Laser Photofragment Spectroscopy Research Articles

Absolute photolysis cross-sections for NaHCO3 , NaOH, NaO, NaO2and NaO3 : implications for sodium chemistry in the upper mesosphere

The compounds NaHCO3, NaOH, NaO and NaO2 are formed from meteor-ablated Na in the upper mesosphere. This paper reports the absolute photolysis cross-sections of these species at selected wavelengths between 193 and 423 nm, measured by laser photofragment spectroscopy. The sodium species were produced in both a slow flow reactor and a fast flow tube, by the addition of appropriate reagents to a flow of atomic Na from a heat-pipe oven. The pulsed photolysis source was either a Raman-shifted excimer laser or a Nd:YAG laser, and the resulting Na atoms were probed by laser induced fluorescence at 589.0 nm (Na(3 2P3/2–2S1/2)). Measurements are reported at 200 and 300 K. A limited set of cross-sections for NaO3 was also obtained at 300 K. In the case of NaHCO3, NaOH and NaO, the onset of photolysis is close to the thermodynamic threshold calculated from quantum theory. Calculations using the CI-singles method predict excited states whose relative energies are in sensible accord with the bands in the experimental spectra, but which are too high in energy with respect to their respective ground states. Photodissociation coefficients are then derived which are appropriate for modelling sodium chemistry in the upper mesosphere. Photolysis of the major sodium reservoir species, NaHCO3, is shown to be an important route for recycling this compound, and is also predicted to produce the HCO3 radical, which should be stable under mesospheric conditions.

Electronic Spectroscopy and Photodissociation Dynamics of Hydrated Co2+Clusters: Co2+(H2O)n(n= 4−7)

Solvated cluster ions Co2+(H2O)n with n = 4−7 have been generated by electrospray ionization and studied by laser photofragment spectroscopy. The similarity between the spectrum of gas-phase Co2+(H2O)6 and the absorption spectrum of aqueous cobalt(II) suggests that Co2+(H2O)6 (aq) is responsible for the room-temperature solution absorption spectrum. The observed photodissociation spectrum of Co2+(H2O)4 is similar to new bands which appear in aqueous cobalt(II) at high temperatures and have been assigned to Co2+(H2O)4 (aq) by Swaddle and Fabes (Swaddle, T. W.; Fabes, L. Can. J. Chem. 1980, 58, 1418−1426). The hexahydrate was found to dissociate by loss of one or two water molecules, whereas the heptahydrate dissociates by loss of two or three water molecules. In both cases, loss of two water molecules is the preferred dissociation pathway. The tetrahydrate dissociates either by simple loss of water or by charge separation to form CoOH+(H2O)2 and H3O+, with charge separation being the preferred dissociation...

Photodissociation Dynamics of Hydrated Ni2+ Clusters: Ni2+(H2O)n (n = 4−7)

Hydrated divalent nickel clusters, Ni2+(H2O)n (n = 4−7), have been generated through electrospray ionization and studied with laser photofragment spectroscopy. Clusters containing six and seven water molecules dissociate by loss of either one or two water molecules. In contrast, Ni2+(H2O)4 dissociates via charge reduction, producing H3O+ + NiOH+(H2O)2. The modest kinetic energy release in the H3O+ product is in agreement with a salt-bridge dissociation mechanism. Observed dissociation cross-sections indicate that the hexahydrated species is the probable carrier for nickel's aqueous absorption spectrum.

High-resolution laser photofragment spectroscopy of GeH+ (A 1Π—X1Σ+)

High-resolution laser photodissociation spectra of the GeH+ molecular ion in the visible region of the spectrum have been recorded between 541 and 613 nm by the detection of Ge+ fragment ions, using a fast-ion-beam apparatus. Many of the lines have been assigned to rovibronic transitions in the A 1Π—X1Σ+ system, which is observed for the first time. The predissociated levels of the A 1Π state lie between the fine structure dissociation asymptotes, Ge+(2P3/2, 2P1/2) + H (i.e. they are Feshbach resonances), and show a pronounced dependence of the lifetime on the rotational quantum number. This is interpreted in terms of a strong variation in the coupling induced by the radial kinetic energy operator. Measurements of the angular distribution profiles and fragment translational energy releases are reported.

High-resolution laser photofragment spectroscopy of GeH+ (A1Pi-X1Sigma+)

High-resolution laser photofragment spectroscopy of GeH+ (A1Pi-X1Sigma+)

An experimental and theoretical study of shape resonances in CH+

A study of shape resonance energies and widths in the A 1Π state of 12CH+ and 13CH+ is reported. The experimental data were obtained by high-resolution laser photofragment spectroscopy of CH+ in a fast ion beam. Spectroscopic transitions were recorded by tunable laser excitation from levels of the X 1Σ+ state to shape resonances (quasibound rotational levels) of the A 1Π state and detection of photoproduct C+ ions. Some of the dissociation lifetimes are sufficiently short to cause broadening of the spectral lines. The linewidths are interpreted in terms of tunnelling through the centrifugal barrier and the effects of rotational-electronic and radial coupling. The e lambda-doublet component in two of the resonances is found to have a narrower linewidth than the corresponding f component. This is shown to be due to the effect of the rotational-electronic (Coriolis) interaction on the shapes of the rotationally adiabatic potential-energy curves which determine the tunnelling probability. The contributions to the predissociation rates are calculated and are in good agreement with the experimental results.

The c1Π-b1Σ+ system of SH+ detected by high-resolution laser photofragment spectroscopy

The c1Π-b1Σ+ system of SH+ detected by high-resolution laser photofragment spectroscopy

Laser photofragment spectroscopy of near-threshold resonances in SiH +

Near-threshold resonances in the photodissociation cross section of SiH + have been recorded by the detection of photofragment Si + ions. The photodissociation spectrum is found to be dominated by Feshbach resonances. The first evidence for multichannel resonances in the photodissociation spectrum of a diatomic molecule is presented. Laser photodissociation spectra between 15600 cm -1 and 18750 cm -1 were recorded at a resolution of 0.0012 cm -1 by coaxial laser irradiation of a fast ion beam of SiH + . Over 70 transitions were observed, the majority of which involve excited-state levels (resonances) that lie between the Si + (2P|) + H ( 2 S) and Si+(2Pi) + H ( 2 S) dissociation limits, which are separated by 287 cm -1 . The assignments were made by combining experimental information with predictions of the vibrational and rotational energy levels obtained by numerical solution of the radial Schrodinger equation and calculations of the rotational line intensities. The experimental data were obtained by measurement of the transition frequencies, line intensities, line widths and hyperfine splittings and through examination of the effect of laser power on the linewidth. The kinetic energy released on dissociation and the photofragment angular distribution were also determined.

High-resolution laser photofragment spectroscopy of the c 1Π-b 1Σ + system of OH +

The first observation of transitions in the c 1Π-b 1Σ + system of the OH + molecule has been achieved by fast-ion-beam laser photofragment spectroscopy. The spectrum was recorded by the detection of O + ions which arise from predissociation of the c 1Π state. The assignment of P, Q and R branches of the 3-0 band is described and molecular parameters for these two vibrational levels are reported. The rotational constant for ν=0 agrees well with the value obtained by Merer et al. (Can. J. Phys. 53 (1975) 251) from an analysis of perturbations in the A 3Π state. Molecular parameters for the c 1Π and b 1Σ + states are in good agreement with those obtained from ab initio calculations of the potential energy curves.

The triplet 3s,3d complex of HD

High-resolution laser photofragment spectroscopy of the 3s,3d←2p c 3Π−u transitions in HD has been performed in a fast mass and state-selected beam. Transition wave numbers and rotational energy levels are given for v=0–3 of the h3∑+g, g 3∑+g, i 3Πg, and j 3Δg states. The data are represented by an effective Hamiltonian which accounts for the rotational breakdown of the Born–Oppenheimer approximation (L-uncoupling). The model parameters indicate a vibrational Born–Oppenheimer breakdown in the i 3Πg state. Good agreement with the most accurate ab initio calculations is obtained.

Laser photofragment spectroscopy of jet-cooled CClF 2NO

Photofragment yield spectra of jet-cooled CClF 2NO in the visible region have been recorded using pulsed laser dissociation in conjunction with two-photon LIF probing of the nascent NO fragment. Photodissociation of the cold parent at 646.6 nm yields NO with a statistical rotational state distribution but a colder than statistical spin-orbit distribution. A dissociation energy of 13500 ± 350 cm −1 has been evaluated for the C-N bond of the parent nitroso compound.

The n=2,3 triplet Rydberg states of the HD molecule observed by fast neutral-beam photofragment spectroscopy

The four states of the 1s3s, 1s3d triplet gerade complex and the metastable 1s2p c 3Π−u state of HD are studied by laser photofragment spectroscopy in a fast mass- and state-selected neutral beam. The spectra are assigned using a model for the l uncoupling in the 3s, 3d complex. Precise experimental molecular constants are obtained for the c state.

High-resolution laser spectroscopy of H 2 in a fast beam

High-resolution laser photofragment spectroscopy is performed in a fast beam of H 2c 3Π u − formed by near resonant charge exchange. The observed linewidths allow partial resolution of the fine and hyperfine structure in the 1s3d g 3Σ + g Rydberg state. Simulations of the spectra indicate that the hyperfine splittings are very close to those of H + 2 as expected for Rydberg states.

High resolution laser photofragment spectroscopy of PH+

High resolution electronic spectra of the 1–2 and 0–1 bands of the A 2Δ-X 2Π system of PH+ have been recorded by laser photofragment spectroscopy. Nuclear hyperfine structure due to the 31P and 1H nuclei is resolved and hyperfine parameters for both nuclei in the ground and excited electronic states are obtained. The parameters are discussed in terms of the electronic structure of the PH+ molecule.

High-resolution laser photofragment spectroscopy of CH+

High-resolution laser photofragment spectra of the CH+ ion in the range 16 125–18 708 cm–1 have been recorded by the detection of C+ ions, using a laser/ion-beam apparatus. Twenty nine transitions from levels of the ground X1Σ+ state to quasibound rotational levels (shape resonances) of the A1Π state were observed at a resolution of ca. 100 MHz. Predissociation linewidths were measured and are compared with the results of recent calculations. Additional resonances involving levels with triplet character were also recorded and their origin is discussed.

Electronic spectroscopy and rotationally resolved competitive predissociation of SiH2+

An electronic spectrum of the SiH2 + ion arising from the à 2 B 1(Π) ← [Xtilde] 2 A 1 transition has been recorded by laser photofragment spectroscopy. The molecular geometry in both states is determined. Rotationally resolved competitive predissociation into two distinct chemical channels, Si+ + H2 and SiH+ + H is observed.

A b i n i t i o investigation of the 3Π states of NO+

MCSCF+CI calculations on the five lowest 3Π states of NO+ strongly indicate transitions to the b 3Π state have been observed in the laser photofragment spectroscopy experiment by Cosby and Helm [J. Chem. Phys. 75, 3882 (1981)]. The 2 3Π state is shown to have a long range minimum bound with respect to separated atoms and a local minimum near R=1.2 Å which gives rise to a quasibound state 0.81 eV above the N+(3P)+O(3P) asymptote or 21.84 eV above ground state NO. This quasibound state is responsible for the prominent feature at 21.7 eV in the photoelectron spectrum of NO, which was previously assigned to a c 3Π state.

Laser photofragment spectroscopy of N+2: Evidence for predissociation of B 2Σ+u

Band progressions of a Σ←Σ system showing predissociation in N+2 are observed using a coaxial fast-ion-beam-laser-photofragment spectrometer. The most consistent interpretation of these spectra is in terms of absorptions in the first negative system of N+2. Thus, the bands originate in five vibrational levels (v′′=32–36) of N+2 X 2Σ+g and populate quasibound states of at least three vibrational levels (v′′≳45) in the B 2Σ+u state which lie above the dissociation limit N+(3P0)+N(4S0). Measurements of the photofragment kinetic energies show that predissociation to the least exothermic fine-structure limit is favored.

Laser photofragment study of isomeric C4H6+ cations at 5145 Å

We present a study of the isomerization of seven isomeric C4H6+ cations using laser photofragment spectroscopy. Parent ions created by electron impact intersect the 5145 Å beam of an argon ion laser and the C3H3+ photofragments, the only fragments observable, are energy-analyzed by an electrostatic analyzer. On the basis of C3H3+ photofragment recoil spectra, we argue that five isomerically distinct neutrals 1,3-butadiene, 1,2-butadiene, cyclobutane, 1-methylcyclopropene, and 3-methylcyclopropene, form parent ions which appear to rearrange to a common precursor prior to fragmentation. Neither 1-butyne nor 2-butyne cations photodissociate at this wavelength. The dienes 1,3-butadiene and 1,2-butadiene show a minor fragmentation channel to a higher energy product, which we hypothesize to be the propargyl cation. The very high kinetic energy release upon fragmentation (most probable values of 100–200 MeV) cannot be rationalized on the basis of a statistical model of vibrational predissociation in the ground electronic state. The results are also inconsistent with PIPECO fragmentation studies of 1,3-butadiene ions at comparable excess energies, in which substantially lower kinetic energy releases, consistent with a statistical theory, were observed [Klots, Mintz, and Baer, J. Chem. Phys. 66, 5100 (1977)]. We argue that the high translational energy release observed here is consistent with excitation to the first excited state of the parent cation on a portion of the potential surface, which is coupled strongly to a repulsive potential whose barrier is preferentially disposed of in product kinetic energy.

Laser photofragment spectroscopy of NO+. I. Predissociation of the 2 3Π state

NO+ ions, produced by electron impact on NO, are observed to predissociate into O+(4S0)+N(4S0) when irradiated by a dye laser at wavelengths between 6600–5650 Å. The highly structured wavelength dependence of the photofragments reflects absorptions from three vibrational levels in a long-lived NO+ electronic state to 20 vibrational levels of a predissociated electronic state. The transitions are tentatively identified as 2 3Π(v′ = 0–19)←b′ 3Σ−(v″ = 8–10). The weakly bound 2 3Π state is found to lie 0.698–0.925 eV above the lowest separated atom limit of NO+ and to adiabatically correlate to the N+(3P)+O(3P) limit. This state is also found to be homogeneously perturbed, most likely by the 3Π state arising from the O+(4S0)+N(2D0) limit, and to be predissociated by the α 5Σ+ state. The predissociation lifetimes for all rotational levels in v′ = 0–8 are ≥1.6 ns. Perturbations in b′ 3Σ−(v = 9) are discussed.