A precise study of the rotational spectrum of formaldehyde H2CO, H2CO, H2CO, H2CO

The rotational spectra of formaldehyde, H₂¹²C¹⁶O and its isotopic species H₂¹³C¹⁶O, H₂¹²C¹⁸O, and H₂¹³C¹⁸O have been investigated in the ground vibrational state in the frequency region between 8 and 460 GHz. For most cases in which measurements of the a-type R- and Q-branch transitions already existed the accuracy of the line position has been improved to about 10 kHz. For H₂¹²C¹⁶O and H₂¹³C¹⁶O a large number of ΔK_a = ±2 transitions were measured with similar accuracy. These new data when combined with all other available data and appropriate weightings lead to a set of ground state parameters which for the first time are compatible with infrared and ultraviolet data. The rotational constants (and 3σ standard deviations) obtained using Watson's A-reduced Hamiltonian are:

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The reaction of H2S with surface oxygen on Ni(110)

The reactions of H₂S with predosed surface oxygen on Ni(110) surfaces were studied for a variety of coverage conditions. The primary reaction product is H₂O, but the details of the water formation and desorption depends on the coverage of both O and H₂S.

For high coverages of oxygen (p(2 × 1)−O; 0.5 ML), the reaction to form water is quantitative. The loss of oxygen from the surface (as measured by AES) is equal to the increase in sulfur coverage. XPS and HREELS measurements indicate the presence of chemisorbed H₂O immediately following large exposures of H₂S on the oxygen predosed surface at 110 K. Deuterium incorporation results suggest that the primary mechanism for these coverage conditions involves direct transfer of hydrogen from SH or H₂S moieties to the oxygen.

A second mechanism involving reaction of surface hydroxyl groups with surface hydrogen was also identified. This mechanism is particularly important for high coverages of oxygen (0.5 ML) and low coverages of H₂S (0.15 ML), where water desorption was observed at 235 K, but was not observed spectroscopically at 110 K. The sequential addition of two surface hydrogen atoms to surface oxygen is not an important mechanism in this system.

These reactions were modeled using a bond-order conservation method, and the model successfully reproduced the important mechanistic conclusions.     

The surface chemistry of H2O on clean and oxygen-covered Cu(110)

Adsorption of water at 100 K. on clean and oxygen-covered Cu(110) has been studied using UPS, TDS, Δφ and LEED measurements. The results indicate that two-dimensional hydrogenbonded islands are formed on the clean surface. The long-range order in these islands is in registry with the substrate lattice and gives rise to a c(2×2) LEED pattern. Upon the formation of multilayer ice, the ordering disappears. The presence of oxygen on the surface disrupts the hydrogen bonding, and composite oxygen-water layers are formed. A model of the arrangement of oxygen atoms and water molecules is presented, based upon the LEED observations for these layers and an estimate of the relative oxygen and water coverages. The intensity variation of a thermal desorption peak at 290 K, attributed to adsorbed OH species, with oxygen coverage is in accordance with this model. For low oxygen coverages, the TDS and Δφ results indicate that small oxygen-water clusters with an enhanced ratio of water molecules per adsorbed oxygen atom are present.     

The decomposition of H2S on Ni(110)

Adsorbed H₂S decomposes on Ni(110) to form primarily surface S and H for coverages of less than 0.5 ML. The hydrogen evolves in two separate TPD peaks, characteristic of hydrogen recombination and desorption from the clean surface and from regions perturbed by chemisorbed sulfur. XPS and HREELS indicate the presence of SH and possibly H₂S groups on the surface at 110 K. The XPS data indicates that for coverages less than about 0.5 ML, the concentration of molecular H₂S is small, but it is difficult to asess the coverage of SH groups. However, all of the molecular species decompose prior to hydrogen desorption (for high coverage, 180 K). Physisorbed H₂S is observed on the surface for coverages greater than about 0.5 ML.

The sulfur Auger lineshape was observed to be a function of both coverage and temperature. The changes in the lineshape were attributed to perturbations in local bonding interactions between the S and the Ni surface, perhaps involving some change in either bonding sites or distances but not involving SH bond scission.

The decomposition reaction was modeled using a bond order conservation method which successfully reproduced the experimental results.     

ΔK = 2 transitions in H2CO and D2CO

Weak transitions of the type ΔJ = ± 1, $\text{Δ}\text{K}{}_{\mathrm{a}}\text{= ? 2}$, ΔK_c = ± 3 have been observed in H₂CO and D₂CO by the millimeterwave double resonance method and also by direct absorption with a Stark modulated spectrometer. The addition of these new transitions in a least-squares analysis, in which all previously known microwave and millimeterwave data are also included, results in an improved set of rotational and distortion constants.     

A study of H2S adsorption kinetics on the clean and oxygen covered (110) surface of copper

The very low pressure adsorption kinetics of H₂S on the clean and oxygen covered Cu(110) face have been examined by Auger Electron Spectroscopy (AES) and Mirror Electron Microscopy (MEM, used for continuous surface potential variations of the copper surface). The AES experimental curves on the clean copper face have been interpreted using a model of island growth by surface diffusion. The presence of an adsorbed oxygen layer on the copper surface changes notably the induction times observed on both AES and MEM measurements.     

The ν2 fundamental band of H2CO

The ν₂ fundamental band of H₂CO has been studied using a combination of sub-Doppler laser Stark spectroscopy and Doppler-limited Fourier transform spectroscopy. A combined analysis of the Stark and Fourier infrared data together with previous microwave data on the ν₂ = 1 state yielded improved molecular parameters for formaldehyde, including the excited state dipole moment. A small perturbation was noted at K′_a = 7 which may be ascribed to a ΔK_a = 2 interaction with the v₃ = 1 state. Precise treatments of ν₂ with K′_a > 6 will thus require a combined analysis taking into account Coriolis interactions among ν₄, ν₆, ν₃, and ν₂.     

Stark spectroscopy with the CO laser: The ν2 fundamentals of H2CO and D2CO

The ν₂ (CO stretching) vibration-rotation bands of H₂CO and D₂CO near 5.8 μm have been studied using the technique of laser Stark spectroscopy. The following vibrational and rotational constants have been determined:

     

9.

Measurement of H₂CO hyperfine structure with a two-cavity maser     

S.G. Kukolich  D.J. Ruben 《Journal of Molecular Spectroscopy》1971

Hyperfine structure on the 1₁₀ → 1₁₁ rotational transition in H₂CO was measured using a two-cavity maser spectrometer with 0.3 kHz resolution. The measured spin-rotation constants are C(1₁₀) = −0.80 ± 0.05 kHz and C(1₁₁) = −3.07 ± 0.05 kHz. The hydrogen spin-spin interaction strength is D = 17.74 ± 0.10 kHz. The line center is at 4 829 659.89 ± 0.12 kHz. Properties of the beam source and computer line resolving methods are discussed briefly.     

10.

The adsorption of CO,H₂, H₂, CO₂ and H₂O on carburized and graphitized Ni(110)     

J.G. McCarty  R.J. Madix 《Surface science》1976,54(1):121-138

A study of the adsorption/desorption behavior of CO, H₂O, CO₂ and H₂ on Ni(110)(4 × 5)-C and Ni(110)-graphite was made in order to assess the importance of desorption as a rate-limiting step for the decomposition of formic acid and to identify available reaction channels for the decomposition. The carbide surface adsorbed CO and H₂O in amounts comparable to the clean surface, whereas this surface, unlike clean Ni(110), did not appreciably adsorb H₂. The binding energy of CO on the carbide was coverage sensitive, decreasing from 21 to 12 $\text{kcal}\text{mol}$ as the CO coverage approached 1.1 × 10¹⁵ molecules cm^?2 at 200K. The initial sticking probability and maximum coverage of CO on the carbide surface were close to that observed for clean Ni(110). The amount of H₂, CO, CO₂ and H₂O adsorbed on the graphitized surface was insignificant relative to the clean surface. The kinetics of adsorption/desorption of the states observed are discussed.     

11.

Dynamics of interaction of H₂ and D₂ with Ni(110) and Ni(110) surfaces     

H.J. Robota  W. Vielhaber  M.C. Lin  J. Segner  G. Ertl 《Surface science》1985,155(1):101-120

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H₂ and D₂ on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s₀, irrespective of surface temperature T_s and mean kinetic energy normal to the surface 〈 E_⊥ 〉. The desorbing molecules exhibit a cos θ_e angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s₀ is only about 0.05 if both the gas and the surface are at room temperature. s₀ is again independent of T_s, but increases continuously with 〈 E⊥ 〉 up to a value of ～0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos⁵θ_e angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.     

12.

Interaction of H₂O with a clean and oxygen precovered Ni(110) surface studied by XPS     

C. Benndorf  C. Nöbl  F. Thieme 《Surface science》1982,121(2):249-259

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

13.

Desorption of H₂ from W(110) and Fe covered W(110) surfaces     

T. -U. Nahm  R. Gomer 《Surface science》1997,380(2-3):434-443

The kinetics of H₂ desorption from H/W(110) and H/Fe₁/W(110) were studied by measuring work function changes Δø vs time at a number of temperatures. Combination with previously determined Δø vs coverage data and differentiation at various fixed coverages gave rate vs T data from which activation energies of desorption could be obtained. E vs coverage results agree well with previously determine ΔH_des results. In the case of H/Fe₁/W(110) this includes a rise from 20 to 30 kcal mol⁻¹ of H₂ at H/Fe = H/W > 0.3. Plots of rate −dθ/dt vs θ (θ being coverage in units of H/W) vary much more steeply than θ² at most coverages for both systems. The θ dependence can be explained almost quantitatively in terms of the variations of ΔH_des and surface entropy S_s with coverage, by assuming that rates of desorption are equal to the equilibrium rates of adsorption. The latter can be formulated thermodynamically, except for a sticking coefficient, s. Values for s(θ, T) can also be obtained and show relatively little temperature dependence.     

14.

The adsorption of H₂O on TiO₂ and SnO₂(110) studied by first-principles calculations     

J. Goniakowski  M. J. Gillan 《Surface science》1996,350(1-3):145-158

First-principles calculations based on density functional theory and the pseudopotential method have been used to investigate the energetics of H₂O adsorption on the (110) surface of TiO₂ and SnO₂. Full relaxation of all atomic positions is performed on slab systems with periodic boundary conditions, and cases of full and half coverage are studied. Both molecular and dissociative (H₂O→OH⁻+H⁻) adsorption are treated, and allowance is made for relaxation of the adsorbed species to unsymmetrica configurations. It is found that for both TiO₂ and SnO₂ an unsymmetrical dissociated configuration is the most stable. The symmetrical molecularly adsorbed configuration is unstable with respect to lowering of symmetry, and is separated from the fully dissociated configuration by at most a very small energy barrier. The calculated dissociative adsorption energies for TiO₂ and SnO₂ are in reasonable agreement with the results of thermal desorption experiments. Calculated total and local electronic densities of states for dissociatively and molecularly adsorbed configurations are presented, and their relation with experimental UPS spectra is discussed.     

15.

TDS and LEED studies of H₂O adsorption on GaAs(110)     

W Mokwa  D Kohl  G Heiland 《Surface science》1984,139(1):98-108

The UHV cleaved (110) face has been exposed to water in the range from 10 L to 2 × 10⁴ L. The main TDS peak in H₂O desorption appears at 350 K, independent of coverage. The low desorption energy of 0.7 eV (16 kcal/mol) is reasonable for oxygen atoms bound via the lone pair orbital to As as was earlier derived from UPS measurements. A broad spur between 450 and 600 K may be related to O-Ga bonds. The sticking probability shows values below 10^-4; only near 4.8 × 10³ L (6 × 10¹⁵ cm^-2 s^-1 H₂O molecules for 300 s) corresponding to a coverage of about 0.4 monolayes a steep maximum appears. At about one monolayer saturation is observed. Exposures to more than 10⁴ L of water quench the intensity of the (10) LEED spot considerably stronger than the intensity of the (11) spot. A comparison of the I(E) curves with existing model calculations suggests that the observed behaviour of the LEED spots is caused by a change in surface structure towards the unrelaxed configuration. The higher sticking coefficient observed near 0.4 monolayers may be connected with this rearrangement of surface atoms.     

16.

The interacting states (030), (110), and (011) of H₂¹⁶O     

C. Camy-Peyret  J.M. Flaud 《Journal of Molecular Spectroscopy》1976,59(3):327-337

A fit of 382 rotational levels of the three vibrational states (030), (110), and (011) of H₂¹⁶O has been performed using 51 effective constants. The Fermi-type interaction between (030) and (110) and the Coriolis-type interaction between (110) and (011) as well as between (030) and (011) are taken into account. The part of the Hamiltonian which is diagonal in the vibrational quantum numbers is a Watson-type Hamiltonian. Considering the wide spread of J and K_a values, the general agreement between experimental and calculated levels is satisfactory. A comparison with the results relative to the states (020), (100), and (001) is given.     

17.

Observation of hot bands in the infrared spectrum of H₂CO     

Ricardo Perez  Yurii Utkin  Jiaxiang Han  Robert F. Curl 《Journal of Molecular Spectroscopy》2006,236(2):151-157

Two hot bands in the infrared spectrum of formaldehyde (H₂CO) have been identified by means of tunable infrared laser spectroscopy using a jet-cooled sample. One band falls in the region 2760-2800 cm⁻¹; it follows a-type selection rules and it has been assigned as the ν₁ + ν₄ − ν₄ hot band. The other band falls in the region 2800-2860 cm⁻¹; it follows b-type selection rules and it has been assigned as the ν₅ + ν₄ − ν₄ hot band. The observations are restricted to low J and K_a levels. It has consequently been possible to ignore the effects of the extensive Coriolis couplings involving these levels in the analysis of the spectra and to model the rotational structure as that of a simple asymmetric top. Least-squares fits of the data have provided values for the band origins: 2774.2706(11) cm⁻¹ for the ν₁ + ν₄ − ν₄ and 2829.2621(8) cm⁻¹ for the ν₅ + ν₄ − ν₄ band. Term values for the upper vibrational levels involved in the transitions have been determined by use of the previously reported term values for the v₄ = 1 level.     

18.

The oxidation of the GaAs (110) surface     

R. Ludeke 《Solid State Communications》1977,21(8):815-818

It is demonstrated that, contrary to previous proposals, Ga surface atoms are already involved in the oxidation process for the lowest observable oxygen coverages (0.01 monolayer). A similar involvement of As atoms could not be readily ascertained experimentally, although it is to be expected from energetic considerations. An oxidation model consisting of multiple bridge bonds to both Ga and As surface atoms is proposed, which is consistent with diverse experimental data for the GaAs(110) surface.     

19.

Experimental microwave collision diameters in the rotational spectrum of H₂CO     

D.V. Rogers  J.A. Roberts 《Journal of Molecular Spectroscopy》1973,46(2):200-206

Collision diameters for some select transitions in the rotational spectrum of H₂CO have been determined using pressure broadening of the spectral lines. Transitions of the type ΔJ = 0, K_?1 = 1, and ΔK₊₁ = 1 with 1 ≤ J ≤ 5 were investigated for both self-broadening and foreign gas broadening (He and H₂) of the spectral lines. Pressure ranges from 0.001 Torr to 0.1 Torr were explored in obtaining the line width parameters Δν_p for each transition. Collision diameters were found to be very nearly constant (14 Å) over the J states studied for H₂COH₂CO interaction, 2.5–5.8 Å for H₂COH₂ interaction and 2.7–3.5 Å for H₂COHe interaction.     

20.

The adsorption and reaction of H₂O on clean and oxygen covered Ag(110)     

E.M. Stuve  R.J. Madix  B.A. Sexton 《Surface science》1981,111(1):11-25

The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H₂O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm^?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H₂O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).     

Constant	H2CO	D2CO	Unit
$\text{ν}{}_0$	1746.011	1701.620	cm?1
$\text{A′}$	281807.8 ± 6.	141696.6 ± 7.	MHz
$\text{B′}$	38608.7 ± 5.	32068.4 ± 7.	MHz
$\text{C′}$	33738.7 ± 3.	25998.6 ± 10.	MHz
μ″	2.328 ± 0.006	2.344 ± 0.006	Debye
μ′	2.344 ± 0.006	2.364 ± 0.005	Debye

Measurement of H2CO hyperfine structure with a two-cavity maser

Hyperfine structure on the 1₁₀ → 1₁₁ rotational transition in H₂CO was measured using a two-cavity maser spectrometer with 0.3 kHz resolution. The measured spin-rotation constants are C(1₁₀) = −0.80 ± 0.05 kHz and C(1₁₁) = −3.07 ± 0.05 kHz. The hydrogen spin-spin interaction strength is D = 17.74 ± 0.10 kHz. The line center is at 4 829 659.89 ± 0.12 kHz. Properties of the beam source and computer line resolving methods are discussed briefly.     

The adsorption of CO,H2, H2, CO2 and H2O on carburized and graphitized Ni(110)

A study of the adsorption/desorption behavior of CO, H₂O, CO₂ and H₂ on Ni(110)(4 × 5)-C and Ni(110)-graphite was made in order to assess the importance of desorption as a rate-limiting step for the decomposition of formic acid and to identify available reaction channels for the decomposition. The carbide surface adsorbed CO and H₂O in amounts comparable to the clean surface, whereas this surface, unlike clean Ni(110), did not appreciably adsorb H₂. The binding energy of CO on the carbide was coverage sensitive, decreasing from 21 to 12 $\text{kcal}\text{mol}$ as the CO coverage approached 1.1 × 10¹⁵ molecules cm^?2 at 200K. The initial sticking probability and maximum coverage of CO on the carbide surface were close to that observed for clean Ni(110). The amount of H₂, CO, CO₂ and H₂O adsorbed on the graphitized surface was insignificant relative to the clean surface. The kinetics of adsorption/desorption of the states observed are discussed.     

Dynamics of interaction of H2 and D2 with Ni(110) and Ni(110) surfaces

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H₂ and D₂ on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s₀, irrespective of surface temperature T_s and mean kinetic energy normal to the surface 〈 E_⊥ 〉. The desorbing molecules exhibit a cos θ_e angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s₀ is only about 0.05 if both the gas and the surface are at room temperature. s₀ is again independent of T_s, but increases continuously with 〈 E⊥ 〉 up to a value of ～0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos⁵θ_e angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

Desorption of H2 from W(110) and Fe covered W(110) surfaces

The kinetics of H₂ desorption from H/W(110) and H/Fe₁/W(110) were studied by measuring work function changes Δø vs time at a number of temperatures. Combination with previously determined Δø vs coverage data and differentiation at various fixed coverages gave rate vs T data from which activation energies of desorption could be obtained. E vs coverage results agree well with previously determine ΔH_des results. In the case of H/Fe₁/W(110) this includes a rise from 20 to 30 kcal mol⁻¹ of H₂ at H/Fe = H/W > 0.3. Plots of rate −dθ/dt vs θ (θ being coverage in units of H/W) vary much more steeply than θ² at most coverages for both systems. The θ dependence can be explained almost quantitatively in terms of the variations of ΔH_des and surface entropy S_s with coverage, by assuming that rates of desorption are equal to the equilibrium rates of adsorption. The latter can be formulated thermodynamically, except for a sticking coefficient, s. Values for s(θ, T) can also be obtained and show relatively little temperature dependence.     

The adsorption of H2O on TiO2 and SnO2(110) studied by first-principles calculations

First-principles calculations based on density functional theory and the pseudopotential method have been used to investigate the energetics of H₂O adsorption on the (110) surface of TiO₂ and SnO₂. Full relaxation of all atomic positions is performed on slab systems with periodic boundary conditions, and cases of full and half coverage are studied. Both molecular and dissociative (H₂O→OH⁻+H⁻) adsorption are treated, and allowance is made for relaxation of the adsorbed species to unsymmetrica configurations. It is found that for both TiO₂ and SnO₂ an unsymmetrical dissociated configuration is the most stable. The symmetrical molecularly adsorbed configuration is unstable with respect to lowering of symmetry, and is separated from the fully dissociated configuration by at most a very small energy barrier. The calculated dissociative adsorption energies for TiO₂ and SnO₂ are in reasonable agreement with the results of thermal desorption experiments. Calculated total and local electronic densities of states for dissociatively and molecularly adsorbed configurations are presented, and their relation with experimental UPS spectra is discussed.     

TDS and LEED studies of H2O adsorption on GaAs(110)

The UHV cleaved (110) face has been exposed to water in the range from 10 L to 2 × 10⁴ L. The main TDS peak in H₂O desorption appears at 350 K, independent of coverage. The low desorption energy of 0.7 eV (16 kcal/mol) is reasonable for oxygen atoms bound via the lone pair orbital to As as was earlier derived from UPS measurements. A broad spur between 450 and 600 K may be related to O-Ga bonds. The sticking probability shows values below 10^-4; only near 4.8 × 10³ L (6 × 10¹⁵ cm^-2 s^-1 H₂O molecules for 300 s) corresponding to a coverage of about 0.4 monolayes a steep maximum appears. At about one monolayer saturation is observed. Exposures to more than 10⁴ L of water quench the intensity of the (10) LEED spot considerably stronger than the intensity of the (11) spot. A comparison of the I(E) curves with existing model calculations suggests that the observed behaviour of the LEED spots is caused by a change in surface structure towards the unrelaxed configuration. The higher sticking coefficient observed near 0.4 monolayers may be connected with this rearrangement of surface atoms.     

The interacting states (030), (110), and (011) of H216O

A fit of 382 rotational levels of the three vibrational states (030), (110), and (011) of H₂¹⁶O has been performed using 51 effective constants. The Fermi-type interaction between (030) and (110) and the Coriolis-type interaction between (110) and (011) as well as between (030) and (011) are taken into account. The part of the Hamiltonian which is diagonal in the vibrational quantum numbers is a Watson-type Hamiltonian. Considering the wide spread of J and K_a values, the general agreement between experimental and calculated levels is satisfactory. A comparison with the results relative to the states (020), (100), and (001) is given.     

Observation of hot bands in the infrared spectrum of H2CO

Two hot bands in the infrared spectrum of formaldehyde (H₂CO) have been identified by means of tunable infrared laser spectroscopy using a jet-cooled sample. One band falls in the region 2760-2800 cm⁻¹; it follows a-type selection rules and it has been assigned as the ν₁ + ν₄ − ν₄ hot band. The other band falls in the region 2800-2860 cm⁻¹; it follows b-type selection rules and it has been assigned as the ν₅ + ν₄ − ν₄ hot band. The observations are restricted to low J and K_a levels. It has consequently been possible to ignore the effects of the extensive Coriolis couplings involving these levels in the analysis of the spectra and to model the rotational structure as that of a simple asymmetric top. Least-squares fits of the data have provided values for the band origins: 2774.2706(11) cm⁻¹ for the ν₁ + ν₄ − ν₄ and 2829.2621(8) cm⁻¹ for the ν₅ + ν₄ − ν₄ band. Term values for the upper vibrational levels involved in the transitions have been determined by use of the previously reported term values for the v₄ = 1 level.     

The oxidation of the GaAs (110) surface

It is demonstrated that, contrary to previous proposals, Ga surface atoms are already involved in the oxidation process for the lowest observable oxygen coverages (0.01 monolayer). A similar involvement of As atoms could not be readily ascertained experimentally, although it is to be expected from energetic considerations. An oxidation model consisting of multiple bridge bonds to both Ga and As surface atoms is proposed, which is consistent with diverse experimental data for the GaAs(110) surface.     

Experimental microwave collision diameters in the rotational spectrum of H2CO

Collision diameters for some select transitions in the rotational spectrum of H₂CO have been determined using pressure broadening of the spectral lines. Transitions of the type ΔJ = 0, K_?1 = 1, and ΔK₊₁ = 1 with 1 ≤ J ≤ 5 were investigated for both self-broadening and foreign gas broadening (He and H₂) of the spectral lines. Pressure ranges from 0.001 Torr to 0.1 Torr were explored in obtaining the line width parameters Δν_p for each transition. Collision diameters were found to be very nearly constant (14 Å) over the J states studied for H₂COH₂CO interaction, 2.5–5.8 Å for H₂COH₂ interaction and 2.7–3.5 Å for H₂COHe interaction.     

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H₂O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm^?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H₂O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).