We have developed a sensitive spectrometer to study astrophysically relevant species in the expansion of supersonic jets. It includes a heated pulsed valve that can be equipped with a discharge device. The sample, which is solid at room temperature in the case of PAHs, can be heated to temperatures higher than 400 °C. The vapor released by the sample is then carried by a flow of argon through a round nozzle and the electrodes of the discharge device. PAH cations are created by soft Penning ionization in the discharge and are cooled down when the gas mixture expands in a supersonic jet [1].

The laboratory technique chosen for recording absorption spectra of molecules in the gas phase is cavity ring-down laser absorption spectroscopy (CRLAS or CRDS) [2]. It is a technique several orders of magnitude more sensitive than the classic absorption spectroscopy. Instead of measuring the intensity attenuation of a light beam which has traveled through an absorbing medium, CRDS measures the decay of the intensity during that travel. Moreover, in the best conditions, CRDS yields a direct measurement of the absorption cross section.

 

CRDS setup

Fig. 1. Setup of the CRDS device.

 

The absorption spectra of several neutral polycyclic aromatic hydrocarbons (PAHs) were studied with a heated valve [3-6]. We also studied the absorption spectra of the cations of naphthalene and anthracene by using a discharge just in front of the heated source [7].

 

studied PAHs

Fig. 2. PAHs studied so far with the heated source. The three PAHs marked by
red frames were used for testing the laser vaporization source.

 

A laser vaporization source was built to study molecules which are difficult to vaporize just by thermal heating (like big PAHs) or which decompose at moderate temperatures (like some biomolecules). The sample in form of a pellet is placed outside of the valve close to the nozzle. When the valve opens the light from a pulsed Nd:YAG laser is directed onto the surface of the pellet. This source was tested by measuring the known spectra of the three PAHs fluorene, anthracene, and phenanthrene. The used laser vaporization source leads to higher internal temperatures. By applying this source to the biomolecule tryptophan, we could show that the decomposition of tryptophan into tryptamine can be avoided [8]. With the heated source, one cannot avoid the decomposition, but the signal to noise ratio is somewhat better compared to the laser vaporization source. To obtain a better signal to noise ratio and lower internal temperatures, the vaporization should take place inside the nozzle (before the expansion starts). We also built such a laser vaporization source and tested it by studying the carbon cluster C3 which is produced during the vaporization of graphite. Further studies regarding biomolecules and bigger PAHs are in progress.

 

antracene - comparision laser vaporization and heating

Fig. 3. Comparison of the absorption spectra of anthracene using the laser vaporization
source (blue and violet curves) and the heated source (red and gray curves).

 

References:

[1] L. Biennier, F. Salama, L. J. Allamandola, and J. J. Scherer: Pulsed discharge nozzle cavity ringdown spectroscopy of cold polycyclic aromatic hydrocarbon ions, J. Chem. Phys. 118, 7863-7872 (2003).
[2] A. O'Keefe and D. A. G. Deacon: Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources, Rev. Sci. Instrum. 59, 2544-2551 (1988).
[3] A. Staicu, O. Sukhorukov, G. Rouillé, T. Henning, and F. Huisken: Cavity ring-down laser absorption spectroscopy of jet-cooled anthracene, Mol. Phys. 102, 1777-1783 (2004). [DOI]
[4] G. Rouillé, S. Krasnokutski, F. Huisken, T. Henning, O. Sukhorukov, and A. Staicu: UV spectroscopy of pyrene in a supersonic jet and in liquid helium droplets, J. Chem. Phys. 120, 6028-6034 (2004). [DOI]
[5] G. Rouillé, M. Arold, A. Staicu, S. Krasnokutski, F. Huisken, Th. Henning, X. Tan, and F. Salama: The S1(1A1) ← S0(1A1) transition of benzo[g,h,i]perylene in supersonic jets and rare gas matrices, J. Chem. Phys. 126, 174311/1-174311/11 (2007). [DOI]
[6] A. Staicu, G. Rouillé, Th. Henning, F. Huisken, D. Pouladsaz, and R. Scholz: S1 ← S0 transition of 2,3-benzofluorene at low temperature in the gas phase, J. Chem. Phys. 129, 074302/1-074302/10 (2008). [DOI]
[7] O. Sukhorukov, A. Staicu, E. Diegel, G. Rouillé, T. Henning, and F. Huisken: D2 ← D0 transition of the anthracene cation observed by cavity ring-down absorption spectroscopy in a supersonic jet, Chem. Phys. Lett. 386, 259-264 (2004). [DOI]
[8] G. Rouillé, M. Arold, A. Staicu, Th. Henning, and F. Huisken: Cavity ring-down laser absorption spectroscopy of jet-cooled L-tryptophan, J. Phys. Chem. A 113, 8187-8194 (2009). [DOI]

 

For more information contact Dr. Cornelia Jäger.

Related funding(s): DFG HU 474/18.

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