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Deceleration of a Supersonic Beam of SrF Molecules

Sreekanth Chirayath Mathavan

Published Jun 13, 2023• 0 citations
Publisher

Abstract

The work depicted in this thesis demonstrate the successful operation of a 4 meter long traveling-wave Stark decelerator. This is a novel tool for deceleration of heavy diatomic molecules to arbitrarily slow beams with a very small velocity spread which can serve as ideal starting point for precision tests of fundamental physics. The first part of the thesis discusses the characterization of a new Supersonic beam source for creation of SrF molecules which serves as an ideal candidate for precision test of nuclear spin dependent parity violation. The later part of the thesis discusses the the results of deceleration of a SrF molecules in the N = 1 rotation state of the SrF(v = 0) electronic ground state from a supersonic beam with an initial velocity of 340 m/s to 265 m/s using a deceleration potential with 3.25 k V voltage amplitude using a the 4 m long traveling-wave decelerator built in our research lab. Increasing the voltage amplitude of the potential to 4.8 kV, we were able to decelerate molecules from 340 m/s to 250 m/s which corresponds to a deceleration strength of 6.6 km /s2. By cooling the valve by flushing cold nitrogen gas through the valve enclosure we were able to decrease the initial velocity of the supersonic beam. With the valve cooled to ~190 K, we demonstrated deceleration of SrF molecules from 290 m/s to 120 m/s which corresponds to the removal of almost 85% of the initial kinetic energy. The thesis described the operation of the decelerator and end with an outlook for precision tests using this novel tool.

DOI
10.33612/diss.674235092

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The work depicted in this thesis demonstrate the successful operation of a 4 meter long traveling-wave Stark decelerator. This is a novel tool for deceleration of heavy diatomic molecules to arbitrarily slow beams with a very small velocity spread which can serve as ideal starting point for precision tests of fundamental physics. The first part of the thesis discusses the characterization of a new Supersonic beam source for creation of SrF molecules which serves as an ideal candidate for precision test of nuclear spin dependent parity violation. The later part of the thesis discusses the the results of deceleration of a SrF molecules in the N = 1 rotation state of the SrF(v = 0) electronic ground state from a supersonic beam with an initial velocity of 340 m/s to 265 m/s using a deceleration potential with 3.25 k V voltage amplitude using a the 4 m long traveling-wave decelerator built in our research lab. Increasing the voltage amplitude of the potential to 4.8 kV, we were able to decelerate molecules from 340 m/s to 250 m/s which corresponds to a deceleration strength of 6.6 km /s2. By cooling the valve by flushing cold nitrogen gas through the valve enclosure we were able to decrease the initial velocity of the supersonic beam. With the valve cooled to ~190 K, we demonstrated deceleration of SrF molecules from 290 m/s to 120 m/s which corresponds to the removal of almost 85% of the initial kinetic energy. The thesis described the operation of the decelerator and end with an outlook for precision tests using this novel tool.
Physicists have observed all the elementary particles predicted by the Standard Model. These particles, however, account for only 5% of the total mass-energy of the Universe. In the quest to solve the mystery of missing matter and to extend our knowledge about nature, a large number of experimental research programs are devoted to find a glimpse of New Physics. Table-top high precision measurements on molecules can contribute to these efforts in a manner complementary to high-energy experiments at colliders. In this thesis, we describe the operation of a traveling-wave Stark decelerator. This device is designed and constructed in order to slow down heavy polar molecules for various future explorations. Deceleration is achieved by manipulation of the molecules in controllable electric fields. We demonstrate in an experiment the successful operation of the Stark decelerator by slowing down a beam of SrF molecules to one-third of their initial velocity, thus removing almost 90% of the initial kinetic energy. We also reveal limiting conditions for this process. These results are of importance for many exciting experiments using heavy molecules in the near future. One relevant and promising example of such experiments is the search for a permanent electric dipole moment of the electron.
Abstract A beam of SrF molecules is decelerated from 290 to 120 m s−1. Following supersonic expansion, the molecules in the X2Σ (ν=0, N=1) low‐field‐seeking state are trapped by the moving potential wells of a traveling‐wave Stark decelerator. With a deceleration strength of 9.6 km s−2 the removal of 85 % of the initial kinetic energy in a 4 m‐long modular decelerator is demonstrated. The absolute amount of kinetic energy removed is a factor of 1.5 higher compared to previous Stark‐deceleration experiments. The demonstrated decelerator provides a novel tool for the creation of highly collimated and slow beams of heavy diatomic molecules, which serve as a good starting point for high‐precision tests of fundamental physics.
Permanent electric dipole moments (EDMs) are signatures of time-reversal and parity violation and act as sensitive probes for physics beyond the Standard Model (BSM). The work in this thesis has been done as part of the NL-eEDM collaboration, which plans to measure the electron EDM using a beam of BaF molecules. The sensitivity of such a table-top precision experiment benefits from utilizing an intense, slow beam of molecules. We plan to achieve that through a combination of Stark deceleration and transverse laser cooling of a cryogenic beam of BaF. The work presented in this thesis has primarily been done to produce intense, slow molecular beams for an electron EDM measurement. We constructed a supersonic beam source of SrF and BaF molecules. The source produces high intensity beams with low translational and rotational temperatures. Time-resolved detection of laser-induced fluorescence from pulsed excitation of electronic states in BaF molecules was performed to determine the lifetimes of the A^{2}Pi_{1/2} and A^{2}Pi_{3/2} states. The lifetimes of these states were determined to be 57.1(3) ns and 47.9(7) ns, respectively. The transition between the A^{2}Pi_{1/2} and X^{2}Sigma^{+} states is the main laser cooling transition in BaF. We decelerated a beam of SrF molecules from a cryogenic source to a standstill inside a traveling-wave Stark decelerator, with the number of decelerated molecules on the order of 1000. BaF molecules will be decelerated with the same technique to perform an electron EDM measurement.
We report on the electrostatic trapping of neutral SrF molecules. The molecules are captured from a cryogenic buffer-gas beam source into the moving traps of a 4.5-m-long traveling-wave Stark decelerator. The SrF molecules in X^{2}Σ^{+}(v=0,N=1) state are brought to rest as the velocity of the moving traps is gradually reduced from 190 m/s to zero. The molecules are held for up to 50 ms in multiple electric traps of the decelerator. The trapped packets have a volume (FWHM) of 1 mm^{3} and a velocity spread of 5(1) m/s, which corresponds to a temperature of 60(20) mK. Our result demonstrates a factor 3 increase in the molecular mass that has been Stark decelerated and trapped. Heavy molecules (mass>100 amu) offer a highly increased sensitivity to probe physics beyond the standard model. This work significantly extends the species of neutral molecules of which slow beams can be created for collision studies, precision measurement, and trapping experiments.
Cold molecules have great scientific significance in high-resolution spectroscopy, precision measurement of physical constants, cold collision, and cold chemistry. Supersonic expansion is a conventional and versatile method to produce cold molecules with high kinetic energies. We theoretically show here that fast-moving molecules from supersonic expansion can be effectively decelerated to any desired velocity with a rotating laser beam. The orbiting focus spot of the red-detuned laser serves as a two-dimensional potential well for the molecules. We analyze the dynamics of the molecules inside the decelerating potential well and investigate the dependence of their phase acceptance by the potential well on the tilting angle of the laser beam. ND 3 molecules are used in the test of the scheme and their trajectories under the impact of the decelerating potential well are numerically simulated using the Monte Carlo method. For instance, with a laser beam of 20 kW in power focused into a pot of in waist radius, ND 3 molecules of 250 m/s can be brought to a standstill by the decelerating potential well within a time interval of about 0.73 ms. The total angle covered by the rotating laser beam is about 5.24° with the distance travelled by the potential well being about 9.13 cm. In fact, the molecules can be decelerated to any desired velocity depending on the parameters adopted. This scheme is simple in structure and easy to be realized in experiment. In addition, it is applicable to decelerating both molecules and atoms.
Cryogenic buffer-gas beams are a promising method for producing bright sources of cold molecular radicals for cold-collision and chemical-reaction experiments. In order to use these beams in studies of reactions with controlled collision energies or in trapping experiments, one needs a method of controlling the forward velocity of the beam. A Stark decelerator can be an effective tool for controlling the mean speed of molecules produced by supersonic jets, but efficient deceleration of buffer-gas beams presents new challenges due to longer pulse lengths. Traveling-wave decelerators are uniquely suited to meet these challenges because of their ability to confine molecules in three dimensions during deceleration and their versatility afforded by the analog control of the electrodes. We have created ground-state $\mathrm{CH}(X{\phantom{\rule{0.16em}{0ex}}}^{2}\ensuremath{\Pi})$ radicals in a cryogenic buffer-gas cell with the potential to produce a cold molecular beam of 1011{10}^{11}1011 molecules/pulse. We present a general protocol for Stark deceleration of beams with a large position and velocity spread for use with a traveling-wave decelerator. Our method involves confining molecules transversely with a hexapole for an optimized distance before deceleration. This rotates the phase-space distribution of the molecular packet so that the packet is matched to the time-varying phase-space acceptance of the decelerator. We demonstrate with simulations and an analytic one-dimensional model that this method can decelerate a significant fraction of the molecules in successive wells of a traveling-wave decelerator to produce energy-tuned beams for cold and controlled-molecule experiments.