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Physical Review A

Two-photon spectroscopy of mercury and velocity-selective double resonances

Thomas Beyer, Daniel Kolbe, Martin Scheid, Frank Markert, J. Walz

Published Nov 12, 2009• 4 citations
Publisher

Abstract

Two-photon laser spectroscopy of the $6\text{ }{^{1}S}_{0}\ensuremath{-}7\text{ }{^{1}S}_{0}$ transition in mercury has been performed using two copropagating continuous-wave laser beams. One laser beam is at 254 nm wavelength and can be tuned to the $6\text{ }{^{1}S}_{0}\ensuremath{-}6\text{ }{^{3}P}_{1}$ resonance. The other laser beam is at 408 nm. Two very different regimes can be distinguished, one far off resonance and one near resonance with the one-photon resonance. A resonance which is not Doppler broadened has been observed for low Rabi frequencies. This velocity-selective double resonance in a three-level ladder system is analogous to the dark resonance in three-level $\ensuremath{\Lambda}$ systems.

DOI
10.1103/physreva.80.053414

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Two-photon laser spectroscopy of the 6 1S0\ensuremath−7 1S06\text{ }{^{1}S}_{0}\ensuremath{-}7\text{ }{^{1}S}_{0}6 1S0​\ensuremath−7 1S0​ transition in mercury has been performed using two copropagating continuous-wave laser beams. One laser beam is at 254 nm wavelength and can be tuned to the 6 1S0\ensuremath−6 3P16\text{ }{^{1}S}_{0}\ensuremath{-}6\text{ }{^{3}P}_{1}6 1S0​\ensuremath−6 3P1​ resonance. The other laser beam is at 408 nm. Two very different regimes can be distinguished, one far off resonance and one near resonance with the one-photon resonance. A resonance which is not Doppler broadened has been observed for low Rabi frequencies. This velocity-selective double resonance in a three-level ladder system is analogous to the dark resonance in three-level \ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ systems.
A two-photon mechanism for cooling atoms below the Doppler temperature is analyzed. We consider the magnesium ladder system $(3{s}^{2})^{1}S_{0}\ensuremath{\rightarrow}(3s3p)^{1}P_{1}$ at 285.2nm285.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}285.2nm followed by the $(3s3p)^{1}P_{1}\ensuremath{\rightarrow}(3s3d)^{1}D_{2}$ transition at 880.7nm880.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}880.7nm. For the ladder system quantum coherence effects may become important. Combined with the basic two-level Doppler cooling process this allows for reduction of the atomic sample temperature by more than a factor of 10 over a broad frequency range. First experimental evidence for the two-photon cooling process is presented and compared to model calculations. Agreement between theory and experiment is excellent. In addition, by properly choosing the Rabi frequencies of the two optical transitions a velocity independent atomic dark state is observed.
In this paper, we present experimental techniques to resolve the closely spaced hyperfine levels of a weak transition by eliminating the residual or partial two-photon Doppler broadening and crossover resonances in a wavelength-mismatched double-resonance spectroscopy. The elimination of the partial Doppler broadening is based on velocity-induced population oscillation (VIPO) and velocity-selective saturation (VSS) effects followed by the subtraction of the broad background of the two-photon spectrum. Since the VIPO and VSS effects are the phenomena for near-zero-velocity group atoms, the subtraction gives rise to Doppler-free peaks and the closely spaced hyperfine levels of the 6P3/26{P}_{3/2}6P3/2​ state in Rb are well resolved. The double-resonance experiment is conducted on the $5{S}_{1/2}\ensuremath{\rightarrow}5{P}_{3/2}$ strong transition (at 780 nm) and $5{S}_{1/2}\ensuremath{\rightarrow}6{P}_{3/2}$ weak transition (at 420 nm) at room temperature.
Isotopically selective resonance ionization of mercury atoms has been accomplished by three-step excitation via the 6 3P1 and 8 1S0 excited states. Three collinear beams, respectively 254, 286, and 532 nm, from two dye lasers and a pulsed neodymium:yttrium aluminum garnet (ND:YAG) pump laser, were passed through a closed mercury vapor cell containing electrodes to which the ions were drawn by an electric field. The ion current and the 254-nm fluorescence radiation were measured as a function of laser wavelengths. Hyperfine structures for the 254- and 286-nm transitions were observed.
Coherent radiation at several wavelengths in the vacuum ultraviolet (VUV) has been generated by four-wave parametric oscillation in mercury vapor. When a powerful ultraviolet pump laser of frequency omega(p) is tuned to a two-photon resonance, VUV signal photons at frequency omega(s), as well as idler photons at frequency omega(i), are generated such that omega(s) + omega(i) = 2omega(p). A frequency-doubled dye laser tuned to two-photon resonance with the 6s6d(1)D(2) level near 280.3 nm produced output at 184.9 and 143.5 nm. A tunable krypton fluoride excimer laser tuned to two-photon resonance with the 6s10s (1)S(0) level near 248.7 nm produced output at 140.1, 130.7, 130.1, 125.9, and 125.0 nm. Approximately 1-microJ, 200-W pulses were observed at 143.5 and 125.9 nm.
High-resolution three-photon laser spectroscopy has been performed for the first time to gain new spectroscopic information. Starting from the $6{s}^{2}^{1}S_{0}$ ground state of Yb, 6s15p1,3P16s15p^{1,3}P_{1}6s15p1,3P1​ Rydberg states were populated by two-photon-resonant, three-photon absorption via the 6s7s1S06s7s^{1}S_{0}6s7s1S0​ intermediate level. As a result of two-photon line narrowing Doppler-free spectra, showing well resolved isotope shifts and hyperfine structures, were obtained. From the hyperfine coupling constants 171,173A(1P1)^{171,173}A(^{1}P_{1})171,173A(1P1​) the singlet-triplet mixing was derived.