Experimental observation of competing pathways in the relaxation of ICl* in a He supersonic expansion

Abstract

Differing caging mechanisms of electronically excited ICl within He...ICl complexes are investigated using laser-induced fluorescence and two-laser, pump-probe spectroscopy. A continuum signal is observed in laser-induced fluorescence spectra recorded with excitation energies that access regions above the I 2P3/2 + Cl 2P3/2 dissociation limit. We show that this signal is associated with transitions of the He ...ICl(X,v" = 0) complex with a linear equilibrium geometry to the continuum of states correlating with the repulsive, inner wall of the ICl(A 3pi1) potential. The dissociating ICl(A) molecule within the complex undergoes kinematic one-atom caging, ejecting the weakly bound He atom, and re-stabilizing the molecule in high lying vibrational levels within the A electronic state with very little rotational excitation. At higher excitation energies, weak LIF features superimposed on the continuum fluorescence signals are observed in preliminary investigations. When fixing an excitation laser on one of these features, at 18,012 cm(-1), I35Cl(B,v' = 2,3) vibrational levels are formed with little rotational excitation. This feature is associated with transitions of the linear He...I35Cl(X,v" = 0) complex to intermolecular states associated with the ICl(B) diabat that are coupled with those in the ICl(B') adiabat formed by an avoided curve crossing. The I35Cl(B,v') products are formed via a non-adiabatic one-atom caging mechanism of ICl* molecules within linear He...I35Cl* complexes.

Combined experimental/theoretical investigation of the He+ICl interactions. I. Rovibronic spectrum of He⋯ICl complexes in the ICl B–X, 3–0 region

“…The very weak I 35 Cl B -X, 3-0 feature is also observed in this spectrum and the dynamics that give rise to this signal are characterized elsewhere. 37 The results from the He¯I 35 Cl B -X, 3-0 action spectroscopy experiments are in accord with those from the previously reported LIF spectroscopy endeavors. 26,31 The same number of rotational lines are observed in the linear He¯I 35 Cl action spectroscopy band as the same feature in the LIF band.…”

“…26,31 The same number of rotational lines are observed in the linear He¯I 35 Cl action spectroscopy band as the same feature in the LIF band. This suggests that the disagreement between the experimental data and the simulations of the linear band previously performed 26 is not due to the overlap of additional He¯I 37 Cl or higher-order He n¯I 35,37 Cl features and the additional rotor lines arise from He¯I 35 Cl transitions. Note that a continuum signal that turns on at the high-energy region of the linear band is observed in the LIF spectrum, Fig.…”

“…In order to observe those spectral features that could be attributed to transitions of He¯I 35 Cl complexes and not to He¯I 37 Cl complexes or higher-order He n¯I 35,37 Cl complexes, two-laser, pump-probe action spectra were recorded in the ICl B -X, 3-0 region in a similar manner as performed on the He¯7 9 Br 2 system. 25 For these experiments, we relied on the previously reported trend that an intermolecular vibrational level of the He¯I 35 Cl(B,vЈϭ3) preferentially dissociates into HeϩI 35 Cl(B,vЈϭ2) fragments, 35,36 while higher order complexes lose additional vibrational quanta when undergoing vibrational predissociation.…”

“…29,30 These simulations consistently underpredicted the number of rotational lines observed in the experimental spectrum, especially at higher transition energies in the band. The presence of additional He¯I 35,37 Cl or higher-order He n¯I 35,37 Cl LIF bands overlapping in this energy region as well as K-type doubling of rotor lines and Coriolis effects on transition energies were proposed as possible sources of discrepancies between the simulations and the experimental data. 26 Although a satisfactory fit was not obtained, the rigid rotor simulations did indicate that it was necessary to invoke a linear geometry in the ground and excited states in order to obtain the overall shape of the band.…”

“…In addition, insights into the HeϩICl interactions that give rise to the complicated rotational structure observed in the higher energy He¯ICl bands are sought. For simplicity, we will refer to the lower energy He¯I 35,37 Cl band observed in each ICl B -X, vЈ -0 region as the T-shaped He¯I 35,37 Cl band and the higher energy band as the linear He¯I 35,37 Cl band throughout this article.…”