K.P. Lawley

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.

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 ...

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.

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.

Analysis of the 350–400 nm oscillatory continuum from I2 (D 1Σ+u)

Abstract The 350–400 nm oscillatory continuum, observed in emission when I 2 is excited to the D 1 Σ + u state (λ = 193 nm, u′ ≈ 134), has been analysed. The lower potential state, which correlates with two ground-state 1 atoms, is found to be purely repulsive between 2.75 and 3.8 A, the range defined by the observed fluorescence. The band is partially overlapped by another system and there are two possible positions for the band origin, leading to very similar lower states.

Determination of the radiative lifetimes of nine ion-pair states of I2

Abstract The radiative lifetimes of 9 out of the 20 possible ion-pair states of I2 have been measured using two-colour optical-optical double-resonance excitation. A pulse-shortened dye laser was used to populate the final state. Lifetimes for all six states in the first cluster - δ(2u), γ(1u), D(0u+), D′(2g) β(1g) and E(0g+) - have been measured again in view of the often d literature values. The lifetimes of three states in the second ion-pair cluster (H(1u), F(0u+) and f(0g+)) are reported for the fir time. No significant pressure dependence was found up to 100 m Torr, with the exception of the longest lived γ state.

High-resolution threshold photoelectron spectroscopy of hydrogen iodide

The threshold photoelectron spectrum of hydrogen iodide has been recorded at high resolution (4–30 meV) using synchrotron radiation and a penetrating-field electron spectrometer over the photon energy range 10.2–31.0 eV. Extended vibrational structure in the first Franck-Condon gap region between the X (2Πi) and A (2Σ+) states of HI+ has been observed and analyzed to yield improved spectroscopic constants for the X (2Πi) state. The Rydberg states responsible for the production of this structure, through resonance autoionization, are identified as [A (2Σ+]nsσ 1Σ+ for n = 6–8. A vibrational progression in the A (2Σ+) band system in HI+ leading up to the dissociation continuum of this state has been identified for the first time, yielding spectroscopic constants for the state. The A (2Σ+) state of HI+ appears to be formed by an avoided crossing between the diabatic A (2Σ12+) and 4Π12 potentials. Spectroscopic constants are derived for the ‘main’ band system for 5sσ−1 inner-valence ionization of HI.

RYDBERG EXCITED IODINE-ARGON VAN DER WAALS COMPLEXES STUDIED BY RESONANCE ENHANCED MULTIPHOTON IONIZATION SPECTROSCOPY

Several new gerade Rydberg states of the I2–Ar van der Waals complex are reported in the region 53 000–69 000 cm−1 above the electronic ground state, using mass resolved (2+1) resonance enhanced multiphoton ionization (REMPI) spectroscopy. The spectra all exhibit anharmonic vibrational progressions in the I2...Ar van der Waals stretching mode, ν3, having typical ωe values in the range 49–62 cm−1, depending upon the principal quantum number of the Rydberg state involved. The observed increase in ωe and spectral red shift which accompanies the increase in principal quantum number is consistent with a progressive strengthening of the van der Waals bond, resulting from less effective shielding of the ion core by the Rydberg electron. The apparent good Franck–Condon overlap in the ν3 mode between the ground state and the Rydberg states together with the absence of any clear evidence of bending mode progressions in the spectra, indicates that the I2–Ar complex retains a T‐shaped geometry in all the Rydberg exci...

High-resolution threshold photoelectron spectroscopy of molecular fluorine

Abstract The threshold photoelectron spectrum of molecular fluorine has been recorded in the 5.6–21.9 eV photon energy range, at resolutions ranging from 3 to 12 meV, using synchrotron radiation and a penetrating-field electron spectrometer. In addition to observing the three known band systems of F 2 + at higher resolution than previously achieved with conventional photoelectron spectroscopy, extensive vibrational structure is found in the Franck-Condon gaps between the main electronic systems of F 2 + . This extended vibrational structure is attributed to resonance autoionization of neutral Rydberg states.

Vibronic coupling between Rydberg and ion-pair states of I2 investigated by (2+1) resonance enhanced multiphoton ionization spectroscopy

Abstract Two striking examples of Rydberg/ion-pair state coupling have been observed in the spectrum of I 2 in the regions 62500–64000 and 56500–59500 cm −1 . The coupling is of intermediate strength and the (2+1) REMPI spectra of the resonant Ry ← X transitions have ion-pair satellites around each Rydberg vibrational level from which they have borrowed intensity. The Rydberg states are identified as the [ 2 Π 3 2 ] c 7 s ;1 g and [ 2 Π 3 2 ] c 5 d ; 2 g and the ion-pair states as the β1g and D′2g. Curve crossings occur close to the ν ′ = 532 level of the β state and ν ′ = 311 of the D′ state respectively. The spectra are modelled by solving the two-state coupling problem in a diabatic basis and in order to deduce approximate values of the coupling matrix elements, extrapolations of the known portions of the potential energy curves of the β1g and D′2g ion-pair states are necessary. The diagnostic value of the magnitude of the electronic part of Rydberg/ion-pair coupling matrix elements in probing the electronic configuration of ion-pair states at short range is discussed.