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University of Helsinki Faculty of Science
 

Molecular spectroscopy and theoretical chemistry

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Contact Information

Laboratory of Physical Chemistry
Department of Chemistry
A.I. Virtasen aukio 1
(P.O. BOX 55)
FI-00014 University of Helsinki
Finland

Group leader
Prof. Lauri Halonen, D.Sc.
Phone: +358(0)2941 50280
email: lauri.halonen helsinki.fi
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Experimental work in laser spectroscopy

Dispersed vibration-rotation fluorescence spectroscopy

The dispersed laser-induced vibration-rotation fluorescence in the ground electronic state is studied with a new method [1]. For this purpose, an intracavity cell equipped with light collecting optics has been built. Vibration-rotation fluorescence in the infrared region is analyzed with our high-resolution infrared interferometer. Rotationally resolved fluorescence spectra induced by molecular collisions have been observed for molecules such as acetylene and water. By this method, vibrational states of acetylene not directly reachable by infrared spectroscopy have been accessed for the first time. The sensitivity of this experiment can be further improved by using the laser locking with the external high-finesse cavity.

Cavity ring-down spectroscopy

We are currently pursuing two different lines of work using the technique of cavity ring-down spectroscopy. For the first line, we have built a "standard" cavity ring-down spectrometer using a cw ring laser and an external high finesse cavity. The cavity length is modulated with a piezoceramic element and every time the interferometer fringe sweeps over the laser frequency an optical switch "turns off" the laser to begin an exponential decay of the amount of light trapped into the cavity. Sample absorption can then be calculated from the time constant of the exponential ring-down function. We have applied this technique to the overtone spectroscopy of acetylene. Currently we are using such a setup for breath analysis

Our second line of work has resulted in the construction of a high-repetition rate cavity ring-down setup in order to increase the maximum sensitivity normally attainable with this technique [2]. The setup is based on the use of an active frequency stabilization system that using the well-known Pound-Drever-Hall scheme permanently locks the laser frequency to the maximum of a transmission fringe of the external interferometer. In this way, light is continously injected into the interferometer. Exponential decays can then be generated by "switching off" the laser with an optical switch. The main advantage of this setup is that in principle the rate of generation of exponential decays is limited only by the characteristics of the cavity mirrors, which determine the time required for the ring-down and ring-up cycles, so that rates of tens of kilohertz are easily achievable. The spectrometer has also a usable automatic scanning range to enable convenient calibration of the spectra. We have already applied the setup to the recording of vibrational overtones of the doubly C-13 subsituted isotopologue of acetylene.

[1] M. Metsälä, S. Yang, O. Vaittinen, and L. Halonen, J. Chem. Phys. 117, 8686-8693, (2002). [2] R. Z. Martinez, M. Metsälä, O. Vaittinen, T. Lantta and L. Halonen, J. Opt. Soc. Am. B, 23, 727-740 (2006).