Theories of intramolecular vibrational energy transfer

▪ Abstract The vibrational relaxation of noble gas–halogen van der Waals clusters has been fertile territory for the development of experimental and theoretical tools for state-to-state dynamics studies. Proceeding through the various combinations of noble gas atoms and halogen molecules, one goes from “simple” direct vibrational predissociation, through sequential relaxation pathways, to the statistical intramolecular vibrational relaxation limit. In some cases the vibrational processes dominate, in others electronically nonadiabatic processes, including chemical reactions, interfere. Thus the noble gas–halogen species provide test cases for most of the dynamical processes available to more “normal” molecules. This review discusses the current state-of-the-art for such studies and points to important problems that remain to be solved.

The dynamics of noble gas - halogen molecules and clusters

Photofragment angular distributions have been measured for HF dimer which show resolved structure that can be assigned to individual fragment rotational channels. This data is used to establish intermolecular scalar correlations between the rotational states of the two HF fragments. The observed angular distributions are strongly dependent upon whether the ‘‘free’’ or ‘‘hydrogen bonded’’ HF stretch is initially excited. Since the infrared spectrum of the parent molecule is highly resolved, these results can be used to determine the relative state‐to‐state photodissociation cross sections. In addition, the zero point dissociation energy (D0 ) of the HF dimer is accurately determined.

Photofragment angular distributions for HF dimer: Scalar J–J correlations in state‐to‐state photodissociation

The state‐to‐state vibrational predissociation dynamics of the hydrogen fluoride dimer has been investigated in detail using a newly developed instrument which gives both initial state selection and photofragment state determination. Results are reported for a wide variety of initial states associated with the ν1 and ν2 vibrations. The final state distributions universally indicate that the preferred dissociation channels correspond to the production of one HF fragment that is highly rotationally excited and another that is not. This is explained in terms of an impulsive dissociation mechanism which proceeds from a geometry close to that of the equilibrium structure of the dimer. We find that nearly degenerate initial states can have rather different final state distributions. In particular, there is a distinct difference between the upper and lower members of the tunneling doublet, which is most likely attributable to their related symmetries. The dissociation energy (D0) is determined to be 1062±1 cm−1.

Initial state effects in the vibrational predissociation of hydrogen fluoride dimer

Author Institution: Department of Chemistry, Harvard University; Bell Laboratories, Harvard University

THE INTERMOLECULAR POTENTIAL BETWEEN AN ATOM AND A DIATOMIC MOLECULE - THE STRUCTURE OF ArC1F

The structure and vibrational predissociation dynamics of HeCl2 are studied by pump–probe spectroscopy and by three‐dimensional quantum mechanical calculations. Parity selected excitation spectroscopy is used to confirm the essential features of the previous analysis of the HeCl2 B←X laser excited fluorescence spectra. Product vibrational and rotational state distributions are measured for the v’=6, 8, 12, 20, and 24 levels of HeCl2 in the B state. For the v’=6 and 8 levels the dependence of the product state distribution on the initially excited rotational state is also measured. Although the dissociation dynamics are dominated by Δv=−1, V→T energy transfer, several interesting effects are revealed by monitoring the product rotational degrees of freedom. Due to the symmetry of the HeCl2 potential, the parity of the initially excited HeCl2 rotational state is conserved during the dissociation dynamics. Even when a single initial rotational state is excited, the observed product rotational state distributi...

State‐to‐state vibrational predissociation dynamics and spectroscopy of HeCl2: Experiment and theory

Product state distributions are reported for the vibrational predissociation of the NeCl2, B state, v’=6 through v’=13 levels For the lower vibrational levels, Δv=−1 dissociation produces a bimodal Cl2 product rotational state distribution with the first maximum at j=4 and a secondary maximum at j=20 Surprisingly, the positions of these maxima are the same for v’=6, 7, 8, 9, and 10 For higher vibrational levels the limited available phase space constricts the observed rotational distribution allowing the Ne–Cl2 bond energy D0 to be determined D0 is 54±2 cm−1 for the B electronic state, and 60±2 cm−1 for the ground electronic state Δv=−2 dissociation produces a rotational distribution which, although not bimodal, is otherwise quite similar to that of the Δv=−1 channel, even though significantly more energy is released to product translation for Δv=−2 This behavior is quite different from what would be predicted by an impulsive half‐collision model for the dynamics Three dimensional quantum calculati

Product state distributions for the vibrational predissociation of NeCl2

Rotational–vibrational distributions are reported for Cl2 product states of the vibrational predissociation of the v=6 through v=12 levels of the ArCl2 molecule in the B electronic state. For v=6 and 7, the Δv=−1 dissociation channel dominates the predissociation dynamics, and the observed rotational distribution appears to be slightly inverted. This inversion does not appear to be due to ‘‘momentum gap’’ constraints but may be due to kinematic symmetry selection rules. Above v=7 the molecule decays via the Δv=−2 channel. Unlike the rotational distributions observed for HeCl2 and NeCl2, those of the ArCl2 Δv=−2 channel are highly structured and have a strong dependence on the initially excited vibrational level. It is argued that the dependence of the observed rotational distribution on the initial vibrational state (for the dissociation of ArCl2) is due to intermediate resonances in a sequential, IVR mechanism, whereas the dissociation of HeCl2 and NeCl2 occurs by a direct mechanism. Although this conclu...

The vibrational predissociation dynamics of ArCl2: Intramolecular vibrational relaxation in a triatomic van der Waals molecule?

The geometry and bond energy of ArCl2 are obtained by the technique of parity selected excitation spectroscopy. This is a pump–probe technique in which the probe laser is set to detect specific rotational levels of the B state of Cl2 while the pump laser is tuned through an ArCl2 B–X band. Since the excitation spectrum obtained depends on the parity of the detected Cl2 rotational level, the ArCl2 complex must have a symmetrical, T‐shaped geometry on average over the time scale of the experiment, ∼100 ps. Simulation of the observed spectra, which show partially resolved rotational structure, yields an argon to chlorine center of mass distance of 3.7 A for both the X and B electronic states of the complex. By measuring the threshold for dissociation to the v=6, j=8 level of the Cl2 B state, the Ar–Cl2 bond energy was determined to be 178 cm−1 in the B state and 188 cm−1 in the X state. These results are quite different from those on the isovalent complex ArClF which is a linear molecule with an Ar–Cl bond l...

The structure and bond energy of ArCl2

Laser‐induced fluorescence excitation spectra of free‐jet expansions of He and Cl2 have been recorded. Spectroscopic features observed ∼3.5 cm−1 to the blue of the free 35Cl2 vibronic origins of the 7–0 through the 12–0 bands of the B 3Π0+u–X 1Σ+g system were attributed to the He35Cl2 molecule. The observed spectra were well simulated by a T‐shaped, rigid, asymmetric top model. The spectra were observed to display homogeneous broadening dependent on the vibrational band excited. Predissociation lifetimes were calculated and structural parameters determined by fitting the observed contours. The distance from the He atom to the Cl2 center of mass is 3.8±0.4 and 3.9±0.4 A for the ground and excited states, respectively.

Dwight D. Evard

Papers

State‐to‐state vibrational predissociation dynamics and spectroscopy of HeCl2: Experiment and theory

Product state distributions for the vibrational predissociation of NeCl2

The vibrational predissociation dynamics of ArCl2: Intramolecular vibrational relaxation in a triatomic van der Waals molecule?

The structure and bond energy of ArCl2

The laser induced fluorescence spectrum of the HeCl2 van der Waals molecule