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The Journal of Chemical Physics

Green bands of the CsHg molecule

Robert Polly, Stoyan Dinev, Laurentius Windholz, Slobodan Milošević, B.A. Hess

Published May 8, 1999• 2 citations

Abstract

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.

DOI: 10.1063/1.478818

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The Journal of Physical Chemistry•Jan 1, 1996•8 citations

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

6{}^{2}{S}_{1/2}--5{}^{2}{P}_{1/2}

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

\mathrm{In}(6{}^{2}{S}_{1/2})

with

\mathrm{Hg}(6{}^{1}{S}_{0}),

and UV atomic fluorescence from the

\mathrm{In}(5{}^{2}D)

states that lie at energies above the

\mathrm{In}(6{}^{2}{S}_{1/2}--5{}^{2}{P}_{1/2})

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.

Lasing on the D(1) transition (6P1/22-->6S1/22) of Cs has been observed by photoassociating Cs-Kr atomic pairs with a tunable, pulsed dye laser. Pumping of the blue or red satellites of the Cs D(2) line (62P3/2<==>62S1/2), peaking at approximately 841.1 nm and approximately 853 nm (respectively) in Cs/Kr/C(2)H(6) gas mixtures, provides a photodissociation laser in which the CsKr excimer parent molecule is not, at any point in the pumping process, in a bound electronic state. Relative to the absorbed pump pulse energy, laser slope efficiencies greater than or approximately 5% have been measured when the Cs number density is in the range of 5x10(14)-1.5x10(15) cm(-3) and the pump wavelength is 841.1 nm. Direct photoexcitation of the Cs 6P3/22 state at 852.1 nm under these conditions is a less efficient pathway for pumping the 894.3 nm laser, presumably as a result of competing nonlinear optical processes such as 1+2 resonantly enhanced multiphoton ionization of the alkali atom.

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.

We have studied in detail the ionization and population of high-lying atomic states which occur in cesium vapor excited at the second resonance transition, 6S-7P. We have developed a simple model which qualitatively explains the laser-power dependences of level populations and electron densities which we observed. Measurements of spatial dependences of fluorescence point out the critical role of radiation trapping in these phenomena. Similar observations on rubidium demonstrate the same type of behavior when the first (5S-5P) or second (5S-6P) resonance is excited. At higher densities and temperatures we found that the fluorescence was dominated by diffuse molecular bands. Some of these bands probably consist of transitions to the repulsive $^{3}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ state. Additionally four bands observed at 817.5, 826.6, 835.3, and 839.6 nm are tentatively identified as the cesium analog of the $^{3}\mathrm{\ensuremath{\Pi}}_{\mathrm{g}}$${\ensuremath{\rightarrow}}^{3}$${\mathrm{\ensuremath{\Sigma}}}_{\mathrm{u}}^{+}$ satellite bands which have previously been observed in the blue wing of the first resonance doublet of the lighter alkali metals.

We report the determination of the complete LiHg shallow bound ground state potential curve by combination of laser-induced fluorescence spectroscopy of bound−bound transitions, photoassociation spectroscopy of bound−free transitions, and ab initio calculations. The results are evaluated by comparison of quantum mechanically simulated emission spectra with experimental data and used to describe spectroscopic features of the 2 2 Π 3/2 − band as well as the photochemical formation via excitation of the lithium dimer.