Lennard Wharton

The fluorescence excitation spectrum of the HeI2 van der Waals complex

The HeI2 van der Waals complex was prepared from a dilute mixture of iodine in helium at a pressure of 100 atm by supersonic expansion through a nozzle into a vacuum. Laser‐induced fluorescence excitation spectra were recorded for the B←X transition of HeI2 as well as corresponding spectra for the He2I2 and I2 molecules in the expanding gas. I2 was found to be cooled by the expansion to a rotational temperature of 0.4 K, and a vibrational temperature of 50 K. Similarly, cold internal temperatures were attained by the van der Waals complexes. Evidence was found for vibrational predissociation of the HeI2 complex in both the X and B electronic states. The vibrational predissociation rate was found to depend weakly upon the degree of excitation of the I–I stretching mode, ν1. For ν1=1 in the X state the predissociation rate was found to be greater than 5×106 s−1. In the B state the vibrational predissociation rate is ∼5×1010 s−1 for ν1=27, decreasing to <5×109 s−1 for ν1⩽7. The small (3.4–4.0 cm−1) blue shifts of the vibronic bands of the HeI2spectrum relative to corresponding bands of I2 indicate (1) the van der Waals complex is slightly more strongly bound in the X state than it is in the B state, and (2) the I–I bonding in both the X and B states of iodine is largely unaffected by the formation of the van der Waals bond with helium.

The fluorescence excitation spectrum of rotationally cooled NO2

The fluorescence excitation spectrum of NO2 was measured in the region 5708–6708 A using a tuneable dye laser as an excitation source. The NO2 was cooled to a rotational temperature of ∼3 K by expansion with argon as a carrier gas through a supersonic nozzle. This cooling drastically reduced the rotational structure and thereby permitted a clear separation and analysis of 140 vibronic bands found in this 1000 A region of the spectrum. The results indicate that most of the fluorescence in this region is due to a 2B2 electronic state, and that this state is so heavily perturbed by high lying levels of the ground electronic state that a substantial fraction of all ground state levels of appropriate vibronic symmetry in this region have appreciable fluorescence intensity.

Measurement of Fast Desorption Kinetics of D2 from Tungsten by Laser Induced Thermal Desorption.

Abstract A laser heating technique for studying fast surface processes has been applied in an initial study to the thermal desorption of D 2 from a polycrystalline tungsten sample. This technique is a means for measuring surface reactions at rates, concentrations, and temperatures that approach conditions of technical interest, but with the high degree of definition and control made possible with an ultrahigh vacuum apparatus. The method is analogous to the fast temperature jump method used for studying reactions in condensed phases, and can sort out elementary processes that have differing activation energies. The variation of total flux desorbed with maximum surface temperature reached and initial surface coverage serves, with the aid of a model kinetic rate expression, to determine the desorption rate parameters. It is shown that the desorption of D 2 from W at rates of 5 × 10 7 monolayers/sec is governed by the same kinetics as obtained by extrapolating previous measurements made at a rate about 10 5 times slower. The surface is subjected to a sufficiently fast and large temperature rise to desorb surface atoms or molecules in a time short compared to the range of flight times to a mass spectrometer detector. In this way the velocity distribution of the desorbing species may be determined. This along with the surface temperature history gives additional information on the reaction rate model and also whether the species are emerging in translational thermal equilibrium with the surface. In the present experiments a significant number of desorbatedesorbate collisions occur. Corrections are made for the collision effects in the interpretation of the data. It is shown how modifications of the technique can be made to substantially eliminate these effects. The present conditions were laser pulse width of 3 × 10 −8 sec and surface temperature rise of 300 to 3000 K.

CO2 product velocity distributions for CO oxidation on platinum

Abstract : It has been inferred from the very peaked angular distribution of the CO2 formed by catalytic oxidation of CO on Pt(111) that the product CO2 is desorbed with excess translational kinetic energy. The first direct measurement of the CO2 velocity distributions and angular distributions for this reaction on polycrystalline Pt are reported. These qualitatively confirm the inference and give quantitative information. For CO2 desorbing in the normal direction the most probable velocity was 116000 cm/sec, implying a kinetic energy of E/k = 3560 K. At larger angles from the surface normal the most probable velocity decreases monotonically. At 45 degrees it is 90000 cm/sec for a kinetic energy of E/k - 2140 degrees. (Author)

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...

Energy distribution in the photodissociation products of van der Waals molecules: Iodine–helium complexes

The process of photodissociation of the van der Waals molecules I2He, I2He2, and I2He3 has been studied. Vibronic state distributions of the electronically excited product I2* fragment produced upon photodissociation have been measured by observing the dispersed fluorescence spectra of these fragments. It was found that for each complex I2Hen, the first observed dissociation channel involved the loss of n vibrational quanta from the I2 stretch even though channels involving the loss of fewer quanta were energetically open. The n quantum channel was by far the dominant channel although some weaker n+1 processes were observed. Branching ratios are reported. It was found that the various complexes can be identified by a band shift rule which accurately predicts the spectral shift between the absorption features due to the complex and the absorption band of uncomplexed iodine. This rule says that the spectrum shifts a constant amount for each additional helium atom in the complex. The cross section for collis...

The photodissociation lifetime of the van der Waals molecule I2He

The photodissociation lifetime of the van der Waals molecule I2He has been measured as a function of v, the iodine stretching vibrational quantum number. Lifetimes were deduced from the measured width of the R‐branch head of the fluorescence excitation spectrum obtained in a supersonic free jet. The linewidth (HWHM) varied from 0.0120 cm−1 for v′=12 to 0.0700 cm−1 for v′=26, corresponding to lifetimes of 221 psec (v′=12) and 38 psec (v′=26). The inferred lifetimes were compared to calculated lifetimes provided by a recent theory of vibrational predissociation.

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...

Fluorescence excitation spectrum of s‐tetrazine cooled in a supersonic free jet: Van der waals complexes and isotopic species

Van der Waals complexes between s‐tetrazine and a number of light gases have been observed and characterized by laser spectroscopic studies of a free supersonic expansion of s‐tetrazine in a helium carrier gas. The observed complexes are of the form X–s‐tetrazine and X2–s‐tetrazine where X=He, H2, and Ar. The spectra are consistent with the X species in both types of complexes being bound on or near the out‐of‐plane C2v axis on the top and/or bottom of the s‐tetrazine ring. For the He and H2 complexes, analysis of the rotational structure indicates that the van der waals bond length is ∼3.3 A. The observed red shift of the O–O band of the 1B3u←1A1g transition of all complexes relative to that of s‐tetrazine indicates that a slighly stronger van der waals bond exists in the excited state. Observed vibrational progressions for these complexes indicates the stretching frequency of the van der waals bond in the excited state is 38.0, 95, and 110 cm−1 for the X=He, H2 and Ar complexes, respectively. The O–O ba...

Laser spectroscopic measurement of weakly attractive interatomic potentials: The Na+Ar interaction

The van der Waals molecule NaAr has been prepared in a supersonic expansion of sodium vapor plus a few percent argon in a helium carrier gas. The fluorescence excitation spectrum of the X2Σ+→A2Π optical transition has been analyzed and the vibrational and rotational spectroscopic constants have been derived from this analysis. The ground state interatomic potential implied by these parameters is consistent with that measured or calculated by other workers. The observed Λ‐doubling of the A2Π state suggests that the coupling between the A2Π and B2Σ states may be stronger than would be expected from previous calculations. The magnitude and sign of the dependence of the fine‐structure splitting of the A2Π state on vibrational quantum number suggests that the van der Waals bond produces an observable perturbation of the core electrons of the sodium atom.