High-resolution rotationally resolved complexes

Dimers. High-resolution spectra obtained by Fourier transform spectroscopy with long path lengths (up to 150 m) and temperatures down to 20 K have shown the bound state-to-bound state bands of a number of van der Waals molecules [267]. Spectra of complexes that contain H2 (e.g., H2Ar) can sometimes fully be resolved and rotationally assigned this provides valuable information concerning molecular interactions. Spectra of heavier complexes (e.g., N2Ar) may not be fully resolved but can still yield useful information. 

Some of the earliest potentials computed by the SRS variant of SAPT were for Ar-H2 [149] and for He-HF [150,151]. An application of the latter potential in a calculation of differential scattering cross sections [152] and comparison with experiment shows that this potential is very accurate, also in the repulsive region. Some other SAPT results are for Ar-HF [153], Ne-HCN [154], CO2 dimer [155], and for the water dimer [129,156]. The accuracy of the water pair potential was tested [130,131] by a calculation of the various tunnehng splittings caused by hydrogen bond rearrangement processes in the water dimer and comparison with high resolution spectroscopic data [132,133]. Other complexes studied are He-CO [157,158], and Ne-CO [159]. The pair potentials of He-CO and Ne-CO were applied in calculations of the rotationally resolved infrared spectra of these complexes measured in Refs. [160,161]. They were employed [162-165] in theoretical and experimental studies of the state-to-state rotationally inelastic He-CO and Ne-CO collision cross sections and rate constants. It was reaffirmed that both potentials are accurate, especially the one for He-CO. 

High-resolution spectroscopy of cluster ions has largely been limited to binary complexes between a molecular ion and a rare-gas atom or a hydrogen molecule. Much of this work had been done by the group of Maier and Dopfer in Switzerland. The spectrum of Ar-H20" given below in Fig. 17 is an excellent recent example of the rotationally resolved spectra that are possible using a tandem quadrupole mass spectrometer. Except for, Rg2-CH3 where Rg = Ar and larger clusters have yielded... 

Direct RKR inversion of vibrationally and rotationally resolved spectroscopic data for diatomics is now a fairly routine procedure. In normal RKR applications, however, the spectral data are exploited in a relatively limited fashion. One simply uses B(v) and E(v), the rotational constant and term value dependence on vibrational quantum v, respectively, to infer the inner and outer classical turning points at each v from a semiclassical analysis. In high resolution spectroscopy of van der Waals complexes, however, there is often far more rotational than vibrational data available. Consequently extensive information exists on very high order centrifugal effects on the radial coordinate, sometimes up to, and by virtue of centrifugal barriers, beyond the dissociation limit The hope is that a simple extension of RKR ideas might be able to extract a 1-D potential directly from rotational data alone. 

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