Vacuum ultraviolet laser spectroscopy. II. Spectra of Xe2 and excited state constants

Relaxation times measurement in single and multiply excited xenon clusters

Direct measurement of the rates of nonradiative relaxation processes in electronically excited xenon clusters was carried out. The clusters were created in a pulsed supersonic beam and two-photon excited by femtosecond laser pulses with a wavelength of 263 nm. The measurements were performed using the pump-probe method and electron spectroscopy. It is shown that relaxation of light clusters Xe_N (N < 15) predominantly occurs by desorption of excited xenon atoms with a characteristic time constant of 3 ps. Heavier electronically excited clusters (N > 10) vibrationally relax to the lowest electronically excited state at a rate of about 0.075 eV/ps. Multiply excited clusters are deactivated via energy exchange between excited centers with the ionization of one of them. The production of electrons in this process occurs with a delay of ∼4 ps from the pump pulse, and the process is completed in 10 ps.

Xe2 gerade Rydberg states observed in the afterglow of a microplasma by laser spectroscopy of a(3)Σ(+)(u)(1(u), O(-)(u)) absorption in the green (545-555 nm) and near-infrared (675-800 nm)

Bound←bound transitions of the Xe dimer at small internuclear separation (R < 4.0 Å) have been observed in the 545-555 nm and 675-800 nm spectral regions by laser spectroscopy in the afterglow of a pulsed Xe microplasma with a volume of ∼160 nl. Transient suppression of Xe2 A(1)Σ(+)(u)(O(+)(u)) --> X(1)Σ(+)(g)(O(+)(g)) emission in the vacuum ultraviolet (∼172 nm), induced by laser excitation of Ω(g) ← a(3)Σ(+)(u)(1(u), O(-)(u)) [Rydberg←Rydberg] transitions of the molecule, has confirmed the existence of structure between 720 and 770 nm (reported by Killeen and Eden [J. Chem. Phys. 84, 6048 (1986)]) but also reveals red-degraded vibrational bands extending to wavelengths beyond 800 nm. Spectral simulations based on calculations of Franck-Condon factors for assumed Ω(g) ← a(3)Σ(+)(u) transitions involving Ω = 0(±),1 gerade Rydberg states suggest that the upper level primarily responsible for the observed spectrum is an Ω = 1 state correlated, in the separated atom limit, with Xe(5p(6) (1)S0) + Xe(5p(5) 6p) and built on a predominantly A(2)Π3/2g molecular ion core. Specifically, the spectroscopic constants for the upper state of the 1(g) ← 1(u), O(±)(u) absorptive transitions are determined to be Te = 13,000 ± 150 cm(-1), ω'(e) = 120 ± 10 cm(-1), ω'(e)x'(e) = 1.1 ± 0.4 cm(-1), De = 3300 ± 300 cm(-1), and ΔR(e) = R'(e) = R''(e) = 0.3 ± 0.1 Å which are in general agreement with the theoretical predictions of the pseudopotential hole-particle formalism, developed by Jonin and Spiegelmann [J. Chem. Phys. 117, 3059 (2002)], for both the (5)1g and (3)O(+)(g) states of Xe2. These spectra exhibit the most extensive vibrational development, and provide evidence for the first molecular core-switching transition, observed to date for any of the rare gas dimers at small R (<4 Ǻ). Experiments in the green (545-555 nm) also provide improved absorption spectra, relative to data reported in 1986 and 1999, associated with Xe2 Rydberg states derived from the Xe(7p) orbital.

Photoelectron imaging of several 5d and 6p Rydberg states Xe2 and improving the Xe2(+) I(1∕2g) potential

Velocity map photoelectron imaging was used to study the photoionization of Xe(2) in several low-lying 5d and 6p Rydberg states. The Rydberg states were prepared by two-photon excitation and ionized by either one additional photon from the pump laser (2+1 ionization), or by one photon of a second color (2+1(') ionization). The 2+1 images and associated photoelectron spectra were consistent with previous results, although some adjustment of previously proposed equilibrium bond lengths was necessary to fit the spectra with Franck-Condon factor calculations. The 2+1(') images provided higher resolution photoelectron spectra and, in conjunction with the Xe(2)(+) potentials reported by Zehnder and co-workers [J. Chem. Phys. 128, 234306 (2008)] and the 6p and 5d Xe(2)∗ potentials calculated by Jonin and Spiegelmann [J. Chem. Phys. 117, 3059 (2002)], provided a means for improving the Xe(2)∗ potentials. New experimental data are also presented for photoionization populating the Xe(2)(+) I(1∕2g) state, and are used to provide a better description of its potential curve.

On the R-dependence of the spin-orbit coupling constant: Potential energy functions of Xe(2) (+) by high-resolution photoelectron spectroscopy and ab initio quantum chemistry

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of Xe(2) has been measured between 97 350 and 108 200 cm(-1), following resonant two-photon excitation via selected vibrational levels of the C 0(u) (+) Rydberg state of Xe(2). Transitions to three of the six low-lying electronic states of Xe(2) (+) could be observed. Whereas extensive vibrational progressions were observed for the transitions to the I(32g) and I(32u) states, only the lowest vibrational levels of the II(12u) state could be detected. Assignments of the vibrational quantum numbers were derived from the analysis of the isotopic shifts and from the modeling of the potential energy curves. Adiabatic ionization energies, dissociation energies, and vibrational constants are reported for the I(32g) and the I(32u) states. Multireference configurational interaction and complete active space self-consistent field calculations have been performed to investigate the dependence of the spin-orbit coupling constant on the internuclear distance. The energies of vibrational levels, measured presently and in a previous investigation (Rupper et al., J. Chem. Phys. 121, 8279 (2004)), were used to determine the potential energy functions of the six low-lying electronic states of Xe(2) (+) using a global model that includes the long-range interaction and treats, for the first time, the spin-orbit interaction as dependent on the internuclear separation.

High-resolution vacuum ultraviolet laser spectroscopy of the C 0+u ← X 0+g transition of Xe2

Rotationally resolved (1 + 1′), resonance-enhanced, two-photon ionization spectra of the C 0+u ← X 0+g transition of several isotopomers of Xe2 have been recorded. Rotational constants have been determined for the v′ = 14–26 levels of the C 0+u Rydberg state and the v′′ = 0 and 1 levels of the X 0+g ground state, and band origins have been determined with an absolute accuracy of 0.015 cm–1 for the transitions to the v′ = 14–26 levels of the C 0+u state of the 129Xe2, 129Xe–132Xe, and 131Xe–136Xe isotopomers. The equilibrium internuclear separation of the X 0+g ground state (Re = 4.3773(49) Å) was determined from the rotational constants of the v′′ = 0 and 1 levels. The analysis of the isotopic shifts of the band origins enabled the confirmation of the absolute numbering of the vibrational levels of the C 0+u state determined by Lipson et al. (R.H. Lipson, P.E. Larocque, and B.P. Stoicheff. J. Chem. Phys. 82, 4470 (1985)). A semiempirical interaction potential for the X 0+g ground state was derived in a nonlinear fitting procedure using the present spectroscopic results, the positions of the v′′ = 2–9 levels determined by Freeman et al. (D.E. Freeman, K. Yoshino, and Y. Tanaka. J. Chem. Phys. 61, 4880 (1974)) and experimental values for the second virial coefficient. The interaction potential is similar to previous semiempirical potentials but the dissociation energy (De = (196.1 ± 1.1) cm–1) differs from the value of 183.1 cm–1 determined in the latest ab initio calculation (P. Slavíček, R. Kalus, P. Paška, I. Odvárková, P. Hobza, and A. Malijevský. J. Chem. Phys. 119, 2102 (2003)).

Key words: high-resolution vacuum ultraviolet laser spectroscopy, rare gas dimers and their cations, photoionisation, Xe2, rotationally resolved electronic spectrum.