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<div><font face="Arial" color="#000000">Seminar on<br>
<b>Modern Optics and Spectroscopy<br>
<br>
<br>
Martin Gruebele</b>,<br>
University of Illinois Urbana-Champaign<br>
<br>
<i><b>STM-assisted single molecule absorption spectroscopy<br>
<br>
</b></i>APRIL 22, 2008</font><br>
<font face="Arial" color="#000000"></font></div>
<div><font face="Arial" color="#000000">12:00 - 1:00 p.m.</font></div>
<div><font face="Arial" color="#000000">Grier Room 34-401</font></div>
<div><font face="Arial" color="#000000"><br>
<br>
</font></div>
<div><font face="Times New Roman" color="#000000">Scanning tunneling
microscopy provides exquisite spatial resolution, but only moderate
energy resolution via current-voltage derivative curves.
Absorption spectroscopy can resolve individual molecular energy
levels, but usually provides no spatial information. We combined
the two techniques by using an STM tip to detect the electron density
change when a molecular absorption line is resonantly excited by a
laser. There are plenty of technical difficulties, beginning
with laser heating effects, and I show how they can be overcome by
combining laser frequency modulation, rear illumination, tip field
enhancement, and lock-in detection of the STM current signal.
The results is a technique that simultaneously returns topographic
images and absorption spectra with sub-molecular resolution. The
ability to tell where a macromolecule actually absorbs can be usefully
applied, for example to detecting defects in the molecule invisible in
ordinary STM scans. Future applications to energy transfer from
one quantum dot to another will also be discussed, along with
ultrathin gold surfaces needed to satisfy the requirements of both STM
and absorption spectroscopy.</font></div>
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