Kenneth C. Janda

Hydrogen desorption from the monohydride phase on Si(100)

The kinetics of the thermal recombinative desorption of hydrogen from the monohydride phase on the Si(100) surface has been studied by laser‐induced thermal desorption (LITD). A rate law that is first order in the atomic hydrogen coverage with an activation energy of 45 kcal/mol gives an accurate fit to the data over a temperature range of 685–790 K and a coverage range of 0.006 to 1.0 monolayer. A new mechanism is proposed to explain these surprising results, namely, that the rate limiting step of the reaction is the promotion of a hydrogen atom from a localized bonding site to a delocalized band state. The delocalized atom then reacts with a localized atom to produce molecular hydrogen which desorbs. Evidence to support these conclusions comes from isotopic mixing experiments. Studies of recombinative desorption from other surfaces of silicon, which had been assumed to obey second‐order kinetics, are discussed in the light of these results.

Benzene dimer: A polar molecule

The electric deflection of molecular beams of (C6H6)2, produced by adiabatic expansion, has been measured. The benzene dimer is observed to be a polar species. It is likely that the structure of this species is that of two perpendicular planes, as is observed for nearest neighbors in crystal and liquid benzene.

Centrifugal distortion in ArHCl

Measurements of centrifugal distortion constants for four isotopic species of the weakly bound, wide amplitude bender ArHCl are reported. Their relationship to an effective radial potential and the associated vibrational wavefunction is discussed. Although the stretching vibrational frequency and the resulting force constant do not have a straight forward relationship to the nonharmonic, angle averaged radial potential, an interpretation of the centrifugal distortion constant in terms of the standard deviation—or width—of the radial wavefunction is proposed. It is shown that a harmonic approximation for the standard deviation of the wavefunction accurately reflects a realistic wavefunction for a complicated potential surface. The derived constants for the four isotopic species are as follows: D0(kHz) ω (cm−1) k (mdyn/A) σ= (〈R2〉−〈R〉2)1/2(A) ArH35Cl 20.0(4) 32.4 0.0117 0.166 ArH37Cl 19.0(4) 31.9 0.0114 0.165 ArD35Cl 17.1(4) 34.4 0.0134 0.160 ArD37Cl 16.4(4) 33.7 0.0132 0.159.

Direct inelastic scattering Ar from Pt(111)

High resolution angularly resolved time of flight distributions are presented for a supersonic argon beam scattering from a clean well‐characterized Pt(111) single crystal. A novel presentation of the resulting velocity and angular flux information in terms of iso‐flux contour maps in Cartesian velocity space allows the scattering process to be decomposed into three mutually independent directions defined by the surface normal (z), parallel to the surface and in the scattering plane (y), and parallel to the surface but perpendicular to y and z(x). The iso‐flux contour maps appear as nested ovals with principal axes oriented parallel to the above defined directions; axis length decreases in the order z, y, x. The corresponding variances in the x, y, and z velocities vary directly with the surface temperature. Three beam energy regimes are evident and are discussed in terms of the diminishing effect of the attractive well which occurs for increasing beam energies and the increasing effect of short range phe...

Hydrogen bonding: The structure of HF–HCl

The structure of the hydrogen bonded complex formed between HF and HCl has been determined by molecular beam electric resonance spectroscopy. The molecule HF–HC1 is a slightly asymmetric prolate top. The spectroscopic constants determined from K=0 spectra of several isotopic species are 〈eqQDa〉 for HF D35Cl is 146(15) kHz. The equilibrium atomic arrangement is HF–HCl with the internal proton colinear with the two heavy atoms spaced 2.12 A from the fluorine atom. The exterior proton is off axis by 50°. The bonding of this complex is discussed with respect to other gas phase complexes and hydrogen halide crystal structures. It appears that a model which involves electron donation from the highest occupied molecular orbital (HOMO), of one submolecule to the lowest unoccupied molecular orbital (LUMO) of the other gives a useful qualitative description of complex formation between closed shell species.

Direct measurement of velocity distributions in argon beam–tungsten surface scattering

Velocity distributions for Ar atoms scattering from a clean, polycrystalline tungsten surface have been measured for a wide range of incident supersonic beam energies 300 K < (1/2) m〈v2〉/k<2000 K, and surface temperatures 350 K<Ts<1900 K. This work studies directly the nature of the scattering process of an intermediate mass atom on a clean metal surface over a very wide range of conditions. Direct inelastic scattering involving a single encounter of the gas atom with the surface is the most important process. No distinct elastic or quasielastic scattering occurs. Only at the lowest temperatures is a trapping–desorption scattering process observed. The direct inelastic scattering process is characterized by the linear proportional relationship 〈KEe〉=0.83 〈KEi〉+0.20 〈KETs〉 over the entire range of energies and temperatures for 45° angle of incidence and observation in the specular direction (KEe, KEi, and KETs are the kinetic energy of the exiting Ar, the incident Ar, and the Ar in equilibrium at the surfa...

State‐to‐state vibrational predissociation dynamics and spectroscopy of HeCl2: Experiment and theory

The structure and vibrational predissociation dynamics of HeCl2 are studied by pump–probe spectroscopy and by three‐dimensional quantum mechanical calculations. Parity selected excitation spectroscopy is used to confirm the essential features of the previous analysis of the HeCl2 B←X laser excited fluorescence spectra. Product vibrational and rotational state distributions are measured for the v’=6, 8, 12, 20, and 24 levels of HeCl2 in the B state. For the v’=6 and 8 levels the dependence of the product state distribution on the initially excited rotational state is also measured. Although the dissociation dynamics are dominated by Δv=−1, V→T energy transfer, several interesting effects are revealed by monitoring the product rotational degrees of freedom. Due to the symmetry of the HeCl2 potential, the parity of the initially excited HeCl2 rotational state is conserved during the dissociation dynamics. Even when a single initial rotational state is excited, the observed product rotational state distributi...

Infrared photodissociation of van der Waals molecules containing ethylene

Vibrational predissociation line shapes in the ν7 region of the ethylene spectrum are measured for van der Waals molecules of ethylene bound to Ne, Ar, Kr, C2H4, C2F4, and larger ethylene clusters. The predissociative rate is very fast for this group of molecules. The vibrationally excited state lifetimes are 0.44, 0.59 and 0.89×10−12 sec for (C2H4)2, Ar⋅C2H4, and C2H4⋅C2F4 respectively. That the observed line shapes are homogeneous is demonstrated by the fact that a low‐power, narrow frequency bandwidth laser can dissociate a large fraction of the initial ensemble of ethylene clusters. The observed transition probability is proportional to the number of ethylene subunits for clusters containing three or fewer ethylene subunits. These observations are interpreted in terms of intramolecular energy flow directly from ethylene ν7 to the weak van der Waals modes of motion.

Three‐dimensional quantum mechanical study of Ne⋅⋅⋅Cl2 vibrational predissociation

Three‐dimensional quantum mechanical calculations for vibrational predissociation of the Ne⋅⋅⋅Cl2 van der Waals complex are presented and compared with experiments. Lifetimes and final rotational state distributions were obtained for the two processes: (i) Ne⋅⋅⋅Cl2(X,v=1) →Ne+Cl2(X,v=0) and (ii) Ne⋅⋅⋅Cl2(B,v=11) →Ne +Cl2(B,v=10,9) where v denotes the vibrational quantum number of Cl2 and X and B specify electronic states of Ne⋅⋅⋅Cl2 which correlate with the X 1∑+0g and B 3∏+0u states of the free Cl2 molecule, respectively. The van der Waals interaction potential was taken to have the same form in the X and B states. At short distances, it is described by a sum of Morse pairwise potentials between the Ne atom and each of the Cl atoms, and between the neon atom and the center of mass of Cl2. At large distances the potential switches to an anisotropic van der Waals interaction with R−6 and R−8 dependence. The parameters were adjusted so that the T‐shaped configuration the potential matched the one determined...

Product state distributions for the vibrational predissociation of NeCl2

Product state distributions are reported for the vibrational predissociation of the NeCl2, B state, v’=6 through v’=13 levels. For the lower vibrational levels, Δv=−1 dissociation produces a bimodal Cl2 product rotational state distribution with the first maximum at j=4 and a secondary maximum at j=20. Surprisingly, the positions of these maxima are the same for v’=6, 7, 8, 9, and 10. For higher vibrational levels the limited available phase space constricts the observed rotational distribution allowing the Ne–Cl2 bond energy D0 to be determined. D0 is 54±2 cm−1 for the B electronic state, and 60±2 cm−1 for the ground electronic state. Δv=−2 dissociation produces a rotational distribution which, although not bimodal, is otherwise quite similar to that of the Δv=−1 channel, even though significantly more energy is released to product translation for Δv=−2. This behavior is quite different from what would be predicted by an impulsive half‐collision model for the dynamics. Three dimensional quantum calculati...