We have used IR excitation to selectively create populations in admixtures of the zeroth-order states comprising the ∼3000 cm−1 ‘‘C–H stretching Fermi triad’’ of benzene. UV spectra of the 260 nm Ã(1B2u)←X̃(1A1g) transition in the IR-excited molecules show several new bands, which we have assigned. Final states in the UV transitions are some vibrational levels which have not been detected before, allowing us to find several excited-state vibrational frequencies. We have determined ν′3 =1327±3 cm−1, ν19 =1405±3 cm−1, and ν′20 =3084±5 cm−1. Also, vibrational structure which was unresolved in IR spectra of the ‘‘Fermi triad’’ was resolved in the UV double resonance spectra, confirming that the C–H stretching admixture is really a tetrad. The 3048, 3079, and 3101 cm−1 states had formerly been given the labels ν″20, ν″8+ν″19, and ν″1+ν″6+ν″19, respectively. Actually, the middle level most nearly resembles ν″1+ν″6+ν″19, and the 3101 cm−1 level is strongly mixed with ν″3+ν″6+ν″15. As predicted by molecular orbital theory, excited-state C–H bending and stretching frequencies are not very different from those in the ground state. Furthermore, we suggest that the four C–H stretching frequencies increase uniformly by ∼20 cm−1 in the excited state; reexamination of the Atkinson and Parmenter 260 nm Ã←X̃ spectrum leads us to reassign ν2 from 3130 to ∼3093 cm−1, which is 19 cm−1 above ν″2. There is a Fermi resonance between the ν6+ν′20 level and another level ∼13 cm−1 lower in energy; the strength of the perturbation is ∼18 cm−1. Possibilities for the perturbing vibrational state are ν6+ν′8+ν14 and ν′6+ν13.

We used rotational cooling of molecules to -5°K by supersonic expansion and state-selective, multilevel saturation spectroscopy to obtain high-resolution spectra of the fundamental and first and second overtone transitions of C-H stretching modes in benzene and its dimer.Greatly reduced linewidths (<3 cm FWHM) in the rich spectra show that previously reported spectra have suffered from inhomogeneous congestion. Our observed spectral widths indicate that the vibrational lifetimes of the C-H stretches are at least a few psec, even at the energy of the second overtone (8800 cm 1 .) The "local mode" picture appears to apply when at least 3 quanta of C-H stretching motion are present. Spectra of the dimer are similar to those of the monomer but show a red shift of a few cm 1 , the appearance of combination bands involving van der Waals vibrational modes, some intensity changes, and a broadening of spectral features that increases with the vibrational energy. The dimer's predissociation lifetime at -3000 cm 1 vibrational energy exceeds -3 psec.

Terahertz time-domain spectroscopy was used to measure the refractive indices of Ih crystalline ice in the frequency range of 0.25–1.0 THz. With increasing frequency, the real part, n′, of the refractive index increases from 1.787 to 1.793 at 243 K, and the imaginary part, n″, increases from 0.005 to 0.020. The temperature dependence of n′ is less than 0.01%/K and that of n″ is ∼1%/K. Our results connect smoothly to the data of Matsuoka and co-workers [T. Matsuoka, S. Fujita, and S. Mae, J. Appl. Phys. 80, 5884 (1996)] in the microwave range and the data in the far IR range, and can be well described by the existing theoretical models.