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The resonance-enhanced multiphoton ionization (REMPI) spectra of atomic chlorine and bromine in the range of 230-245 nm and 250-285 nm were obtained by using REMPI and time of flight (TOF) techniques. There are twenty-six lines for Cl and twenty-three for Br in the spectra, among which five transitions of Cl and four of Br are observed for the first time. These lines are due to transitions from the Cl(3p 2P3/20, 2P1/20) and Br(4p 2P3/20, 2P1/20) states to upper states induced by two-photon excitation. It is found that the Stark shift of different excited states are almost the same, and they are independent of the S, L, and J quantum numbers of the excited state.

In this paper, by using resonance enhanced (2+1) multiphoton ionization (REMPI) of AsH3 and detecting the daughter molecular ions AsH+ and As+, a long progression discrete structure was obtained from 267 to 291 nm, which obeys the formula v0(cm-1)=68875.3+504.1v'2+5.26v'22. Analysis of the spectrum has revealed that it belongs to 4d Rydberg state transition. Our assignment yields the 4d Rydberg state parameters Te'=67881.1 cm-1, we'=495.1 cm-1 and δ=0.962. Finally the 4d Rydberg state structure of AsH3 was discussed.

About 50 spectral lines due to the (2+1) resonance-enhanced multiphoton ionization (REMPI) of atomic sulfur, generated in the sequential multiphoton dissociation of SO2 at probing wavelengths, in the 243—263 nm region has been observed. Besides most of the lines which are due to the two-photon transition from the ground states 3P2,1,0 to the Rydberg states 3P, 3D, 3F until n=10 of S atom, several new lines including the spin forbbiden transition 1P0, 5S0←3P and the two-photon transitions from the excited state 1D2 to the Rydberg states np1,3D, 1,3F are observed. Intensities of some lines have been calculated and compared with the experimental results.

The (2+1) resonance-enhanced multiphoton ionization (REMPI) spectra of the CF2Cl radicals, which are generated by a pulsed dc discharge,were investigated between 332 and 362nm.Two new Rydberg states,4s and 4p,were observed and indentified.The observed states and spectroscopic constants include 4s Rydberg state (band origin ν0—0=55371cm-1,ω3′ (CF2 scissors)=523±7cm-1,ω′4 (OPLA,i.e.,out-of-plane bending mode)=734±4cm-1) and 4p Rydberg state (ν0—0=57601.3cm-1,ω3′=552±10cm-1,ω4′=762±12cm-1).The quantum defect values of 4s and 4p Rydberg states were also estimated.It seems that the radical production by a pulsed dc discharge combined with PEMPI technique is a powerful tool to study the electronic state of radicals.

We present a kinetic study of the reactions of the OH radical with the molecules HCl and DCl over the temperature range 300–700 K in order to investigate the ‘intermediate’ region between room-temperature measurements and flame investigations. Ground-state OH radicals were generated by the repetitive pulsed irradiation of H2O vapour in a flow system, kinetically equivalent to a static system, and monitored in the time-resolved mode by resonance fluorescence at λ= 307 nm [OH(A2∑+→X2Π), (0, 0)] following optical excitation. Decay profiles of the OH radical in the presence of HCl and DCl were constructed following pre-trigger photomultiplier gating, photon counting, signal averaging and computerised analysis. A specially constructed high-temperature stainless-steel reactor, particularly designed for photon-counting measurements on a high-temperature system and critical to the extension of previous rate measurements of such reaction of OH by ca. 250 K, is described in detail with particular emphasis for procedures for minimising large background photon counts normally encountered in this type of system. The resulting rate data obtained in this investigation for the range 300–700 K may be summarised in the Arrhenius forms: OH (X2Π)+ HCl: kR=(2.94 ± 0.48)× 10–12 exp[–(446 ± 32)/T] cm3 molecule–1 s–1, OH (X2Π)+ DCl: kR=(4.04 ± 0.74)× 10–12 exp[–(718 ± 33)/T) cm3 molecule–1 s–1.These data are compared with the results of temperature-dependent studies that have been carried out by direct monitoring of the OH radical, employing both resonance absorption and resonance fluorescence of OH(A–X), on pulsed systems and discharge–flow systems up to temperatures of 460 K. The extrapolation of these results to flame temperatures, for which rate data for the reaction between OH + HCl have been reported from mass-spectrometric sampling of stable molecules, is considered in some detail. The temperature dependence of the diffusional loss of OH(X2Π), which dominates removal of the radical in the absence of HCl or DCl, when fitted to the standard form kdiff=C + n ln (T) yields n= 1.77 ± 0.14. Estimates of the geometrical parameters for light collection of the resonance fluorescence signal, coupled with the measured temperature dependence for diffusional loss, in turn yields D12(OH—He)= 0.28 ± 0.06 cm2 s–1 at s.t.p. The observed temperature dependence is further considered quantitatively in terms of the appropriate Lennard-Jones parameters for interaction.

We present the first direct measurement of the absolute rate constant for the third-order reaction K + OH + He → KOH + He. A complex experimental system is described in which a heat-pipe oven, from which atomic potassium is generated, is coupled to a high-temperature reactor for time-resolved resonance-fluorescence measurements on OH following pulsed irradiation. The apparatus is a slow-flow system kinetically equivalent to a static system. Atomic potassium is monitored in the steady mode using resonance fluorescence of the Rydberg transition at λ= 404 nm K[(5 2PJ)–(4 2S1/2)] coupled with phase-sensitive detection. Ground-state OH(X2Π), generated by the repetitively pulsed irradiation of water vapour, is monitored in the time-resolved mode at λ= 307 nm [OH(A2∑+–X2Π), (0, 0)] using molecular resonance-fluorescence measurements following optical excitation with pre-trigger photomultiplier gating, photon counting and signal averaging. Thus the decay of OH(X2Π) as a function of time is studied both as a function of [K(42S1/2)] and [He], yielding the absolute rate constant k3(K + OH + He)=(8.8 ± 1.8)× 10–31 cm6 molecule–2 s–1(T= 530 K). A full account is given of the isolation of this reaction by the use of a ‘chemical window’ through the control of temperature and the effective elimination of the reaction between K2+ OH, and the use of He as the third body which demonstrates negligible collisional quenching efficiency with OH(A2∑+, v′= 0). We also present a detailed extrapolation of the rate data to the environment of flames using unimolecular-reaction-rate theory developed by Troe and coworkers for dissociation reactions. The agreement between the present results for the isolated reaction extrapolated to flame temperatures and measurements on flames (1800 < T/K < 2200), in which k3(K + OH + M)(where M stands for the burnt gases of a fuel-rich flame) has been extracted by modelling the complex coupled equilibria, is considered highly satisfactory. Quantitative account of the modelling of the dissociation of KOH is presented in the paper.

We present a direct kinetic measurement of the absolute third-order rate constant for the reaction Cs + I + He → CsI + He. (1) I(52P3/2) was generated by the repetitive pulsed irradiation of CsI produced in a flow system from the reaction of CH3I in the presence of excess atomic caesium derived from a heat-pipe oven. The ground-state iodine atom was monitored by time-resolved atomic resonance fluorescence at λ= 178.3 nm {I[5p46s(2P3/2)]–I[5p5(2P3/2)]} using photon counting in the vaccum ultraviolet region. Cs(62S1/2) was monitored by steady atomic resonance fluorescence of the Rydberg doublet at λ= 457 nm [Cs(72PJ)–Cs(62S1/2)] using phase-sensitive detection. We report a measured absolute third-order rate constant of k1(T= 491 K)=(7.9 ± 1.2)× 10–31 cm6 atom–2 s–1, which we believe to be the first measurement of its kind. We also report an estimate of the rate constant for the reaction I + Cs2→ CsI + Cs (3) of k3(T= 491 K)≈ 2 × 10–10 cm3 molecule–1 s–1, which is found to be in accord with previous kinetic measurements on I + Na2 and Br + K2. The crossing between the covalent and the ionic surface, described by the Rittner potential function, is found to take place at 17.3 A, and hence the recombination is governed by the large impact parameters involved on collision. Consideration of the splitting between the two surfaces, coupled with the Landau–Zener formalism and Monte Carlo calculations of trajectories on those surfaces, yields a good quantitative account of the observed kinetic behaviour, not only of the present recombination measurements but also of shock-tube data that have been reported for the dissociation of CsI at elevated temperatures (2400 K). In view of this, despite the clear third-order kinetic behaviour observed here for reaction (1), detailed theoretical consideration of the ‘fall-off’ pressure regime found to satisfy both the present measurements and the shock-tube data leads to the preferred value of k1(T = 491 K)= 9.1 × 10–31 cm6 atom–2 s–1(±25%). The combination of the recombination and dissociation-rate measurements and the modelling calculations lead to the temperature dependence of the form k1(491 < T/K < 2400)= 4.1 × 10–30T–0.24 cm6 atom–2 s–1.

We present the first direct measurement of the absolute third-order rate constant (k1) for the reaction Cs + OH + He → CsOH + He (T= 481 K) completing the series of analogous studies with the present experimental system we have also described for Na, K and Rb. OH(X2Π), generated by the pulsed irradiation of H2O vapour through CaF2 optics in a slow-flow system, kinetically equivalent to a static system, was monitored by time-resolved resonance fluorescence at λ= 307 nm [OH(A2Σ+–X2Π), (0, 0)] in the presence of an excess of atomic caesium derived from a heat-pipe oven and helium buffer gas. The atomic caesium was monitored in the steady mode using atomic resonance fluorescence of the unresolved Rydberg doublet at λ= 457 nm [Cs(72PJ– 62S1/2)] coupled with phase-sensitive detection. The following value of k1 was obtained: k1(481 K)=(10.0 ± 1.5)× 10–31 cm6 molecule–2 s–1. This result was extrapolated to 2000 K using the unimolecular reaction rate theory by Troe, including the quantitative effects of hindered rotation on the low-frequency bending modes of CsOH. We have employed the same formalism to calculate the rate constant for the reaction between Li + OH + He at 500 and 2000 K. The resulting calculated temperature dependences of the third-order rate constants (kR) for the reaction X + OH + He → XOH + He are thus compared: [graphic omitted]. Extrapolation of the rate data to flame conditions is considered with particular emphasis on the role of H2O as a third body in recombination reactions of this type. It is concluded that a reassessment of the standard collisional deactivation efficiency, βc, for H2O at elevated temperatures can be used to reconcile former differences between recombination rate data extrapolated from measurements of the present type with those derived from modelling on flames.