Bibliothek

Notizen: Experimental and theoretical methods have been applied to investigate the effect of internal parent excitation on the ultraviolet photodissociation dynamics of HCl (X 1Σ+) molecules. Jet-cooled H35Cl molecules within a time-of-flight mass spectrometer were prepared by infra-red absorption in the following quantum states: v=1, J=0 and J=5; v=2, J=0 and J=11; v=3, J=0 and J=7. The excited molecules were then photodissociated at λ∼235 nm and the Cl(2Pj) photofragments detected using (2+1) resonance enhanced multiphoton ionization. The results are presented as the fraction of total chlorine yield formed in the spin–orbit excited state, Cl(2P1/2). The experimental measurements are compared with the theoretical predictions from a time-dependent, quantum dynamical treatment of the photodissociation dynamics of HCl (v=1−3, J=0). These calculations involved wavepacket propagation using the ab initio potential energy curves and coupling elements previously reported by Alexander, Pouilly, and Duhoo [J. Chem. Phys. 99, 1752 (1993)]. The experimental results and theoretical predictions share a common qualitative trend, although quantitative agreement occurs only for HCl (v=2).© 2000 American Institute of Physics.

Notizen: The two experimental techniques of cavity ringdown spectroscopy and high-resolution, long-path Fourier transform spectroscopy have been used to measure quantitative absorption spectra and determine the integrated absorption intensity (Sint,B) for the O2 a 1Δg–X 3Σg− (0,0) band. Einstein A-factors and radiative lifetimes for the O2 a 1Δg v=0 state have been derived from the Sint,B values. The two methods give values for the integrated absorption intensity that agree to within 2%. The value recommended from the results of this study is Sint,B=3.10±0.10×10−24 cm molecule−1, corresponding to an Einstein-A coefficient of A=2.19±0.07×10−4 s−1 and a radiative lifetime of τrad=76 min. The measurements are in excellent agreement with the recent absorption study of Lafferty et al. [Appl. Opt. 37, 2264 (1998)] and greatly reduce the uncertainty in these parameters, for which accurate values are required for determination of upper stratospheric and mesospheric ozone concentrations. © 1999 American Institute of Physics.

Notizen: The near ultraviolet (UV) and visible photodissociation dynamics of BrCl have been explored using the technique of photofragment ion imaging at 26 wavelengths in the range 235 to 540 nm. Ion images of the Cl(2P3/2), Cl(2P1/2), Br(2P3/2) and Br(2P1/2) photofragments reveal both the angular distributions of photofragment velocities (characterized by anisotropy parameters, β) and which of the four possible photofragment pathways are active at different wavelengths. The anisotropy parameters show extensive variation both with wavelength and for the different fragmentation channels, and these variations are interpreted largely in terms of excitations to the A 3Π(1), B 3Π(0+), C 1Π(1) and D(0+) states as the wavelength is reduced. At wavelengths between 235 and 262 nm, the Br(2P1/2)+Cl(2P3/2) channel is dominant and β=2.0±0.1 at 235 nm, characteristic of a parallel parent transition (ΔΩ=0) and supporting previous assignments of the absorption in this wavelength range being due to the D(0+)–X 1Σ+(0+) transition. A minor channel forms Cl(2P1/2)+Br(2P3/2) with an anisotropy indicative of the involvement of an underlying perpendicular absorption (ΔΩ=±1) to a state with Ω=1. Br(2P3/2)+Cl(2P3/2) and Br(2P1/2)+Cl(2P1/2) fragmentation channels are not observed. Excitation in the wavelength range 320 nm to 410 nm results in Cl(2P3/2)+Br(2P3/2) products with an anisotropy parameter of β=−1.0±0.1, consistent with assignment of the strong parent absorption to the C 1Π(1)–X 1Σ+(0+) transition. For photolysis wavelengths longer than 400 nm, the Cl(2P1/2)/Cl(2P3/2) branching ratio increases [with β∼1.0 to 1.4 for the Cl(2P1/2)], β for Cl(2P3/2) becomes less negative (and for λ≥450 nm, values lie in the range 0 to −0.2) and Br(2P3/2) β-parameters also increase. No formation of Br(2P1/2) is observed. These observations are, in part, consistent with absorption via the B 3Π(0+)–X 1Σ+(0+) transition, although the nonlimiting β-parameter values imply a significant perpendicular contribution to the absorption spectrum. The measured anisotropy parameters for λ≥410 nm are interpreted in terms of excitation both to an Ω=0 state [B 3Π(0+)] and an Ω=1 state [A 3Π(1) or C 1Π(1)], together with transfer of dissociating flux between states during the dissociation. © 1998 American Institute of Physics.

Notizen: The technique of H Rydberg atom photofragment translational spectroscopy has been applied to investigate the ultraviolet photodissociation dynamics of hydrogen bromide. Branching fractions between the channels forming ground Br(2P3/2) and spin-orbit excited Br(2P1/2) atoms have been determined at 15 independent wavelengths in the range 201–253 nm, and photofragment recoil anisotropies for these two channels have been characterized at six different wavelengths within the same wavelength range. The channel forming ground state products, H+Br(2P3/2), is observed to arise solely from a perpendicular (i.e., ΔΩ=1) transition at all excitation energies, whereas the channel to formation of excited state products, H+Br(2P1/2), has a marked wavelength dependence: at long wavelengths (λ=243 nm), the photofragments are produced by a parallel (i.e., ΔΩ=0) photodissociation mechanism, which becomes more perpendicular in character as the photolysis energy is increased. Within the wavelength range studied, the branching fractions indicate that Br(2P3/2) products are formed in preference to Br(2P1/2) products, with propensities that are relatively invariant to excitation wavelength, although a small, yet pronounced, cusp appears at λ∼235 nm. The observations are discussed with reference to the known behavior of the other hydrogen halides and highlight the influence of spin-orbit interactions in the photofragmentation dynamics of this series of molecules. © 1999 American Institute of Physics.

Notizen: The technique of cavity ring-down spectroscopy has been used to investigate predissociation in the A 2Σ+ state of the SH and SD radicals. Spectra were recorded of the A–X (1,0) band of SH and the (1,0), (2,0) bands of SD. Linewidth measurements of transitions to individual rovibrational levels of the A state revealed increasing predissociation rates with vibrational and rotational quantum number. These and all other available data have been reproduced, quantitatively, by Fermi Golden Rule calculations employing the best (experimentally determined) analytic potential for the A state and ab initio repulsive potentials and spin–orbit coupling matrix elements. © 1997 American Institute of Physics.

Notizen: Predissociation of the B 3Σu− state of S2 has been investigated by a combination of cavity ring-down spectroscopy and model calculations. The experimental spectra of the B 3Σu−−X 3Σg−(v′,0) bands for 10≤v′≤22 span the wavenumber range 35 480–39 860 cm−1. Extensive variation is observed in the degree of rotational structure within the vibrational bands because of lifetime broadening caused by predissociation. Fits to the band contours give homogeneous linewidths for transitions to the B-state vibrational levels for 10≤v′≤17 that vary from ≤1 cm−1 for the (10,0) band to 7±1 cm−1 for the (17,0) band with a maximum linewidth of 14±1 cm−1 for the (13,0) band. For v′≥18, all bands are completely diffuse, indicating linewidths in excess of 15 cm−1. The experimental results are compared with the results of a theoretical model that uses a Rydberg–Klein–Rees (RKR) potential for the B 3Σu− state, ab initio calculations of the repulsive potentials that cross the B state, and Fermi golden rule calculations of the predissociation rates for the different repulsive potentials. Minor adjustments to the ab initio potentials, and an estimate of the spin-orbit coupling between the bound and repulsive states enable us to calculate predissociation rates in excellent agreement with the experimental observations. We deduce that the predissociation for v′≤16 is predominantly via a 1Πu state, whereas for v′≥17, coupling to a second repulsive state, suggested to be either a 5Σu− or 5Πu state, provides the primary mechanism for predissociation. © 1998 American Institute of Physics.

Notizen: This paper extends our knowledge of the higher excited states of the ammonia molecule by presenting detailed measurements of the 2+1 resonance enhanced multiphoton ionization (REMPI) spectrum of both NH3 and ND3 obtained following excitation in the wavelength range 298–242 nm, i.e., at energies up to the first ionization energy. Complementary analyses of the wavelength resolved REMPI spectrum and the accompanying REMPI-photoelectron spectra leads to the identification of ten new Rydberg origins of NH3 (four for ND3) with principal quantum numbers n≤8 and, in most cases, of the accompanying out-of-plane bending vibrational progression. Symmetry assignments for the various newly identified excited states are offered, based on band contour simulation and/or quantum defect considerations. Dominant amongst these are the E˜″ 1A2″ (5sa1′←1a2″) state: ν0=74 118(2) cm−1 [NH3], ν0=74 258(2) cm−1 [ND3], the F˜″ 1E″ (5pe′←1a2″) state: ν0=76 220(50) cm−1 [NH3], ν0=76 240(50) cm−1 [ND3], the F˜′ 1A1′ (5pa2″←1a2″) state: ν0=76 674(1) cm−1 [NH3], ν0=76 770(5) cm−1 [ND3], and the G˜′ 1A1′ (6pa2″←1a2″) state: ν0=78 494(1) cm−1 [NH3]. The present work serves to reinforce the previously noted dominance of np←1a2″ Rydberg excitations in the 2+1 REMPI spectrum of ammonia. In addition, the adiabatic ionization energy of ND3 is estimated to be 82 280±40 cm−1 based on the assumption that analogous Rydberg states of NH3 and ND3 will have very similar quantum defects. © 1998 American Institute of Physics.

Notizen: The spectroscopy and predissociation dynamics of the A˜ 1A′′ state of HNO have been investigated by measurement of line positions and lifetime broadened linewidths in the cavity ring-down (CRD) spectrum. CRD spectroscopy is a technique better suited to studies of molecular predissociation than methods such as laser induced fluorescence in cases where the excited state dissociation lifetime is short compared to its fluorescence lifetime. The CRD spectrum extends well beyond the dissociation limit (we have identified transitions to rotational states lying up to 2400 cm−1 above the dissociation limit of 16450 cm−1). The lifetime-dependent Lorentzian components of the line shapes of numerous rovibrational features of the A˜ 1A′′–X˜ 1A CRD absorption spectrum have been deconvoluted from the Doppler and laser line profiles to obtain lifetimes and predissociation rates for individual |v1v2v3〉|JK〉 states. Here, the labels v1, v2, and v3 denote the number of quanta of the N–H stretch, N(Double Bond)O stretch, and H–N–O bending vibrations, respectively. We have measured line broadening (of up to 0.3 cm−1) in transitions to six vibronic states above the predissociation threshold (the 100 and 020 states, for which the higher K levels are above the dissociation limit, and the 101, 030, 110, and 111 states). For three substates (100 K=5, 101 K=1 and 110 K=4) strongly J-dependent transition linewidths are seen. The 100 K=5 and 101 K=1 substates show maximum transition linewidths midway through the observed spectral transitions while the linewidths for transitions involving the 110 K=4 substate increase with J. Linewidths also generally increase with K. Some lines involving the 100 K=5 state are markedly asymmetric. Linewidths for transitions to states having excitation of the bending mode (the 101 and 111 states) are larger than those for which v3=0. These observations clarify the predissociation mechanism suggested by previous absorption and LIF studies. We attribute the primary predissociation mechanism to a-axis Coriolis coupling of A˜ state levels to discrete quasibound highly vibrationally excited levels of the ground state which in turn are coupled to the electronic ground state continuum corresponding to dissociation to H(2S)+NO(X 2Π). Predissociation of A˜ state levels with K=0 is probably caused by b-axis Coriolis coupling to such quasibound levels of the ground state. The variation of predissociation rates with J and K for the A˜ 110 K=4, 5, and 6 substates cannot be accounted for by this mechanism and we propose the onset of predissociation to the continuum of the a˜ 3A′′ state. Interpretation of our experimental data is assisted by calculations performed using the potential energy surfaces of Guadagnini et al. [J. Chem. Phys. 102, 774 (1995)]. © 1997 American Institute of Physics.

Notizen: 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.

Notizen: The photofragment excitation spectra for O(1D) production from the 317–327 nm photolysis of ozone under supersonic free-jet and low-temperature flow conditions show structure superimposed on an underlying continuum. Doppler profiles of the nascent O(1D) photofragments confirm that the O(1D) formed by photolysis at the wavelengths of the peaks in the photofragment excitation spectrum arises from the hitherto unobserved spin-forbidden predissociation to O(1D)+O2(X 3Σg−) products. © 1996 American Institute of Physics.