Lewandowski Group - Past Experiments
Molecular Spectroscopy
Spectroscopy is the study of how matter interacts with electromagnetic radiation. Molecular spectroscopy generally focuses specifically on the interactions with isolated molecules. One of the best ways to isolate the molecules of interest is by studying them in the gas phase. Generally speaking, gas phase measurements are the gold standard when it comes to high-resolution spectroscopy. Gas phase spectra do not suffer from the frequency shifts that are universally observed for matrix-isolation spectra. Gas phase measurements also allow for the study of species that would not exist long enough to probe in the liquid or solid states. When performing gas phase spectroscopy, it is often desirable to cool the species of interest as cold as possible, which has multiple benefits. At colder temperatures, there is reduced spectral broadening, such as Doppler broadening, resulting in narrower, better resolved bands in the measured spectra. Additionally, cooling also results in fewer initial quantum states of the molecules being populated, leading to fewer total bands observed in the measured spectra. This all results in a simplification of the analysis of the resulting spectra. Using a Cryogenic Buffer-Gas Beam (CBGB) setup enables all of the benefits of measuring cold, gas phase spectra. There are two main portions of this experimental setup – generating the buffer-gas cooled molecules and probing the mid-IR spectra of these cold molecules. Click on the topics below to learn more about each portion of the experiment.
Cryogenic Buffer-gas Cooled Beams
Schematic of the inside of the CBGB cell, with the buffer gas flowing in from the left, the laser ablation (forming the clusters) at the bottom, and the cell exit and probe QCL laser on the right
A Cryogenic Buffer-Gas Beam (CBGB) consists of a cold, inert gas (such as neon or helium) flowing in a specific direction (often through a CBGB cell), with some species of interest entrained in the flowing gas. Many species of interest (such as reactive molecules, clusters, etc.) do not remain gaseous at the temperatures desirable for further scientific study (~4 – 30 K). However, if these species are entrained in a flowing, inert gas that can be held at these temperatures, the species of interest can be cooled to these low temperatures and still remain in the gas phase for further study. In our lab, we are working on producing carbon clusters and metal oxide clusters, produced via laser ablation of a solid target and then cooled to ~25 K in a neon buffer gas beam. These types of clusters have relevance to many different fields of research, including astronomical and interstellar physics and chemistry, combustion chemistry and soot formation, and catalysis, just to name a few.
Mid-IR Molecular Spectroscopy
Schematic of the laser optics setup, with the ablation laser in green and the IR laser in red. After origination in the QCL, a small portion of the IR beam is sent towards the CBGB system for the spectroscopic measurements, and the rest of the IR beam is sent to diagnostics to analyze the laser frequency
Generally speaking, mid-infrared (mid-IR) light consists of electromagnetic radiation with wavelengths ranging from ~2.5 – 20 µm (~500 – 4,000 cm-1), and it induces molecular vibrational motion. Therefore, once we have produced the Cryogenic Buffer-Gas Beam, we use an infrared Quantum Cascade Laser (QCL) to probe the vibrational spectra of these cold clusters. Molecular vibrational frequencies are governed by the strengths of the bonds between the atoms. By studying vibrational spectroscopy, we can learn about the structure and bonding of these types of clusters, leading to a better understanding of how they form and how they can be used for future technology (such as designing new catalysts). Upon collection of a vibrational spectrum, the experimental results are then compared against theoretical predictions, using quantum mechanical calculations. Ideally, this comparison leads to greater insights about the nature of the structure and bonding of these clusters, as well as improving the theory for future calculations.
Stark Deceleration
Our research uses supersonic expansion coupled with Stark deceleration to cool and slow polar molecules. We start by expanding a mixture of Krypton and the molecule of interest through a small aperture into our vacuum system. The resulting pulse of ground-state molecules has a narrow velocity distribution in three dimensions and a mean longitudinal velocity of several hundred meters per second. The next step is to slow the molecules into the rest frame of the laboratory. After the expansion, the molecular pulse propagates through a skimmer, which allows for differential pumping between vacuum chambers. The molecules then fly into the entrance of a Stark decelerator. The geometry of the electrodes that make up the decelerator, creates a maximum of electric field in the longitudinal direction directly between electrodes. As the molecules propagate into an increasing electric field they lose longitudinal kinetic energy and thus slow down. We use a series of repeating electrodes to slow the molecules to rest for low-temperature collision studies.
We have two decelerators with distinct configurations of electrodes to slow cold molecules to rest. The details of deceleration and goals of each experiment are outlined below.
Pulsed-Pin Deceleration
Pulsed deceleration utilizes pairs of oppositely charged rods to create the longitudinal potential hill for cold polar molecules to climb. Before they can fall down the potential hill and regain their kinetic energy, the pin pairs are quickly discharged. This process is repeated with successive stages of electrodes until the molecules have been sufficiently slowed to be loaded into an electrostatic trap. Additionally, transverse guidance of the molecules is achieved because the molecules are attracted to the minimum of electric field along the center of the decelerator. Successive pin pairs are orientated orthogonally to one another, to guide the molecules equally in both transverse dimensions.
Current research involves the trapping of slowed hydroxyl radicals (OH) for cold collision studies with rubidium (Rb).
In the past, we trapped slowed ammonia molecules (ND3) to study density fluctuations and trap loading dynamics. We used Monte Carlo simulations to optimize the trap loading timing. An example of loading the electric trap can be seen in the following phase-space and coordinate-space simulations:
Traveling-wave Deceleration
In-vacuum image of the end of the traveling wave ring decelerator and time-of-flight mass spectrometer plates.
The traveling wave decelerator utilizes ring electrodes charged to voltages that vary sinusoidially both in space and time. The end effect is 3D potential well that is co-moving with cold polar molecules. These packets are slowed continuously to trappable velocities by the end of the 624 ring electrodes that make up the traveling wave ring decelerator.
Currently, we use the traveling wave decelerator to provide molecular beams with controllable mean velocities. In the near future, we will couple this decelerator to our ion trap to investigate cold collision studies. We will be able to vary the collision energy between the reactants and thus tune over collision channel thresholds.
Cold Molecular Collisions
Cold atom-molecule collisions are studied in a co-trapped environment. Rubidium atoms are laser cooled before being trapped in a quadrupole magnetic trap. A cold beam of neutral ammonia molecules (ND3) or hydroxyl radicals (OH) is produced via supersonic expansion and decelerated using a Stark decelerator before being trapped in an electrostatic trap. To initiate interactions, the coils forming the magnetic trap for atoms is translated across the optical table so that the trapped samples of atoms and molecules overlap. The dynamics of the combined system is monitored by measuring the density distribution of molecules using a resonance-enhanced multi-photon ionization scheme to state-selectively ionize the molecules for subsequent detection using a multi-channel plate detector.
Publications
To find our list of published works please click here.