2. Molecular Aggregation and Multibody Interactions

In collaboration with Robert R. Lucchese

Current and past Support: Robert A. Welch Foundation,

National Science Foundation, ARP Texas Higher Education Coordinating Board

One series of systems which we are studying are the small (HX)

_{n}clusters. Investigations of the structures, dynamics, and non-additive multi-body interactions will focus especially on the trimers and tetramers such as (HCl)_{3}, (HCl)_{4}, (HBr)_{3}and (HI)_{3}. High resolution spectroscopic analysis will be combined with theoretical vibrational and electronic structure calculations. Our recently developed submillimeter/Terahertz spectrometer will prove a powerful tool in providing precise data facilitating such analyses. Fully vibrational ab-initio porphed potentials are currently being optimized for such interactions. The structure and dynamics of (HBr)_{2}and (HI)_{2}will also be studied to provide pair-wise interaction potentials that facilitate the investigations of the larger clusters. Recently a four dimensional ground state morphed potential has been developed based on our rovibrational infrared, rotational tunneling and previously investigated microwave analysis of HBrDBr and the second virial coefficient of (HBr)_{2}as shown below. These studies will directly contribute to the understanding and modeling of how multibody interactions contribute to gas phase aggregation and condensation in hydrogen halides.

## The Four Dimensional Ground State Potential of HBr Dimer

Figure 1. A segment of P(2) transitions in the rotational tunneling transition of HBr-HBr showing J',F'1 F'2 F - J",F"1,F"2 components. The center frequencies of each resolved quadrupole component is determined from the average of corresponding frequencies measured for each displaced Doppler component for that transition.

Figure 2. Two dimensional cuts of the morphed interaction potential of (HBr)_{2}(left column) with the corresponding statistical uncertainties from the non-linear least squares fit (right column). All contours are in cm-1 and are relative to the minimum of the potential which occurs at R = 4.04 Å, θ_{1}= 16.9°, θ_{2}= 108.3°, and φ = 180°, with V = -644.0 cm^{-1}. The angles are the Jacobi coordinates of the (H79Br)_{2}isotopomer.

Figure 3. Two dimensional cuts of the vibrational wave functions of (H^{79}Br)_{2}in the ground state (left column) with energy of E = -409.87 cm^{-1}and in the first excited state (v_{5}= 1) (right column) with an energy of E = -394.83 cm^{-1}. Note that the v_{5}= 1 wave function is identically zero when θ_{1}= 45° and θ_{2}= 135°.

Figure 4. Top panel: Cuts through the potential obtained by finding the minimum in the potential with the value of θ_{1}fixed at the value indicated on the abscissa. The height of the barrier is 58.6 cm^{-1}in the ab initio calculation, 57.2 cm^{-1}in the three parameter morphed potential, and 72.8 cm^{-1}in the seven parameter morphed potential. Bottom panel: The value of R at the minima obtained with fixed values of θ_{1}.

Previous dimer studies of clusters Rg

:HBr and Rg:HI have shown unusual structural and dynamical behaviour that predicates the investigation and characterization of solvation effects in clusters of the type Rg_{n}HBr and Rg_{n}HI and their relevance to cage effects in photoinitiated reactions in these clusters will also be investigated form this perspective.