SLIT-JET NEAR-INFRARED DIODE-LASER SPECTROSCOPY OF (DCL)(2)-NU(1),NU(2) DCI STRETCHING FUNDAMENTALS, TUNNELING DYNAMICS, AND THE INFLUENCE OF LARGE-AMPLITUDE GEARED INTERMOLECULAR ROTATION

The first high resolution spectra of (DCl)2 are reported using direct IR laser absorption spectroscopy in a slit supersonic expansion. The s pectral data are analyzed to obtain vibrational frequencies, rotationa l constants, and tunneling (interconversion) level splittings for isot opically symmetric (DCl-35)2 and (DCl-37)2, and mixed DCl-35-DCl-37 di mers. Six dimer absorption bands are observed and analyzed for both (D Cl-35)2 and DCl-35-DCl-37. These include two perpendicular K(a) = 1 <- - 0, v1 = 1 <-- 0 (i. e., ''free'' DCl stretch) bands, one each origin ating from the lower (+) and the upper (-) tunneling sublevels in the ground vibrational state. Four parallel v2 = 1 <-- 0 (i.e., ''bound'' DCl stretch) bands are also observed, one for each of the K(a) = 0 <-- 0 and K(a) = 1 <-- 1 subbands originating from both the lower (+) and upper (-) tunneling components. In addition, two bands are observed o nly for the isotopically mixed dimer (i.e., complexes from DCl-35 and DCl-37), Which acquire oscillator strength by virtue of the breaking o f inversion symmetry. This complete set of bands provides the necessar y data to determine interconversion splittings for the mixed dimer in the ground [5.9595(6) cm-1] and the two DCI vibrationally excited stat es [3.2286(6) cm-1 for v1=1 and 2.9935(6) cm-1 for v2=1], as well as t o make accurate predictions for the symmetric (DCl-35)2 dimer. These e xperimental splittings for the ground state are compared to results fr om (i) a 1D quantum calculation for adiabatic motion over a minimum en ergy tunneling path; and (ii) a 3D variational calculation in a basis set of free DCl rotors which treats all three internal bend and torsio n angles (THETA1, THETA2, and phi1-phi2). These calculations, performe d on an approximate dipole and quadrupole model of the electrostatic p otential surface, reproduce the ground state tunneling splittings to w ithin 15%. The corresponding eigenfunctions provide direct evidence fo r highly correlated, ''geared'' internal rotation of the two DCI subun its over a low barrier. The fivefold decrease in tunneling splitting f or the symmetric (DCI)2 upon v1=1 or v2=1 excitation is qualitatively consistent with previous models of vibrationally diminished tunneling rates due to intramolecular V --> V energy transfer at the C2h transit ion state. However, this decrease is nearly identical to the 4.8-fold decrease observed in (HCl)2, which is quantitatively inconsistent with a simple dipole-dipole vibrational energy transfer mechanism. Measure d linewidths in these dimer spectra are all at the resolution limit of the diode laser apparatus, which translates into vibrational predisso ciation lifetimes in excess of 3 ns.