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The Journal of Chemical Physics

Photoassociation, photoluminescence, and collisional dissociation of the Sr2 dimer

T. H. Bergeman, P. F. Liao

Published Jan 15, 1980• 41 citations
Publisher

Abstract

We report observations and analysis of the laser-excited photoluminescence of Sr2. The spectra contains some discrete molecular line spectra, but is dominated by extensive, deeply modulated continua characteristic of bound–free transitions of excimer systems. Dye laser and argon laser excitation of the Sr2 dimer is used. Some of the spectra is attributed to fluorescence from dimers created by laser induced photoassociation of free Sr atoms. Techniques are developed to obtain Morse potential parameters by computer modeling of the observed spectra. We find for the X 1Σ+g state, De=1100±100 cm−1, ωe=39.6±1 cm−1; for the A 1Σ+u state, De=5460±100 cm−1, ωe=83±3 cm−1; and Re″−Re′=0.49±0.02 Å. Strong collision-induced predissociation is observed and attributed to a potential curve crossing. Cross sections for dissociation by argon are reported as a function of laser excitation wavelength.

DOI
10.1063/1.439244

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Physical Review Letters•Oct 4, 1993•227 citations
We report observations and analysis of the laser-excited photoluminescence of Sr2. The spectra contains some discrete molecular line spectra, but is dominated by extensive, deeply modulated continua characteristic of bound–free transitions of excimer systems. Dye laser and argon laser excitation of the Sr2 dimer is used. Some of the spectra is attributed to fluorescence from dimers created by laser induced photoassociation of free Sr atoms. Techniques are developed to obtain Morse potential parameters by computer modeling of the observed spectra. We find for the X 1Σ+g state, De=1100±100 cm−1, ωe=39.6±1 cm−1; for the A 1Σ+u state, De=5460±100 cm−1, ωe=83±3 cm−1; and Re″−Re′=0.49±0.02 Å. Strong collision-induced predissociation is observed and attributed to a potential curve crossing. Cross sections for dissociation by argon are reported as a function of laser excitation wavelength.
Photoassociation, the bound←free absorption of a photon by a colliding pair of atoms, has been studied experimentally in the Kr–F and Xe–I systems and is demonstrated to be an effective tool for deducing molecular interaction potentials and for determining the dependence of the relative electronic transition moment on internuclear separation (R) in small molecules having dissociative ground states. Specifically, the excitation (‘‘action’’) spectrum for the photoassociation of thermalized Kr(4p6 1S0)–F(2p5 2P) collision pairs, measured at 300 K in the 208–250 nm wavelength region, reveals deeply modulated Franck–Condon structure associated with transitions of Kr–F pairs from the ground state’s thermal and vibrational continua to the lowest 27 bound vibrational states (v′=0–26) of the KrF(B 2Σ) excited state. Under these conditions, photoassociation occurs in a narrow (ΔR=0.7–0.8 Å) Franck–Condon region in which the difference potential is a single valued function of R. Similar spectra have been observed for Xe(1S0)–I(2P) pairs in the 208–258 nm spectral region that arise from bound←free transitions to the lowest ∼40 vibrational levels of the XeI(B) state. Since the Kr–F and Xe–I collision pairs are photoexcited from the vibrational continuum of the ground state, the atomic pair energy distribution is thermalized and the envelope of the excitation spectrum can be interpreted directly in terms of the relative B–X transition moment, μ(R). Consequently, the behavior of μ(R) has been determined over a limited range in R and, for XeI, μ is found to fall by a factor of 2 in the ∼0.4 Å region extending from R−ReB≊0.25–0.62 Å. The XeI ground state (X 2Σ1/2+) potential in the 3.0≤R≤5.0 Å interval, derived from photoassociation spectra, is similar to that deduced from scattering experiments. Excited and ground state structural constants derived from numerical quantum simulations of the experimental spectra are presented.
We report the first observation of laser-induced fluorescence of the CsHg molecule. Cs/Hg vapor mixtures contained in a heat-pipe oven were excited by several different argon ion laser lines. Following these excitations, we observed three distinct peaks at λ=492.8, λ=498.3, and λ=521.5 nm in addition to the well-known fluorescence of the Cs2 molecule. Based on an analysis using ab initio potential energy curves of Cs2 and CsHg, we identified the origin of these three distinct peaks as bound–free transitions of CsHg (λ=492.8: IV3/2→X1/2, λ=498.3: VII1/2→X1/2, and λ=521.5 nm: VI1/2→X1/2). Furthermore, we could show that two different production channels are responsible for the formation of the CsHg molecules in the excited states. The excitation of the Cs/Hg vapor mixtures with wavelengths λexc=476–514 nm of the argon ion laser results in the production of the CsHg molecules via a photoassociation process, whereas the CsHg molecules are produced in a three-body collision following an excitation with λexc=457 nm. These results are confirmed by subsequent quantum-mechanical simulations of the observed laser-induced fluorescence spectra of CsHg.
Laser excitation of In atoms at the 62S1/2−−52P1/26{}^{2}{S}_{1/2}--5{}^{2}{P}_{1/2}62S1/2​−−52P1/2​ atomic resonance transition wavelength of 410.3 nm in the presence of Hg vapor produces several types of emission features: HgIn excimer satellites on the red wings of the In resonance fluorescence transitions at 410.3 and 451.1 nm, bound-bound HgIn molecular fluorescence at 499 and 522 nm resulting from photoassociation of In(62S1/2)\mathrm{In}(6{}^{2}{S}_{1/2})In(62S1/2​) with Hg(61S0),\mathrm{Hg}(6{}^{1}{S}_{0}),Hg(61S0​), and UV atomic fluorescence from the In(52D)\mathrm{In}(5{}^{2}D)In(52D) states that lie at energies above the In(62S1/2−−52P1/2)\mathrm{In}(6{}^{2}{S}_{1/2}--5{}^{2}{P}_{1/2})In(62S1/2​−−52P1/2​) excitation. These various spectral features are interpreted in terms of a proposed HgIn potential-energy level diagram with additional information from measured excited-state lifetimes and the power dependence of the laser excitation at 410.3 and 522 nm. Direct excitation of the 499 and 522 nm HgIn bound-bound transitions also produces emission at 410.3 and 451.1 nm resulting from collisional dissociation of the excited HgIn states.
A continuous excitation band of a free-bound photoassociation transition of ytterbium atoms is observed as a red wing of the $^{1}S_{0}\text{\ensuremath{-}}^{1}P_{1}$ atomic line at 399 nm for a hot thermal vapor. The excitation to the 0u+{0}_{u}^{+}0u+​ molecular state is observed by monitoring fluorescence from the 3P1^{3}P_{1}3P1​ state atoms, which allows us to detect the production of Yb2{\mathrm{Yb}}_{2}Yb2​ molecules with high sensitivity. The photoassociation is characterized in comparison with transitions to atomic Rydberg states. The time profile of the fluorescence signal suggests that the 0u+{0}_{u}^{+}0u+​ molecular state predissociates with states correlating to the 1S0+3D2^{1}S_{0}+^{3}D_{2}1S0​+3D2​ atomic states.
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