State Population Distributions Research Articles

Experimental data vs. 3-D model calculations of HFCVD processes: correlations and discrepancies

An existing 3-D model [Mankelevich et al. Diamond Relat. Mater . 7 (1998) 1133] has been used to explain experimentally measured spatially resolved CH 3 radical number densities in hot filament CVD reactors operating with both CH 4 /H 2 and C 2 H 2 /H 2 process gas mixtures and to examine in detail the process of C 2 ↔C 1 inter-conversion in the gas phase. It has been shown that cooler regions distant from the filament need to be modelled in order to obtain the significant C 2 →C 1 conversion observed in HFCVD reactors with C 2 H 2 /H 2 process gas mixtures. The origin and effect of the non-equilibrated H 2 molecule vibrational state population distribution are studied for the first time in the context of HFCVD reactor models.

Rotational distribution of the HCl products from the reaction of Cl( 2P) atoms with methanol

The rotational quantum state population distribution of the nascent HCl ( v ′=0) products of the reaction of Cl( 2P) atoms with methanol has been measured using photoinitiation of reaction and 2+1 resonance-enhanced multiphoton ionization detection of the products. At an average reagent collision energy of 5.6 kcal mol −1, the HCl ( v ′=0) is formed in a distribution of rotational states which peaks at J ′=3–5, and has a mean rotational energy of 〈E rot 〉=262±25 cm −1 (1 σ errors). The results are interpreted with reference to recent ab initio calculations of the energetics of the reaction pathway [J.T. Jodkowski et al., J. Phys. Chem. A 102 (1998) 9230], and comparisons are drawn with the much colder rotational distributions for the HCl products of reaction of Cl( 2P) atoms with alkanes.

Translational Energy Distributions for Dissociation of the van der Waals Cation Species (C6H6···Arn)+ (n = 1,2) Measured by Velocity Map Imaging

The kinetic energy distributions associated with ejection of Ar from the cation van der Waals species (C6H6···Ar)+ and (C6H6···Ar2)+ have been measured by ion imaging with a velocity mapping configuration. The (C6H6···Ar2)+ dissociation was observed for the isomer with an Ar atom on each side of the benzene ring. The cations were created by (1+1) resonance-enhanced multiphoton ionization via their 610 transitions. The initial cation vibrational state population distributions were deduced from photoelectron spectra of benzene measured with the velocity map imaging spectrometer. The cations are produced with an average vibrational energy ∼1800−1900 cm-1. For dissociation of (C6H6···Ar2)+, on average enough energy remains in the (C6H6···Ar)+ fragment to eject the remaining Ar. The experiment views fragmentation of the subset of the (C6H6···Ar2)+ cations that lose a single Ar and for this reason the initial internal ion energy is significantly lower than 1800 cm-1 for the (C6H6···Ar2)+ dissociations monitored. The average initial energies above dissociation are estimated to be ∼1310 and ∼350 cm-1 for (C6H6···Ar)+ and (C6H6···Ar2)+, respectively. The kinetic energy distributions are well fitted by the function P(E) = E1/2{c1 exp(−k1E) + c2 exp(−k2E)}. The average kinetic energies released were 92 ± 4 and 78 ± 5 cm-1 for (C6H6···Ar)+ and (C6H6···Ar2)+, respectively. The lower average kinetic energy released for (C6H6···Ar2)+ is attributed to its lower initial internal energy. For (C6H6···Ar)+ the kinetic energy released represents only a small fraction of the total energy that requires redistributing. A large proportion of the total energy is therefore taken up as rotational and vibrational energy of the benzene cation fragment.

Photofragmentation of ammonia at 193.3 nm: Bimodal rotational distributions and vibrational excitation of NH2(Ã)

Time-resolved Fourier transform infrared emission spectroscopy is used to measure the nascent rovibrational distribution of low-lying electronically excited NH2(Ã 2A1) produced in the 193.3 nm photolysis of room-temperature and jet-cooled ammonia. Emission is observed predominantly from NH2(Ã) states with rotational motion about the a-axis and without bending excitation, υ2′=0. A bimodal N′=Ka′ rotational state population distribution is observed with up to Ka′=7 in υ2′=0 and with maxima at Ka′=5 and Ka′=1. We suggest that the bimodal rotational distribution may result from the competition between planar and bent geometries during dissociation. Weaker emission from NH2(Ã) with bending excitation, υ2′=1 and 2, is detected; the υ2′=1, N′=Ka′ rotational state population distribution spans from Ka′=0 to the energetic limit of Ka′=4. The vibrational energy partitioning for the formation of NH2(Ã,υ2′=0):NH2(Ã,υ2′=1) is 3:1 and 2:1 in the room-temperature and jet-cooled conditions, respectively. An upper limit of the NH2(Ã,υ2′=2) population is ∼10% of the total NH2(Ã) photofragments. Emission from rotational states with N′>Ka′ (molecules with rotational excitation about the b/c-axes) is also observed. Under jet-cooled conditions the NH2(Ã) b/c-axes rotational temperature of ∼120 K is higher than that expected from the rotationally cold parent species and is attributed to a mapping of the zero-point bending motion in the ν4 H–N–H scissors bending coordinate of the NH3(Ã) predissociative state onto the NH2(Ã,υ2′,N′,Ka′)+H photofragments.

Fast multiquantum vibrational relaxation of highly vibrationally excited O2

A significant fraction of the initial population of highly vibrationally excited oxygen molecules X 3Σg−(v⩾23) prepared by stimulated emission pumping, relaxes to much lower vibrational levels (Δv≈−9). The time scale is much shorter than the known collisional lifetimes of the intervening vibrational levels and thus a sequential single-quantum relaxation mechanism can be explicitly ruled out. State-to-state measurements after preparation of v=28 and 30 provide the final-vibrational state population distribution resulting from relaxation of these two states. For v=28(30), at least 38%(7.9%) of the initially prepared population, undergoes multiquantum vibrational relaxation. The observed multiquantum relaxation explains, at least in part, the previously reported “dark channel” for relaxation of vibrational levels higher than v=26, but does not exclude the possibility of reactive scattering (forming ozone) for the remaining fraction of highly vibrationally excited molecules. We discuss possible explanations of this startling result including V–V energy transfer, V–E energy transfer, and complex formation.

Discrete structure in the 4d photoabsorption spectrum of antimony and its ions

The 4d inner-shell absorption spectra of Sb, Sb+, Sb2+ and Sb3+ have been obtained with the dual-laser-produced plasma technique. Discrete structure has been observed in the 30-40 eV region and identified as originating from 4d-5p transitions by comparison with the results of Hartree-Fock calculations performed within the superposition of configurations method. For comparison of theoretical and experimental spectra, 4d photoabsorption cross sections were calculated which took non-radiative decay rates into account. It was found to be necessary to include absorption from excited states of the ground configurations, especially in the higher ionization stages. Synthetic spectra, which allowed for thermal lower state population distributions, were produced and these are in agreement with the experimental data.

Effect of Al(CO) x complexation on the aluminum oxidation process

A comparison of the AlO B 2 Σ + state population distributions observed for the multiple collision Al+Ar+O 3→AlO *( B 2 Σ + )+Ar+O 2 and Al+ nCO+O 3→AlO *( B 2 Σ + )+ nCO+O 2 reactive encounters suggests that Al(CO) x adducts are formed via the entrainment of aluminum atoms in CO. The subsequent oxidation of these adducts with O 3 produces an AlO B 2 Σ + state population distribution which reflects a smaller reaction exothermicity and shorter range reactive encounter than the oxidation of bare Al atoms. Differences in the observed population distributions are attributed to an Al(CO) 2+O 3 reaction. This conclusion is also based, in part, on the similarity between the AlO B state population distributions associated with the Al(CO) 2+O 3 reaction and that previously measured for the Al+N 2O multiple collision reactive encounter. This assignment is further supported by matrix isolation studies where only Al(CO) 2 has been observed.

Spectroscopy and nonadiabatic predissociation of CN–Ne

The spectroscopy and predissociation dynamics of CN–Ne were investigated using a variety of laser excitation techniques. Properties of the A 2Π state (vibrational levels v=2, 3, and 4) were characterized through studies of the A–X system. Both spin–orbit components of CN(A)–Ne were subject to predissociation. The upper component (Ω=1/2) was predissociated by rapid spin–orbit relaxation (τ≈6 ps, no vibrational dependence). The lower component (Ω=3/2) was predissociated by the nonadiabatic internal conversion process, CN(A 2Π3/2,v)–Ne→CN(X 2 Σ+,v+4)+Ne. Rates for predissociation by internal conversion were found to be exponentially dependent on the energy gap between the initial and final CN levels. These rates were relatively slow, permitting observation of rotationally resolved spectra for bands associated with the monomer Ω=3/2 v=3 and 4 levels. Double resonance techniques were used to simplify the spectra and establish ro-vibronic assignments. Details of the intermolecular potential-energy surfaces were derived from these data. CN final state population distributions resulting from spin–orbit and internal conversion predissociation were characterized. For the former, excess energy was channeled into rotational excitation of CN, and levels of−parity were preferentially populated. The excess energy in predissociation by internal conversion was released primarily to translational recoil. In the accompanying paper, Yang and Alexander present ab initio potential surfaces for CN–Ne. From these surfaces they predict ro-vibronic energies and predissociation rates for levels associated with A, v=3. Results that depended on the A state surface alone were found to be in good agreement with experiment. Comparison of the internal conversion predissociation rates indicates that the ab initio calculations underestimated the coupling between the A and X states.

N2 triplet band systems and atomic oxygen in the dayglow

New spectrographic observations of the Earth's dayglow have been acquired by the Arizona Airglow Experiment (GLO) flown on the space shuttle. GLO is an imaging spectrograph that records simultaneous vertical profiles of prominent Earth limb emissions occurring at wavelengths between 115 and 900 nm. This study addresses the measured emissions from the N2 triplet states (first positive, second positive, and Vegard‐Kaplan band systems) and their excitation by the local photoelectron flux. The triplet state population distributions modeled for aurora by Cartwright [1978] are modified for dayglow conditions by changing to a photoelectron‐flux energy distribution and including resonance scattering by the first positive system. Modeled and observed intensities are in excellent agreement, in contrast to the well‐studied auroral case. This work concentrates on dayglow conditions at 200 km altitude near the subsolar point. Parameters to infer the local photoelectron flux from the emission band intensities are provided. Several atomic oxygen dayglow emission features were analyzed to complement the N2 analysis. The photoelectron‐excited O I(135.6, 777.4 nm) lines were found to be 3 to 4 times weaker than predicted while the O I(630.0, 844.6 nm) lines were in close agreement with the model prediction.

Resonance enhanced multiphoton ionization spectroscopy of carbonyl sulphide

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.

A quantitative investigation of emission from low temperature laser-induced YBa2Cu3Ox plasma plumes

Quantitative emission spectroscopy has been used to study the plume formed following laser ablation of YBCO in an oxygen atmosphere. Excited state population distributions, determined from emission line intensity ratios, are used to investigate spatiotemporal variations in the local Boltzmann temperatures for both neutral and ionic species within the expanding plume. Temperatures, obtained from emission line intensity ratios of both Cu(I) and Y(I), decrease slowly and nonadiabatically in the range 1.0–0.5 eV during plume expansion. Higher initial Boltzmann temperatures of ∼3 eV are however obtained from the emission line intensity ratios of fast ions, which dominate the composition of the highly luminous expanding front of the plume. Quantitative comparison of emission intensities for the different neutral species present indicate that the neutral composition in the luminous region of the expanding plume is increasingly dominated by Cu(I) which has a faster expansion velocity and lower oxidation rate than Y(I) and Ba(I).

Dissociation dynamics of H2O(D2O) following photoexcitation at the Lyman-α wavelength (121.6 nm)

The technique of H(D) atom photofragment translation spectroscopy has been used to investigate the collision free photodissociation of jet cooled H2O(D2O) molecules following excitation to their B̃(1A1) excited state at 121.6 nm. The resolution of the total kinetic energy release spectrum obtained with this technique, allows assignment of the eigenvalues for the individual rotational quantum states and an estimation of the respective quantum state population distributions for the nascent OH(X 2Π) and OH(A 2Σ+) photofragments (and their deuterated analogs). This provides us the first experimental observations of high angular momentum states of OD(X). Analysis of the quantum state population distribution show both the ground (X 2Π) and electronically excited (A 2Σ+) OH(OD) fragments to be formed with little vibrational excitation but with highly inverted rotational distributions. Spectral simulation enables estimation of relative branching ratios for these two dissociation channels, and for the three-body fragmentation yielding ground state atoms. The observed energy disposal has been rationalized by considering the motion of a wavepacket launched on the B̃ state surface at a geometry corresponding to the ground state equilibrium configuration. Electronically excited OH(OD) fragments result from that fraction of the photoexcited molecules that dissociate on the B̃ state surface; their rotational excitation results from the marked angular anisotropy of the B̃ state surface. Ground state OH(OD) fragments can arise as a result of radiationless transfer to the lower Ã(1B1) or X̃(1A1) surfaces. The wavepacket calculations show that B̃■X̃ transfer via the conical intersection linking these two surfaces leads to the most highly rotationally excited OH(OD) fragments. These calculations also show that the contribution made by B̃■Ã radiationless transfer to the overall rotational distribution in the ground state OH(OD) fragments scales with the amount of a-axis rotational excitation in the photoexcited molecules: The detailed form of the OH(OD) product state population distribution is thus predicted to be temperature dependent.

The effect of surface passivation on rotationally inelastic scattering: N2 scattered from W(110), W(110)–(2×2)N, W(110)–(1×1)H, and Pt(111)

The effects of surface reactivity on rotationally inelastic scattering has been examined by comparing the scattering of N2 from the reactive W(110) surface and from the passivated N and H reconstructions, W(110)–(2×2)N and W(110)–(1×1)H, as well as the nonreactive Pt(111) surface. The translational energy, rotational state population distributions, and angular momentum alignment of N2 scattered from these surfaces have been measured as a function of incident energy Ei, incident angle θi, and exit angle θf. The trends in the final translational and rotational energy correlate well with the average atomic mass of each surface. However, some of the greater rotational excitation for N2 scattered from W(110) compared to W(110)–(1×1)H can be attributed to a more anisotropic potential with respect to initial molecular orientation for N2–W(110) compared to N2–W(110)–(1×1)H or N2–Pt(111). The rotational distributions indicate that parallel momentum is not conserved for N2 scattered from W(110) or from W(110)–(2×2)N, but that parallel momentum is partially conserved for N2 scattered from W(110)–(1×1)H. Furthermore, measurements of angular momentum alignment show that significant in-plane forces are present, and that these forces are due to the repulsive part of the N2–surface potential of each system. The results are consistent with the N2–W(110) potential being highly corrugated with respect to both initial molecular orientation and impact parameter and with this corrugation being reduced dramatically by passivating the surface with the adsorption of hydrogen.

Isotope effects and wavelength dependence in the rotational state distributions of the diatomic photofragments SH and SD from the photodissociation of H 2S and D 2S

We employ a semi-empirical model that utilizes a kinematic distribution function to calculate the rotational state population distributions of diatomic photofragments from the photodissociation of isotopically variant triatomic molecules. The numerically evaluated rotational distributions of the photofragments, SH and SD, are compared with experimental data on the nascent rotational state population distributions of the diatomic photofragments resulting from the laser photolysis of H 2S (λ photolysis=193, 222 and 248 nm) and D 2S (λ photolysis=193 and 222 nm).

Energy and momentum distributions versus incident energy in the scattering of CO from Ag(111)

The effect of changing the incident translational energy on the exit translational energy, rotational state population distributions, and angular momentum alignment of monoenergetic, rotationally cold CO scattered from Ag(111) at normal incidence was measured. As the incident energy is raised, a dramatic increase in the fraction of energy transferred to the surface, a shift of the rotational rainbow to higher rotational energy, and a significant increase in the amount of angular momentum alignment in the scattered molecules were observed. In addition, the alignment versus J distribution exhibits positive curvature for CO at all incident energies. The results are consistent with (1) the effect of the surface phonons and/or physisorption well on the final energy of the scattered molecules being small except at the lowest incident energy; (2) the large ∂V/∂θ being the primary source of the greater rotational excitation seen for CO compared to N2 scattered from Ag(111); and (3) the corrugated attractive portion of the CO/Ag(111) potential being primarily responsible for the modest alignment of CO at low incident energy.

Rotationally inelastic scattering of N2 from W(110)

We have measured the translational energy, rotational state population distributions, and angular momentum alignment of monoenergetic, rotationally cold N2 scattered from W(110), as a function of incident energy, incident angle, and exit angle. Translational energy and rotational state distributions indicate that N2 scattering is direct and highly rotationally inelastic. Rotational distributions are well described by Boltzmann fits at all incident energies, incident angles, and exit angles. Rotational distributions do not change with incident angle or exit angle; the distributions scale only with the total incident energy. A comparison between W(110) and Pt(111) shows that the rotational excitation of N2 scattered from W(110) is significantly larger than that N2 scattered from Pt(111). Measurements of angular momentum alignment show that molecules in low J states rotate like helicopters and molecules in high J states rotate like cartwheels for low incident energies (Ei≤0.5 eV). At higher incident energy (Ei=1.0 eV), only cartwheeling rotations are observed.

Dissociation dynamics of HCN(DCN) following photoexcitation at 121.6 nm

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.

The influence of vibrational and translational motion on the reaction dynamics of O(1D)+H2(1Σ+g,v)

The OH product state distribution from the reaction O(1D)+H2(v)→OH(v″,J″,Ω,Λ)+H was determined by laser-induced fluorescence (LIF) in the Δv=−3 band for v″=3 and 4 with resolution of the J″, Ω, and Λ sublevels. The rotational state population distribution is inverted strongly in v″=3, weaker in v″=4. There is a higher propensity for production of OH in the Π(A′) Λ-sublevels. Vibrationally excited H2 was used for a part of the experiments. Excitation was achieved by stimulated Raman pumping (SRP). The population ratio of the vibrational states was determined to be P(v=3)/P(v=4)=3.5 for the reaction with H2(v=0) and 3.0 when there is H2(v=1) in the reaction chamber. Higher OH product states are populated than it would be expected from the mean available energy of the reaction. The translational energy of the reactants is transferred into OH rotation.

Kinetic energy and vibrational state population distribution of Ar 2∗ dimers desorbed from MeV ion-excited solid argon films

Luminescence in the wavelength range 1000 to 1400 Å from electronically excited solid argon films is analyzed in detail. The component of the spectrum due to a luminescent plume of desorbed Ar 2∗ ( 3Σ u +: 1 u, 0 u −) is isolated. Under pulsed excitation of the films, a delayed onset of luminescence is observed when the portion of the plume nearest the argon film is occluded. This delayed onset is analyzed as a time-of-flight, giving the kinetic energy (KE) of desorbed Ar 2∗. The average KE ranges from 0.06 eV for ar 2∗ emitting at 1110 Å, to 0.094 eV for Ar 2∗ emitting at 1210 Å. There is a large spread in the velocity about the average, with a maximum KE of 0.3 eV. Information is obtained on the degree of vibrational relaxation of the desorbed Ar 2∗ by fitting the observed plume luminescence spectrum to a linear combination of quantum mechanically calculated spectra of the vibrational levels of Ar 2∗. The population distribution of the vibrational levels of desorbed Ar 2∗ is broad but peaks at the three lowest states. Models for Ar 2∗ desorption are discussed.

Kinematic distribution function to calculate rotational populations of photofragments from photodissociation of triatomic molecules

A general procedure to evaluate the rotational state population distributions of the nascent photofragments from the photodissociation of polyatomic molecules has been implemented with the use of the kinematic distribution function developed by Chen and Pei [Chem. Phys. Lett. 124, 365 (1986)]. Numerical evaluations of rotational state population distributions of diatomic photofragments from photodissociation of the general class of triatomic molecules are presented. The calculated rotational state population distributions are compared with the most recent experimental data on OH and SH photofragments to obtain the information on the kinematic aspects of the photodissociating H2O and H2S molecules.