The laser-induced fluorescence and action spectra of I2 in a helium supersonic expansion have been recorded in the I2 B-X, 20-0 region. Two features are identified within the spectra. The lower-energy feature arises from transitions between states that are localized in a T-shaped conformation on both the X- and B-state potentials. The higher-energy feature reflects transitions from states that are localized in a linear conformation on the X state to states that have energies that are larger than the barrier for free rotation of the rare gas atom about the I2 molecule on the B-state potential. Ground-state binding energies of 16.6(6) and 16.3(6) cm-1 were determined for the T-shaped and linear conformers, respectively. These spectra are compared to those calculated using the experimentally determined rotational temperatures. Based on the agreement between the experimental and calculated spectra, the binding energies of the J'=0 states with 0 and 2-6 quanta of excitation in the He...I2 bending mode on the B state were determined. Several models for the B-state potential were used to investigate the origins of the shape of the contour of the higher-energy feature in the spectra for He...I2 and He...Br2. The shape of the contours was found to be relatively insensitive to the choice of potential. This leads us to believe that the spectra of these systems are relatively insensitive to the parameters of the B-state potential energy surface and are more sensitive to properties of the halogen molecule.

A simple model based on the assumption of a thermodynamic equilibrium between the populations of the T-shaped and linear HeÁ Á ÁI 35 Cl(X,v 00 ¼ 0) complexes stabilized in a supersonic expansion is utilized to estimate the relative binding energies of the ground state conformers. In this model, the relative intensities of the laserinduced fluorescence features, attributed to transitions of each conformer, are used to track changes in the populations along the expansion. The relative intensities of the features and the I 35 Cl(X,v 00 ¼ 0) rotational temperatures measured at each distance are fit to ratios of the quantum mechanical partition functions for the T-shaped and linear complexes. These are used to determine the difference between the binding energies of the two conformers. The linear HeÁ Á ÁI 35 Cl(X,v 00 ¼ 0) complex is estimated to be 2.5(6) cm À1 more strongly bound than the T-shaped conformer. The validity of this model was in part confirmed by performing the analysis on calculated ro-vibronic spectra of HeÁ Á ÁI 35 Cl, where the J 00 ¼ 0 binding energies of the ground state conformers are known. The results from high-resolution action and two-color pump-probe spectroscopy experiments reveal that the binding energy of the linear HeÁ Á ÁI 35 Cl(X,v 00 ¼ 0) conformer is precisely 22.0(2) cm À1 . The binding energy of the T-shaped HeÁ Á ÁI 35 Cl(X,v 00 ¼ 0) conformer is then 19.5(6) cm À1 .

The He...I (35)Cl intermolecular vibrational levels with n'=0-6 that are bound within the He+ICl(B,v'=3) potential [A. B. McCoy, J. P. Darr, D. S. Boucher, P. R. Winter, M. D. Bradke, and R. A. Loomis, J. Chem. Phys. 120, 2677 (2004)] are identified in laser-induced fluorescence experiments performed at very low temperatures within a supersonic expansion. Comparisons of the positions and intensities of these lines with the excitation spectra, calculated using potential surfaces to describe the interactions between the helium atom and ICl in its ground and excited state, assist in the assignments. Based on these comparisons the excited state potential was rescaled so that the experimental and calculated J'=0 energies agree to within the experimental uncertainties for all but the lowest, n'=0, intermolecular level. Two-laser, action, and pump-probe spectroscopy experiments indicate that the bound He...I (35)Cl(B,v'=3) intermolecular vibrational levels undergo vibrational predissociation forming rotationally excited I (35)Cl(B,v'=2,j') products with distributions that depend upon the initial intermolecular vibrational level excited. Action spectra recorded in the ICl B-X, 2-0 region while monitoring the Deltav=0, I (35)Cl(B,v'=2) channel reveal two additional dissociation mechanisms for the He...I (35)Cl(B,v') excited state complexes: rotational predissociation of discrete metastable states lying slightly above the He+I (35)Cl(B,v'=2) asymptote and direct dissociation that occurs when the linear conformer is excited to the continuum of states above the same asymptote. The rotational predissociation pathway forms I (35)Cl(B,v'=2,j') products in all of the rotational states energetically accessible. The direct dissociation mechanism yields very cold rotational product state distributions; for instance, the average rotational energy in the product state distribution measured when the linear complexes are prepared 20 cm(-1) above the dissociation limit is only 1.51 cm(-1), representing only 7.6% of the available energy.

The experimental results reported here demonstrate that weakly bound van der Waals complexes are not necessarily kinetically trapped in different isomeric forms in a supersonic expansion. We have recorded the laser-induced fluorescence spectra of the near T-shaped and linear He‚‚‚ICl(X 1 Σ + ,V′′ ) 0) complexes in the ICl B-X, 3-0 spectral region at varying distances along the expansion direction to monitor the populations of the two isomers as the local density, collision frequency, and temperature change. The ratio of the T-shaped to linear complex populations monotonically decreases with increasing downstream distances and thus lower ICl(X) rotational temperatures. Conversion from T-shaped to linear complexes is still observed at the furthest distances where a temperature of 0.57(8) K is measured.

Rovibronic transitions of multiple conformers of the He(2)...(79)Br(2)(X, v'' = 0), He(3)...(79)Br(2)(X, v'' = 0), He(2)...I(35)Cl(X, v'' = 0), and He(3)...I(35)Cl(X, v'' = 0) complexes stabilized in a pulsed, supersonic expansion are observed in action spectra recorded in the B-X region of the dihalogens. In addition to features associated with He(2)...(79)Br(2) and He(2)...I(35)Cl complexes with the rare gas atoms localized in the toroidal potential well lying in a plane perpendicular to the dihalogen bond, those associated with a ground-state conformer that has one He atom localized in the toroidal potential and the other He atom localized in the linear well at the end of the dihalogen moiety are also identified. Transitions of at least three conformers of the He(3)...Br(2) complex and two conformers of the He(3)...ICl complex are also observed. The relative populations of the different conformers are found to depend on where along the supersonic expansion the spectra are recorded, and thus on the local temperature regime sampled. The He(2)...(79)Br(2) and He(2)...I(35)Cl conformers with one He atom in each well are found to be the more stable conformers.