Brooks H. Pate
Vibrational Dynamics and the Spectroscopy of Highly Excited Molecules
Our group studies the dynamics of molecules with significant amounts of vibrational energy. The flow of vibrational energy in a molecule, a process known as intramolecular vibrational energy redistribution (IVR), lies at the heart of chemical reactivity. We study the kinetics of energy flow in isolated molecules and molecules in solution. A major emphasis of our work is understanding the spectroscopy of molecules as the IVR process, and possibly reaction, occurs. In particular, we are interested in how coherent excitation of highly excited molecules can be used to influence reaction products.
We have developed a new type of molecular spectroscopy called dynamic rotational spectroscopy to study isomerization reactions of isolated molecules. The basis of rotational (or microwave) spectroscopy is that the geometry determines the measured rotational frequencies. When the molecule has more energy than the barrier to isomerization, reaction can occur and the geometry becomes time dependent. As a result, the rotational frequency is modulated by the reaction rate. When the reaction causes the frequency to switch between the characteristic values of the reactant and product, the spectrum undergoes the phenomenon known as coalescence. Therefore, the isomerization kinetics can be investigated through the changes in the line shape of the rotational spectrum. We have developed new high-resolution, molecular-beam spectroscopy techniques to obtain the rotational spectrum of single quantum states of a highly excited molecule. These measurements combine ultrasensitive infrared laser spectroscopy methods with strong-field microwave excitation. Application of this technique to conformational isomerization reactions has shown that this class of reactions violates the predictions of quantum transition state theory. We are presently developing new techniques to improve our sensitivity for single quantum state measurements and to extend our measurements to higher energy. A more complete description of the projects we are currently pursuing can be found on our research group web pages.
The second area of research in our group investigates the vibrational dynamics of molecules in dilute solution. Using our molecular-beam spectroscopy techniques we can quantitatively measure energy flow rates for the isolated molecule. Our goal for solution phase studies is to understand how solvent molecules modify the dynamics and reactivity of the isolated molecule. This work is performed in the Ultrafast Laser Facility that is part of the university's SELIM program. We have an impressive array of laser tools available for this work including a two-color femtosecond laser system, a two-color picosecond laser system, and a 32-element infrared array detector for multichannel detection. Our first studies have shown that the basic features of isolated molecule IVR dynamics are preserved in solution. In particular, we have found that a new relaxation channel opens for large molecules that correlates with the onset of fast IVR in the isolated system. From this work, we can identify a time window where the molecule in solution retains the energy deposited by the laser (i.e., before interaction with the solvent causes conversion of the internal energy to heat). Knowing this time scale gives us a window of opportunity to study the spectroscopy and kinetics of the vibrationally hot molecule. Examples of our recent work in this area, as well as descriptions of projects planned for the near future, can be found on our research web pages.
Broadband Fourier transform rotational spectroscopy for structure determination: The water heptamer. Perez C, Lobsiger S, Seifert NA, Zaleski DP, Temelso B, Shields GC, Kisiel Z, Pate BH. Chem. Phys Let. 571:1-15 (2013).
High-Resolution Electronic Spectroscopy of the Doorway States to Intramolecular Charge Transfer. Fleisher AJ, Bird RG, Zaleski DP, Pate BH, Pratt DW. J. Phys. Chem B. 117:4231-4240 (2013).
. Pate BH, Seifert NA, Guirgis GA, Deodhar BS, Klaassen JJ, Darkhalil ID, Crow JA, Wyatt JK, Dukes HW, Durig JR. Chemical Physics. 416: 33-42 (2013).
The detection of interstellar ethanimine (CH3CHNH) from observations taken during the GBT PRIMOS survey. Loomis RA, Zaleski DP, Steber AL, Neill JL, Muckle MT, Harris BJ, Hollis JM, Jewell PR, Lattanzi V, Lovas FJ, Martinez O, McCarthy MC, Remijan AJ, Pate BH, Corby JF. Astrophysical Journal Letters. 765:L9 (2013).
Detection of E-cyanomethanimine towards Sagittarius B2(N) in the Green Bank Telescope PRIMOS Survey. Zaleski DP, Seifert NA, Steber AL, Muckle MT, Loomis RA, Corby JF, Martinez O, Crabtree KN, Jewell PR, Hollis JM, Lovas FJ, Vasquez D, Nyiramahirwe J, Sciortino N, Johnson K, McCarthy MC, Remijan AJ, Pate BH. Astrophysical Journal Letters. 765:L10 (2013).