D. Zhao, K. Doney, J. Guss, A. Walsh, H. Linnartz

Sackler Laboratory for Astrophysics, Leiden Observatory, University of Leiden, the Netherlands

(Presented by Mingli Niu)

High Resolution Infrared Spectroscopy of

Plasma Jet-Cooled Polyacetylenes by cw-CRDS

The 68th International Symposium on Molecular Spectroscopy, June 20, 2013

To date, many hydrocarbon molecules and radicals have been detected in the interstellar medium and circumstellar shells.

Highly unsaturated linear hydrocarbon chains, including polyacetylenes (l-HC2nH), are of great astrophysical interest.

Small polyacetylenes (HC4H, HC6H) have been detected from infrared observations in dense atmosphere of Titan and in a protoplanetary nebula CRL 618.

Gas-phase high-resolution (mid-)infrared data of hydrocarbon chains are limited to several species only:Linear carbon chains Cn (n up to 13)

l-C3Si2, C2H, HC2H+ …

HC4H, HC6H

Additional accurate infrared data of unsaturated hydrocarbon-chains are required;.Particularly for those centro-symmetric linear molecules like polyynes, which are radio-silent due to the lack of a permanent dipole moment.

We present here:

Supersonic Plasma InfraRed Absorption Spectrometer (SPIRAS, located at Leiden Observatory) that is used to record high-resolution infrared spectra of hydrocarbon species of astrophysical interest

A hardware-based trigger and timing scheme to apply sensitive cw-CRDS to a pulsed plasma system.

First high-resolution infrared spectra of jet-cooled HC4H and HC6H recorded through a C2H2/He/Ar plasma

Experimental setup

IR light source: single-frequency single-mode cw-OPO,

linewidth < 1 MHz, output power >1 WDetection method: cw cavity ring-down (cw-CRDS)

Pulsed Plasma(refer to the talk p438/WF08)

Schematic overview

• A hardware-based multi-trigger and timing scheme is developed to apply cw-CRDS to a pulsed plasma.

• This is achieved by the use of a boxcar integrator.

Trigger and timing

• Coinciding a plasma pulse to a ring-down event is realized by proper hardware settings.

• The programming and data processing are based on routine cw-CRDS, with only one additional procedure of chec-king whether a plasma pulse occurs.

Detailed description of this trigger and timing scheme is available from: Zhao, Guss, Walsh, Linnartz, Chem. Phys. Lett., 2013, 565, 132-137.

Detection sensitivity is ~ 2*10-7 cm-1, for a planar plasma with effective absorption path length of ~3 cm.

Noise equivalentabsorption

The experimental spectra contain rich spectroscopic information of hydrocarbon species produced in the plasma.

In the C-H stretch region of polyacetylenes, nearly 3000 lines, including many rovibrational bands of HC4H and HC6H, are recorded under jet-cooled conditions (Trot ~ 15-20 K) for the first time. (Zhao et al. Chem. Phys. Lett., 2013, 565, 132-137.)

Absorption lines of the precursor, C2H2

(from HITRAN)

(Oct. 2012, May 2013)

C-H stretch region of polyacetylenes

Ro-vibrational transitions of ground-state and vibrationally excited HC4H and HC6H are assigned and analyzed. In total:

14 bands are analyzed for HC4H, where 8 bands are recorded for the first time;6 bands are analyzed for HC6H, where only the C-H asymmetric stretch fundamental band has been reported previously. The corresponding constants will be available from Zhao et al. (in prep)

In the recent experiments, the production number density ratio of HC4H and HC6H is estimated as

N(HC4H)/N(HC6H) ~ 1

More than 1000 lines in the C-H stretch region of polyacetylenes in our experimental spectra could not be assigned.

The band origin of the C-H asymmetric stretch band of HC8H is found at 3329.4 cm-1 in previous low-resolution gas-phase studies (Shindo et al. 2001), exactly in the region comprising a lot of unassigned features.

Ro-vibrational transitions of HC8H are expected to be detectable.

No observed lines in the present experimental spectra could be tentatively assigned to rovibrational transitions.

Molecules in the (dense) Interstellar Medium or Circumstellar Shells

TiO TiO2

l-C3H+

CH3O+

NHCHCN

To date, about 180 molecules have been discovered (www.cdms.de).

ACKNOWLEDGEMENTS

Financial supports: NWO NOVA SRON The Dutch Astrochemistry Network.

Sackler Laboratory for Astrophysics at Leiden Observatory

Special thanks to M. Niu.

Please email your questions and comments on this talk to: zhao@strw.leidenuniv.nl (Dongfeng Zhao)