Measurements of trap dynamics of cold OH molecules using resonance-enhanced multiphoton ionization

Abstract

Trapping cold, chemically important molecules with electromagnetic fields is a useful technique to study small molecules and their interactions. Traps provide long interaction times that are needed to precisely examine these low density molecular samples. However, the trapping fields lead to non-uniform molecular density distributions in these systems. Therefore, it is important to be able to experimentally characterize the spatial density distribution in the trap. Ionizing molecules in different locations in the trap using resonance enhanced multiphoton ionization (REMPI) and detecting the resulting ions can be used to probe the density distribution even with the low density present in these experiments because of the extremely high efficiency of detection. Until recently, one of the most chemically important molecules, OH, did not have a convenient REMPI scheme identified. Here, we use a newly developed 1 + 1' REMPI scheme to detect trapped cold OH molecules. We use this capability to measure trap dynamics of the central density of the cloud and the density distribution. These types of measurements can be used to optimize loading of molecules into traps, as well as to help characterize the energy distribution, which is critical knowledge for interpreting molecular collision experiments.

Collisions between ultracold atoms and cold molecules in a dual electrostatic-magnetic trap

“…The decelerated molecules are in the weak-field seeking |j, k, m, = |1, 1, 1, u quantum state, where j is the total angular momentum, k and m are respectively the projections of j onto the axis of symmetry and the laboratory field axis, and u denotes that the molecules are in the weak-field seeking upper state of the inversion doublet [38]. The slowed molecules, with a mean forward speed of approximately 26 m/s, exit the decelerator and are loaded into the electrostatic trap by removing the remainder of their forward kinetic energy using the trap electrodes as the final few stages of deceleration [39,40]. Once the molecules are brought to rest, the electrostatic trap is created by applying (typically) {+8,-8,-8,+8} kV to the electrodes, as listed from the closest to farthest position in relation to the Stark decelerator.…”

“…The resulting electric field accelerates the ions onto the MCP, effectively using the electrostatic trap as a timeof-flight mass spectrometer. Because the REMPI laser beam waist in the detection region is significantly smaller than the characteristic trapped molecular cloud width (1 mm), the vertical (x) and horizontal (z) profiles of the molecular cloud can be measured by scanning the laser position [40]. A 2 mm wide and 1 cm tall slit in the last of the four trapping electrodes, as shown in Fig.…”

Characterization of a vacuum ultraviolet light source at 118 nm

“…[21][22][23][24] Additionally, 118 nm photons have been used to perform multi-photon ionization of cold radical molecules. [25][26][27] Since the conversion efficiency to VUV light is low (∼ 10 −5 ) 28 and may constrain the detection sensitivity of an experiment, it is important to understand the limits of the tripling technique in order to experimentally optimize 118 nm light production. The basic theory of 118 nm generation in a xenon-argon gas cell [29][30][31] and experimental setup and challenges 32 have been detailed in a) Electronic mail: lewandoh@colorado.edu previous papers.…”

Spectroscopy of Molecular Ions in Coulomb Crystals

“…The nonadiabatic effects at low temperature have been studied in prior work including molecule reactions with quantum state controlled atomic ions that indicate a submerged barrier . Experiments to demonstrate state-controlled ion–radical reactions are in development. , Combining these reaction studies with spectroscopic studies to measure the quantum states of reaction and products will yield insight into state-selective chemistry.…”

“…68 Experiments to demonstrate state-controlled ion−radical reactions are in development. 21,69 Combining these reaction studies with spectroscopic studies to measure the quantum states of reaction and products will yield insight into state-selective chemistry.…”