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The fragmentation dynamics of allene and propyne molecules following photoexcitation at 203.3, 209.0 and 213.3 nm have been investigated by H (Rydberg) atom photofragment translational spectroscopy methods. Contrary to conclusions reached in previous photochemical studies of these molecules, at a photolysis wavelength of 193 nm, we find the translational energy spectra associated with the H atom product forming channel in both molecules to be essentially identical, and to have a form that is reproduced well by an approximate statistical model that assumes population of all possible vibrational states of the H[sub 2]CCCH partner. Such behavior can be most readily accommodated by assuming that, for both molecules, at the excitation energies used in the present work, internal conversion to, and isomerization on, the ground state potential energy surface precedes fragmentation. [ABSTRACT FROM AUTHOR]

The boundaries of the diamond deposition region in the C-H-O (Bachmann) atomic phase composition diagram have been reproduced successfully for 38 different C, H, and O containing gas mixtures using the CHEMKIN computer package, together with just two criteria—a minimum mole fraction of methyl radicals [CH[sub 3]] and a limiting value of the [H]/[C[sub 2]H[sub 2]] ratio. The diamond growth/no-growth boundary coincides with the line along which the input mole fractions of C and O are equal. For every gas mixture studied, no-growth regions are found to coincide with a negligible (<10[sup -10]) mole fraction of CH[sub 3] radicals, while for gas mixtures lying within the diamond growth region the CH[sub 3] mole fraction is ∼10[sup -7]. Each no-growth→diamond growth boundary is seen to be accompanied by a 2-3 order of magnitude step in CH[sub 3] mole fraction. The boundary between diamond and nondiamond growth is less clearly defined, but can be reproduced by assuming a critical, temperature dependent [H]/[C[sub 2]H[sub 2]] ratio (0.2, in the case that T[sub gas]=2000 K) that reflects the crucial role of H atoms in the etching of nondiamond phases. The analysis allows prediction of the composition process window for good quality diamond growth for all stable input gas mixtures considered in this study. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Microwave plasma enhanced chemical vapor deposition has been used to grow diamond films at substrate temperatures down to 435 °C using CO[sub 2]/CH[sub 4] gas mixtures. An Arrhenius plot of growth rate as a function of substrate temperature yields a value for the activation energy for the growth step of 28 kJ mol[sup -1]. This is lower than that measured previously for CH[sub 4]/H[sub 2] systems and hints at a different gas-surface chemistry when using CH[sub 4]/CO[sub 2] plasmas. Molecular beam mass spectrometry has been used to measure simultaneously the concentrations of the dominant gas phase species present during growth, for a wide range of plasma gas mixtures (0%-80% CH[sub 4], balance CO[sub 2]). The CHEMKIN computer package has also been used to simulate the experimental results in order to gain insight into the major reactions occurring within the microwave plasma. The calculated trends for all species agree well with the experimental observations. Using these data, the model for the gas phase chemistry can be reduced to only four overall reactions. Our findings suggest that CH[sub 3] radicals are likely to be the key growth species when using CO[sub 2]/CH[sub 4] plasmas and provide a qualitative explanation for the narrow concentration window for diamond growth. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The plume accompanying 193 nm pulsed laser ablation of graphite in vacuum has been studied using wavelength, time and spatially resolved optical emission spectroscopy and by complementary Faraday cup measurements of the positively charged ions. The temporal and spatial extent of the optical emissions are taken as evidence that the emitting species result from electron-ion recombination processes, and subsequent radiative cascade from the high n,l Rydberg states that result. The distribution of C neutral emission is symmetric about the surface normal, while the observed C[sup +] emission appears localized in the solid angle between the laser propagation axis and the surface normal. However, Faraday cup measurements of the ion yield and velocity distributions, taken as a function of scattering angle and incident pulse energy, indicate that the total ion flux distribution is peaked along the surface normal. The derived ion velocity distributions are used as input for a two-dimensional model which explains the observed anisotropy of the C[sup +] emission in terms of preferential multiphoton excitation and ionization of C species in the leading part of the expanding plasma ball that are exposed to the greatest incident 193 nm photon flux, prior to electron-ion recombination and subsequent radiative decay. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

ZnO nanorods (NRs) and nanotubes (NTs) are synthesized by a hydrothermal method on Si substrates that was precoated by pulsed laser decomposition (PLD) with a thin ZnO film. High-resolution transmission electron microscopy and selected area electron diffraction analysis confirms that the NTs are ZnO single crystals and that their growth direction is along [0001], the c-axis.

Hydroxyl radicals (OH) play a central role in the interstellar medium. Here, we observe highly rotationally excited OH radicals with energies above the bond dissociation energy, termed OH “super rotors”, from the vacuum ultraviolet photodissociation of water. The most highly excited OH(X) super rotors identified at 115.2 nm photolysis have an internal energy of 4.86 eV. A striking enhancement in the yield of vibrationally-excited OH super rotors is detected when exciting the bending vibration of the water molecule. Theoretical analysis shows that bending excitation enhances the probability of non-adiabatic coupling between the ~B and ~X states of water at collinear O–H–H geometries following fast internal conversion from the initially excited ~D state. The present study illustrates a route to produce extremely rotationally excited OH(X) radicals from vacuum ultraviolet water photolysis, which may be related to the production of the highly rotationally excited OH(X) radicals observed in the interstellar medium. [ABSTRACT FROM AUTHOR]

The H(D) Rydberg atom photofragment translational spectroscopy technique has been applied to a further detailed investigation of the photodissociation dynamics of NH3 and ND3 molecules following excitation to the lowest two (v2=0 and 1) vibrational levels of the first excited (A 1A2″) singlet electronic state. Analysis of the respective total kinetic energy release spectra, recorded at a number of scattering angles Θ [where Θ is the angle between the ε vector of the photolysis photon and the time-of-flight (TOF) axis], enables quantification of a striking, quantum state dependent, μ-v correlation in the NH2(ND2) products. The spatial distribution of the total flux of H(D) atom photofragments is rather isotropic (βlab∼0). However, more careful analysis of the way in which the TOF spectra of the H(D) atom photofragments vary with Θ reveals that each H+NH2(D+ND2) product channel has a different ‘‘partial’’ anisotropy parameter, βlab(v2,N), associated with it: The H(D) atom ejected by those molecules that dissociate to yield NH2(ND2) fragments with little rotational excitation largely appear in the plane of the excited molecule (i.e., perpendicular to the transition moment and the C3 axis of the parent, with β tending towards -1). Conversely, the H(D) atoms formed in association with the most highly rotationally excited partner NH2(ND2) fragments tend to recoil almost parallel to this C3 axis (i.e., β→+2). Such behavior is rationalized in the context of the known potential energy surfaces of the A and X states of ammonia using a classical, energy and angular momentum conserving impact parameter model in which we assume that all of the product angular momentum is established at the ‘‘point’’ of the conical intersection in the H–NH2(D–ND2) dissociation coordinate. We conclude by reemphasizing the level of care needed in interpreting... [ABSTRACT FROM AUTHOR]

The technique of H Rydberg atom photofragment translational spectroscopy has been applied to a high resolution study of the primary photochemistry of methanethiol (CH3SH) following excitation at a wide range of wavelengths in the near ultraviolet. In accord with previous studies of this molecule, excitation within its first (1 1A‘-X 1A’) absorption continuum is shown to result in S–H bond fission. Spectral analysis yields a refined value for the bond dissociation energy: D00(CH3S–H)=30 250±100 cm-1. The resulting CH3S(X) fragments are deduced to carry only modest vibrational excitation, distributed specifically in the ν3 (C–S stretching) mode and in one other mode having a wave number of ∼1040 cm-1. We associate this latter mode with bending of the CH3 moiety in the plane containing the C and S nuclei and the lobe of the unpaired electron which was originally involved in the S–H bond. Decreasing the excitation wavelength (while remaining within the first absorption continuum) results in an increase in both the vibrational and rotational excitation of the CH3S(X) fragments, but a decrease in the relative yield of the upper (2E1/2) spin–orbit component. Excitation at still shorter excitation wavelengths accesses the second (2 1A‘-X 1A’) absorption band of CH3SH. The CH3S fragments resulting from S–H bond fission at these excitation wavelengths are observed to carry very much higher levels of vibrational excitation in the above two modes. The observation of H atoms attributable to secondary photolysis of SH(X) fragments indicates increased competition from the alternative C–S bond fission channel at these shorter excitation wavelengths. Additional peaks in the H atom time-of-flight spectrum, most clearly evident following excitation at wavelengths in the range 213–220 nm, are interpretable in terms of secondary photolysis of the primary CH3S(X) fragments yielding... [ABSTRACT FROM AUTHOR]

Five new triplet excited states of the ND radical (three in the case of NH) in the wave-number range 85 000–91 000 cm-1 have been identified through analysis of the two-photon resonance enhancements they provide to the wavelength-resolved multiphoton ionization spectrum of X 3Σ- state NH(ND) radicals. The lowest energy of these, the B 3Π state, is found to be a ‘‘regular’’ Rydberg state which, on the basis of its observed quantum defect and its deduced rotational and spin–orbit coupling constant, is surmised to be the 3Π state derived from a 3pσ electron built on the 2Π ground-state ion core. Perturbations are evident in the B 3Π–X 3Σ- origin bands of both NH and ND. In the case of ND the perturbing state provides its own resonance enhancements, the analysis of which enables its definitive identification as the C 3Σ- state. The very small spin–orbit splitting found for the D 3Π state is taken to indicate that (at least in the Franck–Condon region) its wave function is dominated by the configuration involving one 3pπ Rydberg electron and a 4Σ- ion core. To still higher wave number we identify two more 3Σ- excited states, the upper of which (the F 3Σ- state) has a very small rotational constant which we take to imply that it has substantial valence character. Further indications that the F 3Σ- (and C 3Σ-) states possess significant valence character is provided by the observation that both parent and daughter (N+) ions contribute to the overall ion yield when the multiphoton ionization proceeds via these two states. Daughter-ion formation is considered to occur via an overall four-photon excitation process in which the coherent two-photon excitation to the 3Σ- state of interest is followed by a one-photon excitation to a ‘‘superexcited’’ state of the neutral. This is then presumed to absorb a... [ABSTRACT FROM AUTHOR]

The rovibrational state distribution of the nascent NH2(A 2A1) fragments generated by 193.3 nm photodissociation of a room temperature sample of NH3 is determined through an analysis of a major portion (6000–13 000 cm-1) of the NH2(A 2A1→X 2B1) near infrared emission spectrum obtained by time-resolved Fourier transform infrared emission spectroscopy. The NH2(A) fragments are observed to be formed predominantly in their zero-point vibrational level, with substantial rotational excitation about their a-inertial axis up to the limit of the available energy, ∼3150 cm-1, but with little excitation about the other axes. The pattern of this energy disposal is discussed within the framework of existing knowledge regarding the form of the NH3 A state potential energy surface on which the dissociation occurs. The essential features are entirely consistent with a direct carry over, into the fragment, of the out-of-plane bending vibrational motion introduced in the parent molecule by the photoexcitation process. [ABSTRACT FROM AUTHOR]

Rydberg excited states of the OCS molecule in the energy range 70500–86000 cm-1 have been investigated via the two and three photon resonance enhancements they provide in the mass resolved multiphoton ionization (MPI) spectrum of a jet-cooled sample of the parent molecule. Spectral interpretation has been assisted by companion measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. The present study supports the earlier conclusions of Weinkauf and Boesl [J. Chem. Phys. 98, 4459 (1993)] regarding five Rydberg origins in the 70500–73000 cm-1 energy range, attributable to, respectively, states of 3Π, 1Π, 3Δ, 1Δ and 1Σ+ symmetry arising from the 4pλ←3π orbital promotion. We also identify a further 21 Rydberg origins at higher energies. These partition into clumps with quantum defects ca. 3.5 and 4.5, which we associate with the orbital promotions npλ←3π (n=5,6), and others with near integer quantum defect which are interpretable in terms of excitation to s,d and (possibly) f Rydberg orbitals. We also identify MPI resonances attributable to CO(X 1Σ+) fragments and to S atoms in both their ground (3P) and excited (1D) electronic states. Analysis of the former resonances confirms that the CO(X) fragments resulting from one photon dissociation of OCS at excitation wavelengths ca. 230 nm are formed with a highly inverted, bimodal rotational state population distribution, whilst the latter are consistent with previous reports of the wavelength dependence for forming ground and excited state S atoms in the near uv photolysis of OCS. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The technique of H(D) Rydberg atom photofragment translational spectroscopy has been used to investigate the photodissociation dynamics of the mixed isotopomers NH2D and NHD2 following the excitation to the v2′=0 and 1 levels of their lowest lying A 1B1 (C2v) excited electronic states. Peaks in the resulting total kinetic energy release (TKER) spectra are assigned to levels of the NH2, NHD, or ND2 fragments with a wide range of quantum numbers Ka for rotation about their a inertial axes, and with N=Ka, N=Ka+1, or N=Ka+2 as appropriate. These data provide the first measurements of high rotational levels for the ground electronic state of the NHD radical. The least squares fitting of all these spectra, and those resulting from NH3 and ND3, to the best calculated NH2, NHD, and/or ND2 rotational term values provides accurate estimations of the respective N–H and N–D bond dissociation energies D00 across the whole series. These values are D00(H–NH2)=37 115±20 cm-1 (4.602±0.002 eV); D00(H–NHD)=37 240±50 cm-1; D00(H–ND2)=37 300±30 cm-1; D00(D–NHD)=37 880±60 cm-1; and D00(D–ND2)=38 010±20 cm-1. The differences between these values are fully consistent with differences in zero-point energies and lead to a mean value of De=40 510±25 cm-1. Dissociation of NH2D or NHD2 through their (A-X) 210 bands to give an NHD product leads to TKER spectra with a much higher statistical character than those leading to an NH2 or ND2 product, and to those obtained following excitation through the 000 bands. This is rationalized in a semiquantitative manner in terms of a varying contribution to the dissociation rate of the parent molecules from internal conversion (IC) to high levels of their respective ground states. Nuclear permutation symmetry appears to play an important role both for the IC rates and for the subsequent branching between product channels. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Rydberg excited states of the CS2 molecule in the energy range 56 000–81 000 cm-1 have been further investigated via the two and three photon resonance enhancements they provide in the mass resolved multiphoton ionization (MPI) spectrum of a jet-cooled sample of the parent molecule. Spectral interpretation has been aided by parallel measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. Thus we have been able to extend, and clarify, previous analyses of the tangled spin–orbit split vibronic structure associated with the 3Πu and 1Πu states derived from the configuration [2Πg]4pσu and the 3Δu, 1Δu, and 1Σ+u states resulting from the configuration [2Πg]4pπu, and to deduce an approximate wave number for the origin of the hitherto unidentified 3Σ+u state derived from this same configuration. Moving to higher energies we are able to locate, unambiguously, the origins of the next (n=5) members of four of these [2Πg]np Rydberg series, and to identify extensive series based on the presumed Rydberg configurations [2Πg]nsσg and [2Πg]nfλu with, in both cases, n≤10. We also identify MPI resonances attributable to CS(a 3Π) fragments, to ground state C atoms, and to S atoms in both their ground (3P) and excited (1S) electronic states. Analysis of the former resonances indicates that the CS(a 3Π) fragments resulting from two photon dissociation of CS2 at excitation wavelengths around 300 nm are formed with substantial rovibrational excitation. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of HCN(DCN) at 121.6 nm. Analyses of the H(D) atom time-of-flight spectra reveal the partner CN fragment to be formed predominantly in its A 2Π excited electronic state. Branching into the H/D+CN(B 2∑+) product channel accounts for a few percent of the total fragment yield, but we discern no evidence for any contribution from the product channel yielding H/D atoms in conjunction with ground state CN(X 2∑+) fragments. The majority of the CN (A) fragments are formed in their v’=0 level but with a markedly bimodal rotational state population distribution. This bimodality has been rationalized in the light of the available information regarding the form of the potential energy surface of the excited 1Π state of HCN(DCN) populated following photoexcitation at 121.6 nm. [ABSTRACT FROM AUTHOR]