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Following adsorption of atomic hydrogen on Si(100)–(2×1), the surface is heated and the desorbed H2 is detected via (2+1) resonance‐enhanced multiphoton ionization (REMPI). H2 desorption correlated with the decomposition of dihydride groups on the surface (SiH2) is detected at a surface temperature Ts near 660 K, and with the monohydride species (SiH) near Ts=780 K. Although the H2 rotational distributions are nearly identical for the mono‐ and dihydride species, the vibrational distributions differ with roughly 0.2% and 1% of the population in H2(v=1) for the monohydride and dihydride, respectively. The enhancement in the [H2(v=1)]/[H2(v=0)] population ratio over that of a thermal distribution at Ts is, however, roughly 20 times for both mono‐ and dihydride species. The results are interpreted within a model that assumes desorption proceeds through a common intermediate, which is identified as the dihydride.

The dynamics of recombinative hydrogen desorption from the Si(100)‐(2×1) and Si(111)‐(7×7) surfaces have been compared using (2+1) resonance‐enhanced multiphoton ionization to probe the desorbed H2. After dosing the surface with disilane (Si2H6), we performed temperature programmed desorption in a quantum‐state‐specific manner. The rovibrational‐state distributions of H2 desorbed from both Si(100)‐(2×1) and Si(111)‐(7×7) are found to be the same within experimental accuracy. The rotational distribution is non‐Boltzmann and has an average energy significantly lower than kTs, where Ts is the surface temperature. In contrast, superthermal energy is observed in the vibrational degree of freedom, and the v=1 to v=0 population ratio is approximately 20 times higher than that predicted by Boltzmann statistics. Our results imply that the details of the recombinative desorption process that affect the product state distribution are remarkably insensitive to the structural differences between the surfaces. We sugge...

The desorption of hydrogen from the monohydride species on Si(100) has been studied state specifically using (2+1) resonance‐enhanced multiphoton ionization. The monohydride phase was prepared by dosing the surface with either disilane (Si2H6) or atomic hydrogen. Adsorption of disilane with subsequent desorption of H2 leads to the growth of an epitaxial silicon film, based on evidence obtained with scanning electron microscopy and low energy electron diffraction. We report that the rovibrational‐state distribution for hydrogen desorbed from Si(100) is the same after both disilane and atomic‐H adsorption. Hydrogen desorbs with low average rotational energy but with a population in the v=1 state enhanced by roughly 20 times over a thermal distribution at the temperature of the surface. The agreement between internal‐state distributions for both adsorption schemes indicates that the desorption of hydrogen during epitaxial growth of Si after Si2H6 adsorption proceeds in the same manner as that for a hydrogen‐...

We have measured vibrational‐ and rotational‐state distributions for H2, D2, and HD thermally desorbed from the monohydride phase on Si(100) surfaces using resonance‐enhanced multiphoton ionization detection. The ν=1 to ν=0 population ratio is roughly 20 times higher than that predicted by Boltzmann statistics at the surface temperature, Ts≊780 K. In contrast, the average rotational energies of the desorbed molecules are significantly lower than kTs, exhibit no isotopic dependence within experimental error, and are not significantly different in the ν=0 and ν=1 vibrational states. In the vibrational ground state, we find 〈Erot〉 =345±83 K, 451±77 K, and 332±57 K for H2, HD, and D2, respectively. The degree of vibrational excitation suggests that the H–H interatomic distance in the transition state is elongated compared with the gas‐phase equilibrium bond distance. The low average rotational energy clearly rules out recombination from a highly asymmetric transition state or recombination from high‐impact‐pa...

We have measured rotational distributions for H2 and D2 thermally desorbed from Si(100) surfaces using resonance‐enhanced multiphoton ionization (REMPI) for detection. These distributions are described by average rotational energies that are significantly lower than kTs (Ts=surface temperature) and exhibit slight, if any, isotopic dependence, i.e, 〈Erot(H2)〉=368±67 K and 〈Erot(D2)〉=348±65 K. The low average rotational energy clearly rules out recombination from a highly asymmetric transition state or recombination from high‐impact‐parameter collisions. The rotational distributions indicate that some dynamical constraint causes very little torque to be applied to molecular hydrogen during recombination. Our data may be interpreted as resulting from an exaggerated preference for reactive trajectories that are characterized by low‐impact parameters and/or a high degree of symmetry of the bond axis relative to the potential energy surface, followed by prompt desorption of the newly formed molecular hydrogen f...

Vibrational energy relaxation of molecules adsorbed at single crystal metal surfaces is measured by populating excited vibrational levels with picosecond infrared pump pulses and probing the population by infrared-visible sum frequency generation. Energy relaxation of the v=1 levels of the C-O stretching mode in CO/Cu(100) and of the symmetric C-H stretching mode of CH3S/Ag(111) are discussed.

The title oligomers have been incorporated in Zr-based layers on quartz substrates. Absorbance and emission spectra and fluorescence decays have been measured on these films and compared with data from solutions, powders, and PPV. The samples may be divided into those with “liquid-like” behavior and “solid-like” behavior; the latter is characterized by blue-shifted absorbance, red-shifted emission, and more complex decay dynamics than the former. By these criteria, the layers and PPV itself are decidedly “solid-like’.

The primary photodissociation channels of CH2BrI following excitation at 193.3, 210, and 248.5 nm have been studied with the crossed laser‐molecular beam technique. Product translational energy distributions and polarization dependences were derived for the primary dissociation processes observed. The data demonstrate bond selective photochemistry as well as some selective formation of electronically excited photofragments in bond fission and concerted dissociation. Excitation at 248.5 nm, which is assigned to excitation of primarily a n(I)→σ*(C–I) transition with some contribution from an overlapping n(Br)→σ*(C–Br) transition, results in both C–I and C–Br bond fission. C–I bond fission is the dominant channel, producing I atoms in both the 2P3/2 and spin‐orbit excited 2P1/2 states in a ratio of 1.0:0.75. Excitation at 193.3 nm, assigned to a transition to primarily predissociated Rydberg levels on the I atom, leads to C–Br bond fission, some C–I bond fission, and significant concerted elimination of IBr. Analysis of the product translational energy distributions for the dissociation products indicates that the IBr is formed electronically excited and that the halogen atom products are spin‐orbit excited. Excitation at 210 nm, of the transition assigned as n(Br)→σ*(C–Br) based on comparison with CH3Br, results in selective breaking of the stronger C–X bond in the molecule, the C–Br bond, and no fission of the C–I bond. Some concerted elimination of IBr also occurs; the IBr velocity distribution indicates it is probably formed electronically excited as in photolysis at 193.3 nm. The selective breaking of the C–Br bond over the weaker C–I bond is discussed in contrast to previous photolysis studies of polyhalomethanes.

Rotationally resolved experiments on the NO/Pt(111) system explore the mechanisms of inelastic scattering and trapping/desorption. The rotational dynamics associated with these two regimes are markedly different. A neat supersonic NO beam is scattered at normal incidence from a Pt(111) crystal at 375–475 K. The non‐Boltzmann rotational population distribution of the scattered species exhibits considerable rotational excitation beyond the energy available from the incident beam. Thus, a surface vibration to rotational energy transfer mechanism must be operative. The accompanying rotational alignment data reveal that highly excited rotational states exhibit predominantly ‘‘cartwheel’’ motion. In contrast, rotationally excited molecules that desorb from a 553 K Pt(111) surface show a preference for ‘‘helicopter’’ motion. The opposite preferences for rotational alignment in the two dynamical regimes provide insight into the anisotropy of molecule–surface interactions.

Using photofragment translational spectroscopy, we have monitored the dissociation of CH2BrCH2I at 248, 266, and 308 nm, and CF2BrCF2I and CF2ICF2I at 308 nm. The primary fragments are I(2P3/2) and I(2P1/2) and the corresponding haloethyl radicals. The I(2P3/2) contribution decreases upon fluorination, but it is dominant for CH2BrCH2I at 308 nm. The electronic absorption dipole lies roughly along the C–I bond axis in every case. Stable CF2CF2Br and CF2CF2I radicals can be readily generated through photodissociation of the parent compounds, while stable CH2CH2Br could not be unambiguously observed. Upper limits to the reaction enthalpy at 0 K for CF2ICF2Br(I)→C2F4+I+Br(I) are 75±1(59±1) kcal/mol. The TOF spectra and related data suggest that there is a barrier to decomposition for CF2CF2I→C2F4+I that exceeds the C–I bond energy in the radical.