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Dynamic Rotational Spectroscopy
FTMW-Detected IR Spectroscopy
Broadband FTMW Spectrometer

FTMW-Detected Infrared Spectroscopy

We have implemented a Fourier transform microwave (FTMW) spectrometer as a highly sensitive, quantum-state resolved, time-efficient IR detector. This spectrometer is ideally suited for uncovering fast intramolecular vibrational energy redistribution (IVR) rates in vibrationally excited molecules. In the ground state depletion (GSD) technique, we monitor a single pure rotational transition with our FTMW spectrometer while the IR laser is scanned in the area of interest. A key feature of this spectrometer is that the FTMW signal is proportional to the population difference between the two pure rotational levels. When the laser is resonant with a vibrational transition originating in the lower rotational level, the population difference between the two pure rotational levels decreases, and the FTMW signal goes down. Conversely, when the laser is resonant with a vibrational transition out of the upper rotational level, the population difference increases, and the FTMW signal goes up. Thus, our IR spectrum shows positive and negative peaks resulting from transitions out of the upper and lower rotational level, respectively.
A typical FTMW-detected IR spectrum is shown in Figure 1. Here the 203-102 (JKaF notation) ground state rotational transition of the acetylene-ammonia complex is monitored, while the laser is scanned over the acetylenic C-H stretch of the complex. The P and R rovibrational transitions are labeled. We are able to measure the IR spectra of several complexes using this technique.

Our FTMW-detected IR spectrometer is ideally suited for studying fast IVR dynamics in molecules with highly perturbed spectra. The combination of fast scanning of the IR laser and high sensitivity to weak IR transitions gives us the ability to uncover much dynamic information in the IR spectrum. Figure 2 shows the IR spectrum of cyclopropylacetylene (CPA) in the area of the acetylenic C-H stretch.

 

Figure 3 shows slower IR scans with increased signal averaging over the bands discovered in the faster scan (figure 2). The P and R branches are labeled for one of the IR absorption bands in the bottom panel of figure 3. From the perturbations of the v = 0-1 transition of the acetylenic C-H stretch, we can learn important dynamical information about how long the energy stays in the C-H stretch before being redistributed to other modes in the molecule.

The dynamics uncovered from our frequency domain experiments can be compared to similar information measured in our time domain experiments. We have recently completed comprehensive single-molecule IVR dynamics studies of both cyclopropylacetylene and 2-methyl-1-but-3-yne.

 

 
Another advantage to our FTMW-detected IR spectrometer is its ability to monitor species in low natural abundance. We have recently completed a full study of the IR spectra in the region of the acetylenic C-H stretch of all of the 13C species of CPA. Figure 4 shows these spectra, as measured on the 13C species as they exist in natural abundance. The molecular cartoons next to each 13C spectrum are color-coded, with red denoting the position of the 13C atom.