Lifetimes of the 4s4p 3 P 1 and 4s3d 1 D 2 states of Ca I

The lifetimes of the 4s4p ³ P ₁ and 4s3d ¹ D ₂ metastable states of Ca have been studied using the time-of-flight technique. Two kinds of observations were performed. First, the exponential decay of the fluorescence, using a (continuous) dc discharge for excitation and then the velocity distribution of the radiating atoms, using a pulsed discharge, were measured. From the combined results of these measurements the lifetimes were derived. The lifetimes of the 4s4p ³ P ₁ and 4s3d ¹ D ₂ states of Ca are determined to be 0.57±0.03 ms and 1.5±0.4 ms, respectively.     

Crossed-beám chemiluminescence. II. Kinetics and mechanisms for reactions of group IIA Metals in ground and metastable states with fluorine

Visible chemiluminescence technique under crossed-beam conditions has been applied to the study of the reactions of the group IIA metal atoms Mg, Ca, Sr and Ba in their ground state (¹S₀) or in an excited metastable state (³P_i, ³D_i or ¹D₂) with F₂. The monofluoride emission bands are the most prominent features in the chemiluminescence spectrum. Using higher fluorine densities radiation is observed from excited alkaline earth difluorides, which are shown to principally originate from secondary reactions. Formations of MF* are determined to be first order with respect to both the metal atom and the fluorine molecule. Total cross sections for removal of ground state metal atoms from the beam by F₂ are 115 ± 15Ų for Ca, 125 ± 15Ų for Sr, and 160 ± 15Ų for Ba, which is consistent with an electron jump model. Chemiluminescence cross sections are reported for the reactions involving electronically excited reactants M*. Photon yields of 12 ± 3% for Mg*, 18 ± 5% for Ca*, 20 ± 5% for Sr*, and 15 ± 8% for Ba* reacting with F₂ are measured. These high photon yields are remarkable when compared with absolute photon yields for the ground state reactions which indicate that less than 2% of the products are MF* molecules. It was possible to obtain vibrational state distributions for some of the excited monofluorides which are found to be populated in a non-thermal manner. This strongly suggests that the dynamics of the reactions are governed by a direct mechanism. From the crossed-beam chemiluminescence spectra the dissociation energies of the ground state monofluorides are estimated. In addition, improved spectroscopic constants, dissociation energies and dissociation products of some of the excited electronic states of MF are given.     

Lifetimes and Quenching of the Ā 2A′(υ3′ = 0 – 2) → X̄ 2A″ Fluorescence of HSO

Fluorescence lifetimes of vibrational levels of the first electronically excited state (ā ² A′) of HSO, determined by the technique of discharge flow and laser-induced fluorescence, are 81 ± 10, 74 ± 8, and 76 ± 14 μs for υ₃′ = 0, 1 and 2, respectively. The rate coefficients of quenching of each level by He, N₂, O₂ and O₃ were measured; the coefficients for HSO (ā, υ₃′ = 0 – 2) quenched by O₃ arc 35–50 times as great as those of HSO by He.     

Investigation of the molecular chemiluminescence SrCl(A2Π1/2,3/2, B2Σ+ → X2Σ+) and the atomic resonance fluorescence Sr(53P1 → 51S0) in the time-domain following the pulsed dye laser generation of Sr(53 PJ) in the presence of CH2Cl2

Time-resolved investigations of the atomic resonance fluorescence Sr(5³P₁ → 5¹S₀) and the molecular chemiluminescence from SrCl(A²Π_1/2,3/2, B²Σ⁺ → X²Σ⁺) are reported following the reaction of the electronically excited strontium atom, Sr(5s5p(³P_J)), 1.807 eV above its 5s²(¹S₀) electronic ground state, with CH₂Cl₂. The optically metastable strontium atom was generated by pulsed dye-laser excitation of ground state strontium vapor to the Sr(5³P₁) state at λ = 689.3 nm (Sr(5³P₁ ← 5¹S₀)) at elevated temperature (850 K) in the presence of excess helium buffer gas in which rapid Boltzmann equilibration within the 5³P_J manifold takes place. Sr(5³P_J) was then monitored by time-resolved atomic fluorescence from Sr(5³P₁) at the resonance wavelength together with chemiluminescence from electronically excited SrCl resulting from reaction of the excited atom with CH₂Cl₂. The molecular systems recorded in the time-domain were SrCl(A²Π_1/2 → X²Σ⁺) (Δν = 0, λ = 674 nm), SrCl(A²Π_3/2 → X²Σ⁺) (Δν = 0, λ = 660 nm), and SrCl(B²Σ⁺ → X²Σ⁺) (Δν = 0, λ = 636 nm). Both the A²Π (179.0 kJ mol^?1) and (B²Σ⁺(188.0) kJ mol^?1) states of SrCl are energetically accessible on collision between Sr(³P) and CH₂Cl₂. Exponential decay profiles for both the atomic and molecular (A,B – X) chemiluminescence emission are observed and the first-order decay coefficients characterized in each case. These are found to be equal under identical conditions and hence SrCl(A²Π, B²Σ⁺) are shown to arise from direct Cl-atom abstractions on reaction with this halogenated species. The combination of integrated molecular and atomic intensity measurements, coupled with optical sensitivity calibration, yields estimations of the branching ratios into the A_1/2,3/2, B, and X states arising from Sr(5³ P_J) + CH₂Cl₂ which are found to be as follows: A_1/2, 3.0 × 10^?3; A_3/2, 1.7 × 10^?3; B, 4.4 × 10^?4 yielding ΣSrCl(A_1/2 + A_3/2 + B) = 5.1 × 10^?3. As only the X, A and B states of SrCl are accessible on reaction, this indicates an upper limit for the branching ratio into the ground state of 0.995. The present results are compared with previous time-resolved measurements on SrF, Cl, Br(A²Π,B²Σ⁺ ? X²Σ⁺) that we have reported on various halogenated species and with analogous chemiluminescence studies on Sr(³P) with other halides obtained from molecular beam measurements. The results are further compared with those from a series of previous analogous investigations in the time-domain we have presented of molecular emissions from CaF, Cl, Br, I (A,B – X) arising from the collisions of Ca(4³P_J) with appropriate halides and with branching ratio data for Ca(4³P_J) obtained in beam measurements. © 1995 John Wiley & Sons, Inc.     

Collisional Deactivation of K(7s2S) and K(5d2D) by No

Radiative lifetimes and total deactivation cross sections of K(7²S) and K(5²D) by collision with NO are studied. The K atomic vapor in either the 7²S or the 5²D state was prepared by two- photon absorption using a dye laser. The decay signal of the time-resolved fluorescence from the 7²S – 4²P_1/2 or 5²D – 4²P_3/2 transition was then monitored. Based on a Stern-Volmer analysis, the radiative lifetimes are 155 ±8 ns and 561 ± 18 ns for the K(7²S) and K(5²D) states, respectively. The total deactivation cross sections are 88 ±1Ų and 70 ±2Ų for the K(7²S)-NO and K(5₂D)-NO collisions, respectively. In the absence of NO collisions the radiative lifetimes obtained in this work show excellent agreement with those previously reported. The quenching cross sections for NO have been measured for the first time, and have values in a reasonable range, when compared with Na-N₂ collisions.     

Observation of a long-lived nitrogen beam afterglow and lifetime measurements on the N2(W3Δu) state

Nitrogen molecules in a nozzle excited by electron using a strong dc discharge through the beam itself. Intense emission of the first positive band system of N₂, B³Π_g → A³Σ⁺_u, was found to be emitted well downstream from the discharge. The intensity decrease with distance s from the source was measured from s = 27 to 56 cm for various isolated bands. Together with time-of-flight measurements, this yielded effective lifetimes for the B-state vibrational levels υnt' = 1 to 5 of 300-100 μs. Also the spectral distributions were measured as a function of s. Both sets of data can be consistently and quantitatively explained by radiative cascades populating N₂(B) from the long-lived N₂(W³Δ_u) precursor state. Recent ab initio lifetimes for W → B transitions are in very good agreement with the observations, while older lifetime data can be ruled out. The N₂(W) flux density is determined to be (5–10) × 10¹² cm⁻²s⁻¹     

Analysis of the molecular bands of D2H and H2D at 5600 Å

Spectral simulation was used to analyze the molecular rovibrational bands of D₂H and H₂D at 5600 Å. These bands were previously measured by the ion beam neutralization method. They were assigned to the electronic 3p²B₁ ? 2s²A₁ and vibrational (ν - ν″) = (0, 0, 0,-0, 0, 0) transitions. Least squares fits to the experimental line-positions were made to determine the asymmetric rotator constants A, B and C for the 2s²A₁ and 3p²B₁ ν = 0 states of D₂H and H₂D, hitherto unknown. Lorentz line-profiles were assumed for the D₂H and H₂D rotational lines, whose widths are mainly governed by the lifetimes of the lower states. The bands at 5600 Å were simulated and the 2s²A₁ state lifetimes were estimated to be σ ≥ 0.5 ± 0.2 ps for D₂H and σ ≥ 0.4 ± 0.2 ps for H₂D. Vibrational constants of D₃ and D₂H in the 2s²A₁ states are determined from the positions of the 0-0 and 0-1 vibrational bands given in respective experimental spectra previously measured. For the first time the vibrational constants ω₁ and ω₂ of the 2s²A₁ state of H₂D were estimated from the positions of the 0-0 and 0-1 band maxima. These vibrational constants are compared with the corresponding vibrational constants of their ions.     

Fluorescence lifetimes and predissociation of the A3Π state of SH+

The high frequency deflection (HFD) technique has been used to obtain fluorescence lifetimes of the SH⁺ A³Π state. The A³Π?X³∑^? electronic transition moment function has been calculated with the complete active space SCF(CASSCF) method. Radiative lifetimes calculated with this transition moment function are compared with measured lifetimes and found to be in good agreement (τ_exp(v′=0)=1.4±0.3 µs; τ_calc=1.6 µs). Deviations between measured and calculated lifetimes forv′=0,J′ ≥ 15 andv′=1,J′=1?7 are interpretated as due to a predissociation caused by a repulsive⁵∑^? state. A simple model, based on a perturbation approach to the predissociation, is used to obtain quantitative estimates of predissociation rates. These calculated rates are in reasonable agreement with the predissociation rates that are inferred from the measured and calculated lifetimes.     

Lifetimes of the vibronic Ã2A1 states of H2O+ and of the 3Πi (υ′ = 0) state of OH+

Using the delayed coincidence technique, lifetimes have been measured for some Σ and Π vibronic òA₁ states of H₂O⁺ and for the ³Π_i (υ′ = 0) state of OH⁺ by analysing the decay curves of the òA₁(0, υ′₂, 0) ? X?²B₁ (0, υ″₂, 0) and the ³Π_i(υ′ = 0) ? ³Σ^?(υ″ = 0) emission intensities respectively. The excited molecular ionic states are produced via excitation of H₂O molecules by 200 eV electrons. For H₂O⁺(òA₁) the vibronic Σ levels with υ′₂ = 13 and 15 and the vibronic Π levels with υ′₂ = 12 and 14 have been considered. The radiative lifetimes obtained for these levels have about the same value, namely 10.5(±1) × 10^?6 s. The radiative lifetime for the OH⁺(³Π_iυ′= 0) state is 2.5(±0.3) × 10^?6 s. The lifetimes found in this work for H₂O⁺(òA₁) and OH⁺(³Π_i,υ′= 0) are about ten and three times longer respectively than the corresponding lifetimes given by other investigators [1,2]. The probable reason for this discrepancy is that in the other experiments no attention has been paid to the presence of a large space charge effect. This effect is caused by the positive ions which are created by the primary electron beam.     

Potential energy curves and spectral properties for electronic states of F2 and F+2

Potential energy (PE) curves for the Rydberg states of F₂, and for the ground and lowest two electronic states each of symmetry ²Π_g,u, ²Δ_g,u and ²Σ^±_g,u of F⁺₂, have been obtained using modest-sized configuration-interaction calculations. These PE curves have been used to calculate spectroscopic constants for the electronic states and the results agree reasonably well with the limited experimental and theoretical results previously reported. The theoretical PE curves for the Rydberg states of F₂ are found to be strongly perturbed by valence-Rydberg-ionic interactions and these perturbations appear to be responsible for certain features in recently reported electron energy-loss spectra in F₂. The corresponding electronic wavefunctions have been used to calculate the electronic transition moment, as a function of the internuclear distance, for dipole-allowed transitions between the lowest excited electron state of each symmetry and the appropriate ground electronic state. The radiative emission probabilities, natural lifetimes, and absorption oscillator strengths, for each band system, are also reported here. The predicted lifetimes for vibrational levels of the A ²Π_u of electronic state in F⁺₂ vary from 1.3–1.5 μs and agree reasonably well with the single available set of measurements. The predicted radiative lifetimes for the higher electronic states of F⁺₂ are substantially longer and fall into the range 5–100 ms.     

Isotope shifts and hyperfine structures investigation of doubly excited levels in SrI

We measured isotope shifts and hyperfine structure of visible transitions of stable strontium isotopes by means of Doppler-free saturated absorption spectroscopy. In particular, we investigated transitions between excited states where the upper level involves two excited electrons. We report hyperfine coupling constants for the levels 5p4d¹D₂, 5p4d³F₂, 5s4d³D₁, 5s6s³S₁, 5s5p³P₁ and, for some of the studied transitions, we separate the specific mass and volume contribution to the isotope shifts.     

Lifetimes of the vibronic Ã2A1 states of H2S+

Lifetimes have been measured for the Σ and Π vibronic òA₁ states of H₂S⁺ by studying the decay curves of the òA₁ (0, υ′₂, 0) → X? ²B₁ (0, υ″₂, 0) emission bands. The vibronic òA₁ states are produced via excitation of H₂S molecules by 150 eV electrons. The Σ sublevels 1 ? υ′₂ ? 7 and the Π sublevels 3 ? υ′₂ ? 6 have been considered. Predissociation occurs in the Σ sublevels for υ′₂ ? 7 and in the Π sublevels for υ′₂ ? 6. The obtained radiative lifetimes for the non-predissociated Σ and Π sublevels are around 4.2(±0.4) × 10^?6 s and 5.6(±0.5) × 10^?6 s respectively. For the predissociated Σ(0, 7, 0) and Π(0, 6, 0) levels the corresponding lifetimes are 2.3(±0.3) × 10^?6 s and 1.6(±0.3) × 10^?6 s respectively. The rate constant for collisional deactivation (quenching) of the vibronic òA₁ states by H₂S molecules was found to equal 2.3(±0.3) × 10^?9 cm³ mol^?1 s^?1.     

Spectroscopy and kinetics of the 1P1 and 3P2 states of Hg 6s6p in xenon

The spectra and kinetics observed following excitation of Hg 6s6p(¹P₁) in xenon show the occurence of complex attachment and relaxation processes. The ¹P₁ attaches to Xe in termolecular collisions to produce HgXe E¹1, which emits a broad band with λ_max ≈ 2150 Å. Addition of krypton to Hg, Xe mixtures enhances the E-state emission by atom exchange with an HgKr^* complex. The E state also undergoes collisional deactivation by Xe, rate coefficient (1.75 ± 0.25) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, to generate ³P₂. However, the predominant route for ³P₂ formation is via collision of ¹P₁ with Xe, rate coefficient ≈ 4 × 10⁻¹² cm³ molecule⁻¹ s⁻¹. With [Xe] at 760 Torr, about 75% of the generated ³P₂ is attached in the form of the HgXe(C ³2) complex; lifetimes up to 600 μs have been observed. Two new emission bands occur when ³P₂ is prepared in xenon. A sharp feature, slightly blue-shifted from the forbidden ³P₂:¹S₀ line, results when ³P₂ and Xe approach on the D ³1 surface. The other band is broad with λ_max ≈ 2520 Å; the carrier is assigned to an HgXe₂(³2_u) complex. Rate coefficients for deactivation of ³P₂ to lower ³P_J states have been measured.     

Spectra and emission lifetimes of H2CS(Ã 1A2)

Fluorescence spectra and lifetimes of single vibronic levels of the first excited singlet state of H₂CS have been measured under effusive flow conditions. Fluorescence lifetimes of the single vibronic levels decrease from 140±3 μs (0°) to 68 μs(3¹4¹5¹) with increasing excitation energies. The promoting vibrational modes for the non-radiative transition are considered to be the out-of-plane bending (ν₄) and the asymmetric rocking (ν₆) modes rather than the asymmetric stretching mode (ν₅).     

Two-step laser optogalvanic spectroscopy of highJ momentum 4dnd and 4dng autoionizing states of strontium

We have measured the energies of about two hundred even parityJ=3–5 autoionizing 4dnd and 4dng Rydberg states of strontium (Sr) using an optogalvanic method. These states are reached by a two-step dye laser excitation from the 4d5s metastables through the 4d5p ³ P ₂,¹ F ₃,³ F ₄ intermediates. The 4d5s are populated in a d.c. glow discharge through a Sr heated cell. The electronic configuration of the observedJ=3, 4 states is deduced from the Lu-Fano plots of their quantum defect values and the spectral characteristics of the corresponding transitions.     

Detection of the even parity,J=0–3, autoionizing 4dnl Rydberg states of strontium by two-step laser optogalvanic spectroscopy

We have measured the energies of a few hundreds of even parity,J=0–3 autoionizing 4dnl Ry states of strontium using an optogalvanic spectroscopy technique. These states are reached by a two-step pulsed dye laser excitation from the 4d 5s metastables through the 4d 5p ³P_0,1,2 intermediate states. Electronic collisions populate the 4d 5s states in a d.c. glow discharge through a Sr plus He vapour in a heated quartz cell. The electronic configuration for the majority of the observed Sr states is deduced from their quantum defect values and other characteristics of the corresponding transitions.     

The production and subsequent relaxation of vibrationally excited OH in the reaction of atomic oxygen with HBr

A fast discharge flow apparatus equipped for EPR detection of radicals has been used to investigate the reaction O + HBr → OH + Br. At 295°K, measurements showed that more than 97% of all OH produced in this reaction was formed initially in its first vibrationally excited state. Rate constants for physical deactivation of OH(v = 1) by O(³P), Br(²P_3/2), H₂O, and HBr were measured as (1.45 ± 0.25) × 10^?10, (6.4 ± 2.4) × 10^?11, (1.35 ± 0.50) × 10^?11, and < 10^?12 cm³/molec·sec, respectively.     

Ab initio transition probabilities,band strengths and lifetimes for the lowest-lying vibrational states of Li+3

By calculating the dipole moments at 65 discrete points on a dipole moment hypersurface and by fitting a power series analytical function to these points, we have determined the ab initio transition probabilities, band strengths and lifetimes for the ten lowest-lying states of ⁷Li⁺₃ (⁶Li⁺₃). Of these, the fourth and tenth states were found to have long lifetimes of the order 4907 s(3344 s) and 1648 s(1093 s) respectively which is in accordance with the fact that transitions from these states to the ground state are dipole forbidden.     

Laser fluorescence spectroscopy and lifetime of the 2B1 (K′ ⪢ 0) electronic state of NO2

Fluorescence of NO₂ excited by HeCd 442 nm laser radiation is found to exhibit a spectrum characteristic of perpendicular transitions from several levels belonging to ²B₁ (K′ ? 0) vibronic states. The lifetime of a level K′ = 4, N′ = 16 ± 1 is 36 μs, substantially greater than lifetimes given previously for K′ = 0 levels of the ²B₁ state. This result supports the mechanism of lifetime lengthening by the Renner interaction of the ²B₁ and ²A₁ components of the linear ²Π_u state.     

Lifetime of the 4D 3/2 and 4D 5/2 metastable states in Sr II

Sr⁺ ions were confined in a r.f. quadrupole trap for times of the order of 30 min. The metastable 4D states were populated via laser excitation of the 5P states. The weak quadrupole transition rate into the 5S _1/2 ground state at 674 and 687 nm was deduced from observation of the exponential decay. At background pressures above 10^?7 mbar the radiative decay is dominated by collisional quenching. Extrapolation of the observed decay rate to zero background pressure yields the radiative lifetimes. At pressures around 10^?6 mbar fine structure mixing collisions between the 4D states have been observed, which lead to corrections of the extrapolated lifetimes. As the final result we obtain 395±38 ms for 4D _3/2 and 345±33 ms for 4D _5/2. These results are somewhat higher than theoretical predictions.