3618 (1994); paper I] are analyzed to extract quantitative data on the collision-free intramolecular vibrational dynamics of the molecules. Attention is focused on the spectra and dynamics of the excited C–O stretch vibration (ν3) states at energies between 3 000 cm−1≤E≤10 000 cm−1. The spectra are found to exhibit a rather sudden transition at E≊5000 cm−1 from the regular, assignable properties at low energies to the ‘‘statistical’’ region at high energies. This IVR ‘‘threshold’’ is approximately 2000 cm−1 below the asymptotic H–H2CO dissociation limit of the molecule. The observed densities of states at these energies approach the full J- and symmetry-sorted rovibronic densities estimated from harmonic state counts. The nearest-neighbor level spacing distribution in the spectra and the magnitude of the inferred root mean square level coupling matrix elements support the assumption of nearly ‘‘ergodic’’ molecular properties in this region. From computed time autocorrelation functions, the IVR lifetimes of the highly excited C–O stretch states are found to be of the order of τ≊0.2–0.3 ps (v3=5 to 6). Evidence is obtained from some spectra for two-tier IVR processes with characteristic time scales of 0.2 and 2 ps, respectively. The highest excited C–O stretch states at energies above ≊8000 cm−1 (v3≥8) appear to be coupled only weakly to neighboring background states. This last observation hints at the existence of quasistable periodic orbits, which are embedded in the quasicontinuum of dissociative states at these energies.

Rotation–vibration state resolved unimolecular dynamics of highly vibrationally excited CH3O (X̃ 2E). I. Observed stimulated emission pumping spectra

Using the technique of stimulated emission pumping (SEP) spectroscopy, highly excited vibration–rotation states of the CH3O (X̃ 2E) molecule were probed up to energies of E≤10 000 cm−1. The highest excitation energies exceed the asymptotic H–H2CO dissociation limit of the molecule [ΔrH00(H–H2CO)≊6900 cm−1]. Work was carried out at different experimental resolutions. First, low resolution survey SEP spectra were found to exhibit persistent vibrational structure up to energies far above the dissociation limit. The observed main features were found to be assignable, in a zero-order picture that leaves aside possible mode-to-mode couplings, to the progression of the excited C–O stretch vibration states (ν3). The widths of the respective features correspond to localized short-time vibrational motion for times of ≥0.3 ps (≥10 C–O vibrational periods). Second, in high resolution scans over the coarse vibrational features, characteristic clumps of individual vibration–rotation eigenstates were revealed. These clumps are ascribed to distinctive Franck–Condon active bright zero-order levels which are mixed with the large number of Franck–Condon inactive dark bath states. Under carefully selected conditions, the clumps could be attributed to states with defined and well known values of the total angular momentum quantum number J, which remains as a good quantum number in different coupling cases. These clump spectra will be analyzed quantitatively in the following paper with respect to their bearing for the intramolecular vibrational dynamics of highly excited CH3O (X̃) as a function of vibrational and rotational excitation. From the observed spectra, quantitative data can be obtained on the rate and extent of collision-free intramolecular vibrational and rovibrational energy redistribution (IVR and IRVR) processes, which would result after coherent ultrashort pulse excitation of the molecules.

Intramolecular dynamics. III. Theoretical studies of the CH overtone spectra for benzene

The CH overtone spectra for vCH=1, 2, and 3 for an intermediate sized molecule, benzene, and the related intramolecular vibrational dynamics are treated theoretically. For this purpose, an artificial intelligence (AI) search technique is employed, using the evaluation function developed in Part II. The curvilinear local-normal mode coordinate system discussed in Part I is also used. The main features of the theoretical spectra are in reasonable accord with those of the experimental ones. Results reflecting the important role of the symmetry of molecules even in their high energy states are described.

Novel photon echo for studying intramolecular vibrational dynamics

A new two-pulsed three-level photon echo is proposed for measuring the dephasing process due to intramolecular vibrational relaxation. An explanation of this echo is given, its applicability to molecular vibrational spectra is considered, the intensity of the echo signal is calculated, and some actual experimental schemes are discussed.

Intramolecular dynamics. I. Curvilinear normal modes, local modes, molecular anharmonic Hamiltonian, and application to benzene

The Hamiltonian based on curvilinear normal modes and local modes (CNLM) is discussed using Wilson’s exact vibrational Hamiltonian as basis, the CNLM representation diagonalizing only the normal mode block of FG matrix in curvilinear internal coordinates. Using CNLM the kinetic and potential energy operators for benzene are given, including cubic and quartic anharmonicity in the potential energy and cubic and quartic terms in the kinetic energy expansion in curvilinear coordinates. Using symmetrized coordinates and cubic and higher force constants the number and identity of the independent symmetry allowed (A1g) such force constants are obtained. The relation to conventional anharmonic force constants is then given and the allowed contributions of the latter are obtained. The results are applied to CH overtone spectra and intramolecular vibrational dynamics in Part III of this series.

Theoretical and computational studies of highly vibrationally excited acetylene

A consistent interpretation of the intramolecular vibrational dynamics of highly excited acetylene on the ground potential energy surface is proposed. Classical trajectory computations in the time domain (and their Fourier transform) and quantum-algebraic computations of the spectrum provide a picture which is in accord with the results of an analysis of the experimental stimulated emission pumping. A separation of time scales suggested by the algebraic Hamiltonian is verified in the classical simulations. There are four distinct, nested relaxation stages following the initial state preparation, each corresponding to energy exchange amongst a bigger subset of states. Initially (<1 ps) only the two CH stretch motions are effectively coupled. It is suggested that the two CH quanta states (which are primarily local mode in character) are responsible for the broad features in the spectra at ≈26000 cm −1. Next the CH stretches and the (Franck-Condon) excited CC stretch share energy. This stage is yet to be identified in the spectrum. By ≈ 10 ps, the CH trans bend excitation is also fully coupled, resulting in the “clumps” in the experimental spectrum. Finally, by about 15 ps there is complete relaxation corresponding to a concerted orbiting motion of the H atoms about the CC bond with a wide frequency band centered at ≈6 cm −1.

Modeling the rotational and vibrational structure of the i.r. optical spectrum of NH 3

Current progress on the rotational assignment of NH 3 vibrational bands from 4800 cm −1 to 18 000 cm −1 is reported. We discuss a vibrational Hamiltonian for regions containing overtones or combinations of the three normal modes, excluding the symmetric inversion motion. The Hamiltonian is capable of modeling the normal mode to local mode transition observed in the spectrum as well as the observed band origins, rotational constants and intensifies. It includes the Darling—Dennison (local mode—normal mode) coupling, the (ν 1 ν 3)-2ν 4 Fermi resonance, and one significant (ν 1 ν 3)-ν 2 interaction. We illustrate the intramolecular vibrational dynamics predicted by the model Hamiltonian.

The use of artificial intelligence methods in studying quantum intramolecular vibrational dynamics

Artificial intelligence methods are used to treat the time evolution in intramolecular quantum dynamics. Comparison is made of several AI search algorithms and evaluation functions in an application to the study of quantum intramolecular vibrational redistribution. A combination of a beam search and a best-first search is used, in conjunction with an accumulated evaluation function which encourages both searching and ultimately satisfying the uncertainty principle. The methods developed are applied to an 11-coordinate heavy central mass problem and used to treat both quantum beats and ‘‘dissipative’’ intramolecular energy transfer. Good agreement is obtained with the ‘‘exact’’ quantum dynamics.

An effective Golden Rule decay rate expression for quasidissipative IVR processes

An effective Golden Rule rate expression is derived for the intramolecular vibrational decay dynamics of an initially prepared nonstationary state in an isolated polyatomic molecule. It is shown that the decay rate of the initial state depends on the properties of those states which are directly coupled to the initial state. In many cases, there may be a suppression of the exponential decay rate. An explanation for the insensitivity of some experimental C–H overtone linewidths with increasing excitation energy is also proposed.

Local-mode overtones. Ultrashort pulse excitation, intramolecular relaxation and unimolecular reactions

The dynamics of the preparation of local-mode overtone states in model systems have been considered where the excitation competes with relaxation of the initial state into either a reaction continuum or an intramolecular quasi-continuum. The overtone excitation process is many orders of magnitude slower than either vibrational relaxation or unimolecular decay, so that the prepared state is actually not localized. We demonstrate that when a continuum or quasi-continuum is present the prepared state is a sensitive function of the duration of the excitation pulse. Correspondingly, the absorption spectrum depends on the pulse duration, particularly when the pulse length is comparable to the IVR or reaction time. As such, ultrashort laser pulses are capable of probing the detailed intramolecular vibrational dynamics associated with decay of localized excitations. In the case of intramolecular relaxation into a quasi-continuum of vibrational bath states, the pulse-length-dependent spectrum reveals a sequential appearance of states which contribute to IVR. We have labelled these states as IVR resonances. In the case of unimolecular decay into a reactive continuum, the photochemical yield can be a non-monotonic function of the pulse length. This indicates the simultaneous competition amongst ultrashort pulse excitation, intramolecular dynamics and unimolecular reaction.

Vibrational quasicontinuum of polyatomic molecules: New problems for laser spectroscopy

Generally, the subject of the paper is related to the structure of highly excited vibrational states of polyatomic molecules. The problem is treated in terms of intramolecular vibrational dynamics as well as spectroscopy of vibrational transitions. Theoretical approximations which allow us to calculate spectra starting from dynamics are discussed. A microscopic treatment of spectroscopic relaxation timesT 1 andT 2 is given. Also, various experimental approaches to study the problem are outlined including the measurements of stochasticity onset in molecules.

Rotational decoupling of vibrational modes due to rotational excitation: Application to a model of the HDO stretching motions

AbstractRotational motion can have a significant effect on intramolecular vibrational dynamics. In this paper, we explore the recently discovered phenomenon of rotational decoupling through both quantum mechanical and classical methods. It is found in a model system for the stretching motions of HDO, that rotational motion about the c axis can decouple the stretches. This fact can be understood as a cancellation between kinetic and centrifugal coupling as illuminated using classical resonance analysis and quantum matrix element calculations. The significance of this phenomenon is discussed in infrared multiple photon absorption experiments with specific application to isotope separation in HDO.

Intramolecular vibrational dynamics including rotational degrees of freedom: Chaos and quantum spectra

A criterion for chaos in quantum spectra developed by Heller et al. is applied to the vibration-rotation spectrum at about 8000 cm−1 of the X̃ 1A1 state of formaldehyde. It is found that the observed increase in complexity in the spectrum as the angular momentum quantum numbers are increased corresponds nonetheless with decreasingly stochastic behavior. This can be understood in terms of an available phase space volume that expands more rapidly than the occupied phase space volume in J increases.