J. Le Calvé

De-excitation cross sections of metastable argon by various atoms and molecules

The rate constants for the de‐excitation of Ar(3P2) metastable atoms by many gases were measured, and the corresponding cross sections are calculated. These rate constants were measured in a static system using the N2(C 2Πu→ B 3Πg) emission band at 3371 A to detect the Ar(3P2). The results are compared with those obtained for the quenching of He(2 3S) metastable atoms and for photoabsorption at the argon resonance wavelengths. From these comparisons the de‐excitation cross section of metastable atoms appears to be a function of the long range forces and thus it can be presented as a function of the polarizability.

Pulse radiolysis study of argon‐nitrogen mixtures. Measurement of the rate constant of metastable argon de‐excitation by nitrogen

The postluminescence of N2(C 3Πu→ B 3Πg) induced by the transfer of energy to nitrogen by 3P2 metastable argon atoms obtained by the pulsed radiolysis of argon‐nitrogen mixtures has been studied spectrophoto‐electrically. The emission decay lifetime τ is controlled by the destruction of metastable argon atoms. The variation of 1/τ as a function of the argon or nitrogen pressure is used to determine the rate constants k1(Ar*+N2)=3± 0.3 × 10−11cm3sec−1 and k2(Ar*+2Ar)=6± 0.4 × 10−33cm6sec−1. The general reaction process is discussed. The results obtained seem to show that the metastables are formed mainly from the dissociative recombination of Ar2+ ions.

De‐excitation of the Ar 3P2, 3P1, and 1P1 states in pure argon and in Ar/H2 or Ar/CO mixtures

The de‐excitation of the 3P2, 3P1, and 1P1 states of argon was studied in pure argon between 10 and 200 torr and in Ar+CO and Ar+H2 mixtures. These states, which were followed by absorption spectroscopy, are populated after excitation of the gas by a short (20 ns) pulse of 500 keV electrons. The three body rate constants k2, which were not considered in previous low pressure studies, were measured for the two resonant states 3P1(k2=1.7×10−32 cm6 s−1) and 1P1 (k2=0.9×10−32 cm6 s−1). For the metastable 3P2, the measured value of k2 (1.6×10−32 cm6 s−1) is in good agreement with those found in the literature. However, our two body rate constant k1 for the 3P2 state is about ten times higher than values obtained previously from low pressure measurements. The rate constants for the quenching by CO and H2 were measured for the metastable state 3P2 (1.85 and 10.5×10−11 cm3 s−1) and for the resonant states 3P1 (4.5 and 20×10−11 cm3 s−1) and 1P1 (8.5 and 33×10−11 cm3 s−1).

The second conformer of the benzene—argon2 van der Waals complex

Abstract From a rotational contour study of UV vibronic bands of the benzene—Ar 2 complex as well as from theoretical structures modelling, conclusive arguments are presented, demonstrating the existence of a second (2, 0) stable conformer of C s symmetry, in addition to the already known symmetric (1, 1) species.

Formation of KrCl* and ArCl* molecules and radiative lifetimes of their B states investigated with selective synchrotron radiation excitation

Abstract Monochromatic pulsed VUV excitation of Cl2 or inert gas atoms in Cl2 doped Kr and Ar leads to formation of KrCl* and ArCl*. The radiative lifetimes of the B states (KrCl* 19 ns, ArCl* 9 ns) and rate constants for excimer formation, quenching and collisional mixing of B and C states are given. The radiative lifetime and the quenching rate increase with vibrational excitation of the B state (KrCl*).

Synchrotron radiation study of the molecular xenon fluorescence around 2000 Å

Time‐gated fluorescence spectra have been obtained in xenon excited by synchrotron radiation around the first resonance line. Evidence of the emission from the left turning point of the Xe2 excited Ou+ (3P1) potential curve is obtained between 2000 and 2100 A for different excitation wavelengths. From our results, we propose an experimental potential curve for the Ou+ state with an equilibrium distance Re of 3.25 A and a well depth De of 5000 cm−1. Franck–Condon factors have been calculated with this curve for the bound–free transitions between different vibrational levels of the Ou+ excited state and the Og+ ground state. They are in excellent agreement with experiment.

Vacuum ultraviolet spectroscopy of the Cl2 molecule trapped in pure neon, pure argon, or mixed neon–argon matrices

Synchrotron radiation excitation and emission spectra with lifetime measurements are reported for the first time in the VUV region for systems consisting of Cl2 molecules trapped in a neon matrix, an argon matrix, and mixed Ar/Ne matrices. In pure neon, the emission spectrum of the D’→A’ ‘‘laser’’ transition at 4.7 eV of the Cl2 molecule is vibrationally well resolved and constitutes an interesting example of UV spectroscopy of a matrix ‘‘isolated’’ molecule. In pure argon or mixed Ar/Ne matrices, new broad emissions at 4.1, 3.8, and 3.5 eV are clearly identified, which result from the specific interaction between Cl*2 and Ar and are attributed to different charge–transfer states of the ArCl+Cl− entity. The Ar concentration dependence and the time‐gated spectra are shown to be especially useful in interpreting the large differences observed between the pure neon and the pure argon matrix case.

Spectral and kinetic studies of the second continuum fluorescence of xenon excited by synchrotron radiation

Abstract Synchrotron radiation at LURE has been used to excite xenon in the pressure range of 50 to 200 torr. The subsequent second continuum fluorescence at 1720 A has been spectrally and temporally analyzed. The observed two-component decay of this emission, as well as various spectral features, are shown to be in disagreement with some results previously reported in the literature.

NEW ASSIGNMENTS IN THE UV SPECTROSCOPY OF THE SMALL BENZENE-ARGONN CLUSTERS : THE EFFECTS OF A STRUCTURE-SELECTIVE VIBRATIONAL PREDISSOCIATION

In this paper, new experimental results using UV spectroscopy of the small benzene–Arn clusters are presented. We have found evidence for the vibrational predissociation of the S1 state of some of these species on a nanosecond time scale and we propose a new assignment for the UV spectral features of this system. This assignment is consistent with other experimental data and it accounts additionally for the previously reported spectral anomalies, in particular the ionization potential measurements of these species. The two-color R2PI spectra performed can thus be considered as nanosecond pump–probe experiments that allow us to estimate the relaxation rate of the S1state of these clusters. The vibrational relaxation is found to be strongly size and structure dependent: The two isomers [(1/1) and (2/0)] of the n=2 species exhibit different lifetimes differing by at least one order of magnitude. The size dependence of the relaxation process within the one-sided isomer series appears also paradoxical since th...

Observation of the intermediate states in the (Xe-Cl2)*→ XeCl* (B,C) + Cl reaction

Abstract The Xe-Cl 2 van der Waals complex formed in a supersonic expansion is excited in a two-photon process. The XeCl(B,C) reaction spectra (excitation of the complex and detection of the XeCl (B→X) and (C→A) fluorescence) are obtained in the region 290–310 nm. In addition to a diffuse band also obtained in the C action spectrum, the B action spectrum presents a vibronic structure similar to the Xe-Cl (B←X) absorption band. Xe-Cl 2 ( 1 Π u ) resonantly enhanced two-photon process with two chromophores is proposed to account for the results.