Real-time dissociation dynamics of the Ne(2)Br(2) van der Waals complex.

We have characterized the vibrational predissociation (VP) of the Ne(2)Br(2) van der Waals complex using time- and frequency-resolved pump-probe spectroscopy. After exciting Br(2) within the complex to a vibrational level 1619. We also report vibrational product state distributions for direct excitation to NeBr(2) 16

NeCl2 and ArCl2: transition from direct vibrational predissociation to intramolecular vibrational relaxation and electronic nonadiabatic effects.

Pump-probe results are reported for NeCl(2) excited to the Cl(2) B state, undergoing vibrational predissociation, and then probed via E <-- B transitions. Intensities, lifetimes and product vibrational branching ratios are reported for 16 < or = v' < or = 19 Cl(2) stretching quanta. The intensity of the signal rapidly decreases above v' = 17. Detailed wave packet calculations of the vibrational predissociation dynamics are performed to determine if the experimental results can be explained by the onset of IVR dynamics. The calculations and the experiment are in close accord for low vibrational levels. For higher levels, some, but not all, of the loss of experimental signal can be attributed to IVR. To test whether electronic relaxation dynamics are important for NeCl(2) and ArCl(2), excited state potential surfaces that incorporate spin orbit coupling effects are calculated. These surfaces are then used in a wave packet calculation that includes both vibrational predissociation and electronic predissociation dynamics. The results show that electronic predissociation is important for ArCl(2) levels above v' = 12. For NeCl(2) the calculation suggests that the onset of electronic predissociation should occur for levels as low as v' = 13 but may not contribute markedly to the observed loss of signal above v' = 17. Suggestions are made for further studies of this puzzling problem.

Product state resolved excitation spectroscopy of He-, Ne-, and Ar-Br2 linear isomers: experiment and theory.

Valence excitation spectra for the linear isomers of He-, Ne-, and Ar-Br2 are reported and compared to a two-dimensional simulation using the currently available potential energy surfaces. Excitation spectra from the ground electronic state to the region of the inner turning point of the Rg-Br2 (B,nu') stretching coordinate are recorded while probing the asymptotic Br2 (B,nu') state. Each spectrum is a broad continuum extending over hundreds of wavenumbers, becoming broader and more blueshifted as the rare gas atom is changed from He to Ne to Ar. In the case of Ne-Br2, the threshold for producing the asymptotic product state reveals the X-state linear isomer bond energy to be 71+/-3 cm(-1). The qualitative agreement between experiment and theory shows that the spectra can be correctly regarded as revealing the one-atom solvent shifts and also provides new insight into the one-atom cage effect on the halogen vibrational relaxation. The measured spectra provide data to test future ab initio potential energy surfaces in the interaction of rare gas atoms with the halogen valence excited state.

Time and frequency resolved dynamics of ArBr2.

We report the first spectroscopic observation of and vibrational predissociation dynamics for ArBr(2). Measurements are reported for the linear and T-shaped isomers with time and frequency resolution near the Fourier limit of a 15 ps pulse. For the T-shaped isomer, the Ar-Br(2) bond energy D(0) for the B state, nu(')=19, is found to be 200 cm(-1), yielding a D(0)(X) value of 213 cm(-1). Product appearance rates are determined for nu(')=16-25 of the B state of ArBr(2). While the rate generally increases with increasing vibrational quantum number, the trend is not monotonic. Also, obtaining reproducible rates for any given vibrational level requires very careful control of the experimental conditions. The data suggest that ArBr(2) undergoes vibrational predissociation (VP) in the sparse intramolecular vibrational relaxation regime. These observations are consistent with theoretical calculations that show that T-shaped ArBr(2) undergoes VP in the sparse regime, such that lifetimes are strongly dependent on both the vibrational and rotational quantum numbers. As for ArI(2), a linear isomer of ArBr(2) is found to contribute a quasicontinuous background to the excitation spectra. Direct excitation of this continuum results in a very broad product vibrational distribution.

Time-dependent pump-probe spectra of NeBr2.

Time- and frequency-resolved pump-probe measurements on NeBr2 have been performed to better characterize its fragmentation dynamics on the B electronic state for vibrational levels in the energy region of the transition from direct vibrational predissociation to intramolecular vibrational relaxation dynamics. Above nu'=20 of the Br2 stretching mode, it was observed that the dependence of lifetime on the vibrational quantum number deviates from the energy-gap law by leveling off in the range of 10 psE transitions of the complex. These transitions are shifted 20 cm(-1) to lower energy from the free Br2 resonances, indicating an E state Ne-Br2 bond energy of 82 cm(-1). Measurements of NeBr2 vibrational predissociation via the delta nu=-2 channel were also performed for nu'=27, 28, and 29. The closing of the delta nu=-1 channel leads to an increase in the lifetimes of these vibrational levels. A new Nd:yttrium aluminum garnet pumped dual optical parametric oscillator/optical parametric amplifier system is described that allows us to conveniently record time-delayed pump-probe spectra with 2-cm(-1) spectral resolution and 15-ps time resolution.