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The mechanism and time scale of hydrolysis reaction of chlorine nitrate (ClONO2) on a single crystalline basal ice surfaces under pressure and temperature conditions prevailing in the stratospheric layer of atmosphere are investigated. The surface second harmonic generation analysis revealed that the submonolayer ClONO2 reaction with water on basal ice (Ih) surfaces at 185K occurs autocatalytically, with the product molecule HOCl acting as the autocatalyst, while the other product HNO3 acts to delay the reaction.

An electrochemical template synthesis method is used to prepare centro- and noncentrosymmetric gold particle structures in porous anodic alumina hosts. Linear polarization spectra of the gold particle/anodic alumina composites show that both are dichroic under p-polarization for incidence angles theta > 0. A strong linear dichroism is also observed in noncentrosymmetric gold particle pair structures due to the close proximity of spherical and rodlike segments in the porous host.

The study on the molecular oxygen in supersonic seeded beams by V. Aquiliani et al. is problematic. Their findings disagree with those in the study by Harich and Wodtke in terms of the alignment of CO in a supersonic beam. Moreover, the highest polarizations reported by V. Aquiliani et al. are physically impossible and the lower alignment polarization cases they presented give improbable distributions.

The vinyl radical (C[sub 2]H[sub 3])òA″ ← &Xtilde;² A′ spectrum has been measured between 530 and 385 nm using cavity-ringdown spectroscopy. The active vibrational progressions involve C-C stretching and alpha H-C-C bending vibrations. Optimal rotational constants and linewidths were determined for the first four vibrational bands by modeling the spectrum as an asymmetric top. The best-fit rotational constants obtained for the excited electronic state are consistent with the molecular geometry predicted by ab initio calculations. The lifetime of the vibrationless level in the excited electronic state is estimated to be a few picoseconds, and increasing vibrational excitation leads to a decrease in the lifetime, based upon an increasing linewidth. Various possibilities for the predissociation mechanism are discussed. The most likely is judged to be a conical intersection or seam of intersections. A preliminary CASSCF calculation has found the point on the relevant potential energy surfaces at which the ground and electronically excited states are closest. While the geometry and other properties of this crossing point are in accord with the experimental results, the calculated position of the point of closest approach of the two electronic states lies considerably (> 1 eV, including zero-point energy) above the already predissociative òA″ state origin. Other mechanisms are also discussed to account for the observed rapid predissociation. Clearly there is a need for a higher level theoretical work on this problem. [ABSTRACT FROM AUTHOR]

An approach for detecting the vibrational spectrum of transient species is demonstrated on the vinyl radical. Photodissociation of carefully chosen precursors at selected photolysis wavelengths produce highly vibrationally excited radicals. Infrared (IR) emission from these radicals is then measured by time-resolved Fourier transform spectroscopy with nanosecond time resolution. All nine vibrational bands of the vinyl radical, generated from four different precursors, are obtained and reported here for the first time. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The laser induced fluorescence spectra of XeAr and XeNe have been measured near the Xe 6s[3/2]°1-1S0 transition at 68 045.663 cm-1. In XeAr, nine discrete peaks were observed, attributed to excitation to vibrational levels of the excited Ω=0+ electronic state, merging into a continuum. From a Franck–Condon analysis, the potential for the excited Ω=0+ state of XeAr was determined. It was found that this potential has a shallow minimum at long range [De′=28(1) cm-1, re′=5.44(7) Å] with a shallow repulsive wall. The shallow repulsive wall is attributed to the influence of the XeAr+ ion core at short range. The role of the 6s Rydberg electron of Xe in determining the shape of the interatomic potentials of excited XeRg (Rg=a rare gas atom) is discussed on the basis of a simple model potential, in which the interatomic potential is described as the sum of a diatomic ion core (XeRg+) potential term and a term representing the exchange repulsion between the Rydberg electron and the rare gas moiety. For XeNe, only continuum spectra were observed, indicating that the upper states are not bound. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The laser induced fluorescence spectrum of jet-cooled XeKr has been measured in the vicinity of the Xe 6s[3/2]01–1S0 atomic transition at 68 045.663 cm-1. The spectrum consists of two band systems, corresponding to transitions to the Ω=0+,1 electronic states from v‘=0 of the ground electronic state. By using the observed band positions and intensities, we have constructed model potentials for both excited electronic states. The Ω=0+ state has a double minimum potential [inner well, re’ = 3.09(3) Å, De’ = 624(3) cm-1; outer well, re’ = 5.1(2) Å, De’ = 101(1) cm-1] while the Ω=1 state potential has only a shallow van der Waals potential [re’ = 5.24(4) Å, De’ = 52.2(7) cm-1]. The double minimum potential for the Ω=0+ state and the difference between the potentials for the Ω=0+ and Ω=1 states are understood in terms of the dominance of two different types of bonding interactions over different ranges of the internuclear distance. At long range, the interaction is dominated by weak dispersion and overlap repulsion between the closed shell Kr atom and the excited Xe atom, giving rise to shallow minima at r≊5 Å in both states. At short range, the XeKr interaction is better described by a XeKr+ ion-core with an excited 6sσ Rydberg electron.The Ω=0+ state is associated with the strongly bound 2Σ+1/2 XeKr+ ion-core, while the Ω=1 state corresponds to the weakly bound 2Π3/2 XeKr+ ion-core. The dual nature of the bonding which gives rise to the double minimum potential in the Ω=0+ state is similar to the bonding seen in excited states of HgAr and HgNe [Duval et al., J. Chem. Phys. 85, 6324 (1986); Okunishi et al., ibid. 98, 2675 (1993); Onda et al., ibid. 101, 7290 (1994); Onda and Yamanouchi, ibid. (submitted)] or the long range s–s, short range d–d bonding seen in the ground state of Cr2 [Casey and Leopold, J. Chem.... [ABSTRACT FROM AUTHOR]

Photofragment excitation (PHOFEX) spectroscopy of OCS in the vacuum ultraviolet (VUV) has been carried out using tunable VUV excitation of jet-cooled OCS, followed by ultraviolet laser induced fluorescence (LIF) detection of the S (1S) photofragment via the S (3D°1–1S) transition. The PHOFEX spectrum near 154 nm is better resolved than previous absorption spectra, and new features are visible. The LIF spectrum of the S (1S) photofragment may be successfully modeled using the product recoil anisotropy (β=1.8±0.2) and CO (v,J) product state distribution previously measured for photodissociation of OCS at 157 nm [Strauss et al., J. Chem. Phys. 90, 5364 (1989)]. [ABSTRACT FROM AUTHOR]

We report the observation of discrete and continuous laser induced fluorescence (LIF) spectra of the B’ 1u–X 0+g transition of Xe2, near 68 000 cm-1. The discrete features continue 5 cm-1 above the predicted atomic asymptote, which indicates the presence of a barrier to dissociation in the excited state. The dissociation energy (De’=48±12 cm-1), and excited state constants (re’=5.46±0.05 Å, ωe’=5.9±0.7 cm-1, and ωexe’=0.17±0.02 cm-1) for the B’ state were obtained from a Franck–Condon fit to the spectrum. The resulting potential is more shallow and has a longer equilibrium bond length compared with a previous experimentally derived potential. The barrier to dissociation (2 cm-1≤h≤10 cm-1, r≊10 Å) is attributed to the presence of a long-range (∝1/r3) repulsion, arising from a dipole–dipole resonant interaction. [ABSTRACT FROM AUTHOR]

The dissociation of formaldehyde into molecular products is known to proceed through vibrationally excited, quantum ergodic levels of the ground electronic state (H2CO*) and then a transition state (H2CO‡) according to the following mechanism: H2CO(S0)+hν→H2CO(S1)→H2CO*(S0) →H2CO‡→H2+CO. This paper reports the distribution of rotational energy in the CO (v=0) fragment following excitation to single rotational levels of the S1 2141 band of formaldehyde. The maxima of the rotational distributions resulting from photolysis on different rotational lines are observed to vary between J=38 and 49. Variations in the widths of the distributions are also observed. These variations show no systematic trends in the quantum indices or radiative lifetime of the parent molecule. Distributions calculated using an infinite-order-sudden approximation-based algorithm show fluctuations similar to those observed experimentally when each of the vibrational levels of the transition state through which the molecule can decay is randomly weighted. Thus it is understood that the ergodic character of each H2CO* S0 eigenstate uniquely projects onto the vibrational levels of the transition state, and since different levels of the transition state give rise to differing product distributions, decay through different S0 levels produces differing rotational distributions. [ABSTRACT FROM AUTHOR]

The room temperature rate constants for molecular angular momentum reorientation of H2, HD, and D2 (B 1∑+u, v’=0, J’=1, MJ’=0) in collisions with He, Ne, Ar and H2 (X 1∑+g) have been measured. The effective cross sections for changing MJ’ in collisions of H2, HD, D2 with He and Ne were found to be about 30 Å2 and were nearly the same for each isotope and with He and Ne as collision partners. The measured He–H2(B) reorientation cross section is about 50% larger than the cross section calculated with a simple semiclassical model using a potential that approximates the ab initio data for the H2(B)–He potential energy surface. The cross sections for reorientation of HD and D2 in collisions with Ar were found to be 10.6±2.0 and 13.9±3.0 Å2, respectively. The smaller cross section is due to the dominant role played by quenching of the electronic energy of molecular hydrogen in collisions with Ar. The reorientation of D2(B) in collisions with room temperature H2(X) occurs with a 7.6±3.4 Å2 cross section. The small cross section for reorientation of the angular momentum is again due to the dominance of quenching in the collision dynamics. [ABSTRACT FROM AUTHOR]

The room temperature rate constants for quenching of the fluorescence of H2, HD, and D2 B1Σ+u by 4He have been measured as a function of the initially excited rotational and vibrational levels of the hydrogen molecule. The effective quenching cross sections increase with increasing vibrational energy from about 1 Å2 up to a maximum of about 6 Å2. The effective cross sections for D2 (B, v’ = 0) were independent of the rotational level excited for 0 < J’ ≤ 7, and the cross sections for (v’ = 0, J’ = 0) were about 80% of the values for (v’ = 0, J’ >= 0) for all three isotopes studied. Quenching occurs via formation of an electronically excited (H2He)* collision complex followed by crossing to the repulsive H2(X)–He potential energy surface. The vibrational state dependence of the quenching cross sections fits a vibrationally adiabatic model for complex formation. From the vibrational state dependence of the quenching cross section, the barrier height for the quenching reaction is found to be 250±40 cm-1, and the difference in the H–H stretching frequencies between H2(B) and the H2–He complex at the barrier to reaction is 140±80 cm-1. Both values are substantially smaller than results from ab initio calculations. The rotational state dependence of the quenching cross sections suggests that quenching occurs with H2 rotating in a plane perpendicular to the relative velocity vector, in qualitative agreement with the rotational anisotropy of the H2(B)–He ab initio electronic potential energy surface. [ABSTRACT FROM AUTHOR]

Demonstrates the nanosecond time-resolved Fourier transform infrared emission spectroscopy. Combination of the Fourier Transform spectromer with an HgCdTe detector; Measurement of the emission spectrum through the rapid time response of the system; Determination of the instantaneous energy distribution of the excited molecules.

The rotational distributions of CO products from the dissociation of ketene at photolysis energies 10 cm-1 below, 56, 110, 200, 325, 425, 1107, 1435, 1720, and 2500 cm-1 above the singlet threshold (30 116.2 cm-1 ), are measured in a supersonic free jet of ketene. The CO(v‘=0) rotational distributions at 56, 110, 200, 325, and 425 cm-1 are bimodal. The peaks at low J’s, which are due to CO from the singlet channel, show that the product rotational distribution of CO product from ketene dissociation on the singlet surface is well described by phase space theory (PST). For CO(v‘=0) rotational distributions at higher excess energies (1107, 1435, 1720, and 2500 cm-1 ), the singlet and triplet contributions are not clearly resolved, and the singlet/triplet branching ratios are estimated by assuming that PST accurately predicts the CO rotational distribution from the singlet channel and that the distribution from the triplet channel changes little from that at 10 cm-1 below the singlet threshold. The singlet yield shows a rapid increase in the low excess energy region (0–300 cm-1 ), and a slower increase above. The singlet and triplet rate constants are derived from the directly measured total rate constants using the singlet yields. The triplet rate constant increases monotonically with increasing photolysis energy through the singlet threshold region. The singlet rate constant is accurately established in the threshold region and found to increase much less rapidly than predicted by phase space theory. At 2500 cm-1 excess energy, the CO(v‘=1) rotational distribution is obtained, and the ratio of CO(v‘=1) to CO(v‘=0) products for the singlet channel is measured to be 0.045±0.017. This ratio is close to the variational Rice–Ramsberger–Kassel–Marcus (RRKM) calculation 0.038, and the separate statistical ensembles (SSE) prediction 0.041, but much greater than the PST prediction, 0.016. [ABSTRACT FROM AUTHOR]

Reports that high resolution transient absorption spectra have been measured for the vinyl radical (C[sub 2]H[sub 3]) A[sup 2]A''...X[sup 2]A' transition between 540 and 415 nm using cavity ring-down laser absorption spectroscopy. Production of the light for the ring-down spectroscopy.