Structural Spectroscopy
When studying molecules of the size and flexibility that we investigate, several conformations are often present in the jet or beam. When obtaining an electronic spectrum, all conformations contribute. We make use of several "double resonance" techniques in order to obtain conformation specific spectra. We then know how many conformations are present in the expansion and also gain clues as to what the conformational preferences are by studying these spectra.
UV-UV Holeburning
In order to obtain conformation specific electronic spectra, we employ a technique known as UV-UV hole-burning (UV-UV HB).
The schematic for UV-UV HB is shown below. A hole-burn laser is fixed on a transition seen in the electronic spectrum. The HB laser is overlapped spatially and the timing adjusted so that it intersects the jet or beam approximately 200 ns before the probe. The probe laser is scanned, and when resonant with a transition that shares the same ground state as the probe laser, a dip in the signal from the probe laser is observed (observed as either a dip in fluorescence or ion signal, depending upon the detection method employed). Typically we run the HB laser at 10 Hz and the probe at 20 Hz and perform the experiment using active baseline subtraction (ABS). Thus, if the HB laser is resonant with a transition that does not share the same ground state as the probe, ABS returns zero. However, if the HB laser is resonant with a transtion that shares the same ground state as the probe ABS returns a negative value.
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Stimulated Emission Pumping
Stimulated Emission Pumping (SEP) is a method of not only of obtaining specific information about low frequency ground state vibrational levels (which we could also get by employing dispersed fluorescence, but at lower resolution), but also a method of preparing the molecule with a known amount of energy. The schematic for SEP is shown on the right. A pump laser is fixed on the origin of a particular conformation. The second laser, referred to as the dump laser, is spatially overlapped and placed temporally 5-20 ns after the probe, depending upon the lifetime of the transition. If the dump laser is resonant with a transtion from the zero-point level of the excited state to a ground state vibrational level, then a dip in either the ion or fluorescence signal is observed. We can perform the experiment using ABS or without with both lasers running at the same repetition rate.
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Infrared Spectroscopy
One of our most powerful techniques for making conformational assignments is infrared spectroscopy. The method for obtaining conformation specific IR spectra is analogous to UV-UV HB, but we replace the first UV laser with an IR laser. The schematic is shown below. We fix the UV laser on transition in our electronic spectrum, spatially overlap the IR with the UV and place the IR temporally approxiamately 200 ns before the UV. The IR laser is scanned, and if it is resonant with a transition that shares the same ground state as the transition on which our UV laser is fixed then we see a depletion in the UV signal. If we perform the experiment using fluorescence as our detection method then we refer to the experiment as Fluorescence Dip Infrared Spectroscopy (FDIRS). In an ion chamber, the experiment is called Resonant Ion Dip Infrared Spectroscopy (RIDIRS). This experiment performed using ABS with the IR laser at 10 Hz and UV at 20 Hz.
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IR-UV Holeburning
Another method we can employ is IR-UV Holeburning (IR-UV HB). It is analogous to UV-UV HB, but we use an IR laser instead of an UV laser to do the holeburning. The schematic is the same as for obtaining infrared spectra, shown above. We fix the IR laser on an infrared transition in the IR spectrum and scan the UV probe. Any UV transitions sharing the same ground state as the IR transition will show a depletion in signal. As with our other double resonance techniques, we use ABS with the IR at 10 Hz and UV at 20 Hz.
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