Rotational State Population Distributions Research Articles

State-selective photodissociation dynamics of formaldehyde: Near threshold studies of the H+HCO product channel

The laser-induced photodissociation of formaldehyde in the wavelength range 309<lambda<330 nm has been investigated using H (Rydberg) atom photofragment translational spectroscopy. Photolysis wavelengths corresponding to specific rovibronic transitions in the A (1)A2<--X (1)A1 2(0)(1)4(0)(3), 2(0)(2)4(0)(1), 2(0)(2)4(0)(3), 2(0)(3)4(0)(1), and 2(0)(1)5(0)(1) bands of H(2)CO were studied. The total kinetic energy release spectra so derived can be used to determine partial rotational state population distributions of the HCO cofragment. HCO product state distributions have been derived following the population of various different N(Ka) levels in the A (1)A2 2(2)4(3) and 2(3)4(1) states. Two distinct spectral signatures are identified, suggesting competition between dissociation pathways involving the X (1)A1 and the a (3)A2 potential energy surfaces. Most rovibrational states of H(2)CO(A 1A(2)) investigated in this work produceH+HCO(X (2)A') photofragments with a broad kinetic energy distribution and significant population in high energy rotational states of HCO. Photodissociation via the A (1)A2 2(2)4(3) 1(1,1) (and 1(1,0)) rovibronic states yields predominantly HCO fragments with low internal energy, a signature that these rovibronic levels are perturbed by the a (3)A2 state. The results also suggest the need for further careful measurements of the H+HCO quantum yield from H(2)CO photolysis at energies approaching, and above, the barrier to C-H bond fission on the a (3)A2 potential energy surface.

Improved characterisation of C 2 and CH radical number density distributions in a DC arc jet used for diamond chemical vapour deposition

Modelling studies of the plasma chemistry prevailing in CH 4/H 2/Ar mixtures in a DC arc jet reactor used for diamond chemical vapour deposition are reported, together with complementary new experimental data. Gas temperatures, T gas, close to the substrate have been determined via analysis of the measured rotational state population distribution in C 2( a) radicals and found to be ∼3200 K—similar to the temperature established previously in the free plume region. These, and previous (J. Appl. Phys. 92 (2002) 4213), T gas and number density measurements are in good accord with the first results from a full 2D ( r, z) modelling of the plasma chemical transformations and heat and mass transfer processes within the evolving plume and the periphery of the reaction chamber. The modelling shows formation of a shock front in the supersonic expansion, a few millimeters downstream from the nozzle exit. The spatial distributions of the various species number densities are predicted to display localised maxima and minima within the reaction chamber, reflecting the complex balance between gas flow, diffusive transfer and chemical transformations in the widely varying range of local conditions (notably T gas and the H and H 2 concentrations). The calculations provide clear evidence of the importance of gas flow re-circulation in transporting the hydrocarbon feedstock gas (methane) from the injection ring to the hot plume. C 2H 2, C 2H, C, CH, C 2 and C 3 species are all predicted to be present at number densities >5×10 12 cm −3 in the plume incident on the substrate; it is suggested that all of the major C containing radical species (i.e. most notably C 2H, C, CH, C 2 and C 3) must make some contribution to material growth in order to satisfy the experimentally measured film deposition rate.

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′&amp;gt;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.

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.

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.

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.

Photodissociation dynamics of H2S at 121.6 nm and a determination of the potential energy function of SH(A 2Σ+)

A new and improved version of the technique of H atom photofragment translational spectroscopy has been applied to a study of H2S photodissociation at 121.6 nm. The primary fragmentation pathways leading to H+SH(A) fragments and H+H+S(1D) atoms are observed to dominate the product yield; the yield of H atoms formed in conjunction with ground state SH(X) fragments is undetectably small. The majority of the SH(A) fragments are formed in their v=0 level with a rotational state population distribution that spans all possible bound and quasibound rotational levels. The experimental determination of the energies of these hitherto unobserved high rotational states has enabled a refinement of the SH(A) potential energy function, an improved estimate of the SH(A) well depth (9280±600 cm−1), and thus of the SH(X) ground state bond dissociation energy D00 (S–H)=3.71±0.07 eV. All aspects of the observed energy disposal in the title photodissociation process may be understood, qualitatively, if it is assumed that (i) the primary fragmentations occur on the B̃ 1A1 potential energy surface and (ii) Flouquet’s ab initio calculations of portions of this surface [Chem. Phys. 13, 257 (1976)] correctly predict its gross topological features.

The structure and dissociation dynamics of the Ne2Cl2 van der Waals complex

The structure and dynamics of Ne2Cl2 and Ne3Cl2 are studied by laser pump–probe spectroscopy. Analysis of a rotationally resolved B←X excitation band shows that Ne2Cl2 has a distorted tetrahedral structure with a Ne–Ne bond length of 3.23 Å and Ne2 center of mass to Cl2 center of mass distance of 3.12 Å. This structure is very close to that predicted by summing the atom–atom interactions. Excitation spectral shifts suggest a Ne3Cl2 structure with the neon atoms encircling the Cl2 bond axis. The total van der Waals binding energy of Ne2Cl2 is found to be between 145.6 and 148.6 cm−1, which is 20 cm−1 greater than 2*D0(Ne–Cl2)+D0(Ne2). For Cl2 stretching levels below υ′=10, transfer of one Cl2 vibrational quantum to the van der Waals vibrational modes is sufficient to dissociate both neon atoms from the complex. This indicates that the two neon atoms need not dissociate via independent, impulsive ‘‘half-collisions’’ which would require two Cl2 vibrational quanta. Observation of a NeCl2 dissociation fragment, however, indicates that such a sequential mechanism competes with the direct dissociation. Cl2 fragment rotational state population distributions for different initial vibrational levels are characterized using a simple rotational surprisal analysis. Comparison of these surprisal plots to those of the NeCl2 dissociation shows that as the size of the complex increases, so does the degree of statistical redistribution during the reaction. Even for Ne2Cl2, however, the extent of product rotational excitation is only weakly dependent upon the amount of energy available to the products and is always less than predicted by a statistical distribution between the translational and rotational product degrees of freedom.

Ultraviolet photodissociation dynamics of H2S and D2S

Nascent SH(X 2Πi, v″=0,1) and SD(X 2Πi, v″=0,1) rotational state population distributions, spin–orbit state population ratios, and Λ-doublet state population ratios have been measured following the UV excimer laser photodissociation of H2S (λ=193, 222, and 248 nm) and D2S (λ=193 and 222 nm), respectively. Nascent SH(X 2Πi, v″=0) rotational state distributions following 193 nm photodissociation of cold H2S in a free jet expansion vs 300 K H2S in a flowing gas cell were essentially the same, indicating that photofragment angular momentum must be originating predominantly in the dissociation event, and not from rotational energy contained in the parent triatom. Laser excitation spectra of SH(X 2Πi, v″=1) and SD(X 2Πi, v″=1) have been recorded for the first time. Rotational state distributions for SH(X 2Πi, v″) and SD(X 2Πi, v″) are independent of v″. Λ-doublet population ratios of the nascent photofragments are essentially unity for all the cases measured. The nascent rotational state distributions are consistent with an impact parameter model for the dissociating triatomic molecule.

Direct inelastic scattering of N2 from Ag(111). I. Rotational populations and alignment

The rotational state populations and the quadrupole and hexadecapole alignment moments of N2 scattered off clean Ag(111) are determined by resonance enhanced multiphoton ionization (REMPI). The scattered N2 is found to be highly aligned with its rotational angular momentum vector J parallel to the surface. The degree of alignment is found to increase with increasing rotational excitation. We see less than perfect alignment at intermediate J values indicating that the surface is not completely flat. The alignment is relatively insensitive to incident energy, incident angle, or surface temperature Ts. However, the rotational state population distributions show pronounced rainbows for higher incident energy and/or more grazing exit angle. The rotational state distributions are found to depend strongly on the final scattering angle at low Ts; this effect is markedly reduced at higher Ts. Time-of-flight measurements are used to determine the average velocity of the scattered N2 as a function of rotational level. It is found that higher rotational excitation correlates with lower average velocity and that the incident molecules lose 20%–30% of their translational energy to the solid. No correlation is found between velocity and alignment. A comparison is made with published results for the NO/Ag(111) system and a variety of theoretical models found in the literature.

Rotational distribution of recombinatively desorbing H 2 from polycrystalline Pd

Rotational state distributions have been determined for hydrogen molecules recombinatively desorbing from a polycrystalline Pd surface by laser-induced fluorescence. A tunable vacuum ultraviolet (VUV) laser at λ = 106 nm has been used to excite the hydrogen molecules in the B 1Σ u + (ν′ = 3, J′) ← X 1Σ g + (ν″ = 0, J″) Lyman band. The rotational state population distribution shows a non-Boltzmann characteristic with an average rotational energy less than the surface temperature.