Session P4: Cooling and Spectroscopy of Molecules

A molecular fountain

The resolution of any spectroscopic or interferometric experiment is ultimately limited by the total time a particle is interrogated. Here we present the first molecular fountain, a development which permits hitherto unattainably long interrogation times with molecules. In our experiments, ammonia molecules are decelerated and cooled using electric fields, launched upwards with a velocity between 1.4 and 1.9~m/s and observed as they fall back under gravity. A combination of quadrupole lenses and bunching elements is used to shape the beam such that it has a large position spread and a small velocity spread (corresponding to a transverse temperature below 10~$\mu$K and a longitudinal temperature below 1~$\mu$K) when the molecules are in free fall, while being strongly focused at the detection region. The molecules are in free fall for up to 266 ms, making it possible, in principle, to perform sub-Hz measurements in molecular systems and paving the way for stringent tests of fundamental physics theories.   

Laser cooling diatomic molecules to below the Doppler limit

Ultracold molecules are useful for testing fundamental physics, studying strongly-interacting quantum systems, and exploring collisions and chemistry in the ultracold regime. We produce ultracold CaF by the following steps. First, we produce a beam of CaF molecules, with an average velocity of 140 m/s, by laser ablation of Ca into a flow of cryogenic helium gas mixed with SF$_6$. This beam is slowed via a chirped, counter-propagating laser beam to below the capture velocity of a magneto-optical trap (MOT). The molecules are then trapped and Doppler cooled in the MOT where they reach an equilibrium temperature of 12mK. We cool the molecules further to about 960$\mu$K by decreasing the intensity of the MOT beams. Finally, we load the molecules into a three-dimensional blue-detuned molasses where they cool to 50$\mu$K, well below the Doppler limit.   

One dimensional magneto-optical compression of a cold CaF molecular beam

We demonstrate one dimensional, transverse magneto-optical compression of a cold beam of calcium monofluoride (CaF). By continually alternating the magnetic field direction and laser polarizations of the magneto-optical force (RF-MOT), a photon scattering rate of 2$\pi $ x 0.4 MHz is achieved. A 3D model for this RF-MOT, validated by agreement with data, predicts a 3D RF-MOT capture velocity for CaF of 5 m/s.   

Doppler and Sisyphus Laser Cooling of the Polyatomic Molecule SrOH

Ultracold polyatomic molecules hold promise for many applications in physics and chemistry due to their complex internal structures and strong interactions. While the triatomic free radical SrOH has a linear geometry in the vibronic ground state, it serves as a useful test candidate for the feasibility of laser cooling complex, nonlinear isoelectronic species like strontium monoalkoxides, where hydrogen is replaced by a more complex group (e.g. CH$_3$). We perform Doppler and Sisyphus laser cooling of SrOH in a cryogenic buffer-gas beam. The transverse temperature of the molecular beam is reduced in one dimension from 50 mK to 700 $\mu$K, leading to an order of magnitude increase in phase-space density [1]. Our results open a path towards creating a variety of ultracold polyatomic molecules [2]; we will outline our approach to laser cooling of a symmetric-top radical, SrOCH$_3$. [1] I. Kozyryev, L. Baum, K. Matsuda, B. L. Augenbraun, L. Anderegg, A. Sedlack, and J. M. Doyle, arXiv:1609.02254 (2016). [2] I. Kozyryev, L. Baum, K. Matsuda, and J. M. Doyle, ChemPhysChem 17, 3641 (2016).   

Characterization of a cryogenic buffer-gas beam using matrix isolation infrared spectroscopy

Cryogenic buffer-gas beams have many advantages over traditional supersonic jet sources including the ability to produce intense beams of exotic molecular clusters and radicals. We report on the characterization of a cryogenic buffer-gas beam used as a source of cold molecules in a matrix isolation Fourier transform infrared spectroscopy experiment. Using laser ablation of a solid target inside a buffer-gas cell, carbon clusters are produced, cooled, and entrained in a cryogenic beam of neon gas. This beam is directed towards an IR transparent window where it freezes, effectively trapping the molecules in a solid, inert matrix from which vibrational modes in the range of 800 cm$^{\mathrm{-1}}$ to 4000 cm$^{\mathrm{-1}}$ can be investigated. In addition to the characterization of our apparatus with carbon clusters, we report on efforts to investigate transition metal oxide molecules (i.e. VO) as well as cold chemical reactions involving CH.   

Continuous all-optical deceleration of molecular beams and demonstration with Rb atoms

Ultracold samples of molecules are desirable for a variety of applications, such as many-body physics, precision measurement and quantum information science. However, the pursuit of ultracold molecules has achieved limited success: spontaneous emission into many different dark states makes it hard to optically decelerate molecules to trappable speed. We propose to address this problem with a general optical deceleration technique that exploits a pump-dump pulse pair from a mode-locked laser. A molecular beam is first excited by a counter-propagating ``pump'' pulse. The molecular beam is then driven back to the initial ground state by a co-propagating ``dump'' pulse via stimulated emission. The delay between the pump and dump pulse is set to be shorter than the excited state lifetimes in order to limit decays to dark states. We report progress benchmarking this stimulated force by accelerating a cold sample of neutral Rb atoms.   

Quantum-state controlled radical-ion reactions

Radicals and ions frequently play an important role in gaseous media such as the Interstellar Medium (ISM), the upper atmosphere, flames, plasmas, etc. Although collisions in the ISM between ions and radicals are very rare events, the long timescales involved mean such reactions make important contributions to the pathways for assembly and destruction of complex chemical species. Unfortunately, experimental measurements of the rates and particularly the dynamics of reactions \textit{between} ions and radicals are very few and far between. Our system overcomes some of the experimental challenges by using trapped molecular ions and Stark decelerated neutral radicals. Here, we can study reactions between molecules in single quantum states down to millikelvin temperatures. Our very high sensitivity allows us to study reactions where the reaction rate can be as low as one reaction per minute.   

Rovibronic spectroscopy of sympathetically cooled $^{40}$CaH$^{+}$

CaH$^{+}$ is an astrophysically relevant molecule with proposed applications in fundamental physics. We use CaH$^{+}$ co-trapped with Doppler cooled Ca$^{+}$ to perform spectroscopy using two photon photodissociation with a frequency doubled mode locked Ti:Sapph laser. This method was used to measure the vibronic spectrum of the 1$^{1}\Sigma, v = 0 \rightarrow 2^{1}\Sigma, v' = 0, 1, 2, 3$ transition \footnote{R. Rugango, \textit{et al.} \textbf{Chem. Phys. Chem.} 17, 3764–-3768 (2016)}. Measurements of the same transition with the deuterated isotopologue confirmed the assignment and showed an 687 cm$^{-1}$ mismatch with theory \footnote{J. Condoluci, \textit{et al.} article in preparation}. The broad bandwidth of the pulsed Ti:sapph provided an advantage for the initial search for transitions, but did not allow spectral resolution of rotational transitions. Here, we use femtosecond pulse shaping to spectrally narrow the linewidth of the femtosecond laser. This allowed us to obtain rotational constants for the $2^{1}\Sigma, v' = 0, 1, 2, 3$ and $1^{1}\Sigma, v = 0$ states \footnote{A. Calvin, \textit{et al.} article in preparation}.   

Achieving Single-Molecule Spectroscopy with Fast State Regeneration

Single atomic or molecular ions provide well-isolated systems suitable for high-precision spectroscopy, but require a large number of measurements in order to probe fundamental physics. However, this drawback can be mitigated by implementing high repetition rates using fast optical state preparation techniques. These techniques are available for atomic ions, but remain a challenge to implement for molecules. Following our previous demonstration of optical rovibrational cooling, we report our progress toward demonstrating fast rovibrational spectroscopy of a single $AlH^+$ ion. Adapting the recipe from quantum logic spectroscopy, we co-trap a single $AlH^+$ ion alongside a $Ba^+$ ion. $Ba^+$ serves as a coolant ion for ground motional state preparation and a means to detect the internal state of $AlH^+$. The internal state of $AlH^+$ can be transferred to $Ba^+$ through a series of momentum kicks induced by multiple absorption events, a process made possible by the highly diagonal Franck-Condon factors in $AlH^+$. After state readout, $AlH^+$ can then be returned to its ground rovibrational state via optical pumping for the next measurement. Since we are relying on fast optical manipulations, we aim to reach a repetition rate of at least several Hertz.   

Preparation and coherent manipulation of pure quantum states of a single molecular ion

We demonstrate control of individual molecules based on quantum-logic spectroscopy [1, 2]. In our experiment, we drive the motional sidebands of Raman transitions in a molecular ion and probe the secular motion with a co-trapped atomic ion. Detection of motional excitation projects the molecule into a pure internal state. The state of the molecule can subsequently be coherently manipulated, as demonstrated by Rabi oscillations between magnetic sublevels of rotational states. We need only one far off-resonant continuous-wave laser to manipulate the molecule. This makes our approach applicable to coherent control and precision measurement of a vast class of molecular ions.\\ \\ \mbox{[1]} C. W. Chou, submitted for publication, arXiv:1612.03926\\ \mbox{[2]} D. Leibfried, New J. Phys. \textbf{14}, 023029 (2012)