One of the “big picture” goals of the research in the Pate lab is to gain understanding of molecular dynamics across a wide range of molecular environments.
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The experiments performed in the frequency domain give insight into the dynamics occurring when a molecule is ultracold and isolated. But what happens when a molecule is at room temperature? And what, if anything, changes about the dynamics when interactions with other molecules such as solvent can play a part? These are questions that we address using ultrafast time-domain techniques. Our goal in this work is to understand how solvent modifies the intramolecular dynamics of the isolated molecule. Our ability to study the dynamics of isolated molecules in gas phase and then these same molecules in a solvated environment provides a powerful way to assess the role that solvent plays in vibrational energy relaxation. |
The Role of Intramolecular Vibrational Energy (IVR) in Vibrational Energy Relaxation (VER) in Solution |
| Here’s a simple diagram illustrating what can be called a CASCADE effect for energy flow dominated by solvent effects. After preparation of the first excited vibrational level of (in this case) a C-H stretch, a certain amount of energy can be transferred to to the solvent and the rest would couple into the lower energy vibrational modes of the solute. In this case, the internal energy of the solute molecule is decreased. This picture is not incorrect by any means, but it is incomplete in describing energy flow in solution because what it fails to take into account is the fact that the solute molecules, when exceeding the threshold state density for IVR, can redistribute vibrational energy without coupling to solvent molecules. |
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This redistribution of vibrational energy within a solute molecule can be described by this standard model. Excitation into the bright state of a molecule having sufficient vibrational state density to overcome the "IVR threshold" (~10 states/cm-1) is followed by a coupling to dark, or bath, vibrational states according to an interaction term. The combination of the bright state and dark states leads to the molecular eigenstates, the bright state character of which is seen in an absorption spectrum. This is the only available process for energy relaxation in isolated molecules and the rate of this process is molecule specific. The key feature of this process is that the total vibrational energy of the molecule is unchanged. We are able to measure the rate of this relaxation mechanism via room temperature gas-phase transient absorption spectroscopy in a pump-probe scheme. |
In terms of chemical reactivity, it is important to be able to distinguish these mechanisms to understand whether the molecule has enough energy for reaction. Because so much of chemistry happens in solution, it is important to know what effects, if any, the solvent has on vibrational energy flow. Incorporating the knowledge of isolated molecule dynamics into the picture of energy flow in solution is key to gaining comprehensive insight into solvent effects on vibrational energy relaxation. When molecules are no longer isolated, but surrounded by solvent, both the intramolecular process (IVR) and the intermolecular process (VER) contribute to the dynamics, as indicated in the diagram below. |
Following excitation into the v=1 state, energy flow is determined by two processes. IVR: Energy flows from the v=1 state to other vibrational states within the molecule according to the standard model described above. VER: Energy is coupled into the solvent molecules due to interactions with the solute. The vibrational energy remaining in the solute is coupled to lower energy vibrational states. |
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kIVR : IVR Rate (Intramolecular) kVER : VER Rate (Intermolecular) Extensive studies on a series of terminal acetylene molecules in several solvents have shown that the total relaxation rate measured in solution is simply the sum of the intramolecular relaxation rate measured in gas and the intermolecular relaxation rate due to interactions with the solvent. This relationship is described by the following expression: kTOT = kIVR + kVER |
Our group uses the CAMOS Ultrafast Laser Facility at the University of Virginia. This state-of-the-art facility provides us with an excellent set of spectroscopy tools for unraveling the complicated dynamics of large molecules both in gas phase and in solution. The systems available for this work include a picosecond laser system (tpulse ~ 1.4 ps at 3330 cm-1), which uses a high-power Ti:sapphire regenerative amplifier system to simultaneously pump two tunable optical parametric amplifiers (OPAs). A similar system is available with femtosecond pulse duration (tpulse ~ 100 fs at 3330 cm-1). The picosecond and femtosecond regenerative amplifiers are synched in time to provide four time-synched, independently tunable infrared laser pulses with broad tunability (1 – 10 micron). |
Pump-Probe Experimental Setup A basic experimental pump-probe setup is shown below. For simplicity, the two-color setup is shown and the laser systems pumping the TOPAS OPAs are not included. The pump and the probe light travel along separate beam paths to the sample, which can be either gas phase or solution phase. The chopper in the pump beam path blocks every other laser shot to provide a "pump-on versus pump-off" transmitted intensity signal at the InSb detector. There is a half wave plate in the beam path to adjust the polarization of the light to magic angle relative to the probe light, which eliminates any rotational dephasing contributions to the transient absorption signal. The variable delay stage in the probe beam path gives a scan in delay time of arrival at the sample between the pump and probe pulses. The resulting data is a transient absorption spectrum that follows the population of the probed state after excitation by the pump. To ensure that the lasers are emitting the correct frequencies for the experiment, the beams are each checked with the monochromator before data collection.
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We are currently working on several projects, including studying the dynamics of a series of terminal acetylenes at both the fundamental of the acetylenic C-H stretch as well as at higher energies such as the first overtone of the acetylenic C-H stretch. For more information on these and other current time domain experiments, please follow the project links on this page. |