R.J. Donovan

Vacuum ultraviolet fluorescence excitation spectrum of I2

A detailed study of the vacuum ultraviolet fluorescence excitation spectrum of I2 in the region 110–210 nm is reported. Both absorption and fluorescence excitation spectra were recorded simultaneously using continuously tunable synchrotron radiation. At long wavelengths (176–210 nm) the fluorescence excitation spectrum is dominated by the D(0+u) ion‐pair state. Perturbations between the D(0+u) state and isoenergetic levels of the c6 Rydberg state lead to pronounced resonances (dips) in the fluorescence excitation spectrum. Weaker absorption to the F(0+u) and F’(0+u) ion‐pair states is identified at 170 and 150 nm, respectively. Below 149 nm electronically excited iodine atoms (I 6s 4P5/2 and I 6s 2P3/2) are formed by predissociation of high‐lying Rydberg and ion‐pair states.

Time-resolved kinetic studies of electronically excited CH radicals II. Quenching efficiencies for CH(A 2Δ)

Abstract Quenching of CH(A 2 Δ) by the molecules H 2 , CO, NO, O 2 , N 2 O, C 2 H 2 , C 2 H 4 , C 2 H 6 , C 3 H 8 and C 4 H 10 has been studied using direct, time-resolved, techniques. CH(A 2 Δ) was produced by ultraviolet laser (193 nm) multiphoton excitation of (CH 3 ) 2 CO and CHBr 3 : fluorescence from CH(A 2 Δ → X 2 Π), at 430 nm, was then monitored as a function of time using a fast (10 ns per channel) transient recorder. The kinetic data obtained with both precursor molecules agree closely. The rate constants for total removal of CH(A 2 Δ) are compared with those for the ground state, CH(X 2 Π), and are shown to be smaller in every case, except for CO where slow termolecular removal of CH(X 2 Π) takes place. A broadly parallel kinetic behaviour for CH(A 2 Δ), CH(X 2 Π) and the equivalent united atom state, N( 2 D), is found: the order of reactivity is CH(X 2 Π) > CH(A 2 Δ) > N( 2 D).

Double resonance ionisation nozzle cooled spectroscopy (DRINCS) of the E(3P2), f(3P0) and f′(1D2) O+g ion-pair states of I2

Abstract The double resonance ionisation nozzle cooled spectroscopy (DRINCS) technique has been used to extend the existing vibrational constants for the lowest three ion-pair states of I 2 with 0 + g symmetry using the B state as the first resonant intermediate. The E( 3 P 2 ), f( 3 P 0 ) and f′( 1 D 2 ) states now cover the range ν′ = 0–422, ν′ = 0–228 and ν′ = 0–166 respectively. Potential energy curves beyond the previously existing RKR potentials have been generated using only the vibrational Dunham coefficients and the known asymptotic behaviour of the Coulombic branch of the potentials. These potentials have been used to simulate the DRINCS spectra and correctly reproduce the observed intensity minima in the envelope of the vibrational progression below 56000 cm −1 . Above this the band intensities are very irregular because of homogeneous interactions with Rydberg states. The lifetimes of the f′(ν′ = 0) state has been determined as 142±6 ns. The reason for this unusually long lifetime is discussed in terms of the electronic structure of the I + ( 1 D 2 ) cation and the B( 3 Π u O + u ) state. The electronic transition dipole moment function, μ f′ → B ( R ), over the range 3.51–4.23 A has also been determined by simulation of the dispersed fluorescence from the ν′ = 11 level of the f′(O + g ) ion-pair state and shows a maximum at 4.05 A.

Two-photon excitation of h2o and D2O with a KrF laser (248 nm): photofragment fluorescence from OH and OD(A2∑+)

Photofragment fluorescence. From OH and OD(A 2∑+→X2π), has been observed following two-photon excitation of H2O and D2O in the gas phase with a KrF laser (248 nm). The rotational band contour of the OH fluorescence is the same as that observed following single-photon vacuum ultraviolet photolysis of H2O at 123.6 nm.

Zero kinetic energy pulsed field ionization (ZEKE‐PFI) spectroscopy of electronically and vibrationally excited states of I+2: The A 2Π3/2,u state and a new electronic state, the a 4Σ−u state

Both one‐ and two‐color zero kinetic energy‐pulsed field ionization (ZEKE‐PFI) spectra of the first electronically excited state of I+2 (A 2Π3/2,u) as well as a new electronic state, the a 4Σ−u state, have been recorded for the first time. In the one‐color (two photon) experiment, transitions to the quartet state are formally spin‐forbidden and this is reflected in the weak transition intensity observed compared with the A 2Π3/2,u state. However, in the two‐color (1+2′) experiment, which used the valence B 3Π0+u state as an intermediate, transitions into both A 2Π3/2,u and a 4Σ−u states are fully allowed and appear in the spectrum with comparable intensity. The a 4Σ−u state appears in the one‐color experiment by virtue of spin–orbit coupling with excited electronic states for which direct ionization from the neutral ground state is fully allowed. Values for ωexe of 0.46±0.01 cm−1 for the A 2Π3/2,u state and 0.38±0.02 cm−1 for the a 4Σ−u state were derived, together with lower limits for ωe of 138±2 and ...

Resonance fluorescence study of electronically excited sulphur atoms: reactions of S(31D2)

Abstract The direct observation of S(3 1 D 2 ) using time-resolved resonance fluorescence is reported. Rate data for the removal of this excitcd state by CS 2 , OCS and CH 4 are presented, and compared with data obtained using absorption techniques. S(3 1 D 2 ) was produced by photolysis of CS 2 at 193 nm (O 1 D) ≈ 15XXX) using an ArF laser.

Efficient laser action on the 342-nm band of molecular iodine using ArF laser pumping

Strong laser action on the 342‐nm band of I2 has been obtained by transverse pumping I2/SF6 mixtures at 193 nm with an ArF laser. The highest output energy obtained was 230 mJ at an intrinsic energy conversion efficiency of approximately 30% equivalent to a photon efficiency of greater than 50%.

ZERO KINETIC ENERGY PHOTOELECTRON SPECTROSCOPY OF RYDBERG EXCITED MOLECULAR IODINE

Two color (2+1’) zero kinetic energy pulsed field ionization (ZEKE‐PFI) spectra of I2 ionized via a number of gerade Rydberg excited states have been recorded for the first time. Ionization into both X 2Π3/2,g and X 2Π1/2,g spin–orbit ionic substates was achieved by selective excitation of Rydberg states having the appropriate spin–orbit ionic core. Adiabatic ionization energies have been determined as 75 069±2 cm−1 for the X 2Π 3/2,g state and 80 266±2 cm−1 for the X 2Π1/2,g state with ωe values for the two states determined as 240±1 and 229±1 cm−1, respectively. The ZEKE‐PFI spectra recorded for the lower spin–orbit state exhibit non‐Franck–Condon behavior which has been interpreted in terms of an autoionization mechanism. The ZEKE‐PFI technique has also been used to probe the extent of Rydberg and ion‐pair character in a strongly mixed Rydberg/ion‐pair state.

Direct kinetic study of CH(A 2Δ)

Abstract Time-resolved kinetic studies of CH(A 2 Δ), following formation by ultraviolet multiphoton excitation of (CH 3 ) 2 CO and CH 2 I 2 , are reported. Absolute rate constants (300 K) for removal of CH(A 2 Δ) by H 2 , O 2 and CH 4 are determined as (8.5 ± 0.4)× 10 −12 , (1.10 ± 0.04)× 10 −11 and (2.0 ± 0.1)× 10 −11 cm 3 molecule −1 s −1 , respectively. In each case the rate constant for removal of CH(A 2 Δ) is less than that for the ground electronic state. CH(X 2 Π).

Photofragment fluorescence following ultraviolet laser multiple-photon excitation of CH3X molecules (X = OH, Br, I)

Abstract Photofragment fluorescence following excitation of the molecules CH 3 I, CD 3 I, CH 3 Br and CH 3 OH, with an inert-gas halogen exciplex laser, is reported. Strong fluorescence from the A 2 Δ and B 2 Σ − states of CH and CD, and atomic iodine fluorescence, is observed. The mechanism for the excitation process is discussed and it is suggested that simultaneous two-photon absorption is the dominant mechanism in these systems.