Thomas Allison, Assistant Professor
Joint appointment between the Department of Chemistry and the Physics Department
B. S. Cornell University, 2003
M.S. University of California at Berkeley, 2006
Ph. D. University of California, Berkeley, 2010
Phone: (631) 632-7767 and 632-8199 | Fax: (631) 632-7960
Broadly speaking, our group develops and utilizes new light sources and techniques to follow the motions of molecular systems in real-time. Developing new technologies and physics ideas go hand in hand with gaining insight into chemical dynamics.
Frequency comb lasers, recognized with the 2005 Nobel prize in physics, have revolutionized atomic clocks and precision measurement. However, their enormous potential for ultrafast time-resolved measurements has been largely unexplored. The exquisite coherence of the pulse train of a frequency comb enables the signals from successive pulses to be coherently added and stored. This leads to orders of magnitude improvement in the attainable signal to noise of time-resolved experiments, enabling new and exciting directions that simultaneously push the boundaries of our ability to probe matter in the time and frequency domains. Now mature in the visible in near-infrared, recent breakthroughs have pushed frequency comb technology into more spectroscopically interesting regions of the electromagnetic spectrum. Mid-IR combs enable the probing and excitation of vibrational dynamics and UV combs enable the probing of electronic dynamics.
Ultrafast Imaging of Molecular Electronic Structure
The interaction of molecules with extreme ultraviolet (XUV) light involves the high frequency dipole moment between bound electronic orbitals and continuum electronic states with Angström scale de Broglie wavelengths. By recording the XUV dipole moment in the molecular frame, one can resolve the structure of the participating electron density. This dipole can be measured via the ionization of molecules with XUV light (e.g. molecular frame photoelectron angular distributions), or generation of XUV light via High Order Harmonic Generation. Our group pursues both approaches using high-repetition rate XUV frequency combs sources based on intra-cavity HHG.
Using ultrashort pulses allows `snapshots' of a molecule in motion to be recorded with femtosecond (or shorter) temporal resolution, and repeating the experiment many times allows a 'movie' to be reconstructed. These time-resolved imaging methods allow new insight into non-trivial molecular dynamics, such as non-adiabatic dynamics where electronic and nuclear motions are strongly coupled and the Born-Oppenheimer approximation breaks down.
Ultrafast Dynamics of Clusters
Many liquids, with the bonds that hold them together only a few times larger than the available thermal energy of kT, are intrinsically dynamic, with bonds constantly being broken and reformed on picosecond to femtosecond time scales. Spectroscopy of small clusters, containing a few molecules weakly bound together, has allowed for these bonds and their interaction to be explored in extraordinary detail. Bridging the gap between isolated molecules and bulk materials, small clusters offer us the opportunity to obtain deep insight into chemical bonding by building up interactions one monomer at a time, and allow for detailed comparisons to theory.
However, time-resolved studies of clusters have been largely limited to methods using photoionization for detection, due to the low densities of clusters available from molecular beams. These methods take the system away from the electronic ground state, making it difficult to study purely vibrational dynamics. New techniques using mid-IR frequency combs allow us to directly observe vibrational dynamics and couplings while still on the most chemically relevant electronic ground state.
X-ray and optical wave mixing. T. E. Glover, D.M. Fritz, M. Cammarata, T. K. Allison, Sinisa Coh, J. M. Feldkamp, H. Lemke, D. Zhu, Y. Feng, R. N. Coffee, M. Fuchs, S. Ghimire, J. Chen, S. Shwartz, D. A. Reis, S. E. Harris, and J. B. Hastings. Nature 498, 603 (2012). PDF, DOI
Ultrafast internal conversion in ethylene II. Mechanisms and pathyways for quenching and hydrogen elimination. T. K. Allison, H. Tao, W. J. Glover, T. W. Wright, A. M. Stooke, C. Khurmi, J. van Tilborg, Y. Liu, R. W. Falcone, T. J. Martínez, and A. Belkacem, J. Chem. Phys 136, 124317 (2012). PDF, DOI
Ultrafast internal conversion in ethylene. I. The excited state lifetime. H. Tao, T. K. Allison, T. W. Wright, A. M. Stooke, C. Khurmi, J. van Tilborg, Y. Liu, R. W. Falcone, A. Belkacem, and T. J. Martinez. J. Chem. Phys. 134, 244306 (2011). PDF, DOI
Femtosecond Spectroscopy with Vacuum Ultraviolet Pulse Pairs. T. K. Allison, T. W. Wright, A. M. Stooke, C. Khurmi, J. van Tilborg, Y. Liu, R. W. Falcone, and A. Belkacem. Opt. Lett. 35, 3664 (2010). PDF, DOI
Controlling x-rays with light. T. E. Glover, M. P. Hertlein, S. H. Southworth, T. K. Allison, J. van Tilborg, E. P. Kanter, B. Krassig, H. R. Varma, B. Rude, R. Santra, A. Belkacem, L. Young. Nature Physics 6, 69 (2010). PDF, DOI
Femtosecond Isomerization Dynamics in the Ethylene Cation Measured in an EUV-pump NIR-probe configuration. J. van Tilborg, T. K. Allison, T. W. Wright, M. P. Hertlein, R. W. Falcone, Y. Liu, H. Merdji, and A. Belkacem. J. Phys. B. 42, 081002 (2009). PDF, DOI
Time resolved measurements of the structure of water at constant density. A. M. Lindenberg, Y. Acremann, D. P. Lowney, P. A. Heimann, T. K. Allison, T. Matthews, and R. W. Falcone. , J. Chem. Phys. 122 204507 (2005). PDF