ALBERT

Notes: Vibrational spectra of CO physisorbed onto well defined NaCl(100) surfaces were studied using a Fourier transform infrared interferometer. Structures of CO starting from the monolayer to multilayers were explored. At 31.5 K and a CO pressure of 1×10−6 mbar only the monolayer is formed. Polarization measurements confirm our earlier study that the monolayer CO molecules are aligned perpendicular to the NaCl(100) surface. Increasing the CO pressure to 7×10−6 mbar produces multilayer adsorption. The multilayer spectra closely resemble that of α-CO absorption previously reported. The near perfect match of crystal structures and lattice constants of α-CO and NaCl is reasoned to force the epitaxial growth of single crystal multilayers in our experiments. At 22 K the monolayer absorption is at 2155.01 cm−1 with a bandwidth (FWHH) of 0.26 cm−1. The two prominent features in the multilayer spectra at 22 K are assigned to the longitudinal optical (LO) mode at 2142.54 cm−1 and the transverse optical (TO) mode at 2138.51 cm−1. Their frequency separation is a consequence of the lowering of the cubic symmetry of the bulk α-CO crystal by the shape, in the form of thin slabs, of our multilayer samples. Their bandwidths depend on the thickness of the sample and are characterized by a bandwidth parameter of 0.25 cm−1 for the LO mode and 0.85 cm−1 for the TO mode. The relative absorbances of these modes depend on the polarization of the infrared radiation. Theoretical formalism to account for the band splitting and absorption profiles of the infrared absorption is reviewed and applied to our measurements. While many features of our data can be explained by the present theory, further work is required to account for all the experimental results.

Notes: The vibrational dependence of the intermolecular potential of Ar–HF is investigated through the spectra of levels correlating with HF(v=3). We have previously reported measurements of the (vbKn)=(3000), (3100), and (3110) levels of Ar–HF using intracavity laser-induced fluorescence in a slit supersonic jet [J. Chem. Phys. 98, 2497 (1993)]. These levels are found to be well reproduced (within 0.1 cm−1) by the Ar–HF H6(4,3,2) potential [J. Chem. Phys. 96, 6752 (1992)]. The second overtone experiments are extended to include the (3002) state which is coupled to (3110) through Coriolis interaction, and the (3210) state which is more sensitive to higher-order anisotropic terms in the potential. The observations establish that the level (3002) lies 0.229 cm−1 below (3110), with upper state rotational constant B=0.085 89 cm−1. This is in good accord with the predictions of the H6(4,3,2) potential. The (3210) state lies at 11 484.745 cm−1 with B=0.099 79 cm−1. The band origin is 1.7 cm−1 higher than predicted, and thus contains important new information on the vibrational dependence of the potential. Several detailed features of the spectra can be explained using the H6(4,3,2) potential. The Q-branch lines of the (3210)←(0000) band show evidence of a weak perturbation, which can be explained in terms of mixing with the (3112) state. The (3210) spectrum exhibits parity-dependent rotational predissociation and the widths of the P- and R-branch lines and the magnitude of the l-type doubling can be explained in terms of coupling to the (3200) state, which is estimated to lie 4 cm−1 below the (3210) state.The Q-branch lines show a predissociation cutoff above Q(16); this is in reasonable agreement with the predictions of the H6(4,3,2) potential, but suggests that the binding energy calculated for the potential may be about 1 cm−1 too large. To examine the potential further, high-level ab initio calculations are performed, with an efficient basis set incorporating bond functions. The calculations give a well depth of 92%–95% of that of the H6(4,3,2) potential at θ=0° for v=0 and v=3, respectively; this is in line with earlier results on rare gas pairs. The calculations also reproduce the anisotropy of the H6(4,3,2) potential and its vibrational dependence. The dependence of the intermolecular potential on HF bond length is found explicitly.

Notes: Laser-induced fluorescence is used to obtain the second overtone spectrum of ArHF. The method exploits intracavity circulating power of a Ti–sapphire ring laser to pump the weakly bound complex generated in a supersonic slit jet from v=0 to v=3. Fundamental (Δv=−1) emission is monitored using an infrared PbS detector. Intense fluorescence allows recording of the rotationally resolved sub-Doppler spectra of (3000)←(0000), (3100)←(0000), and (3110)←(0000) transitions. We determine vibrational band origins of ν0=11 339.034 cm−1, 11 412.438 cm−1, 11 422.378 cm−1 and rotational constants of B=0.103 30 cm−1, 0.102 76 cm−1, 0.101 18 cm−1 for the (3000), (3100), and (3110) bands, respectively. Both the band origins and the rotational constants indicate that the weak Ar–HF van der Waals bond is strengthened as the HF stretch is vibrationally excited to higher states. All the observations are in near perfect accord with extrapolations of related constants in the HF stretching states of v=0–2.

Notes: We complete the study of the HF stretches (v1 and v2) of (HF)2 at N=v1+v2=3. A previous publication [J. Chem. Phys. 98, 9266 (1993)] reported the observations of the free-HF and hydrogen-bonded-HF stretches at (v1,v2)=(3,0) and (0,3). In this paper, second overtone (ΔN=3←0) spectra of the vibrations mixed between the two HF subunits are presented. Spectroscopic constants of the K subbands and tunneling states (A+ and B+) of the two mixed modes (2,1) and (1,2) are determined from their lifetime-broadened but rotationally resolved manifolds. For the (2,1) mode, we observe only a parallel band, K=0←0, and obtain band origins ν0=11 552.897 cm−1 (A+), 11 552.509 cm−1 (B+), rotational constants B¯=0.220 86 cm−1 (A+), 0.220 94 cm−1 (B+). For the (1,2) mode, a perpendicular band, K=1←0, is observed at ν0=11 536.95 cm−1 (A+), 11 536.93 cm−1 (B+) with B¯=0.222 cm−1 for both A+ and B+ states. The hydrogen interconversion tunneling splittings are determined to be 0.387 and 0.02 cm−1 for the K=0 levels of (2,1) and the K=1 levels of (1,2), respectively, demonstrating a strong dependence on K rotation and the importance of transition-dipole coupling in the tunneling process. Based on our present and previous results, we provide an overview of all the four components of the quartet by comparing five unique characteristics: vibrational symmetry, band origin, relative transition strength, hydrogen interconversion tunneling, and vibrational predissociation. Systematic comparison is also made against ab initio calculations of Jensen, Bunker, Karpfen, Kofranek, and Lischka [J. Chem. Phys. 93, 6266 (1990)]. A brief analysis suggests that the pure overtone modes can be described sufficiently by a local mode picture, whereas the mixed modes have strong normal mode characters. It is also concluded that the ab initio calculations do not reproduce the observations correctly and more adequate representation of the high vibrationally excited states of the HF dimer is required.

Notes: Three new combination bands, at the second overtone of the HF intramolecular stretch, 3ν1, with each of the three low frequency intermolecular modes, have been spectroscopically characterized by intracavity laser-induced fluorescence for the N2HF complex. The van der Waals stretching, HF and N2 bending frequencies at vHF=3 are determined to be 98.6, 328.6, and 68.5 cm−1, respectively. State dependent vibrational predissociation is observed in these three bands. The variation in the vibrational predissociation rate in these three bands suggests a strong angular dependence of the intermolecular potential with the HF internuclear distance in the complex. © 1996 American Institute of Physics.