Varberg Lab

Molecules.

Light.

Spectroscopy.

NEWS

VANADIUM HYDRIDE
august 2022

I just published the first detection and analysis of an electronic spectrum of vanadium hydride (VH). This is the last of the 3d transition metal monohydrides for which there are no reported spectra. Several metal hydrides have been found in the photospheres of cooler stars within our Galaxy, so we hope an astronomer will be inspired to look for VH now that an electronic band has been recorded and analyzed.

NSF RENEWAL
AUGUST 2021

The National Science Foundation has fully funded my Research in Undergraduate Institutions (RUI) renewal proposal in the CSDM-A program of the Division of Chemistry. This three-year grant, entitled “Electronic Spectroscopy of Vanadium Hydride and Niobium Hydride” will run through July 2024, enabling me to hire another generation of talented Macalester students who want to receive training in experimental physical chemistry and chemical physics. I am very grateful to the NSF for this critical support.

Honoring Anthony merer
July 2020

I presented our recent work on the electronic spectroscopy of tantalum hydride (TaH) at the 25th International Conference on High Resolution Molecular Spectroscopy at Bilbao, Spain. My co-authors are Sam Gleason, Paul Reischmann and Dalir Kellett. This work has now been published in a special issue of the Journal of Molecular Spectroscopy honoring Anthony Merer, who is a long-time collaborator of mine and an inspiration to all spectroscopists.

Professor Varberg in front of research poster

Tom Varberg

DeWitt Wallace Professor of Chemistry

I have been teaching Physical and General Chemistry at Macalester College since 1993. My research is focused on the electronic spectroscopy of metal-containing free radicals in the gas phase, particularly their fine and hyperfine structures. We utilize pulsed and continuous-wave lasers (both dye and Ti:sapphire), often working at very high resolution. We seek to unravel and assign the complicated spectra of these open-shell free radicals, thus deciphering their Hamiltonians and illuminating their electronic structure. Some of these molecules are of astronomical interest, as this image of the Orion Molecular Cloud Complex suggests.

I received my B.A. degree in chemistry in 1985 from Hamline University and my Ph.D. degree in physical chemistry from MIT in 1990. I was a postdoctoral scholar at the National Institute of Standards and Technology from 1990–92 and a NATO Postdoctoral Fellow at the University of Oxford from 1992–93. In this training, I explored the spectroscopy of free radicals using a variety of experimental techniques and wavelengths (microwave, far-infrared and visible).

I have had five sabbatical leaves from Macalester College, taken in (1) Boulder, Colorado, (2) Vancouver, British Columbia, (3) Sydney, Australia, (4) Zürich, Switzerland & Florence, Italy, and (5) Cape Town, South Africa, Bologna, Italy & London, England, as well as two research summers spent at the University of Oxford, England.

I take as my professional motto the sentiment expressed by the 19th-century French chemist and statesman Jean-Baptiste Dumas, who once wrote that “the greatest joy of my life has been to accomplish original scientific work, and next to that, to lecture to a group of intelligent students.“ I feel fortunate to be able to do the same here at Macalester College.

RECENT RESEARCH PROJECTS

We have recently recorded two electronic transitions in the previously unreported gas-phase molecule vanadium hydride. The first of these bands, that of the D5Pi–X5Delta (0,0) band at 654 nm, has now been published in the Journal of Chemical Physics. Both states rapidly spin uncouple from Hund's case (a) to case (b), which made the spectrum an interesting challenge to unravel and assign. We have recorded another transition further to the red which is still being analyzed. Work on VH and VD will continue in the summer of 2023 with new student collaborators.

We have recorded and analyzed (one last!) band of a favorite molecule of ours, TaO. This spectrum is of the D2Pi–X2Delta (0,0) transition near 696 nm, which had been previously detected by other workers but never assigned or analyzed. The band has fearsome hyperfine structure, as seen in the figure, which shows the eight-fold splitting by the I = 7/2 spin of the 181Ta nucleus. At the Doppler-limited resolution of our experiment, there were many overlapping lines, but in the final analysis we were able to assign essentially every feature in the spectrum. My collaborator in this work is Kevin Tovar ('23).

I have developed a laboratory experiment for a Quantum Chemistry course in which students record the Raman spectrum of para-difluorobenzene and then use group theory and computational chemistry to assign all of the observed Stokes lines. An innovative feature is the assignment of several overtone and combination bands in the dense spectrum of this organic molecule.

We have worked hard for a few years now on the challenging electronic spectrum of tantalum hydride (TaH). My collaborators on this project include Siddhant Singh, Zach Fried, Sam Gleason, Paul Reischmann, Dalir Kellett, Stephanie Lee and Casey Christopher. The molecule displays ferocious homogeneous (spin-orbit) and heterogeneous (rotational) perturbations, but we have made good progress. The figure shows all of the electronic states we have identified, including six low-lying states within 0.5 eV of each other, as determined from a combination of laser excitation and dispersed fluorescence experiments.

We were the first to study the electronic spectrum of the molecule gold sulfide, AuS. The gold–sulfur bond is of fundamental interest in the nanoelectronics and molecular self-assembly communities. We have analyzed transitions from the ²Π ground state to three excited doublet states and a low-lying quartet state, all at hyperfine resolution. This work has been carried out by several students, including Austin Parsons, Sam Gleason, Brad Pearlman, Ian Wyse and Kaarin Evens and has led to three publications. We have also collaborated with Tim Steimle's group at ASU on this work.

We have recently developed new laboratory exercises for the undergraduate physical chemistry lab, published as three different articles in the Journal of Chemical Education. These experiments comprise powder x-ray diffractometry, acoustic interferometry with an Apple iPad, and the measurement of gas compressibilities. Collaborators include research students Andrew Bendelsmith, Kacper Skakuj, Brad Pearlman, Ian Wyse, Sam Gleason and Dalir Kellett as well as my colleagues Keith Kuwata and Ken Moffett.

Tantalum oxide has been a favorite molecule of ours. We have published two studies on TaO, with a particular focus on the hyperfine structures of the ground and several excited states. Macalester students Kara Manke, Tyson Vervoort, Casey Christopher, Stephanie Lee, Francis Gwandu, Andrew Matsumoto, Ben Knurr, Tom Mahle and Zachary Morrow have all worked on this interesting molecule.

Our interest in gold chemistry and spectroscopy first began by recording unbelievably strong spectra of gold fluoride in the visible region, made from the reaction of sputtered gold with sulfur hexafluoride. The molecule produced beautiful “textbook” spectra such as the one shown here. By recording Doppler-free spectra we were able to resolve and analyze hyperfine splittings from both the Au and F nuclei. This work was carried out by Ben Knurr, Elissa Butler, Kara Manke and Tyson Vervoort.

Macalester student Andrew Bendelsmith and I analyzed two electronic bands of tantalum sulfide (TaS) at high resolution in order to understand the hyperfine structure of the molecule. As you can see in this sub-Doppler spectrum, the spin of the Ta nucleus is 7/2 producing eight hyperfine components. My colleague Keith Kuwata used density functional theory to obtain values of the magnetic hyperfine parameters that are in excellent agreement with our experimental results.

FUNDING

The Varberg research group is grateful to the following funding agencies for support of the work in our lab.