A combined experimental-theoretical study of the vibrational predissociation and product rotational distributions for high vibrational levels of He79Br2

Abstract

Pump-probe spectra of HeBr 2 in vibrational states vЈϭ10 and 39 through 48 of the B electronic state are reported and the fragment rotational distributions from vibrational predissociation of the cluster are extracted from the measured E(0 g ϩ )←B( 3 ⌸ 0u ϩ ) spectra of Br 2 . The experimental results are compared to theoretical calculations on the B←X spectra using atom-atom model potentials and performing a thermal average over transitions that contribute to the net excitation. Very good agreement between experiment and theory is obtained, except in the region of vЈϭ44, where the ⌬vϭϪ1 channel closes, and in the region of vЈϭ48 where the ⌬vϭϪ2 channel closes. For vЈ ϭ43, and vЈϭ44, the agreement is less satisfactory because the dynamics are extremely sensitive to details of the potential energy surface due to threshold effects associated with the ⌬vϭϪ1 channel closing. Similar sensitivity to the potential due to the ⌬vϭϪ2 channel closing impairs the agreement between experiment and theory for vЈϭ48. Below vЈϭ43, the rotational distributions for ⌬vϭϪ1 and ⌬vϭϪ2 are quite similar. Above vЈϭ43 the peaks of the rotational distributions for ⌬vϭϪ2 move to higher values of j. These results are compatible with the theoretical conclusion that dissociation shifts from a direct mechanism to one involving intramolecular vibrational distribution in the region of the closing of the ⌬vϭϪ1 channel. Although the simple additive potential model used in this work succeeds in reproducing most of the experimental data for this system, further improvements in the potential energy surface will be required to achieve precise agreement between experiment and theory for large Br-Br separations.

Theoretical simulations of the He79Br2 B, v=8←X, v″=0 excitation spectrum: Spectroscopic manifestation of a linear isomer?

“…4, the agreement between both approaches is excellent despite minor discrepancy more evident for the transitions to the n B ϭ1 level. This proves that at low v's the discrete-discrete approximation works very well not only for shifts and widths, 39,41 but also for transition intensities.…”

“…Rigorous discrete-continuum ͑or lineshape͒ 49 calculations were performed for the V X (67.5°) IDIM PT1 pair of PESs as described elsewhere. 39,41 In this case the shifts and integrated intensities of individual transitions were determined from the continuous excitation profile using Lorentzian fits to the corresponding peaks. As is seen in Fig.…”

“…agreement with experimental data on the vibrational predissociation dynamics [36][37][38] achieved even at high v's, where the intramolecular vibrational relaxation causes extreme sensitivity of the dynamics to the properties of the PESs. [39][40][41][42] We try to assign these two spectrum features to transitions from the distinct isomers having similar energies. Implementing the family of the ground-state PESs and comparing the simulated spectra with the observed one, we aim to support a new assignment which involves the coexistence of T-shaped and linear isomers and deduce the qualitative estimation of their energies which can be compared with available ab initio data.…”

“…41,49 The sign of p j defines which rotational angular momenta of the Br 2 fragment j, even or odd, contribute to the total wave function of the complex. In what follows we will designate the rotational levels J p i p j as tЉ and t for the X and B electronic states, respectively.…”

An empirical potential-energy surface for the He–I2(BΠu3) van der Waals complex including three-body effects

“…19 Good agreement was found between the calculated and measured Br 2 rotational distributions. 32 The above results seem to indicate that fitting the potential parameters under the assumption of J = 0 is a reasonably good approximation.…”