George E. Ewing

Infrared Spectrum, Structure, and Heat of Formation of Gaseous (NO)2

Gaseous nitric oxide was examined from 4000 to 1600 cm−1 at temperatures between 77 and 150°K with a 3.9‐m and a 6‐cm cell. Features found at 1860 and 1788 cm−1 exhibited a quadratic pressure dependence and were assigned to (NO)2. The band shape of these features indicated the presence of a cis‐(NO)2 isomer. The heat of formation of (NO)2 from monomeric NO is ΔH = − 2450 cal/mol. Other bands due to (NO)2 are reported.

Spectroscopic investigation of van der Waals molecules. I. The infrared and visible spectra of (O2)2

The infrared and visible spectra of gaseous oxygen have been examined at temperatures around 90°K using a long path absorption cell. At all temperatures the infrared and visible spectra show a broad band which can be assigned as collision‐induced absorption. However, at low temperatures small but discrete features appear with integrated intensities dependent on the square of the gas density. These features are assigned to bound state van der Waals molecules of the type (O2)2. The visible absorption of (O2)2 studied corresponds to the 1 Δg (ν=0)+1 Δg(ν=1) ←3Σg−(ν=0) simultaneous transition. The part of the spectrum attributed to bound dimers shows a progression of eight fine structure bands superimposed on the broad simultaneous transition absorption. The fine structure has been assigned to combinations of electronic and vibrational transitions involving the stretching mode of the van der Waals bond of (O2)2. In the ground state each oxygen molecule is in the 3 Σg−(ν=0) state, while in the excited state on...

Vibrational predissociation in hydrogen bonded complexes

A vibrationally excited hydrogen bonded complex, A–H*⋅⋅B, is metastable since the vibrational energy of the A–H* chemical bond exceeds that needed to sever the hydrogen bond. After a time the excited complex spontaneously breaks up and fragments A–H and B are produced. We show that this vibrational predissociation process is inefficient when there is unfavorable Franck–Condon overlap between the vibrational wave functions which characterize the bound complex A–H*⋅⋅⋅B and the wave functions which describe the fragments A–H+B. We also show that the vibrational predissociation process is most efficient when the fragments are produced in rotationally or vibrationally excited states. Analytical expressions are presented which allow numbers for the lifetime of A–H*⋅⋅⋅B to be easily obtained with a hand electronic calculator. These lifetimes are exceedingly sensitive to the parameters which characterize the multidimensional potential surface of the hydrogen bonded complex. Since the surface is poorly understood, the resulting lifetimes have only a qualitative significance. Sample calculations of vibrational predissociation of HF*⋅⋅⋅HF and its isotopes together with figures of potential surfaces, wave functions, and coupling terms serve to illustrate the ideas of this paper.A vibrationally excited hydrogen bonded complex, A–H*⋅⋅B, is metastable since the vibrational energy of the A–H* chemical bond exceeds that needed to sever the hydrogen bond. After a time the excited complex spontaneously breaks up and fragments A–H and B are produced. We show that this vibrational predissociation process is inefficient when there is unfavorable Franck–Condon overlap between the vibrational wave functions which characterize the bound complex A–H*⋅⋅⋅B and the wave functions which describe the fragments A–H+B. We also show that the vibrational predissociation process is most efficient when the fragments are produced in rotationally or vibrationally excited states. Analytical expressions are presented which allow numbers for the lifetime of A–H*⋅⋅⋅B to be easily obtained with a hand electronic calculator. These lifetimes are exceedingly sensitive to the parameters which characterize the multidimensional potential surface of the hydrogen bonded complex. Since the surface is poorly understood,...

On the contributions of van der Waals interactions to vibrational level mixing. Torsion–vibration coupling in p‐fluorotoluene

As an example demonstrating the importance of van der Waals interactions to vibrational level mixing, a model is developed for the intramolecular coupling of internal methyl torsion to aromatic ring vibrations in p‐fluorotoluene (pFT). The coupling is a consequence of van der Waals interactions between the methyl rotor and the ring. Fermi resonance matrix elements are generated in a manner analogous to previous calculations for collision‐induced vibrational energy transfer and van der Waals molecule predissociation. Using these matrix elements, the extent of vibrational mixing is calculated for the level 6a1 in S1 pFT. The calculated result is in agreement with spectroscopic observations. Arguments for the generality of van der Waals interactions as mediators of intramolecular vibrational level mixing are presented.

A guide to the lifetimes of vibrationally excited van der Waals molecules: The momentum gap

A guide is offered towards understanding the lifetimes of vibrationally excited Van der Waals molecules. A correlation of the theoretically calculated lifetimes, to interpret experimental lifetimes is also given. (AIP).

Nuclear‐Spin Conversion and Vibration‐Rotation Spectra of Methane in Solid Argon

The ν3 and ν4 infrared spectra of methane in an argon matrix have been studied. The assignment of vibration‐rotation features enables the methane rotational energies to be determined. The spacings of these levels, which are discussed in terms of crystal‐field effects, suggest that methane is a hindered rotor in its solid matrix. The vibration‐rotation features exhibit time‐dependent absorption changes. These changes which are evidence of triplet →quintet nuclear‐spin conversion that accompanies the J = 1→J = 0 rotational relaxation, follow first‐order kinetics with a half‐life of 90 min. The half‐life is unchanged by addition of up to 1% N2 to the matrix but decreases to 3 min with only 0.2% O2. Two mechanisms are proposed which are in order of magnitude agreement of observed relaxation rates. In the absence of paramagnetic impurities, spin‐spin interaction within the molecule mixes spin states and allows J = 1→J = 0 to relax into the crystal lattice. In the presence of O2 the spin states are mixed by the...

The infrared spectrum of the (N2)2 van der waals molecule

Abstract The infrared spectrum of gaseous nitrogen has been examined at 77°K using a long path absorption cell. The spectrum shows a relatively strong diffuse band which can be assigned as collision-induced absorption. Weak but discreate features atop the collision-induced band are assigned to vibration-rotation absorption by (N2)2 van der Waals molecules. The interpretation of these absorption features provides an estimate of 3.7A for the (N2)2 van der Waals bond length. It has a floppy structure with a barrier to internal rotation of N2 within (N2)2 estimated to be 15-30 cm−1. The equilibrium Configuration is believed to be “T” shaped.

Spontaneous desorption of vibrationally excited molecules physically adsorbed on surfaces

Abstract The energy within a vibrationally excited physisorbed molecule often exceeds that needed to break its bond to the surface. Energy transfer from the vibrating chemical bond to the surface bond causes the surface bond to rupture and the vibrationally relaxed adsorbate is released from the surface. We present a theoretical model which allows an estimation of the residence time of a vibrationally excited adsorbate on a surface. Because of uncertainties in the nature of the surface bond, the lifetimes obtained from the analytical expressions presented have only qualitative significance. The results are interpreted in terms of Franck-Condon overlaps between the wavefunctions which describe the adsorbate-substrate complex and the released adsorbate. Lifetimes are calculated for hydrogen isotopes adsorbed on sapphire surfaces. Guide-lines are given for estimating lifetimes of other systems in terms of a few easily calculated parameters. Let us summarize this guide to spontaneous desorption of physically adsorbed vibrationally excited molecules. The most efficient desorption processes will occur for adsorbates with a small number of bound states ( d 0 small) and when released the adsorbate has small translational momentum (small q m ). This momentum gap correlation is most succinctly revealed by fig. 3. Smaller translational momentum will be achieved if the adsorbate can take up energy into its internal motions. Absorption of energy into lattice modes of the substrate will also serve to reduce the translational momentum and provide for more efficient desorption. However, if the vibrational frequency of the adsorbate is in near resonance with surface polarons or plasmons of the substrate, energy transfer to the solid will be so efficient that desorption will be quenched. A test of these possible relaxation channels awaits the first experimental measurements of desorption of vibrationally excited molecules.

Infra-red spectrum, structure and properties of the N2-Ar van der Waals molecule

The first spectroscopic observation of bound N2-Ar van der Waals molecules has been achieved with a cryogenic long path cell maintained at 87 K. The infra-red spectrum exhibits prominent fine structure near the N2 stretching frequency which is assigned to hindered internal rotation of N2 within the weakly bound complex. An analysis of this fine structure yields a T-shaped equilibrium geometry in which the N2 bond axis is perpendicular to the N2-Ar van der Waals bond axis. The observed spectrum is shown to be consistent with an internal rotational barrier of 20 cm-1 (57 cal/mole). Approximately 20 per cent of the bound species are trapped by this rotational barrier and acquire a locked semi-rigid structure. The remaining 80 per cent have ill-defined geometry and undergo hindered internal rotation. The rotational envelope of an infra-red fundamental is analysed to give an estimate of the N2-Ar bond length as 3·9 A.

Adsorption of water on the NaCl(001) surface. II. An infrared study at ambient temperatures

Water adsorbed on the (001) face of NaCl under ambient conditions has been studied by infrared spectroscopy. From these measurements, combined with recent Monte Carlo calculations, we find evidence of two structures for the adsorbed water. At low coverages, the water molecules aggregate into islands on the surface. When a critical concentration is reached, multilayer growth becomes favorable, creating a thin film on the surface with properties similar to liquid water.