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Correct parameterization of the Collision-induced Absorption (CIA) phenomena is essential for accurate modeling of planetary atmospheres. The HITRAN spectroscopic database provides these parameters in a dedicated section. Here, we significantly revise and extend the HITRAN CIA data with respect to the original effort described in Richard et al. [JQSRT 113, 1276 (2012)]. The extension concerns new collisional pairs as well as wider spectral and temperature ranges for the existing pairs. The database now contains CIA for N2 N2, N2 H2, N2 CH4, N2 H2O, N2 O2, O2 O2, O2 CO2, CO2 CO2, H2 H2, H2 He, H2 CH4, H2 H, H He, CH4 CH4, CH4 CO2, CH4 He, and CH4 Ar collision pairs. The sources of data as well as their validation and selection are discussed. A wish list to eliminate remaining deficiencies or lack of data from the astrophysics perspective is also presented.

In the version of this Article originally published, Figures 3 and 4 were erroneously swapped, this has been corrected in all versions of the Article.

In the version of this Article originally published, Figures 3 and 4 were erroneously swapped, this has been corrected in all versions of the Article.

Collision-induced absorption is the phenomenon in which interactions between colliding molecules lead to absorption of light, even for transitions that are forbidden for the isolated molecules. Collision-induced absorption contributes to the atmospheric heat balance and is important for the electronic excitations of O

The laboratory measurement of processes in- volved in terrestrial airglows is essential in developing di- agnostic tools of the dynamics and photochemistry of the upper atmosphere. Dissociative electron recombination of O + in the ionospheric F-region is expected to produce both O( 1 D) and O( 1 S) which are the sources of the 630.0 nm red airglow and the 557.7 nm green airglow lines, respectively. We present both theoretical and experimental evidence, the latter from a heavy ion storage ring technique, that the O( 1 S) quantum yield from O + (v = 0) is a strong function of the electron temperature due to a molecular resonance phenomenon. At present the O + (v = 0) theoretical and laboratory recombination data cannot explain rocket obser- vations of the ionospheric green and red airglows (Takahashi et al. 1990; Sobral et al. 1992).

Contains fulltext : 166095.pdf (Publisher’s version ) (Open Access) We present far infrared spectra of the conformer A of tryptamine in the 200 to 500 cm-1 wavenumber range along with resonant photoionization spectra of the far-infrared excited conformer A of tryptamine. We show that single-far-infrared photon excited tryptamine has highly structured resonance enhanced multi-photon ionization spectra, revealing the mode composition of the S1-state. Upon multiple-far-infrared photon absorption, the resonance enhanced multi-photon ionization spectrum broadens allowing ion gain spectroscopy to be performed. In the ion gain spectrum we detect the fundamental far-infrared modes but also combination and overtone bands with high efficiency. The implications to dip spectroscopy using a free electron laser compared to more conventional light sources are discussed.

International audience; This paper reports on the effect of collisions between nitrogen and oxygen on absorption in the A-band near 760 nm under atmospheric conditions relevant for satellite retrieval studies. We use pulsed laser cavity ring-down spectroscopy with a narrow bandwidth laser and use pressure scans to increase the accuracy of the measured oxygen extinction coefficients. We use the so-called Adjustable Branch Coupling model to describe line mixing in the magnetic allowed A-Band dipole absorption and we retrieve the collision induced absorption spectrum due to N$_2$-O$_2$ collisions.

The first fully allowed spectroscopic transition in O2 is the transition comprising the well-known Schumann–Runge bands and continuum. We report a velocity-map imaging study in which the O(1 D) angular momentum polarisation and O(3 P) spin–orbit branching ratios arising from this process have been measured. We show that direct 157 nm excitation into the Schumann–Runge continuum leads to extremely strong angular momentum polarisation in the O(1D) product. Comparison with previous studies indicates that this is a general feature of dissociation via the B state. The fine structure branching ratios in the co-fragment O(3P J=2,1,0) were measured to be 88.5 ± 1.6 : 9.7 ± 1.4 : 1.9 ± 0.4. Based on a consideration of the Massey parameter for the system, the data were modelled using theoretical calculations based on adiabatic and diabatic models of the dissociation. While both models were able to describe some aspects of the dissociation accurately, neither was able to predict both the fine structure branching rat...

We report data on a study into the vibrational dependence in the dissociative recombination of O+2 (X2Πg). The experiment has been performed with a merged-beam apparatus in the heavy-ion storage ring, CRYRING. We present the total rate coefficients as function of collision energy for five different vibrational populations of the ion beam and the partial rate coefficients and branching fractions near 0 eV collision energy for the vibrational levels v = 0, 1 and 2. Furthermore, we report on the control over and characterization of vibrational populations. The total rate coefficient is found to be weakly dependent on the vibrational excitation. The rate decreases upon increasing vibrational excitation, though only within an order of magnitude. The partial rate coefficients are strongly dependent on the vibrational level; with the v = 0 oxygen ions recombining the fastest. The partial branching fractions are also found to be strongly dependent on the vibrational level, specifically the O(1S) quantum yield which increases strongly upon increasing vibrational level.

We have studied the dissociation dynamics of NO+ ions in their ground, X 1Σ+, and first excited metastable, a 3Σ+ states, induced by the capture of electrons of variable collision energy in the dissociative recombination (DR) process. The branching over the different dissociation channels has been measured in a merged-beam experiment on the heavy-ion storage ring, CRYRING. In accord with previous observations, NO+ (X 1Σ+,v=0) ions dissociate dominantly to the N(2D)+O(3P) product limit at 0 and 1.2 eV collision energies. In contrast to earlier reports, the spin-forbidden N(4S)+O(1D) dissociation limit contributes 0(±2)% at 0 eV. At 5.6 eV a new channel coupled to the production of ground-state atoms becomes more important, but no increase in the production of ground-state product atoms was observed. All observed branching fractions compare very favorably with predictions from a simple statistical model, which is based on the multiplicity of each dissociation limit in combination with spin conservation duri...

The highly excited states of gerade symmetry in molecular nitrogen were investigated in a resonance enhanced XUV+Vis transition scheme. A two photon laser excitation study, with subsequent photo-ionization by a third laser pulse was reported. Tunable XUV laser light was generated by frequency doubling the output of a pulsed dye laser into ultraviolet in a KDP crystal, and subsequently frequency tripling the output ion xenon gas jet. Nineteen bands were observed and level energies were determine with an accuracy of ≈ 0.20 cm

The conformational preferences of peptides are mainly controlled by the stabilizing effect of intramolecular interactions. In peptides with polar side chains, not only the backbone but also the side chain interactions determine the resulting conformations. In this paper, the conformational preferences of the capped dipeptides Ac-Phe-Ser-NH2 (FS) and Ac-Phe-Cys-NH2 (FC) are resolved under laser-desorbed jet cooling conditions using IR-UV ion dip spectroscopy and density functional theory (DFT) quantum chemistry calculations. As serine (Ser) and cysteine (Cys) only differ in an OH (Ser) or SH (Cys) moiety; this subtle alteration allows us to study the effect of the difference in hydrogen bonding for an OH and SH group in detail, and its effect on the secondary structure. IR absorption spectra are recorded in the NH stretching region (3200-3600 cm(-1)). In combination with quantum chemical calculations the spectra provide a direct view of intramolecular interactions. Here, we show that both FS as FC share a singly γ-folded backbone conformation as the most stable conformer. The hydrogen bond strength of OH···O (FS) is stronger than that of SH···O (FC), resulting in a more compact gamma turn structure. A second conformer is found for FC, showing a β turn interaction.

The rotationally resolved magnetic dipole absorption spectrum of the oxygen A-band b(1)Σ(g)(+)(v=0)

Photodissociation of singlet oxygen, O2 a(1)Δg, by ultraviolet radiation in the region from 200 to 240 nm has been investigated using velocity map imaging of the atomic oxygen photofragments. Singlet oxygen molecules are generated in a pulsed discharge and studied by one-laser photodissociation and detection around 226 nm as well as two color photodissociation at various wavelengths in the range from 200 to 240 nm. A simple model of the discharge on and off signal indicates efficient conversion of O2 X(3)Σg(-)(v = 0) in the parent beam to O2 a(1)Δg(v = 0-2). Minute amounts of highly excited vibrational levels of ground state O2 X(3)Σg(-)(v0) are detected but no evidence is found for production of the O2 b(1)Σg(+) state. Over the decreasing wavelength range 240-200 nm the a(1)Δg-state signal relative to the X(3)Σg(-)(v = 0) signal decreases strongly. Around 226 nm the a(1)Δg(v = 0-2) states averaged branching ratio percentage for O((3)Pjj = 2 : 1 : 0) is found to be 56 : 36 : 8 (±5%), respectively. The anisotropy parameter for photodissociation of a(1)Δg(v = 0-2) averages to β = 1.3 ± 0.4. The a(1)Δg(v = 0) photodissociation cross section is found to 3-10 times stronger than theory predicts. Furthermore, the photodissociation image shows a strong parallel character, (i.e., transition moment parallel to the molecular axis) while theory predicts a predominantly negative character.

The interaction of waveguide modes and consequences on laser operation are studied numerically in a THz free electron laser (FEL) driven by short electron bunches. The considered FEL cavity configuration is represented by a parallel-plate waveguide extending over the complete distance between cylindrical cavity mirrors with energy out-coupling through a rectangular slit in one of the mirrors. We describe the multimode FEL cavity desynchronization dynamics and predict strong enhancement in FEL power at special cavity configurations, when modes are degenerate. Furthermore, we predict that simultaneous excitation of several waveguide modes can occur but do not negatively influence the lasing process.

We present experimental data on the dissociative recombination (DR) and the dissociative excitation (DE) of O2+ in its electronic and vibrational ground state using a heavy ion storage ring. The absolute DR cross section has been determined over an electron collision energy range from 1 meV to 3 eV. The thermal DR rate coefficient is derived; α(Te)=2.4×10−7(300/Te)0.70±0.01 cm3 s−1, for T>200 K. The threshold for DE was observed near its energetic threshold of 6.7 eV. The DE cross section curve has a maximum of 3×10−16 cm2 near 15 eV. We have determined the branching fractions to the different dissociation limits and present atomic quantum yields for the DR process between 0 to 300 meV collision energy. The quantum yield of O(1D) is found to be 1.17±0.05, largely independent of the electron energy. Arguments are presented that the branching fraction to O(3P)+O(1S) is negligible. The branching fraction to the O(1S)+O(1D) is smaller than 0.06 and varies strongly as a function of collision energy. The O(1S) quantum yield is a strong function of electron temperature. Hence, the relative strength of the green, O(1S), and the red, O(1D), airglows may be used as a measure of the electron temperature of the upper atmosphere. A qualitative explanation is given of the consequences of nonadiabatic interactions in the dissociation step of the DR process.

Differential Optical Absorption Spectroscopy (DOAS) has proven to be an extremely useful technique for retrieving trace gas columns from atmospheric measurements of reflected solar irradiance. We show that the use of DOAS is strongly constrained by the complexity of the absorption spectral structure that may be sampled by individual pixels in a detector array (for example, in the vibration-rotation bands of many species in the visible and infrared). To this end we introduce a Spectral Structure Parameterization (SSP) modification to DOAS which depends primarily on structure in the absorption cross-section of the atmospheric species intended for retrieval. We show that DOAS-SSP may be used to determine which absorption spectra are suitable, in terms of complexity of absorption structure, for use in DOAS retrievals as well as how to extend DOAS trace gas retrieval to those species with highly-structured absorption spectra.

Electron spin and nuclear spin reduce the anisotropy of photofragments in relatively long-lived levels of the g3Σg+ and i3∏g states of the n = 3 triplet gerade system of molecular hydrogen. The intramolecular forces due to the spin result in a precession of the plane of rotation around the total angular momentum vector. The photofragment anisotropy reveals whether the excited state lifetime allows for this precession to occur, hence whether the fine and hyperfine structure are resolvable. In partially fine structure resolved experiments, using fast beam photofragment spectroscopy, this fine and hyperfine structure depolarization is observed. The theory behind the depolarization is presented as well.

The influence of a magnetic field on the angular distribution of the atomic fragments resulting from photodissociation is studied both theoretically and experimentally. In the theoretical section we present a quantum mechanical description of photo-predissociation of diatomic molecules in a magnetic field, indicating the analogy with the classical Hanle effect observed in fluorescence, and showing how in a similar way lifetimes of predissociative states can be measured. The experimental section describes photodissociation experiments on the (4p)e1Πu(v=0) Rydberg state of N2, where the dissociation process occurs in the presence of a magnetic field. We use the Hanle effect to determine the lifetimes of several rotational levels of this state. They range from 0.2 to 12 ns, in agreement with lower limits that were determined from high resolution spectroscopy. Lifetime information further elucidates the complex predissociation mechanism of the e1Πu state.

The user FELIX Facility Nijmegen offers four FEL beam lines, FLARE from 0.2 to 3 THz, FELIX-2 from 12 to 75 THz, FELIX-1 from 2 to 12 THz and FELICE, a special beam line for intra-cavity experiments from 3-75 THz.

We report experiments on dissociative recombination (DR) of HD{sup +} and H{sub 2}{sup +}. Product state information has been obtained over a wide range of electron energies with a position sensitive detector consisting of a graded absorber in combination with a surface-barrier detector. At low electron energies ({lt}3 eV) hydrogen atoms are formed preferentially in highly excited states ({ital n}{gt}2); at high electron energies ({gt}12 eV) both hydrogen fragments are excited. The dissociative recombination rate of H{sub 2}{sup +} has been measured also as a function of storage time in the energy range of 0 eV to 20 eV. We show that the H{sub 2}{sup +} beam is still vibrationally excited after 20 s at our experimental conditions. The H{sub 2}{sup +} ions relax vibrationally through interaction with electrons in the electron cooler. Vibrationally excited levels ({ital v}{ge}5) have DR rates that exceed the DR rate of the lower vibrational levels by two orders of magnitude. The latter observation has important consequences for the interpretation of previous DR experiments on H{sub 2}{sup +}. {copyright} {ital 1996 The American Physical Society.}

A quantum-mechanical treatment describing photofragmentation of a homonuclear molecule is presented. The treatment is developed to describe results on photofragmentation in molecular hydrogen [preceding paper, Phys. Rev. A {bold 54}, 522 (1996)] in which photoexcitation takes place to an excited state that can dissociate using different channels, all of which are accompanied by fluorescence. The kinetic energy of the photofragments, which is directly related to the fluorescence spectrum, and their anisotropy were measured. The theoretical treatment incorporates electronic-state mixing due to nonadiabatic effects in the excited state. Fluorescence may occur as molecular fluorescence, that is, before dissociation, or atomic fluorescence, after dissociation has taken place. Both are treated simultaneously and the branching is determined. A remarkable result is the occurrence of interference when molecular fluorescence is degenerate with atomic fluorescence. It is shown that in case of nonadiabatic effects the anisotropy of the photofragments can depend on the decay channel; the actual anisotropy does not represent the anisotropy of the ensemble of excited molecules directly. {copyright} {ital 1996 The American Physical Society.}

Photodissociation of quasibound levels in the i${\mathrm{}}^{3}$${\mathrm{\ensuremath{\Pi}}}_{\mathit{g}}$ state of molecular hydrogen is studied using fast-beam photofragment spectroscopy. The molecules dissociate either via molecular fluorescence to a repulsive state or by barrier tunneling followed by atomic fluorescence. We measured the branching between the two pathways and the anisotropy of the photofragment distribution. The branching ratios were calculated from the potential-energy curve (see the theoretical treatment given in the following paper [Phys. Rev. A 54, 531 (1996)]). The observed and calculated branching ratios are found to be in good agreement. Small discrepancies can be accounted for by the electronic mixing with neighboring states. The photofragment anisotropy is a diagnostic tool for the dissociation continuum and the lifetime of the excited state. The experimentally found anisotropy parameters corroborate the electronic mixing and also show that the fine and hyperfine structure are resolvable only for the lowest three rotational levels examined. \textcopyright{} 1996 The American Physical Society.

In this paper we present results of numerical studies on the cavity desynchronization in the short-pulse waveguided free electron laser (FEL) oscillator FLARE, which is a new THz FEL at the Radboud University Nijmegen, with the emphasis to investigate the influence of the waveguide dispersion. In particular, the peaks of the detuning curves are predicted to be essentially broader. We also predict that the extent of the peak broadening increases as the waveguide dispersion becomes more significant.

Predissociation of the (v=7–10) levels of the a 1Πg state in N2 is studied with translational spectroscopy. These levels are produced by the charge‐transfer neutralization of a 4 keV energy N+2 beam in sodium vapor and the dissociation fragments are monitored by a time‐ and position‐sensitive detector. Lifetimes of 300, 300, 100, and 230 (±20) ns are measured for a 1Πg(v=7, 8, 9, and 10), respectively. Quantum mechanical calculations based on Fermi’s golden rule give evidence that the predissociation is caused by direct coupling to the continuum of the A’ 5Σ+g state rather than by an indirect mechanism. The a 1Πg–A’ 5Σ+g electronic coupling matrix element is estimated to be 0.44 (±0.03) cm−1. The required change in spin multiplicity suggests that this coupling arises from a spin–spin interaction.

Time-dependent calculations concerning delayed photodissociation of diatomic molecules are presented. The formalism is used to calculate the time-dependent anisotropy of the fragments resulting from photodissociation of H 2 via tunneling through the barrier of the i 3 Π g state. Only the rotational motion is treated time-dependently. The method presented is relatively simple and is non-perturbative in the treatment of the interaction with the photon field. Oscillations in the photofragment anisotropy and in the photofragment yield are predicted at all photon field strengths upon switching on the interaction. It is argued that these effects are amenable to experimental verification with present day pulsed laser systems.

An experimental study is presented on the predissociation of perturbed n = 3 Rydberg systems in N 2 converging to the X 2 Σ g + and A 2 Π u ion states. The Rydberg states are formed in collisions of N 2 + with cesium at energies between 4.8 and 7 keV. The dissociation fragments are recorded using translational spectroscopy. Assignment of the Rydberg states is realized with a selective laser excitation technique. The laser excitation also reveals that ion core excitation concurrent with electron transfer is a significant mechnism in the charge-transfer formation of the (3sσ g A + Rydberg states. The (3pπ u )X + G 3 Π u (υ=0), (3pσ u )X + D 3 Σ u + (υ=0), )3sσ g A + F 3 Π u (υ=0–3), and (3sσ g A + o 1 Π u (υ=0–4) states are observed with all levels predissociating to N( 2 D) + N( 4 S) atoms. The vibrational numbering of the F 3 Π u state is reassigned with respect to earlier work.

A semiclassical multichannel approach has been used to calculate absolute cross-sections for charge exchange collisions between doubly charged molecular oxygen and alkali (Cs, K and Na) atoms at 12 keV. In the charge exchange reaction the n =3, 4 Rydberg states of O + 2 are populated. The system is described as an active electron in the field of two inert cores. A one-centre description for the molecular oxygen core has been used. The present theoretical results agree with the available experimental data, while results from other two-state models do not. We show that there is population sharing among the final states during the collision and that the charge exchange reaction is not merely localized at curve crossings.

We present experimental and numerical studies on control of rotational population transfer of NO($J=1/2$) molecules to higher rotational states. We are able to transfer 57$%$ of the population to the $J=5/2$ state and 46$%$ to $J=9/2$, in good agreement with quantum mechanical simulations. The optimal pulse shapes are composed of pulse sequences with delays corresponding to the beat frequencies of states on the rotational ladder. The evolutionary algorithm is limited by experimental constraints such as volume averaging and the finite laser intensity used, the latter to circumvent ionization. Without these constraints, near-perfect control ($g$98$%$) is possible. In addition, we show that downward control, moving molecules from high to low rotational states, is also possible.

Larmor precession of a quantum mechanical angular momentum vector about an applied magnetic field forms the basis for a range of magnetic resonance techniques, including nuclear magnetic resonance spectroscopy and magnetic resonance imaging. We have used a polarized laser pump-probe scheme with velocity-map imaging detection to visualize, for the first time, the precessional motion of a quantum mechanical angular momentum vector. Photodissociation of O(2) at 157 nm provides a clean source of fast-moving O((1)D(2)) atoms, with their electronic angular momentum vector strongly aligned perpendicular to the recoil direction. In the presence of an external magnetic field, the distribution of atomic angular momenta precesses about the field direction, and polarization-sensitive images of the atomic scattering distribution recorded as a function of field strength yield 'time-lapse-photography' style movies of the precessional motion. We present movies recorded in various experimental geometries, and discuss potential consequences and applications in atmospheric chemistry and reaction dynamics.

We have determined the collision-induced absorption (CIA) spectrum in the O(2) B-band in pure oxygen. We present absolute extinction coefficients of the minimums in between rotational lines using cavity ring-down spectroscopy. The measured extinction is corrected for the B-band magnetic dipole absorption using a model which includes line-mixing. The remaining extinction consists of collision-induced absorption and Rayleigh scattering. We retrieve the magnitude of the Rayleigh scattering and the CIA spectrum based on their individual different behavior with density. The CIA spectrum of the B-band resembles that of the oxygen A-band in shape but not in magnitude. The contribution of CIA to the total B-band absorption is 40% higher in comparison to that of the A-band.

An experimental and theoretical study is presented on the (4s) 2Σ+g, (3p) 2Σ+u, 2Πu, and (3d) 2Σ+g, 2Πg, 2Δg Rydberg states in O+2. Their spectroscopic properties (T0,ωe,ωexe) have been determined using translational spectroscopy. The Rydberg molecules are formed in collisions of fast O2+2 ions with atomic Cs, K, and Na. The potential energy curves of the Rydberg states have been calculated using a self‐consistent field/averaged coupled pair functional (SCF/ACPF) method and agree well with the experimental results. The (3p) 2Σ+u state is strongly perturbed by the lowest valence state of the same symmetry. The Rydberg‐valence interactions leading to the predissociative decay are discussed.

We report on the vibrational population of NO 2+ ions in the X 2 Σ + and A 2 Π states formed by electron impact in a low-pressure ion source. The theoretical analysis includes ionization in the source as a vertical process and radiative decay of the excited A state during the flight of the resulting ion beam. The presented theoretical results allow for a consistent interpretation of high-resolution translation energy spectroscopy data of transitions between the X and A states in NO 2+ during collisions with He reported previously.

This paper reports on the absorption of molecular oxygen in the region of the A-band near 760 nm under atmospheric conditions relevant for satellite retrieval studies. We use pulsed laser cavity ring-down spectroscopy with a narrow bandwidth laser and use pressure scans to increase the accuracy of the measured oxygen extinction coefficients. Absolute binary absorption coefficients in minima between absorption lines of the A-band spectrum have been measured and tabulated. We use the so-called adjustable branch coupling model including line mixing to calculate the magnetic dipole absorption in order to determine the contribution of collision induced absorption. The line mixing model has been optimized such that the collision induced absorption spectrum is smooth.

Rotationally resolved electronic spectra of the vibrationless origin and of eight vibronic bands of 5-methoxyindole (5MOI) have been measured and analyzed using an evolutionary strategy approach. The experimental results are compared to the results of ab initio calculations. All vibronic bands can be explained by absorption of a single conformer, which unambiguously has been shown to be the anti-conformer from its rotational constants and excitation energy. For both anti- and syn-conformers, a (1)L(a)/(1)L(b) gap larger than 4000 cm(-1) is calculated, making the vibronic coupling between both states very small, thereby explaining why the spectrum of 5MOI is very different from that of the parent molecule, indole.

Sliced velocity-map imaging has been used to measure photofragment scattering distributions for the O((3)P(2)) and O((3)P(1)) products of O(2) photolysis following laser excitation into the Herzberg continuum between 205 and 241 nm. The images were analysed to extract the photofragment spatial anisotropy parameter, β, together with the alignment parameters a(∥), a(⊥), a(⊥), and Re[a(∥, ⊥)]. Our alignment measurements bridge the gap between the recent 193 nm measurement of Brouard et al., Phys. Chem. Chem. Phys., 2006, 8, 5549 and those of Alexander et al., J. Chem. Phys., 2003, 118, 10566 at 222 and 237 nm, and extend out to the threshold at 241 nm. Our measured parameters show no strong dependence on photolysis wavelength. Near the threshold we were able to separate the contributions from the O((3)P(2)) + O((3)P(2)) and O((3)P(2)) + O((3)P(1)) channels, and found significantly different photofragment alignments for the two cases.

In this letter, we propose a mechanism to extend the spectral range of a waveguided free electron laser (FEL) oscillator driven by a rf LINAC toward significantly longer wavelengths without changing the undulator or accelerator design parameters. This mechanism involves selective amplification of a lower-frequency branch supported in a FEL due to the waveguide dispersion. Based on simulations performed for the terahertz FEL under construction at the Radboud University Nijmegen, we conclude that these long wavelengths can be efficiently amplified via stimulated super-radiance as the electron bunches are shorter than the radiation wavelength.

Contains fulltext : 35400.pdf (Publisher’s version ) (Open Access) The size distribution of semitransparent irregularly shaped mineral dust aerosol,samples is determined using a commonly used laser particle-sizing technique. The size distribution is derived from intensity measurements of singly scattered light at various scattering angles close to the forward-scattering direction at a wavelength of 632.8 nm. We analyze the results based on various light-scattering models including diffraction theory, Mie calculations for spheres with various refractive indices, and T-matrix calculations for spheroidal particles. We identify systematic errors of the retrieved size distribution when the semitransparent and nonspherical properties of the particles are neglected. Synthetic light-scattering data for a variety of parameterized size distributions of spheres and spheroids are used to investigate the effect of simplifying assumptions made when the diffraction model or Mie theory is applied in the retrieval. (C) 2006 Optical Society of America.

[ 1] The possibility of using shape mixtures of randomly oriented spheroids for modeling desert dust aerosol light scattering is discussed. For reducing calculation time, look-up tables were simulated for quadrature coefficients employed in the numerical integration of spheroid optical properties over size and shape. The calculations were done for 25 bins of the spheroid axis ratio ranging from similar to 0.3 ( flattened spheroids) to similar to 3.0 ( elongated spheroids) and for 41 narrow size bins covering the size parameter range from similar to 0.012 to similar to 625. The look-up tables were arranged into a software package, which allows fast, accurate, and flexible modeling of scattering by randomly oriented spheroids with different size and shape distributions. In order to evaluate spheroid model and explore the possibility of aerosol shape identification, the software tool has been integrated into inversion algorithms for retrieving detailed aerosol properties from laboratory or remote sensing polarimetric measurements of light scattering. The application of this retrieval technique to laboratory measurements by Volten et al. ( 2001) has shown that spheroids can closely reproduce mineral dust light scattering matrices. The spheroid model was utilized for retrievals of aerosol properties from atmospheric radiation measured by AERONET ground-based Sun/sky-radiometers. It is shown that mixtures of spheroids allow rather accurate fitting of measured spectral and angular dependencies of observed intensity and polarization. Moreover, it is shown that for aerosol mixtures with a significant fraction of coarse-mode particles ( radii >= similar to 1 mu m), the nonsphericity of aerosol particles can be detected as part of AERONET retrievals. The retrieval results indicate that nonspherical particles with aspect ratios similar to 1.5 and higher dominate in desert dust plumes, while in the case of background maritime aerosol spherical particles are dominant. Finally, the potential of using AERONET derived spheroid mixtures for modeling the effects of aerosol particle nonsphericity in other remote sensing techniques is discussed. For example, the variability of lidar measurements ( extinction to backscattering ratio and signal depolarization ratio) is illustrated and analyzed. Also, some potentially important differences in the sensitivity of angular light scattering to parameters of nonspherical versus spherical aerosols are revealed and discussed.

Contains fulltext : 32424.pdf (Publisher’s version ) (Open Access) Dissociative recombination (DR) of the dimer ion (NO)(2)(+) has been studied at the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm. The experiments were aimed at determining details on the strongly enhanced thermal rate coefficient for the dimer, interpreting the dissociation dynamics of the dimer ion, and studying the degree of similarity to the behavior in the monomer. The DR rate reveals that the very large efficiency of the dimer rate with respect to the monomer is limited to electron energies below 0.2 eV. The fragmentation products reveal that the breakup into the three-body channel NO+O+N dominates with a probability of 0.69 +/- 0.02. The second most important channel yields NO+NO fragments with a probability of 0.23 +/- 0.03. Furthermore, the dominant three-body breakup yields electronic and vibrational ground-state products, NO(upsilon=0)+N(S-4)+O(P-3), in about 45% of the cases. The internal product-state distribution of the NO fragment shows a similarity with the product-state distribution as predicted by the Franck-Condon overlap between a NO moiety of the dimer ion and a free NO. The dissociation dynamics seem to be independent of the NO internal energy. Finally, the dissociation dynamics reveal a correlation between the kinetic energy of the NO fragment and the degree of conservation of linear momentum between the O and N product atoms. The observations support a mechanism in which the recoil takes place along one of the NO bonds in the dimer.

We demonstrate an intricate combination of strong and weak interactions of a broadband half-cycle pulse with Rydberg atoms. A transition, which is resonant with a frequency that was taken out of the frequency spectrum, can be enhanced. In the experiment, the broadband ultrashort terahertz half-cycle pulse propagates through water vapor, which absorbs on discrete lines in the spectrum, thus removing frequencies from the spectrum. The resulting pulse interacts with a rubidium Rydberg atom, which has a resonant transition with one of the missing frequencies. Under certain conditions, the transition probability is enhanced although the associated frequency was missing in the spectrum.

Contains fulltext : 60171.pdf (Publisher’s version ) (Open Access) We present single-photon spectroscopy in molecular hydrogen starting from the metastable c(3)Pi(u)(-) state to a number of triplet nd-Rydberg states (v=0-4, n=12-20). Using fast beam spectroscopy both the autoionization channel and the predissociation channel are quantified, field free, as well as with small electric fields. Coupling with the i(3)Pi(g) state is assumed to be responsible for field-free predissociation of the v=0 Rydberg levels. The stronger observed predissociation channel of the v=1 Rydberg levels is due to the nonadiabatic interaction with the h(3)Sigma(g)(+) state in combination with l mixing due to an external electric field. No direct evidence is found for possible electric field induced predissociation of the gerade Rydberg states by low lying ungerade states. The competition between autoionization and predissociation is discussed in terms of possible consequences for dissociative recombination involving low energy electron collisions with the H-2(+) molecular ion. (C) 2004 American Institute of Physics.

Item does not contain fulltext We present an experiment on the photodestruction of the NO2- anion at 266 nm. We have quantified the competition between photodetachment and photodissociation and have identified the nature of the photodissociation process from the photofragment angular distribution. This study involves a novel technique; time resolved multicoincident detection photofragment spectroscopy. A three-dimensional double exposure CCD camera is employed. The system provides the position (x,y) and the relative arrival time (t) of all fragments. In the case of photodissociation events, NO and O- fragments are detected for each event. The detection of multiple events per laser shot is made possible by using center-of-mass selection. (C) 2004 Elsevier B.V. All rights reserved.

Using fast beam photofragment spectroscopy, we have studied the competition between photodissociation and photodetachment in O2−. The photodissociation fraction is ∼10% and increases somewhat between 285 and 266 nm. High resolution photofragment spectroscopy shows that the O2− beam is composed of v=0 ions only. The photofragment angular distribution has an anisotropy parameter β=1.9, indicating that the process of dissociation has a parallel character, attributed to the A 2Πu←X 2Πg transition. A detailed analysis of the kinetic energy release of the photofragments shows the structure related to the distribution over the six fine-structure dissociation limits O− (2P3/2,1/2)+O (3P2,1,0). The results are compared with an analytical diabatic dissociation model, taking into account the long range charge–quadrupole interactions; the A 2Πu is described in a Born–Oppenheimer electrostatic aproximation and projected onto the different fine-structure states at large internuclear separation using analytical expressi...

Dissociative Recombination (DR) has been a subject of research for more than 50 years.1 In a DR event, a molecular ion captures an electron forming an intermediate electronically excited neutral molecule that rapidly dissociates into fragments: AB++e(E)→AB**→A+B*+KER. The symbol ‘KER’ stands for the kinetic energy released to the fragments during the reaction and E is the electron’s energy. Among the numerous species studied so far, the molecular ions H2 +, H3 +, and HeH+ and their isotopomers have been studied in most detail. DR of molecular hydrogen serves as the prototypical DR reaction. In spite of its apparent simplicity, being a one-electron system, DR of H2 + is theoretically challenging. Although theory is making progress,2,3,4 theory cannot yet be used as an a priori reliable standard. In H2 the direct process, as formulated by Bates, is prominent. The lowest neutral doubly excited state, Q1 1Σg +, crosses the ionic state close to the outer turning point of the ground vibrational level. The lowest triplet doubly excited state, Q1 3Πg, does not have such a favorable crossing.5 As a consequence, theory generally assumes that the direct process is very slow for triplet collisions. In systems in which the direct process is impossible, such as HeH+, an alternative mechanism has been formulated in which the ion plus free electron is coupled with bound Rydberg states through the radial part of the kinetic energy operator.6,7 In HeH+, this mechanism explains a substantial DR cross section at low electron collision energies.

Water vapor, arguably the most important trace gas constituent of Earth atmospheric physics, is also both a retrieval goal and a hindrance in the retrievals of other trace gases from nadir-measuring satellite spectrometers. This is because the atmospherically-attenuated solar spectrum in the visible and shortwave infrared is littered with water vapor bands. The recent plethora of water vapor spectroscopy databases in this spectral region has prompted us to study their utility in satellite retrievals. We consider water vapor spectroscopy compiled from four sources including new spectroscopy due to University College Lon-don and Imperial College London. Radiative transfer models of satellite measurements, in combination with accurate retrieval techniques, are quite sensitive to the accuracy and completeness of the water vapor spectroscopy. Notwithstanding the high degree of variability of a number of different factors in satellite measurements we show that retrievals are sensitive to database differences which suggests that our knowledge of water vapor spectroscopy is not as yet complete. In addition, new laboratory measurements indicate that the role of both the far-line wings of water vapor and the cumulative effect of many weak lines each have an important role to play in forming the so-called continuum.