Transcript Ohio2009TC05

LABORATORY STUDIES OF THE FORMATION OF
INTERSTELLAR DUST FROM MOLECULAR
PRECURSORS
IAU 251 - February 2008
G. Pascoli, A. Polleux, A&A (2000)
Cesar Contreras, International Symposium on Molecular Spectroscopy 2009
Characteristics of PAH Bands in UIBs
UIBs 3.29, 6.2, 7.7, 8.6, 11.3, 12.7 m with some weaker features at 3.1-3.7, 6.0-6.9, and
15-20 m (continuum plateau observed in the near infrared region)
PAH band assignments:
3.29 = Aromatic C-H stretch
3.40 = Aliphatic C-H stretch (anti-symmetric)
3.51 = Aliphatic C-H stretch (symmetric)
6.2 = C-C skeletal deformation
7.7-7.9 = C-C skeletal deformations
8.6 = C-H in-plane bend
11.3 = C-H out-of-plane bend (singlet)
11.9 = C-H out-of-plane bend (doublet)
12.7 = C-H out-of-plane bend (triplet)
GC Sloan et al., ApJ (1993,1994), LJ Allamandola, AGGM Tielens et al. Planet. Space Sci.
(1995)
PAHs mixture of radicals, ions, and neutral molecules as well as hydrogenated and
dehydrogenated species that can include aliphatic type carbon bonding
Properties of molecules under study: isolated, cold, high energy UV photon interactions
Polycyclic Aromatic Hydrocarbons in the
Interstellar Medium
PAH structures
Salama et al. ApJ (1996)
Hubble, Spitzer observations show
that PAHs are ubiquitous in ISM
Carbon nanoparticles
Courtesy from R. Ruiterkamp
http://www.spitzer.caltech.edu/features/articles/20050627.shtml
Pulsed Nozzle Discharge
High energy discharge setup
Electron
Cathode Density
(-500V) (m-3)
Anode
(0 V)
High
pressure
reservoir
Simulation of the plasma
insulator
Gas flow
plasma
Vacuum
chamber
B. Broks et al., Phys. Rev. E, (2005)
Davis et al., J. Chem. Phys. (1997)
PDN-CRDS
Pulsed plasma
planar expansion
Simulation
Chamber
10 cm long
slit
Electrodes
Heater
plates
PAH
reservoir
Vacuum
chamber
Cavity Ring Down Spectroscopy
http://www.chem.ualberta.ca/~xu/research/crds.htm
Schematic of the NASA Ames setup
Pulsed Nozzle Discharge
Gas pulse 1.0 – 1.2 ms
Discharge 0.55 ms (T~1200K @ -500V)
Laser probe pulse at center of gas pulse
Cavity Ring Down Spectroscopy
Detection of ring down signal
(laser pulse decay in cavity)
Average 8-20 decays at specific
wavelength
Obtain an absorption spectrum within
tolerances of laser dye mixture
and high reflectivity mirrors
(12% of nominal wavelength)
PDN-CRDS of Perylene Neutral
Previous results using Cavity ring-down spectroscopy for neutral Perylene,
C20H12, in the 400 - 417 nm range by Xiaofeng Tan and Farid Salama (JCP
2005). The spectrum was recorded at 217 C, 4 mm downstream of the PDN
slit nozzle. TROT is of the order of 50 K.
PDN-CRDS of Naphthalene Cation
Gas phase absorption spectrum of the four stronger vibronic bands of the
D2  D0 electronic transition band system of Np+.
Least squares fit of the PAH spectra in the Ames’ database to the ISO
SWS spectrum of the H II Region IRAS 23133. From Cami et al. (2009)
Monitoring Fragment Formation & Detection of Carbon Particles
Spectrum of Pyrene (C16H10+) seeded plasma versus discharge energy
Observation of soot
on the electrodes
+
CH
-825 V
0.05
Plasma energy
(discharge voltage)
Absorbance
0.10
-800 V
-750 V
-700 V
-650 V
0.00
-550 V
430
435
440
445
450
455
Wavelength (nm)
IAU 251 - February 2008
Optical Diagnostics
Upon ignition of the PAH-seeded plasma, observed:
Linear growth of the global losses of the cavity decay signal as the applied voltage is increased
Detailed time-resolved CRDS measurements of the extinction, at a fixed wavelength, show that
the extra losses can be decomposed into two major components caused by:
1) Plasma-induced desorption of the molecular species adsorbed on the electrodes and the walls
of the chamber followed by recondensation on the mirrors (slow drift of the decay time).
2) Broadband absorption from small carbon nanoparticles/ clusters (sharp increase/decrease).
Mass Spectrometry studies confirm the
formation of larger particles: LMS of
soot formed from C12H10 (154 amu)
precursor.
15
Arbitrary Units
0
.4
10
Background signal
0
.3
0
.2
Signal(mV)
Extinction (ppm/pass)
on resonance (ionized precursor)
off resonance
5
0
Sample
0
.1
0
.0
400
500
600
Voltage (Volt)
700
0
1
0
0
2
0
0
m
/e
3
0
0
4
0
0
M. Hammond et al. , 2006
PDN-CRDS-ReTOF-MS
The setup now combines:
• Pulsed discharge nozzle
• Cavity Ring down spectrometer
• Reflectron Time-of-Flight Mass Spectrometer
ReTOF-MS
PDN-CRDS-ReTOF-MS
Jordan TOF Products, Inc.
Intensity (arbitrary units)
EI-ReTOF-MS of Naphthalene
C10
C2
C4
C3
C5
C6
C8
Mass/Charge (m/z)
Initial Spectrum of the PAH Naphthalene with the
ReTOF-MS using an electron ionization source
Conclusions
Use of PDN-CRDS to study PAH provides
more detailed spectra to use for comparison with ISM
Soot formation observed in plasma discharge
Preliminary analysis indicated that formation of species with mass > 300 amu formed
from a precursor of 154 amu (C12H10)
Dynamic behavior of soot formation can be studied with PDN-CRDS-ReTOF-MS
Future Work
Current PDN limited by temperature threshold used to vaporize PAHs (~300° C)
One method to study larger PAHs would incorporate a
laser desorption source in place of the PDN to vaporize large,
intact PAHs and carbon (sub-nanoparticles)
The larger PAHs can then be subjected to CRDS and
ReTOF-MS analysis
This work is supported by the NASA Astronomy and
Physics Research and Analysis (APRA) Program of
the Science Mission Directorate and by the
NASA/ORAU Post Doctoral Program (NPP)
Nd/YAG laser
skimmer
pulsed
valve
Graphite wheel