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Physical Review A

Photoassociative spectroscopy of ultracold metastable argon

Michael K. Shaffer, Gambhir Ranjit, Charles I. Sukenik, M. Walhout

Published May 24, 2011• 7 citations
Publisher

Abstract

We present results of photoassociative spectroscopy performed on ultracold metastable argon atoms in a magneto-optical trap. Ion spectra are obtained with laser detuning up to a few gigahertz below the $4s[3/2]{}_{2}\ensuremath{\rightarrow}4p[5/2]{}_{3}$ trapping transition at 811 nm and with intensities in a range of $~({10}^{2}\ensuremath{-}{10}^{5}){I}_{\mathrm{Sat}}$. We also compute dipole-dipole potentials for both singly and doubly excited diatomic molecules and use a Leroy-Bernstein analysis to determine the approximate vibrational spacings in the ($s+p$) and ($p+p$) manifolds. Based on this theoretical framework, we explain a broad background feature in our data and suggest that double-excitation mechanisms are likely responsible for sharp dips in the ion signal.

DOI
10.1103/physreva.83.052516

Related papers

Photoassociative Spectroscopy of Ultracold Metastable Argon and Study of Dual Species Trap Loss in a Rubidium-Metastable Argon MOT

Michael K. Shaffer

This dissertation presents the findings of two experimental investigations in ultracold atomic and molecular physics: The study of the dual species trap loss in a rubidium - metastable argon magneto-optical trap and the photoassociative spectroscopy of ultracold metastable argon. The interspecies trap loss rate coefficients have been measured for ultracold collisions between 85Rb and 40Ar* in a dual-species magneto-optical trap (MOT) and the two rates have been found to be approximately equal over the range of intensities studied with values of β'Rb–Ar* = 3.0 ± 1.3 × 10-11 cm3/s and β'Ar*–Rb = 1.9 ± 0.9 × 10-11 cm3/s where β' Rb–Ar* is the trap loss coefficient for Rb in the presence of Ar* and β'Ar*–Rb is the reciprocal term. In addition, the trap loss rate coefficient for cold collisions in a metastable argon MOT alone have been measured with an average value of β Ar* = 5.2 ± 1.6 × 10-10 cm3/s. Using a quadrupole mass spectrometer, the production of Ar +, Ar+2, Rb+, and RbAr+ ions in the dual MOT have been observed, clearly identifying heteronuclear Penning and associative ionization as trap loss mechanisms. In the second experiment, the first ever investigation of the photoassociative (PA) spectroscopy of Ar* has been made. The exploratory study focuses on PA spectroscopy near the 4s[3/2]2 + 4p[5/2]3 asymptote of the Ar2* diatomic molecule over a 10 GHz range red detuned of the atomic resonance. With a range of probe laser intensities from ∼ 102–105ISat, 12 resonances have been observed. The spectra have been analyzed using the near-dissociation LeRoy-Bernstein method. Through this analysis, the spectra seem to best correlate with excitations to the 5g state.

•Jan 1, 2008•0 citations

Photoassociative Spectroscopy of Ultracold Argon and Krypton Confined in a Magneto Optical Trap

Maha Khaled Omar

Creating ultracold molecules has attracted considerable interest in the last decade. Once created, such molecules can be used for precision spectroscopy or to study chemical reactions at ultracold tem peratures. Several techniques have been developed to produce ultracold molecules; the most common is photoassociation where two ultracold atoms collide in the presence of light that induces a free-bound transition to an excited molecular state. Photoassociation can also be used to perform spectroscopy in order to map out the ro-vibrational levels of a molecular state. In this dissertaiotn, we report on our Photoasociative Spectroscopy (PAS) studies conducted separately in argon and krypton. For each species, we have studied transitions near two different atomic limits ns[3/2]2 —> np[5/2]2 and ns[3/2]2 —» np[5/2]3where n = 4 for argon and n = 5 for krypton. The former atomic transition is called the “quench transition” since it forms a strong coupled channel to the ground state. As a result for this strong coupling the original atomic sample is degraded and quenched. The latter transition is the one used to cool and confine the atoms and is known as the “trapping transition.” Spectroscopy near the trapping transition in argon was studied previously in our group [1] and specific features were observed in the spectrum. At the time, the features were not definitively identified, but more than one explanation was suggested for further testing. One possible explanation was that these features were a result of resonances happening at “doubly-excited” molecular levels. The population of such states could take place as a result of absorbing two photons of the same frequency which causes photoassociation. In this dissertation we report on a series of experiments that were performed to study and conclusively identify the origin of those features. These experiments led to the conclusion that the features in the spectra were an artifact of otherwise undetectable frequency sidebands on our semiconductor diode laser. Once identified as such, a new laser was constructed to repeat the spectroscopy measurements in argon and take new measurements in krypton that would be free from laser artifacts. In those spectra, no specific vibrational features were observed and resolved in either species. Results were consistent with published results obtained by other groups using other noble gases. Finally, we report on our attempts to perform PAS on a quench transition in order to confirm results obtained for krypton by another group [2]. We were not able to observe any photoassociation signal for both argon and krypton on this transition. We were able to observe similar effects to those reported in Ref. [2] but we attribute these to artifacts from an acousto-optic modulator and not as arising from molecular structure.

•Oct 21, 2019•0 citations

Photoassociative Spectroscopy of Ultracold Argon and Krypton

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We report photoassociative spectroscopy performed separately on Kr84 and Ar40. The spectra were obtained as a function of laser intensity with the spectroscopy laser tuned around the ns[3/2]2→np[5/2]3 trapping transition, where n=4 for argon and n=5 for krypton. The resulting spectra are the first ever obtained for krypton on this important transition. This transition in argon was previously studied in our group. At that time, discrete features were observed in the spectrum, but their origin was not conclusively identified. We have now identified these features as artifacts from laser sidebands. We discuss the origin of these sidebands, how they imprint their presence on the spectra, and present spectra for argon with the artifacts removed. For both krypton and argon, while we observe photoassociation, we do not observe any resolved vibrational structure. We present a theoretical analysis of the expected vibrational level spacings based on calculated potential curves and show that the absence of resolved structure in the spectra of our experiments (as configured) is not surprising. Finally, we discuss the requirements of future experiments in which resolution of vibrational structure in this system might be obtained.

Bulletin of the American Physical Society•May 25, 2016•0 citations

Photoassociative spectroscopy of ultracold argon and krypton

Maha Khaled Omar, W. D. Williams, Charles I. Sukenik

We report photoassociative spectroscopy performed separately on Kr84 and Ar40. The spectra were obtained as a function of laser intensity with the spectroscopy laser tuned around the ns[3/2]2→np[5/2]3 trapping transition, where n=4 for argon and n=5 for krypton. The resulting spectra are the first ever obtained for krypton on this important transition. This transition in argon was previously studied in our group. At that time, discrete features were observed in the spectrum, but their origin was not conclusively identified. We have now identified these features as artifacts from laser sidebands. We discuss the origin of these sidebands, how they imprint their presence on the spectra, and present spectra for argon with the artifacts removed. For both krypton and argon, while we observe photoassociation, we do not observe any resolved vibrational structure. We present a theoretical analysis of the expected vibrational level spacings based on calculated potential curves and show that the absence of resolved structure in the spectra of our experiments (as configured) is not surprising. Finally, we discuss the requirements of future experiments in which resolution of vibrational structure in this system might be obtained.

Journal of the Optical Society of America B•Oct 18, 2016•0 citations

Photoassociation spectroscopy of ultracold Cs below the 6P1/2 limit

Marin Pichler, Hongmin Chen, William C. Stwalley

We have performed high precision photoassociation spectroscopy of ultracold cesium gas. Using trap-loss fluorescence detection and controlling the background cesium pressure we were able to photoassociate atoms into excited states of ultracold molecules with large detunings up to 56 cm(-1) below the Cs(6S(1/2)) + Cs(6P(1/2)) atomic asymptote. Vibrational progressions are assigned to 0(g)(-), 0(u)(+), and 1(g) long-range states. By fitting the spectral data to the LeRoy-Bernstein expression, the effective coefficients of the leading long-range interactions and the vibrational quantum number at dissociation are obtained. In addition we have observed spectral perturbations between states of the same symmetry belonging to different asymptotes (6P(1/2) and 6P(3/2)). The perturbations are manifested through irregular vibrational level spacings and are especially pronounced in the 0(u)(+) symmetry. Many observed rotational levels indicate d- and higher partial wave contributions to the photoassociation cross section in the presence of trapping laser light, while spectral regions with only weak features suggest nodes in the lower state wave functions corresponding to the two ground state atoms asymptote.

The Journal of Chemical Physics•Jul 8, 2004•30 citations

PHOTOASSOCIATION OF ULTRACOLD ATOMS: A NEW TOOL FOR MOLECULAR SPECTROSCOPY

William C. Stwalley

The field of laser cooling and trapping of atoms has had a profound impact on many areas of atomic, molecular and optical science. In particular, ultracold atoms colliding in magneto-optical traps typically have relative kinetic energies of $300\\mu K (\\sim 7 MHz)$, so free-bound absorption lines are as sharp as bound-bound lines. Moreover, because of very long range centrifugal barriers, only a few collisional partial waves are important ($J^{\\prime\\prime} = 0, 1$ and 2 in our work on $^{39}K$). Such photoassociative free-bound absorption allows observation of ``pure long-range molecule'' states ($R_{e} = 28$ and 39 {\\AA} in $^{39}K_{2}$) as well as long-range levels of known short range states with outer classical turning points of $\\sim 20 - 250 {\\AA}$. Such spectra have been observed by ``trap loss'' (decrease in atomic fluorescence), molecular ionization, and fragment ionization. Optical-optical double resonance (OODR), e.g. through the ``Franck-Condon windows'' of the ``pure long-range'' states, produces readily assignable spectra at higher asymptotes as well. Further extensions of ultracold photoassociative spectroscopy and applications to determination of long range potentials, atomic properties, and Bose-Einstein condensate stability will be presented.

•Jan 1, 1998•0 citations
We present results of photoassociative spectroscopy performed on ultracold metastable argon atoms in a magneto-optical trap. Ion spectra are obtained with laser detuning up to a few gigahertz below the 4s[3/2]2\ensuremath→4p[5/2]34s[3/2]{}_{2}\ensuremath{\rightarrow}4p[5/2]{}_{3}4s[3/2]2​\ensuremath→4p[5/2]3​ trapping transition at 811 nm and with intensities in a range of  (102\ensuremath−105)ISat~({10}^{2}\ensuremath{-}{10}^{5}){I}_{\mathrm{Sat}} (102\ensuremath−105)ISat​. We also compute dipole-dipole potentials for both singly and doubly excited diatomic molecules and use a Leroy-Bernstein analysis to determine the approximate vibrational spacings in the (s+ps+ps+p) and (p+pp+pp+p) manifolds. Based on this theoretical framework, we explain a broad background feature in our data and suggest that double-excitation mechanisms are likely responsible for sharp dips in the ion signal.
This dissertation presents the findings of two experimental investigations in ultracold atomic and molecular physics: The study of the dual species trap loss in a rubidium - metastable argon magneto-optical trap and the photoassociative spectroscopy of ultracold metastable argon. The interspecies trap loss rate coefficients have been measured for ultracold collisions between 85Rb and 40Ar* in a dual-species magneto-optical trap (MOT) and the two rates have been found to be approximately equal over the range of intensities studied with values of β'Rb–Ar* = 3.0 ± 1.3 × 10-11 cm3/s and β'Ar*–Rb = 1.9 ± 0.9 × 10-11 cm3/s where β' Rb–Ar* is the trap loss coefficient for Rb in the presence of Ar* and β'Ar*–Rb is the reciprocal term. In addition, the trap loss rate coefficient for cold collisions in a metastable argon MOT alone have been measured with an average value of β Ar* = 5.2 ± 1.6 × 10-10 cm3/s. Using a quadrupole mass spectrometer, the production of Ar +, Ar+2, Rb+, and RbAr+ ions in the dual MOT have been observed, clearly identifying heteronuclear Penning and associative ionization as trap loss mechanisms. In the second experiment, the first ever investigation of the photoassociative (PA) spectroscopy of Ar* has been made. The exploratory study focuses on PA spectroscopy near the 4s[3/2]2 + 4p[5/2]3 asymptote of the Ar2* diatomic molecule over a 10 GHz range red detuned of the atomic resonance. With a range of probe laser intensities from ∼ 102–105ISat, 12 resonances have been observed. The spectra have been analyzed using the near-dissociation LeRoy-Bernstein method. Through this analysis, the spectra seem to best correlate with excitations to the 5g state.
Creating ultracold molecules has attracted considerable interest in the last decade. Once created, such molecules can be used for precision spectroscopy or to study chemical reactions at ultracold tem peratures. Several techniques have been developed to produce ultracold molecules; the most common is photoassociation where two ultracold atoms collide in the presence of light that induces a free-bound transition to an excited molecular state. Photoassociation can also be used to perform spectroscopy in order to map out the ro-vibrational levels of a molecular state. In this dissertaiotn, we report on our Photoasociative Spectroscopy (PAS) studies conducted separately in argon and krypton. For each species, we have studied transitions near two different atomic limits ns[3/2]2 —> np[5/2]2 and ns[3/2]2 —» np[5/2]3where n = 4 for argon and n = 5 for krypton. The former atomic transition is called the “quench transition” since it forms a strong coupled channel to the ground state. As a result for this strong coupling the original atomic sample is degraded and quenched. The latter transition is the one used to cool and confine the atoms and is known as the “trapping transition.” Spectroscopy near the trapping transition in argon was studied previously in our group [1] and specific features were observed in the spectrum. At the time, the features were not definitively identified, but more than one explanation was suggested for further testing. One possible explanation was that these features were a result of resonances happening at “doubly-excited” molecular levels. The population of such states could take place as a result of absorbing two photons of the same frequency which causes photoassociation. In this dissertation we report on a series of experiments that were performed to study and conclusively identify the origin of those features. These experiments led to the conclusion that the features in the spectra were an artifact of otherwise undetectable frequency sidebands on our semiconductor diode laser. Once identified as such, a new laser was constructed to repeat the spectroscopy measurements in argon and take new measurements in krypton that would be free from laser artifacts. In those spectra, no specific vibrational features were observed and resolved in either species. Results were consistent with published results obtained by other groups using other noble gases. Finally, we report on our attempts to perform PAS on a quench transition in order to confirm results obtained for krypton by another group [2]. We were not able to observe any photoassociation signal for both argon and krypton on this transition. We were able to observe similar effects to those reported in Ref. [2] but we attribute these to artifacts from an acousto-optic modulator and not as arising from molecular structure.
We report photoassociative spectroscopy performed separately on Kr84 and Ar40. The spectra were obtained as a function of laser intensity with the spectroscopy laser tuned around the ns[3/2]2→np[5/2]3 trapping transition, where n=4 for argon and n=5 for krypton. The resulting spectra are the first ever obtained for krypton on this important transition. This transition in argon was previously studied in our group. At that time, discrete features were observed in the spectrum, but their origin was not conclusively identified. We have now identified these features as artifacts from laser sidebands. We discuss the origin of these sidebands, how they imprint their presence on the spectra, and present spectra for argon with the artifacts removed. For both krypton and argon, while we observe photoassociation, we do not observe any resolved vibrational structure. We present a theoretical analysis of the expected vibrational level spacings based on calculated potential curves and show that the absence of resolved structure in the spectra of our experiments (as configured) is not surprising. Finally, we discuss the requirements of future experiments in which resolution of vibrational structure in this system might be obtained.
We report photoassociative spectroscopy performed separately on Kr84 and Ar40. The spectra were obtained as a function of laser intensity with the spectroscopy laser tuned around the ns[3/2]2→np[5/2]3 trapping transition, where n=4 for argon and n=5 for krypton. The resulting spectra are the first ever obtained for krypton on this important transition. This transition in argon was previously studied in our group. At that time, discrete features were observed in the spectrum, but their origin was not conclusively identified. We have now identified these features as artifacts from laser sidebands. We discuss the origin of these sidebands, how they imprint their presence on the spectra, and present spectra for argon with the artifacts removed. For both krypton and argon, while we observe photoassociation, we do not observe any resolved vibrational structure. We present a theoretical analysis of the expected vibrational level spacings based on calculated potential curves and show that the absence of resolved structure in the spectra of our experiments (as configured) is not surprising. Finally, we discuss the requirements of future experiments in which resolution of vibrational structure in this system might be obtained.
We have performed high precision photoassociation spectroscopy of ultracold cesium gas. Using trap-loss fluorescence detection and controlling the background cesium pressure we were able to photoassociate atoms into excited states of ultracold molecules with large detunings up to 56 cm(-1) below the Cs(6S(1/2)) + Cs(6P(1/2)) atomic asymptote. Vibrational progressions are assigned to 0(g)(-), 0(u)(+), and 1(g) long-range states. By fitting the spectral data to the LeRoy-Bernstein expression, the effective coefficients of the leading long-range interactions and the vibrational quantum number at dissociation are obtained. In addition we have observed spectral perturbations between states of the same symmetry belonging to different asymptotes (6P(1/2) and 6P(3/2)). The perturbations are manifested through irregular vibrational level spacings and are especially pronounced in the 0(u)(+) symmetry. Many observed rotational levels indicate d- and higher partial wave contributions to the photoassociation cross section in the presence of trapping laser light, while spectral regions with only weak features suggest nodes in the lower state wave functions corresponding to the two ground state atoms asymptote.
The field of laser cooling and trapping of atoms has had a profound impact on many areas of atomic, molecular and optical science. In particular, ultracold atoms colliding in magneto-optical traps typically have relative kinetic energies of 300muK(sim7MHz)300\\mu K (\\sim 7 MHz)300muK(sim7MHz), so free-bound absorption lines are as sharp as bound-bound lines. Moreover, because of very long range centrifugal barriers, only a few collisional partial waves are important (Jprimeprime=0,1J^{\\prime\\prime} = 0, 1Jprimeprime=0,1 and 2 in our work on 39K^{39}K39K). Such photoassociative free-bound absorption allows observation of ``pure long-range molecule'' states (Re=28R_{e} = 28Re​=28 and 39 {\\AA} in 39K2^{39}K_{2}39K2​) as well as long-range levels of known short range states with outer classical turning points of sim20−250AA\\sim 20 - 250 {\\AA}sim20−250AA. Such spectra have been observed by ``trap loss'' (decrease in atomic fluorescence), molecular ionization, and fragment ionization. Optical-optical double resonance (OODR), e.g. through the ``Franck-Condon windows'' of the ``pure long-range'' states, produces readily assignable spectra at higher asymptotes as well. Further extensions of ultracold photoassociative spectroscopy and applications to determination of long range potentials, atomic properties, and Bose-Einstein condensate stability will be presented.