My research is in the area of molecular spectroscopy. I study the spectra and energy levels of free radicals in the gas phase. Free radicals are highly reactive chemical species that contain one or more unpaired electrons. Such molecules have a nonzero electron spin, which interacts in interesting ways with the electrons' orbital angular momenta and any nuclear spins that are present. When I first arrived at Macalester, I studied the rotational spectra of molecules using radiation in the microwave and far-infrared (THz) regions. Since 2003, with the acquisition of our continuous-wave ring laser, we have turned our attention to the electronic spectra of diatomic free radicals that contain a transition metal atom. These molecules are of interest in other fields as well, such as astronomy, catalysis and computational chemistry. In the sections below I outline some current and past projects we have undertaken in our research group.
Current projects in our laboratory
Spectroscopy of gold sulfide (AuS)
Brad Pearlman ('15), Ian Wyse ('15) and I worked very hard this past summer (2014) on the electronic spectrum of AuS. This work is being carried out in collaboration with Prof. Tim Steimle and Dr. Damian Kokkin of Arizona State University. We have now recorded spectra over most of the visible region using our Nd:YAG-pumped pulsed dye laser. We have made vibrational assignments of many cold and hot bands to several different excited electronic states out of both spin-orbit components of the ground 2Π state. We have also recorded some of these bands at high resolution using our cw ring dye laser. These high resolution data will enable us to analyze the rotational and hyperfine structure of the excited and ground electronic states. Much work remains to be done.
A new laser ablation molecular beam source for making transition metal compounds
Robin Forslund ('13), Ian Wyse ('15) and I completed the construction and testing of our new laser ablation molecular beam chamber in the summer of 2013. In November, we saw our first signal (of the molecule YO) with this new spectrometer—thanks to some timely help from our collaborator Prof. Tim Steimle (ASU). The new chamber is differentially pumped with two 6" diffusion pumps, with the detection side backed by a Roots blower/mechanical pump combination. With our new BigSky Nd:YAG ablation laser, we can collect data at a 20 Hz repetition rate. We will detect transitionst by laser induced fluorescence acquired by photon counting. The rotational temperature of the molecular beam is about 10 K, and under optimum conditions our line widths are a bit less than 100 MHz. We are excited to begin high resolution studies of AuS with this new chamber.
Recently completed projects and publications
The E2Π1/2–X12Δ3/2 (1,0) band of tantalum oxide (TaO)
Andrew Matsumoto ('13), Francis Gwandu ('15) and I recorded the (1,0) band of the E–X system of TaO in the summer of 2012. On a recent visit to collaborator Tim Steimle's lab at Arizona State University, Mac McCreary ('12), Kacper Skakuj ('14) and I had attempted to measure the electric dipole moment of TaO (in collaboration with Tim and Anh Le) using the E–X (0,0) band. We were unsuccesful, but are hopeful that the (1,0) band will prove more fruitful. Francis, Andrew and I have now analyzed the (1,0) band, including its hyperfine structure recorded at sub-Doppler resolution. Therefore, we now have accurate information on the v = 0 and 1 levels of the excited state. We have just published this work in the Journal of Molecular Spectroscopy [Publication #32].
Spectroscopy of tantalum hydride (TaH and TaD)
Kara Manke ('07), Tyson Vervoort ('07) and I discovered this molecule in our laboratory in the summer of 2006 using our low-resolution pulsed dye laser system. In 2009, Stephanie Lee ('10) and Casey Christopher ('10) recorded high-resolution spectra of one vibrational band in TaH and one in TaD, including at sub-Doppler resolution using intermodulated fluorescence spectroscopy. These two students are shown in the photo to the left. In this work, we determined accurate values for the ground state rotational constants (thereby determining the bond length of the molecule for the first time) and hyperfine constants. Both bands are Ω = 2←2. We recently published this work in the journal Molecular Physics [Publication #31].
In the fall of 2011, four seniors—Matt Weyer ('12), Natalee Raymond ('12), Audrey Mills ('12) and Kevin Sullivan ('12)—enrolled in my Research in Molecular Spectroscopy course, in which we recorded another band of TaH at high resolution. The upper state is Ω = 1, and we have determined accurate values for its rotational (B), centrifugal distortion (D) and Ω-doubling (q) constants.We have also recorded some sub-Doppler spectra of this band in order to understand and fit its hyperfine structure. This work is still in progress. The picture shows these four students presenting their work at the Spring 2012 ACS National Meeting in San Diego, CA.
Spectroscopy of the E2Π1/2–X12Δ3/2 system of tantalum oxide (TaO)
We have recorded high-resolution spectra of the E2Π1/2–X12Δ3/2 (0,0) band of TaO. This work was first begun by Ben Knurr ('09), Tom Mahle ('09), and Zak Morrow ('08) in the fall of 2008 in my Research in Molecular Spectroscopy course. Tom and Zak are shown to the left. Casey Christopher ('10) and Stephanie Lee ('10) later recorded and analyzed the band at sub-Doppler resolution. The upper state was originally observed by Richard Barrow at the University of Oxford, who was unable to complete a rotational analysis but assigned the state as E2Φ5/2. Our work shows that the upper state is in fact a 2Π1/2 state with very large lambda-doubling. We have now also completed a full analysis of the (1,0) band and published this work in the Journal of Molecular Spectroscopy [Publication #32].
Electric dipole moment of gold fluoride (AuF)
We have collaborated with Tim Steimle and Ruohan Zhang in measuring the electric dipole moment of AuF in Prof. Steimle's research laboratory at Arizona State University. Mac McCreary ('12), Kacper Skakuj ('14) and I visited ASU in January, 2012 to begin these experiments. Mac and Kacper are shown in the center of the next photograph. We recently published this work in the Journal of Physical Chemistry A [Publication #30].
Hyperfine structure in the electronic spectrum of tantalum sulfide (TaS)
Andrew Bendelsmith ('13), who is shown on the left in the photograph, worked with me in investigating the hyperfine structure of the molecule tantalum sulfide. This hyperfine structure arises from the 181Ta nucleus. We made these measurements in both spin-orbit components of the ground 2Δ state, via the [16.8]2.5–X12Δ3/2 and [19.6]2.5–X22Δ5/2 (0,0) bands. My colleague Prof. Keith Kuwata also performed density functional theory (DFT) calculations on the ground state in order to determine values for the magnetic hyperfine parameters. These calculated values agreed within the uncertainty limits of our experimental values. This work was recently published in the Journal of Molecular Spectroscopy [Publication #29].
The electronic spectrum of gold fluoride (AuF)
Elissa Butler ('10), Ben Knurr ('09) and I recorded several vibrational bands of the [17.8]0+–X1Σ+ and [17.7]1–X1Σ+ system of AuF in the yellow region and the [14.0]1–X1Σ+ (0,0) in the red. In doing so, we determined accurate values for the rotational and fine-structure (lambda-doubling) constants. We also recorded the hyperfine structure (197Au and 19F) of the [17.7] state using intermodulated fluorescence spectroscopy. We showed that the hyperfine structure of the upper state is consisent with the state having significant 3Δ1 character mixed with the 3Π1 state that is calculated to lie in the same energy region. We have published two papers describing this work [Publication #26], [Publication #27]. Ben and Elissa spent seven weeks with me on sabbatical in Australia, where this photo was taken. We completed our work on AuF at the University of Sydney, in the laboratory of Prof. Scott Kable and Prof. Tim Schmidt.
The B–X and C–X systems of tantalum oxide (TaO)
Kara Manke ('07), Tyson Vervoort ('07) and I carried out an extensive investigation of the B2Φ5/2–X12Δ3/2 (0,0) and C2Δ3/2–X12Δ3/2 (0,0) bands of TaO. This molecule was first studied by Richard Barrow and coworkers at the University of Oxford back in the days of grating spectroscopy. Our contribution has been to study the prominent 181Ta (I = 7/2) hyperfine structure of the molecule. We showed that the ground state hyperfine structure is consistent with a δσ2 configuration, where the δ electron is in a pure Ta 5dδ orbital. Prof. Keith Kuwata carried out complementary DFT calculations of the hyperfine structure that agree very well with experiment. This work has now been published in the Journal of Chemical Physics [Publication #24].
Hyperfine structure of rhenium oxide (ReO)
Melanie Roberts ('05), Guillermo Alfonzo ('05), Kara Manke ('07) and I recorded and analyzed the hyperfine structure of ReO. Walter Balfour and coworkers at the University of Victoria had previously studied this molecule using grating spectroscopy. Our contribution has been to record and measure the hyperfine structure in this molecule, arising from the Re nucleus. This work was published in a special issue of the journal Molecular Physics dedicated to Prof. John M. Brown [Publication #23].